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Using the Android SQLite Database. This tutorial describes how to use the SQLite database in Android applications. It also demonstrates how to use existing ContentProvider and how to define new ones. It also demonstrates the usage of the Loader framework which allows to load data asynchronously.

1. SQLite and Android

1.1. What is SQLite?

SQLite is an Open Source database. SQLite supports standard relational database features like SQL syntax, transactions and prepared statements. The database requires limited memory at runtime (approx. 250 KByte).

SQLite supports the following data types:

  • TEXT(similar to String in Java)

  • INTEGER(similar to long in Java)

  • REAL (similar to double in Java). All other types must be converted into one of these fields before getting saved in the database. SQLite itself does not validate if the types written to the columns are actually of the defined type. This means you can write an integer into a string column and bvice versa.

Access to an SQLite database involves accessing the file system. This can be slow. Therefore it is recommended to perform database operations asynchronously.

More information about SQLite can be found on the SQLite website: http://www.sqlite.org.

1.2. SQLite in Android and the Room framework

Using an SQLite database in Android does not require a setup procedure or administration of the database.

You can manually manage your database with SQL statements for creating and updating the database. 2017 Google published the Room framework which simplifies the handling of databases.

2. Using Room as SQL object mapping library

Room is an annotation processing based SQL object mapping library provided by Google. To setup the database tables, you define Java objects with additional annotations.

Room is designed to abstract away the underlying database tables and queries. It is based on best-practices for persisting data in databases. For example, Room does, by default, not allow database access in the main thread.

For every table in your database you define a Java class, annotated with @Entity. The primary key must be annotated with @PrimaryKey.

For each Java annotated with @Entity, you define a database access object (DAO) annotated with @Dao.

The generator creates readable error messages, if the database statements contains typos.

To connect your data to your UI, you can use LiveData. LiveData is an observable data holder. It notifies observers when data changes so that the user interface can be updated.

3. Exercise using Room

Create a project called com.vogella.android.persistence with the same top-level package name. Use the Empty Activity template.

Add maven.google.com to the main build.gradle file.

allprojects {
    repositories {
        jcenter()
        maven {
            url "https://maven.google.com"
        }
    }
}

Add the following dependencies to your app/build.gradle file.

    compile 'android.arch.lifecycle:extensions:1.0.0-alpha1';
    compile 'android.arch.persistence.room:runtime:1.0.0-alpha1';
    annotationProcessor 'android.arch.lifecycle:compiler:1.0.0-alpha1';
    annotationProcessor 'android.arch.persistence.room:compiler:1.0.0-alpha1';
}

Create the data object.

package com.vogella.android.persistence;

import android.arch.persistence.room.Entity;
import android.arch.persistence.room.PrimaryKey;

import java.util.Date;

@Entity
public class Task {

    @PrimaryKey
    public final long id;
    public String summary;
    public String description;
    public boolean done;
//    public Date dueDate;


    public Task(long id, String summary, String description, boolean done) {
        this.id = id;
        this.summary = summary;
        this.description = description;
        this.done = done;
//        this.dueDate = dueDate;
    }

    public static TaskBuilder builder(){
        return new TaskBuilder();
    }

    public static class TaskBuilder {
        private long id;
        private String summary = "";
        private String description = "";
        private boolean done = false;
        private Date dueDate;

        public TaskBuilder setId(long id) {
            this.id = id;
            return this;
        }

        public TaskBuilder setSummary(String summary) {
            this.summary = summary;
            return this;
        }

        public TaskBuilder setDescription(String description) {
            this.description = description;
            return this;
        }

        public TaskBuilder setDone(boolean done) {
            this.done = done;
            return this;
        }

        public TaskBuilder setDueDate(Date dueDate) {
            this.dueDate = new Date(dueDate.getTime());
            return this;
        }

        public Task build() {
            return new Task(id, summary, description, done);
        }
    }

    @Override
    public String toString() {
        return "Task{" +
                "id=" + id +
                ", summary='" + summary + '\'' +
                ", description='" + description + '\'' +
                ", done=" + done +
                '}';
    }
}

Create a DAO object.

package com.vogella.android.persistence;

import android.arch.persistence.room.Dao;
import android.arch.persistence.room.Insert;
import android.arch.persistence.room.OnConflictStrategy;
import android.arch.persistence.room.Query;
import android.arch.persistence.room.Update;

import java.util.List;

@Dao
public interface TaskDao {

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void addTask(Task task);

    @Query("select * from task")
    public List<Task> getAllTasks();

    @Query("select * from task where id = :id")
    public List<Task> getTask(long id);

    @Update(onConflict = OnConflictStrategy.REPLACE)
    void updateTask(Task task);

    @Query("delete from task")
    void removeAllTasks();
}

Create the AppDatabase class.

package com.vogella.android.persistence;

import android.content.Context;

import android.arch.persistence.room.Database;
import android.arch.persistence.room.Room;
import android.arch.persistence.room.RoomDatabase;

@Database(entities = {Task.class}, version = 1, exportSchema = false)
public abstract class AppDatabase extends RoomDatabase {

    private static AppDatabase INSTANCE;

    public abstract TaskDao taskModel();

    public static AppDatabase getDatabase(Context context) {
        if (INSTANCE == null) {
            INSTANCE =
                    Room.databaseBuilder(context, AppDatabase.class, "tasks")
//                    Room.inMemoryDatabaseBuilder(context.getApplicationContext(), AppDatabase.class)
                    // To simplify the exercise, allow queries on the main thread.
                    // Don't do this on a real app!
                    .allowMainThreadQueries()
                    .build();
        }
        return INSTANCE;
    }

    public static void destroyInstance() {
        INSTANCE = null;
    }
}

Adjust your main activity to use the database.

package com.vogella.android.persistence;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

import java.util.List;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        AppDatabase database = AppDatabase.getDatabase(getApplicationContext());
        for (int i = 0; i <10; i++) {
            Task build = Task.builder().setId(i).setSummary("Testing " + i).setDescription("More ..." + i).build();
            database.taskModel().addTask(build);
        }
        List<Task> allTasks = database.taskModel().getAllTasks();
        TextView textView = findViewById(R.id.result);
        textView.setText(allTasks.toString());

    }

    @Override
    protected void onDestroy() {
        AppDatabase.javadestroyInstance();
        super.onDestroy();
    }
}

4. Working directly with SQLite

While it is possible to work directly with SQLite, using Using Room as SQL object mapping library should be preferred. The usage of Room largely simplifies the handling of databases.

