1. Android library projects and Java libraries

Android project can use code contained in JAR files (Java libraries). It is also possible to create libraries modules which can be used as dependencies in Android projects. These modules allow you to store source code and Android resources which can be shared between several other Android projects.

2. Using JAR files in Android

2.1. How to use JAR files

To use a Java library (JAR file) inside your Android project, you can simple copy the JAR file into the folder called <filename class="directory">libs_ in your application. Depending on your IDE this automatically makes your JAR available or your have to do an additional step.

2.2. Adding a module dependency

In Android Studio you have to

  • Right-click the JAR file in the libs folder and select Add as library.

  • Check that you have a new entry compile files('libs/YOURJAR.jar' in your build.gradle file.

  • To ensure that the change is picked up, perform a clean build.

2.3. Restrictions in using Java libraries

If you want to use libraries, these must only use API available in Android. For example, the Android libraries do not contain the java.awt and javax.swing user interface libraries, as Android has its own user interface toolkit.

Library projects cannot be compiled to Android applications and started without another project using them.

Using library projects helps you to structure your application code. Also more and more important Open Source libraries are available for Android. Understanding library projects is therefore important for every Android programmer.

3. Custom Android library modules

3.1. Using custom library modules

If an Android application project uses an Android library module, the Android development tools include the code and resources from the library project into the build result of the Android project. This means, that the components, code and resources of the library project are compiled and packaged into the .apk file of the compiled application.

Therefore a library module can be considered to be a compile-time artifact. An Android library module can contain Java classes, Android components and resources. Only assets are not supported.

To create a library project, set the Mark this project as library flag in the Android project generation wizard.

The library project must declare all its components, e.g., activities , service , etc. via the AndroidManifest.xml file. The application which uses the library must also declare all the used components via the AndroidManifest.xml file.

3.2. Priorities for conflicting resources

The Android development tools merges the resources of a library project with the resources of the application project. In the case that a resource’s ID is defined several times, the tools select the resource from the application, or the library with highest priority, and discard the other resource.

4. Creating custom Android library modules in Android Studio

To create a new libary module in Android Studio, select File ▸ New Module and select Android Library.

5. Android library project

The Android team introduced a new binary distribution format called Android ARchive(AAR) . The .aar bundle is the binary distribution of an Android Library Project.

An AAR is similar to a JAR file, but it can contain resources as well as compiled byte-code. This allows that an AAR file is included in the build process of an Android application similar to a JAR file

This AAR format is currently directly support by the Eclipse IDE. Stare on the following issue so that Google solves this issue: https://code.google.com/p/android/issues/detail?id=59183 Allows Eclipse ADT plugin to work with .AAR files.
It is still possible to use AAR files with Eclipse. You can either convert them to Android library projects as described in http://commonsware.com/blog/2014/07/03/consuming-aars-eclipse.html - Consuming AARs from Eclipse blog post about it. Or use the https://code.google.com/p/maven-android-plugin/wiki/AAR - Android Maven plug-in for your build.

6. The support libraries from Google

7. Prerequisite

The following example assumes that you have created a normal Android project called com.example.android.rssfeed based on the http://www.vogella.com/tutorials/AndroidFragments/article.html#fragments_tutorial - Android Fragments tutorial.

8. Exercise: Create Android library module

8.1. Target

Our library project will not contribute Android components but a data model and a method to get the number of instances. The library provides access to (fake) RSS data. An RSS document is an XML file which can be used to publish blog entries and news. The format of the XML file is specified via the RSS specification.

Our library project will not contribute Android components but a data model and a method to get the number of instances. We will provide RSS-feed data. You can find an introduction to RSS under: http://www.vogella.com/tutorials/RSSFeed/article.html#rssoverview

8.2. Create library module

For Android Studio each library is a module. To create a new library module in Android Studio, select File ▸ New Module and select Android Library.

Selection for creating a library project

Use com.example.android.rssfeedlibrary as module name and Rssfeed Library as library name.

Setting the library property

If prompted for a template select that no activity should be created. As a result Android Studio shows another module.