Using the database APIs as described in this section, provides several challenges:

  • The API is relatively low-level, and requires a large amount of development time and effort.

  • Raw SQL queries can not verified at compile-time.

  • A lot of boilerplate code is required to convert between SQL queries and data objects.

The android.database package contains all necessary classes for working with databases. The android.database.sqlite package contains the SQLite specific classes.

To create and upgrade a database in your Android application you subclass the SQLiteOpenHelper class. An SQLiteDatabase object is the Java representation of the database.

In the constructor of your subclass, you specify the the name and the version of your database in the SQLiteOpenHelper.super() call.

In your SQLiteOpenHelper subclass class you need to override the following methods:

  • onCreate() - is called by the framework, if the database is accessed but not yet created.

  • onUpgrade() - called, if the database version is increased in your application code. This method allows you to update an existing database schema or to drop the existing database and recreate it via the onCreate() method.

The SQLiteOpenHelper class provides the getReadableDatabase() and getWriteableDatabase() methods. They allow read or write access to the SQLiteDatabase database.

The database tables should use the identifier _id for the primary key of the table. Several Android functions rely on this standard.

It is good practice to create a separate class per table. This class defines static onCreate() and onUpgrade() methods. These methods are called in the corresponding methods of SQLiteOpenHelper. This way your implementation of SQLiteOpenHelper stays readable, even if you have several tables.

4.1. SQLiteDatabase

SQLiteDatabase is the base class for working with a SQLite database in Android and provides methods to open, query, update and close the database.

More specifically SQLiteDatabase provides the insert(), update() and delete() methods.

In addition it provides the execSQL() method, which allows to execute an SQL statement directly.

The object ContentValues allows to define key/values. The key represents the table column identifier and the value represents the content for the table record in this column. ContentValues can be used for inserts and updates of database entries.

Queries can be created via the rawQuery() and query() methods or via the SQLiteQueryBuilder class .

rawQuery() directly accepts an SQL select statement as input.

query() provides a structured interface for specifying the SQL query.

SQLiteQueryBuilder is a convenience class that helps to build SQL queries.

4.2. rawQuery() Example

The following gives an example of a rawQuery() call.

Cursor cursor = getReadableDatabase().
    rawQuery("select * from todo where _id = ?", new String[] { id });

4.3. query() Example

The following gives an example of a query() call.

return database.query(DATABASE_TABLE,
    new String[] { KEY_ROWID, KEY_CATEGORY, KEY_SUMMARY, KEY_DESCRIPTION },
    null, null, null, null, null);

The method query() has the following parameters.

Table 1. Parameters of the query() method
Parameter Comment

String dbName

The table name to compile the query against.

String[] columnNames

A list of which table columns to return. Passing "null" will return all columns.

String whereClause

Where-clause, i.e. filter for the selection of data, null will select all data.

String[] selectionArgs

You may include ?s in the "whereClause"". These placeholders will get replaced by the values from the selectionArgs array.

String[] groupBy

A filter declaring how to group rows, null will cause the rows to not be grouped.

String[] having

Filter for the groups, null means no filter.

String[] orderBy

Table columns which will be used to order the data, null means no ordering.

If a condition is not required you can pass null, e.g. for the group by clause.

The "whereClause" is specified without the word "where", for example a "where" statement might look like: "_id=19 and summary=?".

If you specify placeholder values in the where clause via ?, you pass them as the selectionArgs parameter to the query.

4.4. Cursor

A query returns a Cursor object. A Cursor represents the result of a query and basically points to one row of the query result. This way Android can buffer the query results efficiently; as it does not have to load all data into memory.

To get the number of elements of the resulting query use the getCount() method.

To move between individual data rows, you can use the moveToFirst() and moveToNext() methods. The

isAfterLast() method allows to check if the end of the query result has been reached.

Cursor provides typed get*() methods, e.g. getLong(columnIndex), getString(columnIndex) to access the column data for the current position of the result. The "columnIndex" is the number of the column you are accessing.

Cursor also provides the getColumnIndexOrThrow(String) method which allows to get the column index for a column name of the table.

A Cursor needs to be closed with the close() method call.

4.5. ListViews, ListActivities and SimpleCursorAdapter

ListViews are Views which allow to display a list of elements.

ListActivities are specialized activities which make the usage of ListViews easier.

To work with databases and ListViews you can use the SimpleCursorAdapter. The SimpleCursorAdapter allows to set a layout for each row of the ListViews.

You also define an array which contains the column names and another array which contains the IDs of Views which should be filled with the data.

The SimpleCursorAdapter class will map the columns to the Views based on the Cursor passed to it.

To obtain the Cursor you should use the Loader class.

5. Tutorial: Using SQLite

5.1. Introduction to the project

The following demonstrates how to work with an SQLite database. We will use a data access object (DAO) to manage the data for us. The DAO is responsible for handling the database connection and for accessing and modifying the data. It will also convert the database objects into real Java Objects, so that our user interface code does not have to deal with the persistence layer.

The resulting application will look like the following.

dbexample10

Using a DAO is not always the right approach. A DAO creates Java model objects; using a database directly or via a ContentProvider is typically more resource efficient as you can avoid the creation of model objects.

I still demonstrate the usage of the DAO in this example to have a relatively simple example to begin with. Use the latest version of Android 4.0. This is currently API Level 15. Otherwise I would have to introduce the Loader class, which should be used as of Android 3.0 for managing a database Cursor. And this class introduces additional complexity.

5.2. Create Project

Create the new Android project with the name de.vogella.android.sqlite.first and an activity called TestDatabaseActivity.

5.3. Database and Data Model

Create the MySQLiteHelper class. This class is responsible for creating the database. The onUpgrade() method will simply delete all existing data and re-create the table. It also defines several constants for the table name and the table columns.

package de.vogella.android.sqlite.first;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class MySQLiteHelper extends SQLiteOpenHelper {

    public static final String TABLE_COMMENTS = "comments";
    public static final String COLUMN_ID = "_id";
    public static final String COLUMN_COMMENT = "comment";

    private static final String DATABASE_NAME = "commments.db";
    private static final int DATABASE_VERSION = 1;

    // Database creation sql statement
    private static final String DATABASE_CREATE = "create table "
            + TABLE_COMMENTS + "( " + COLUMN_ID
            + " integer primary key autoincrement, " + COLUMN_COMMENT
            + " text not null);";

    public MySQLiteHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase database) {
        database.execSQL(DATABASE_CREATE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        Log.w(MySQLiteHelper.class.getName(),
                "Upgrading database from version " + oldVersion + " to "
                        + newVersion + ", which will destroy all old data");
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_COMMENTS);
        onCreate(db);
    }

}

Create the Comment class. This class is our model and contains the data we will save in the database and show in the user interface.

package de.vogella.android.sqlite.first;

public class Comment {
    private long id;
    private String comment;

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getComment() {
        return comment;
    }

    public void setComment(String comment) {
        this.comment = comment;
    }

    // Will be used by the ArrayAdapter in the ListView
    @Override
    public String toString() {
        return comment;
    }
}

Create the CommentsDataSource class. This class is our DAO. It maintains the database connection and supports adding new comments and fetching all comments.

package de.vogella.android.sqlite.first;

import java.util.ArrayList;
import java.util.List;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;

public class CommentsDataSource {

    // Database fields
    private SQLiteDatabase database;
    private MySQLiteHelper dbHelper;
    private String[] allColumns = { MySQLiteHelper.COLUMN_ID,
            MySQLiteHelper.COLUMN_COMMENT };

    public CommentsDataSource(Context context) {
        dbHelper = new MySQLiteHelper(context);
    }

    public void open() throws SQLException {
        database = dbHelper.getWritableDatabase();
    }

    public void close() {
        dbHelper.close();
    }

    public Comment createComment(String comment) {
        ContentValues values = new ContentValues();
        values.put(MySQLiteHelper.COLUMN_COMMENT, comment);
        long insertId = database.insert(MySQLiteHelper.TABLE_COMMENTS, null,
                values);
        Cursor cursor = database.query(MySQLiteHelper.TABLE_COMMENTS,
                allColumns, MySQLiteHelper.COLUMN_ID + " = " + insertId, null,
                null, null, null);
        cursor.moveToFirst();
        Comment newComment = cursorToComment(cursor);
        cursor.close();
        return newComment;
    }

    public void deleteComment(Comment comment) {
        long id = comment.getId();
        System.out.println("Comment deleted with id: " + id);
        database.delete(MySQLiteHelper.TABLE_COMMENTS, MySQLiteHelper.COLUMN_ID
                + " = " + id, null);
    }

    public List<Comment> getAllComments() {
        List<Comment> comments = new ArrayList<Comment>();

        Cursor cursor = database.query(MySQLiteHelper.TABLE_COMMENTS,
                allColumns, null, null, null, null, null);

        cursor.moveToFirst();
        while (!cursor.isAfterLast()) {
            Comment comment = cursorToComment(cursor);
            comments.add(comment);
            cursor.moveToNext();
        }
        // make sure to close the cursor
        cursor.close();
        return comments;
    }

    private Comment cursorToComment(Cursor cursor) {
        Comment comment = new Comment();
        comment.setId(cursor.getLong(0));
        comment.setComment(cursor.getString(1));
        return comment;
    }
}

5.4. User Interface

Change your main.xml layout file in the <filename class="directory">res/layout_ folder to the following. This layout has two buttons for adding and deleting comments and a ListView which will be used to display the existing comments. The comment text will be generated later in the activity by a small random generator.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <LinearLayout
        android:id="@+id/group"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" >

        <Button
            android:id="@+id/add"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Add New"
            android:onClick="onClick"/>

        <Button
            android:id="@+id/delete"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Delete First"
            android:onClick="onClick"/>

    </LinearLayout>

    <ListView
        android:id="@android:id/list"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
     />

</LinearLayout>

Change your TestDatabaseActivity class. to the following. We use here a ListActivity for displaying the data.

package de.vogella.android.sqlite.first;

import java.util.List;
import java.util.Random;

import android.app.ListActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;

public class TestDatabaseActivity extends ListActivity {
    private CommentsDataSource datasource;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        datasource = new CommentsDataSource(this);
        datasource.open();

        List<Comment> values = datasource.getAllComments();

        // use the SimpleCursorAdapter to show the
        // elements in a ListView
        ArrayAdapter<Comment> adapter = new ArrayAdapter<Comment>(this,
                android.R.layout.simple_list_item_1, values);
        setListAdapter(adapter);
    }

    // Will be called via the onClick attribute
    // of the buttons in main.xml
    public void onClick(View view) {
        @SuppressWarnings("unchecked")
        ArrayAdapter<Comment> adapter = (ArrayAdapter<Comment>) getListAdapter();
        Comment comment = null;
        switch (view.getId()) {
        case R.id.add:
            String[] comments = new String[] { "Cool", "Very nice", "Hate it" };
            int nextInt = new Random().nextInt(3);
            // save the new comment to the database
            comment = datasource.createComment(comments[nextInt]);
            adapter.add(comment);
            break;
        case R.id.delete:
            if (getListAdapter().getCount() > 0) {
                comment = (Comment) getListAdapter().getItem(0);
                datasource.deleteComment(comment);
                adapter.remove(comment);
            }
            break;
        }
        adapter.notifyDataSetChanged();
    }

    @Override
    protected void onResume() {
        datasource.open();
        super.onResume();
    }

    @Override
    protected void onPause() {
        datasource.close();
        super.onPause();
    }

}

5.5. Running the apps

Install your application and use the Add and Delete button. Restart your application to validate that the data is still there.

6. Content provider and sharing data

6.1. What is a content provider?

If you want to share data with other applications you can use a content provider (short provider). Provider offer data encapsulation based on URI’s. Any URI which starts with content:// points to a resources which can be accessed via a provider. A URI for a resource may allow to perform the basic CRUD operations (Create, Read, Update, Delete) on the resource via the content provider.

A provider allows applications to access data. The data can be stored in an SQlite database, on the file system, in flat files or on a remote server.

While a content provider can be used within an application to access data, its is typically used to share data with other application. As application data is by default private, a content provider is a convenient to share you data with other application based on a structured interface.

A content provider must be declared in the manifest file for the application.