Setting the library property

8.3. Create the model class

Create an RssItem class which can store data of an RSS entry.

Generate the getters and setter, the constructor and a toString() method. The result should look like the following class:

package com.example.android.rssfeedlibrary;

public class RssItem {
        private String pubDate;
        private String description;
        private String link;
        private String title;

        public RssItem() {
        }

        public RssItem(String title, String link) {
                this.title = title;
                this.link = link;
        }
        public String getPubDate() {
                return pubDate;
        }

        public void setPubDate(String pubDate) {
                this.pubDate = pubDate;
        }

        public String getDescription() {
                return description;
        }

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

        public String getLink() {
                return link;
        }

        public void setLink(String link) {
                this.link = link;
        }

        public String getTitle() {
                return title;
        }

        public void setTitle(String title) {
                this.title = title;
        }

        @Override
        public String toString() {
                return "RssItem [title=" + title + "]";
        }

}

8.4. Create instances

Create a new class called RssFeedProvider with a static method to return a list of RssItem objects. This method does currently only return test data.

package com.vogella.android.rssfeedlibrary;

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

public class RssFeedProvider {
    public static List<RssItem> parse(String rssFeed) {
        List<RssItem> list = new ArrayList<>();
        Random r = new Random();
        // random number of item but at least 5
        Integer number = r.nextInt(10) + 5;
        for (int i = 0; i < number; i++) {
            // create sample data
            String s = String.valueOf(r.nextInt(1000));
            RssItem item = new RssItem("Summary " + s, "Description " + s);
            list.add(item);
        }
        return list;
    }
}

8.5. Define dependency to the library project

To use the library add it as a dependency in your project select File ▸ Project Structure. Select the app entry. Switch to the Dependencies tab and select Module dependencies via the + sign.

Define dependency in Android Studio - Selecting dependency
Define dependency in Android Studio - Select module
Define dependency in Android Studio - Select module

8.6. Use library project to update detailed fragments

Use the static method of RssFeedProvider to get the list of RssItem objects and display the number in your DetailFragment instead of current system time.

To send the new data, change the updateDetail method in your MyListFragment class.

package com.example.android.rssreader;

import android.app.Activity;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

import com.example.android.rssfeedlibrary.RssFeedProvider;
import com.example.android.rssfeedlibrary.RssItem;

import java.util.List;

public class MyListFragment extends Fragment {

        private OnItemSelectedListener listener;

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                        Bundle savedInstanceState) {
                View view = inflater.inflate(R.layout.fragment_rsslist_overview,
                                container, false);
                Button button = (Button) view.findViewById(R.id.button1);
                button.setOnClickListener(new View.OnClickListener() {
                        @Override
                        public void onClick(View v) {
                                updateDetail("fake");
                        }
                });
                return view;
        }

        public interface OnItemSelectedListener {
            public void onRssItemSelected(String link);
          }

        @Override
          public void onAttach(Context context) {
            super.onAttach(context);
            if (context instanceof OnItemSelectedListener) {
              listener = (OnItemSelectedListener) context;
            } else {
              throw new ClassCastException(context.toString()
                  + " must implement MyListFragment.OnItemSelectedListener");
            }
          }

    // triggers update of the details fragment
    public void updateDetail(String uri) {
        List<RssItem> list = RssFeedProvider
                .parse("http://www.vogella.com/article.rss");
        String text = String.valueOf(list.toString());
        listener.onRssItemSelected(text);
    }


}

8.7. Validate implementation

Start your application and ensure that the toString value of the (at the moment randomly generated) list of RssItems is displayed in the detailed fragment.

9. Creating custom Android library modules in Eclipse ADT

9.1. Defining Libraries modules in Eclipse ADT

To use such a library project in Eclipse, select the generated project, right-click on it and select Properties. On the Android tab, add the library project to it.

If you define components, e.g., activities in your library project, you need to re-define these components in the consuming Android application. If you want to automatically add all components from your library project to your application, add the manifestmerger.enabled=true to your project.properties file of your application project.

Copyright © 2012-2016 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.