6.2. Base URI of the content provider

The base URI to access a content provider is defined via the combination of the content:// schema and the name space of the provider. This name space is defined

in the manifest file via the android:authorities attribute of the receiver registration. This can for example be: content://test/

The base URI represents a collection of resources. If the base URI is combined with an instance identifier, e,g., content://test/2, it represents a single instance.

6.3. Accessing a content provider

As it is required to know the URIs of a provider to access it, it is good practice to provide public constants for the URIs to document them to other developers.

Many Android data sources, e.g. the contacts, are accessible via content providers.

6.4. Custom content provider

To create your custom content provider you have to define a class which extends android.content.ContentProvider. You must declare this class as content provider in the Android manifest file. The corresponding entry must specify the android:authorities attribute which allows identifying the content provider. This authority is the basis for the URI to access data and must be unique.

<provider
       android:authorities="de.vogella.android.todos.contentprovider"
       android:name=".contentprovider.MyTodoContentProvider" >
</provider>

Your content provider must implement several methods, e.g. query(), insert(), update(), delete(), getType() and onCreate(). In case you do not support certain methods its good practice to throw an UnsupportedOperationException().

The query() method must return a Cursor object.

6.5. Security and content provider

Until Android version 4.2 a content provider is by default available to other Android applications. As of Android 4.2 a content provider must be explicitly exported.

To set the visibility of your content provider use the android:exported=false|true parameter in the declaration of your content provider in the AndroidManifest.xml file.

It is good practice to always set the android:exported parameter to ensure correct behavior across Android versions.

6.6. Thread Safety

If you work directly with databases and have multiple writers from different threads you may run into concurrency issues.

A content provider can be accessed from several programs at the same time, therefore you must implement the access thread-safe. The easiest way is to use the keyword synchronized in front of all methods of the provider, so that only one thread can access these methods at the same time.

If you do not require that Android synchronizes data access to the provider, set the android:multiprocess=true attribute in your <provider> definition in the AndroidManifest.xml file. This permits an instance of the provider to be created in each client process, eliminating the need to perform interprocess communication (IPC).

7. Tutorial: Using ContentProvider

7.1. Overview

The following example will use an existing ContentProvider from the People application.

7.2. Create contacts on your emulator

For this example we need a few maintained contacts. Select the home menu and then the People entry to create contacts.

Selecting the

The app will ask you if you want to login. Either login or select "Not now". Press ""Create a new contact". You can create local contacts.

Create a new contact
Details for a maintained contact

Finish adding your first contact. Afterwards the app allows you to add more contacts via the + button. As a result you should have a few new contacts in your application.

7.3. Using the Contact Content Provider

Create a new Android project called de.vogella.android.contentprovider with the activity called ContactsActivity.

Change the corresponding layout file in the <filename class="directory">res/layout_ folder. Rename the ID of the existing TextView to contactview. Delete the default text.

The resulting layout file should look like the following.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/contactview"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

Access to the contact ContentProvider requires a certain permission, as not all applications should have access to the contact information. Open the AndroidManifest.xml file, and select the Permissions tab. On that tab click the Add button, and select the Uses Permission. From the drop-down list select the android.permission.READ_CONTACTS entry.

Change the coding of the activity.

package de.vogella.android.contentprovider;

import android.app.Activity;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.provider.ContactsContract;
import android.widget.TextView;

public class ContactsActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_contacts);
        TextView contactView = (TextView) findViewById(R.id.contactview);

        Cursor cursor = getContacts();

        while (cursor.moveToNext()) {

            String displayName = cursor.getString(cursor
                    .getColumnIndex(ContactsContract.Data.DISPLAY_NAME));
            contactView.append("Name: ");
            contactView.append(displayName);
            contactView.append("\n");
        }
    }

    private Cursor getContacts() {
        // Run query
        Uri uri = ContactsContract.Contacts.CONTENT_URI;
        String[] projection = new String[] { ContactsContract.Contacts._ID,
                ContactsContract.Contacts.DISPLAY_NAME };
        String selection = ContactsContract.Contacts.IN_VISIBLE_GROUP + " = '"
                + ("1") + "'";
        String[] selectionArgs = null;
        String sortOrder = ContactsContract.Contacts.DISPLAY_NAME
                + " COLLATE LOCALIZED ASC";


        return managedQuery(uri, projection, selection, selectionArgs,
                sortOrder);
    }

}

If you run this application the data is read from the ContentProvider of the People application and displayed in a TextView. Typically you would display such data in a ListView.

8. Loader

8.1. Purpose of the Loader class

The Loader class allows you to load data asynchronously in an activity or fragment. They can monitor the source of the data and deliver new results when the content changes. They also persist data between configuration changes.

The data can be cached by the Loader and this caching can survive configuration changes. Loaders have been introduced in Android 3.0 and are part of the compatibility layer for Android versions as of 1.6.

8.2. Implementing a Loader

You can use the abstract AsyncTaskLoader class as the basis for your custom Loader implementations.

The LoaderManager of an activity or fragment manages one or more Loader instances. The creation of a Loader is done via the following method call.

# start a new loader or re-connect to existing one
getLoaderManager().initLoader(0, null, this);

The first parameter is a unique ID which can be used by the callback class to identify the Loader. The second parameter is a bundle which can be given to the callback class for more information.

The third parameter of initLoader() is the class which is called once the initialization has been started (callback class). This class must implement the LoaderManager.LoaderCallbacks interface. It is common practice that an activity or the fragment which uses a Loader implements the LoaderManager.LoaderCallbacks interface.

The Loader is not directly created by the getLoaderManager().initLoader() method call, but must be created by the callback class in the onCreateLoader() method.

Once the Loader has finished reading data asynchronously, the onLoadFinished() method of the callback class is called. Here you can update your user interface.

8.3. SQLite database and CursorLoader

Android provides a Loader default implementation to handle SQlite database connections, the CursorLoader class.

For a ContentProvider based on an SQLite database you would typically use the CursorLoader class. This Loader performs the database query in a background thread so that the application is not blocked.

The CursorLoader class is the replacement for Activity-managed cursors which are deprecated now.

If the Cursor becomes invalid, the onLoaderReset() method is called on the callback class.

9. Cursors and Loaders

One of the challenges with accessing databases is that this access is slow. The other challenge is that the application needs to consider the life cycle of the components correctly, e.g. opening and closing the cursor if a configuration change happens.

To manage the life cycle you could use the managedQuery() method in activities prior to Android 3.0.

As of Android 3.0 this method is deprecated and you should use the Loader framework to access the ContentProvider.

The SimpleCursorAdapter class, which can be used with ListViews, has the swapCursor() method. Your Loader can use this method to update the Cursor in its onLoadFinished() method.

The CursorLoader class reconnect the Cursor after a configuration change.

10. Tutorial: SQLite, custom ContentProvider and Loader

10.1. Overview

The following demo is also available in the Android Market. To allow more users to play with the app, it has been downported to Android 2.3. If you have a barcode scanner installed on your Android phone, you can scan the following QR Code to go to the example app in the Android market. Please note that the app looks and behaves differently due to the different Android versions, e.g. you have an OptionMenu instead of the ActionBar and the theme is different.

AndroidTodoQRCode

We will create a "To-do" application which allows the user to enter tasks for himself. These items will be stored in the SQLite database and accessed via a ContentProvider.

The tasks are called "todo items" or "todos" in this tutorial.

The application consists out of two activities, one for seeing a list of all todo items and one for creating and changing a specific todo item. Both activities will communicate via Intents.

To asynchronously load and manage the Cursor the main activity will use a Loader.

The resulting application will look similar to the following.

Todo Example application OverviewActivity
Todo Example application DetailsActivity

10.2. Project

Create the project de.vogella.android.todos with the activity called TodosOverviewActivity. Create another activity called TodoDetailActivity.

10.3. Database classes

Create the package de.vogella.android.todos.database. This package will store the classes for the database handling.

As said earlier I consider having one separate class per table as best practice. Even though we have only one table in this example we will follow this practice. This way we are prepared in case our database schema grows.

Create the following class. This class also contains constants for the table name and the columns.

package de.vogella.android.todos.database;

import android.database.sqlite.SQLiteDatabase;
import android.util.Log;

public class TodoTable {

    // Database table
    public static final String TABLE_TODO = "todo";
    public static final String COLUMN_ID = "_id";
    public static final String COLUMN_CATEGORY = "category";
    public static final String COLUMN_SUMMARY = "summary";
    public static final String COLUMN_DESCRIPTION = "description";

    // Database creation SQL statement
    private static final String DATABASE_CREATE = "create table "
            + TABLE_TODO
            + "("
            + COLUMN_ID + " integer primary key autoincrement, "
            + COLUMN_CATEGORY + " text not null, "
            + COLUMN_SUMMARY + " text not null,"
            + COLUMN_DESCRIPTION
            + " text not null"
            + ");";

    public static void onCreate(SQLiteDatabase database) {
        database.execSQL(DATABASE_CREATE);
    }

    public static void onUpgrade(SQLiteDatabase database, int oldVersion,
            int newVersion) {
        Log.w(TodoTable.class.getName(), "Upgrading database from version "
                + oldVersion + " to " + newVersion
                + ", which will destroy all old data");
        database.execSQL("DROP TABLE IF EXISTS " + TABLE_TODO);
        onCreate(database);
    }
}

Create the following TodoDatabaseHelper class. This class extends SQLiteOpenHelper and calls the static methods of the TodoTable helper class.

package de.vogella.android.todos.database;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

public class TodoDatabaseHelper extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "todotable.db";
    private static final int DATABASE_VERSION = 1;

    public TodoDatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    // Method is called during creation of the database
    @Override
    public void onCreate(SQLiteDatabase database) {
        TodoTable.onCreate(database);
    }

    // Method is called during an upgrade of the database,
    // e.g. if you increase the database version
    @Override
    public void onUpgrade(SQLiteDatabase database, int oldVersion,
            int newVersion) {
        TodoTable.onUpgrade(database, oldVersion, newVersion);
    }
}

We will use a ContentProvider for accessing the database; we will not write a data access object (DAO) as we did in the previous SQlite example.

10.4. Create ContentProvider

Create the package de.vogella.android.todos.contentprovider.

Create the following MyTodoContentProvider class which extends ContentProvider.

package de.vogella.android.todos.contentprovider;

import java.util.Arrays;
import java.util.HashSet;

import android.content.ContentProvider;
import android.content.ContentResolver;
import android.content.ContentValues;
import android.content.UriMatcher;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteQueryBuilder;
import android.net.Uri;
import android.text.TextUtils;
import de.vogella.android.todos.database.TodoDatabaseHelper;
import de.vogella.android.todos.database.TodoTable;

public class MyTodoContentProvider extends ContentProvider {

    // database
    private TodoDatabaseHelper database;

    // used for the UriMacher
    private static final int TODOS = 10;
    private static final int TODO_ID = 20;

    private static final String AUTHORITY = "de.vogella.android.todos.contentprovider";

    private static final String BASE_PATH = "todos";
    public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY
            + "/" + BASE_PATH);

    public static final String CONTENT_TYPE = ContentResolver.CURSOR_DIR_BASE_TYPE
            + "/todos";
    public static final String CONTENT_ITEM_TYPE = ContentResolver.CURSOR_ITEM_BASE_TYPE
            + "/todo";

    private static final UriMatcher sURIMatcher = new UriMatcher(
            UriMatcher.NO_MATCH);
    static {
        sURIMatcher.addURI(AUTHORITY, BASE_PATH, TODOS);
        sURIMatcher.addURI(AUTHORITY, BASE_PATH + "/#", TODO_ID);
    }

    @Override
    public boolean onCreate() {
        database = new TodoDatabaseHelper(getContext());
        return false;
    }

    @Override
    public Cursor query(Uri uri, String[] projection, String selection,
            String[] selectionArgs, String sortOrder) {

        // Uisng SQLiteQueryBuilder instead of query() method
        SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();

        // check if the caller has requested a column which does not exists
        checkColumns(projection);

        // Set the table
        queryBuilder.setTables(TodoTable.TABLE_TODO);

        int uriType = sURIMatcher.match(uri);
        switch (uriType) {
        case TODOS:
            break;
        case TODO_ID:
            // adding the ID to the original query
            queryBuilder.appendWhere(TodoTable.COLUMN_ID + "="
                    + uri.getLastPathSegment());
            break;
        default:
            throw new IllegalArgumentException("Unknown URI: " + uri);
        }

        SQLiteDatabase db = database.getWritableDatabase();
        Cursor cursor = queryBuilder.query(db, projection, selection,
                selectionArgs, null, null, sortOrder);
        // make sure that potential listeners are getting notified
        cursor.setNotificationUri(getContext().getContentResolver(), uri);

        return cursor;
    }

    @Override
    public String getType(Uri uri) {
        return null;
    }

    @Override
    public Uri insert(Uri uri, ContentValues values) {
        int uriType = sURIMatcher.match(uri);
        SQLiteDatabase sqlDB = database.getWritableDatabase();
        long id = 0;
        switch (uriType) {
        case TODOS:
            id = sqlDB.insert(TodoTable.TABLE_TODO, null, values);
            break;
        default:
            throw new IllegalArgumentException("Unknown URI: " + uri);
        }
        getContext().getContentResolver().notifyChange(uri, null);
        return Uri.parse(BASE_PATH + "/" + id);
    }

    @Override
    public int delete(Uri uri, String selection, String[] selectionArgs) {
        int uriType = sURIMatcher.match(uri);
        SQLiteDatabase sqlDB = database.getWritableDatabase();
        int rowsDeleted = 0;
        switch (uriType) {
        case TODOS:
            rowsDeleted = sqlDB.delete(TodoTable.TABLE_TODO, selection,
                    selectionArgs);
            break;
        case TODO_ID:
            String id = uri.getLastPathSegment();
            if (TextUtils.isEmpty(selection)) {
                rowsDeleted = sqlDB.delete(
                        TodoTable.TABLE_TODO,
                        TodoTable.COLUMN_ID + "=" + id,
                        null);
            } else {
                rowsDeleted = sqlDB.delete(
                        TodoTable.TABLE_TODO,
                        TodoTable.COLUMN_ID + "=" + id
                        + " and " + selection,
                        selectionArgs);
            }
            break;
        default:
            throw new IllegalArgumentException("Unknown URI: " + uri);
        }
        getContext().getContentResolver().notifyChange(uri, null);
        return rowsDeleted;
    }

    @Override
    public int update(Uri uri, ContentValues values, String selection,
            String[] selectionArgs) {

        int uriType = sURIMatcher.match(uri);
        SQLiteDatabase sqlDB = database.getWritableDatabase();
        int rowsUpdated = 0;
        switch (uriType) {
        case TODOS:
            rowsUpdated = sqlDB.update(TodoTable.TABLE_TODO,
                    values,
                    selection,
                    selectionArgs);
            break;
        case TODO_ID:
            String id = uri.getLastPathSegment();
            if (TextUtils.isEmpty(selection)) {
                rowsUpdated = sqlDB.update(TodoTable.TABLE_TODO,
                        values,
                        TodoTable.COLUMN_ID + "=" + id,
                        null);
            } else {
                rowsUpdated = sqlDB.update(TodoTable.TABLE_TODO,
                        values,
                        TodoTable.COLUMN_ID + "=" + id
                        + " and "
                        + selection,
                        selectionArgs);
            }
            break;
        default:
            throw new IllegalArgumentException("Unknown URI: " + uri);
        }
        getContext().getContentResolver().notifyChange(uri, null);
        return rowsUpdated;
    }

    private void checkColumns(String[] projection) {
        String[] available = { TodoTable.COLUMN_CATEGORY,
                TodoTable.COLUMN_SUMMARY, TodoTable.COLUMN_DESCRIPTION,
                TodoTable.COLUMN_ID };
        if (projection != null) {
            HashSet<String> requestedColumns = new HashSet<String>(
                    Arrays.asList(projection));
            HashSet<String> availableColumns = new HashSet<String>(
                    Arrays.asList(available));
            // check if all columns which are requested are available
            if (!availableColumns.containsAll(requestedColumns)) {
                throw new IllegalArgumentException(
                        "Unknown columns in projection");
            }
        }
    }

}

MyTodoContentProvider implements update(), insert(), delete() and query() methods. These methods map more or less directly to the SQLiteDatabase interface.

It also has the checkColumns() method to validate that a query only requests valid columns.

Register your ContentProvider in your AndroidManifest.xml file.

<application
    <!-- Place the following after the Activity
         Definition
    -->
  <provider
      android:name=".contentprovider.MyTodoContentProvider"
      android:authorities="de.vogella.android.todos.contentprovider" >
   </provider>
</application>

10.5. Resources

Our application requires several resources. First define a menu listmenu.xml ` in the folder `res/menu. If you use the Android resource wizard to create the "listmenu.xml" file, the folder will be created for you; if you create the file manually you also need to create the folder manually.

This XML file will be used to define the option menu in our application. The android:showAsAction="always" attribute will ensure that this menu entry is displayed in the ActionBar of our application.

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android" >

    <item
        android:id="@+id/insert"
        android:showAsAction="always"
        android:title="Insert">
    </item>

</menu>

The user will be able to select the priority for the todo items. For the priorities we create a string array. Create the following file priority.xml in the res/values folder .

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string-array name="priorities">
        <item>Urgent</item>
        <item>Reminder</item>
    </string-array>

</resources>

Define also additional strings for the application. Edit strings.xml under res/values.

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="hello">Hello World, Todo!</string>
    <string name="app_name">Todo</string>
    <string name="no_todos">Currently there are no Todo items maintained</string>
    <string name="menu_insert">Add Item</string>
    <string name="menu_delete">Delete Todo</string>
    <string name="todo_summary">Summary</string>
    <string name="todo_description">Delete Todo</string>
    <string name="todo_edit_summary">Summary</string>
    <string name="todo_edit_description">Description</string>
    <string name="todo_edit_confirm">Confirm</string>
</resources>

10.6. Layouts

We will define three layouts. One will be used for the display of a row in the list, the other ones will be used by our activities.

The row layout refers to an icon called reminder. Paste an icon of type "png" called "reminder.png" into your res/drawable folders ( drawable-hdpi, drawable-mdpi, drawable-ldpi )

If you do not have an icon available you can copy the icon created by the Android wizard (ic_launcher.png in the res/drawable* folders) or rename the reference in the layout file. Please note that the Android Developer Tools sometimes change the name of this generated icon, so your file might not be called "ic_launcher.png".

Alternatively you could remove the icon definition from the "todo_row.xml" layout definition file which you will create in the next step.

Create the "todo_row.xml" layout file in the folder <filename class="directory">res/layout_.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" >

    <ImageView
        android:id="@+id/icon"
        android:layout_width="30dp"
        android:layout_height="24dp"
        android:layout_marginLeft="4dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="8dp"
        android:src="@drawable/reminder" >
    </ImageView>

    <TextView
        android:id="@+id/label"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="6dp"
        android:lines="1"
        android:text="@+id/TextView01"
        android:textSize="24dp"
        >
    </TextView>

</LinearLayout>

Create the todo_list.xml layout file. This layout defines how the list looks like.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <ListView
        android:id="@android:id/list"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >
    </ListView>

    <TextView
        android:id="@android:id/empty"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/no_todos" />

</LinearLayout>

Create the todo_edit.xml layout file. This layout will be used to display and edit an individual todo item in the TodoDetailActivity activity.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <Spinner
        android:id="@+id/category"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:entries="@array/priorities" >
    </Spinner>

    <LinearLayout
        android:id="@+id/LinearLayout01"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >

        <EditText
            android:id="@+id/todo_edit_summary"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:hint="@string/todo_edit_summary"
            android:imeOptions="actionNext" >
        </EditText>
    </LinearLayout>

    <EditText
        android:id="@+id/todo_edit_description"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:gravity="top"
        android:hint="@string/todo_edit_description"
        android:imeOptions="actionNext" >
    </EditText>

    <Button
        android:id="@+id/todo_edit_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/todo_edit_confirm" >
    </Button>

</LinearLayout>

10.7. Activities

Change the coding of your activities to the following. First TodosOverviewActivity.java.

package de.vogella.android.todos;

import android.app.ListActivity;
import android.app.LoaderManager;
import android.content.CursorLoader;
import android.content.Intent;
import android.content.Loader;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.view.ContextMenu;
import android.view.ContextMenu.ContextMenuInfo;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView.AdapterContextMenuInfo;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import de.vogella.android.todos.contentprovider.MyTodoContentProvider;
import de.vogella.android.todos.database.TodoTable;

/*
 * TodosOverviewActivity displays the existing todo items
 * in a list
 *
 * You can create new ones via the ActionBar entry "Insert"
 * You can delete existing ones via a long press on the item
 */

public class TodosOverviewActivity extends ListActivity implements
        LoaderManager.LoaderCallbacks<Cursor> {
    private static final int ACTIVITY_CREATE = 0;
    private static final int ACTIVITY_EDIT = 1;
    private static final int DELETE_ID = Menu.FIRST + 1;
    // private Cursor cursor;
    private SimpleCursorAdapter adapter;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.todo_list);
        this.getListView().setDividerHeight(2);
        fillData();
        registerForContextMenu(getListView());
    }

    // create the menu based on the XML defintion
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.listmenu, menu);
        return true;
    }

    // Reaction to the menu selection
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
        case R.id.insert:
            createTodo();
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    @Override
    public boolean onContextItemSelected(MenuItem item) {
        switch (item.getItemId()) {
        case DELETE_ID:
            AdapterContextMenuInfo info = (AdapterContextMenuInfo) item
                    .getMenuInfo();
            Uri uri = Uri.parse(MyTodoContentProvider.CONTENT_URI + "/"
                    + info.id);
            getContentResolver().delete(uri, null, null);
            fillData();
            return true;
        }
        return super.onContextItemSelected(item);
    }

    private void createTodo() {
        Intent i = new Intent(this, TodoDetailActivity.class);
        startActivity(i);
    }

    // Opens the second activity if an entry is clicked
    @Override
    protected void onListItemClick(ListView l, View v, int position, long id) {
        super.onListItemClick(l, v, position, id);
        Intent i = new Intent(this, TodoDetailActivity.class);
        Uri todoUri = Uri.parse(MyTodoContentProvider.CONTENT_URI + "/" + id);
        i.putExtra(MyTodoContentProvider.CONTENT_ITEM_TYPE, todoUri);

        startActivity(i);
    }



    private void fillData() {

        // Fields from the database (projection)
        // Must include the _id column for the adapter to work
        String[] from = new String[] { TodoTable.COLUMN_SUMMARY };
        // Fields on the UI to which we map
        int[] to = new int[] { R.id.label };

        getLoaderManager().initLoader(0, null, this);
        adapter = new SimpleCursorAdapter(this, R.layout.todo_row, null, from,
                to, 0);

        setListAdapter(adapter);
    }

    @Override
    public void onCreateContextMenu(ContextMenu menu, View v,
            ContextMenuInfo menuInfo) {
        super.onCreateContextMenu(menu, v, menuInfo);
        menu.add(0, DELETE_ID, 0, R.string.menu_delete);
    }

    // creates a new loader after the initLoader () call
    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        String[] projection = { TodoTable.COLUMN_ID, TodoTable.COLUMN_SUMMARY };
        CursorLoader cursorLoader = new CursorLoader(this,
                MyTodoContentProvider.CONTENT_URI, projection, null, null, null);
        return cursorLoader;
    }

    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        adapter.swapCursor(data);
    }

    @Override
    public void onLoaderReset(Loader<Cursor> loader) {
        // data is not available anymore, delete reference
        adapter.swapCursor(null);
    }

}

And TodoDetailActivity.java

package de.vogella.android.todos;

import android.app.Activity;
import android.content.ContentValues;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.text.TextUtils;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.Toast;
import de.vogella.android.todos.contentprovider.MyTodoContentProvider;
import de.vogella.android.todos.database.TodoTable;

/*
 * TodoDetailActivity allows to enter a new todo item
 * or to change an existing
 */
public class TodoDetailActivity extends Activity {
    private Spinner mCategory;
    private EditText mTitleText;
    private EditText mBodyText;

    private Uri todoUri;

    @Override
    protected void onCreate(Bundle bundle) {
        super.onCreate(bundle);
        setContentView(R.layout.todo_edit);

        mCategory = (Spinner) findViewById(R.id.category);
        mTitleText = (EditText) findViewById(R.id.todo_edit_summary);
        mBodyText = (EditText) findViewById(R.id.todo_edit_description);
        Button confirmButton = (Button) findViewById(R.id.todo_edit_button);

        Bundle extras = getIntent().getExtras();

        // check from the saved Instance
        todoUri = (bundle == null) ? null : (Uri) bundle
                .getParcelable(MyTodoContentProvider.CONTENT_ITEM_TYPE);

        // Or passed from the other activity
        if (extras != null) {
            todoUri = extras
                    .getParcelable(MyTodoContentProvider.CONTENT_ITEM_TYPE);

            fillData(todoUri);
        }

        confirmButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                if (TextUtils.isEmpty(mTitleText.getText().toString())) {
                    makeToast();
                } else {
                    setResult(RESULT_OK);
                    finish();
                }
            }

        });
    }

    private void fillData(Uri uri) {
        String[] projection = { TodoTable.COLUMN_SUMMARY,
                TodoTable.COLUMN_DESCRIPTION, TodoTable.COLUMN_CATEGORY };
        Cursor cursor = getContentResolver().query(uri, projection, null, null,
                null);
        if (cursor != null) {
            cursor.moveToFirst();
            String category = cursor.getString(cursor
                    .getColumnIndexOrThrow(TodoTable.COLUMN_CATEGORY));

            for (int i = 0; i < mCategory.getCount(); i++) {

                String s = (String) mCategory.getItemAtPosition(i);
                if (s.equalsIgnoreCase(category)) {
                    mCategory.setSelection(i);
                }
            }

            mTitleText.setText(cursor.getString(cursor
                    .getColumnIndexOrThrow(TodoTable.COLUMN_SUMMARY)));
            mBodyText.setText(cursor.getString(cursor
                    .getColumnIndexOrThrow(TodoTable.COLUMN_DESCRIPTION)));

            // always close the cursor
            cursor.close();
        }
    }

    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        saveState();
        outState.putParcelable(MyTodoContentProvider.CONTENT_ITEM_TYPE, todoUri);
    }

    @Override
    protected void onPause() {
        super.onPause();
        saveState();
    }

    private void saveState() {
        String category = (String) mCategory.getSelectedItem();
        String summary = mTitleText.getText().toString();
        String description = mBodyText.getText().toString();

        // only save if either summary or description
        // is available

        if (description.length() == 0 && summary.length() == 0) {
            return;
        }

        ContentValues values = new ContentValues();
        values.put(TodoTable.COLUMN_CATEGORY, category);
        values.put(TodoTable.COLUMN_SUMMARY, summary);
        values.put(TodoTable.COLUMN_DESCRIPTION, description);

        if (todoUri == null) {
            // New todo
            todoUri = getContentResolver().insert(
                    MyTodoContentProvider.CONTENT_URI, values);
        } else {
            // Update todo
            getContentResolver().update(todoUri, values, null, null);
        }
    }

    private void makeToast() {
        Toast.makeText(TodoDetailActivity.this, "Please maintain a summary",
                Toast.LENGTH_LONG).show();
    }
}

The resulting AndroidManifest.xml looks like the following.

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="de.vogella.android.todos"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="15" />

    <application
        android:icon="@drawable/icon"
        android:label="@string/app_name" >
        <activity
            android:name=".TodosOverviewActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".TodoDetailActivity"
            android:windowSoftInputMode="stateVisible|adjustResize" >
        </activity>

        <provider
            android:name=".contentprovider.MyTodoContentProvider"
            android:authorities="de.vogella.android.todos.contentprovider" >
        </provider>
    </application>

</manifest>

10.8. Start your application

Start your application. You should be able to enter a new todo item via the "Insert" button in the ActionBar.

An existing todo item can be deleted on the list via a long press.

Deleting an todo item

To change an existing todo item, touch the corresponding row. This starts the second activity.

11. Accessing SQLite databases directly

11.1. Storage location of the SQLite database

SQlite stores the whole database in a file. If you have access to this file, you can work directly with the data base. Accessing the SQlite database file only works in the emulator or on a rooted device.

A standard Android device will not grant read-access to the database file.

11.2. Shell access to the database

It is possible to access an SQLite database on the emulator or a rooted device via the command line. For this use the following command to connect to the device.

adb shell

The command adb is located in your Android SDK installation folder in the "platform-tools" subfolder.

Afterwards you use the "cd" command to switch the database directory and use the "sqlite3" command to connect to a database. For example in my case:

# Switch to the data directory
cd /data/data
# Our application
cd de.vogella.android.todos
# Switch to the database dir
cd databases
# Check the content
ls
# Assuming that there is a todotable file
# connect to this table
sqlite3 todotable.db

The most important commands are:

Table 2. SQlite commands
Command Description

.help

List all commands and options.

.exit

Exit the sqlite3 command.

.schema

Show the CREATE statements which were used to create the tables of the current database.

You find the complete documentation of SQlite at http://www.sqlite.org/sqlite.html.

12. More on ListViews

Please see Android ListView Tutorial for an introduction into ListViews and ListActivities.

13. Performance

Changes in SQLite are ACID (atomic, consistent, isolated, durable). This means that every update, insert and delete operation is ACID. Unfortunately this requires some overhead in the database processing therefore you should wrap updates in the SQLite database in an transaction and commit this transaction after several operations. This can significantly improve performance.

The following code demonstrates that performance optimization.

db.beginTransaction();
try {
   for (int i= 0; i< values.lenght; i++){
       // TODO prepare ContentValues object values
       db.insert(your_table, null, values);
       // In case you do larger updates
       yieldIfContededSafely()
     }
     db.setTransactionSuccessful();
    } finally {
      db.endTransaction();
}

For larger data updates you should use the yieldIfContededSafely() method. SQLite locks the database during an transaction. With this call, Android checks if someone else queries the data and if finish automatically the transaction and opens a new one. This way the other process can access the data in between.

14. About this website

15. Android SQLite resources

15.1. vogella GmbH training and consulting support

TRAINING SERVICE & SUPPORT

The vogella company provides comprehensive training and education services from experts in the areas of Eclipse RCP, Android, Git, Java, Gradle and Spring. We offer both public and inhouse training. Whichever course you decide to take, you are guaranteed to experience what many before you refer to as “The best IT class I have ever attended”.

The vogella company offers expert consulting services, development support and coaching. Our customers range from Fortune 100 corporations to individual developers.

Copyright © 2012-2017 vogella GmbH. Free use of the software examples is granted under the terms of the EPL License. This tutorial is published under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Germany license.

See Licence.