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This tutorial explains testing with the Hamcrest matcher framework.

1. Purpose of the Hamcrest matcher framework

Hamcrest is a framework for software tests. Hamcrest allows checking for conditions in your code via existing matchers classes. It also allows you to define your custom matcher implementations.

To use Hamcrest matchers in JUnit you use the assertThat statement followed by one or several matchers.

Hamcrest is typically viewed as a third generation matcher framework. The first generation used assert(logical statement) but such tests were not easily readable. The second generation introduced special methods for assertions, e.g., assertEquals(). This approach leads to lots of assert methods. Hamcrest uses assertThat method with a matcher expression to determine if the test was succesful. See Wiki on Hamcrest for more details.

Hamcrest has the target to make tests as readable as possible. For example, the is method is a thin wrapper for equalTo(value).

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.is;
import static org.hamcrest.Matchers.equalTo;

boolean a;
boolean b;

// all statements test the same
assertThat(a, equalTo(b));
assertThat(a, is(equalTo(b)));
assertThat(a, is(b));

The following snippets compare pure JUnit 4 assert statements with Hamcrest matchers.

// JUnit 4 for equals check
assertEquals(expected, actual);
// Hamcrest for equals check
assertThat(actual, is(equalTo(expected)));

// JUnit 4 for not equals check
assertNotEquals(expected, actual)
// Hamcrest for not equals check
assertThat(actual, is(not(equalTo(expected))));

It is also possible to chain matchers, via the anyOf of allOf method.

assertThat("test", anyOf(is("testing"), containsString("est")));

In general the Hamcrest error messages are also much easier to read.

assertTrue(result instanceof String);
// error message:
java.lang.AssertionError
    at org.junit.Assert.fail(Assert.java:86)
    at org.junit.Assert.assertTrue(Assert.java:41)
    at org.junit.Assert.assertTrue(Assert.java:52)
// ...


assertEquals(String.class, result.getClass());
// error message:
java.lang.NullPointerException
    at com.vogella.hamcrest.HamcrestTest.test(HamcrestTest.java:30)
// ....


assertThat(result, instanceOf(String.class));
// error message:
java.lang.AssertionError:
Expected: an instance of java.lang.String
     but: null
    at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
    at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:8)
// ...

Using Hamcrest matchers also provides more type safety as these matchers use generics.

2. Using Hamcrest matchers

2.1. Defining a Hamcrest dependency for Gradle

To use Hamcrest matchers for a project based on the Gradle build system, add the following dependencies to it.

dependencies {
    // Unit testing dependencies
    testCompile 'junit:junit:4.12'
    // Set this dependency if you want to use Hamcrest matching
    testCompile 'org.hamcrest:hamcrest-library:1.3'
}

2.2. Defining a Hamcrest dependency for Maven

To use the library for a Maven based project, the following dependency to your pom file.

<dependency>
<groupId>org.hamcrest</groupId>
<artifactId>hamcrest-library</artifactId>
<version>1.3</version>
<scope>test</scope>
</dependency>

2.3. Adding Hamcrest directly to the classpath in Eclipse

The JUnit distribution included in Eclipse only contain the core Hamcrest matcher. To use all available matchers, download the latest hamcrest-all-*.jar from https://code.google.com/p/hamcrest/downloads/list and add it to your projects classpath.

If you get the following exception "java.lang.SecurityException: class "org.hamcrest.Matchers"'s signer information does not match signer information of other classes in the same package", ensure that the hamcrest jar is before the Junit library in the build path. You an configure the order in the project properties in the Eclipse IDE under Java Build Path on the Order and Export tab.

Changing the build patch order for Hamcrest

3. Using Hamcrest

3.1. Example

The usage of Hamcrest matchers is demonstrates by the following code snippet.

assertThat(Long.valueOf(1), instanceOf(Integer.class));
// shortcut for instanceOf
assertThat(Long.valueOf(1), isA(Integer.class));

3.2. Static import

To make all matchers available in your file add an static import. This also makes it easier to find matchers through code completion.

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.*;

3.3. Hamcrest matchers for lists

The usage of the Hamcrest matchers for lists are demonstrated by the following example.

import org.junit.Test;

import java.util.Arrays;
import java.util.List;

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.contains;
import static org.hamcrest.Matchers.containsInAnyOrder;
import static org.hamcrest.Matchers.greaterThan;
import static org.hamcrest.collection.IsCollectionWithSize.hasSize;
import static org.hamcrest.core.Every.everyItem;

public class HamcrestListMatcherExamples {
    @Test
    public void listShouldInitiallyBeEmpty() {
        List<Integer> list = Arrays.asList(5, 2, 4);

        assertThat(list, hasSize(3));

        // ensure the order is correct
        assertThat(list, contains(5, 2, 4));

        assertThat(list, containsInAnyOrder(2, 4, 5));

        assertThat(list, everyItem(greaterThan(1)));

    }
}
// Check that a list of objects has a property race and
// that the value is not ORC
assertThat(fellowship, everyItem(hasProperty("race", is(not((ORC))))));

3.4. Overview of Hamcrest mather

The following are the most important Hamcrest matchers:

  • allOf - matches if all matchers match (short circuits)

  • anyOf - matches if any matchers match (short circuits)

  • not - matches if the wrapped matcher doesn’t match and vice

  • equalTo - test object equality using the equals method

  • is - decorator for equalTo to improve readability

  • hasToString - test Object.toString

  • instanceOf, isCompatibleType - test type

  • notNullValue, nullValue - test for null

  • sameInstance - test object identity

  • hasEntry, hasKey, hasValue - test a map contains an entry, key or value

  • hasItem, hasItems - test a collection contains elements

  • hasItemInArray - test an array contains an element

  • closeTo - test floating point values are close to a given value

  • greaterThan, greaterThanOrEqualTo, lessThan, lessThanOrEqualTo

  • equalToIgnoringCase - test string equality ignoring case

  • equalToIgnoringWhiteSpace - test string equality ignoring differences in runs of whitespace

  • containsString, endsWith, startsWith - test string matching

To see all matchers, use API reference.

4. Exercise - Writing a custom Hamcrest matcher using FeatureMatcher

4.1. Target

The target of this exercise is to write a custom matcher with Hamcrest.

4.2. Create Hamcrest Matchers

Define a custom matcher for Hamcrest which provides the length matcher for a String. We want to use the class FeatureMatcher. With FeatureMatcher we can wrap an existing Matcher, decide which field of the given Object under test to match and provide a nice error message. The constructor of FeatureMatcher takes the following arguments in this order:

  • The matcher we want to wrap

  • a description of the feature that we tested

  • a description of the possible mismatch

The only method we have to overwrite is featureValueOf(T actual) which returns the value which will get passed into the wrapped matches()/matchesSafely() method.

public static Matcher<String> length(Matcher<? super Integer> matcher) {
    return new FeatureMatcher<String, Integer>(matcher, "a String of length that", "length") {
        @Override
        protected Integer featureValueOf(String actual) {
            return actual.length();
        }
    };
}

4.3. Validate

Use your custom matcher to check that "Gandalf" has a lenght of 8.

@Test
public void fellowShipOfTheRingShouldContainer7() {
        assertThat("Gandalf", length(is(8)));
}
public static  Matcher<String> length(Matcher<? super Integer> matcher) {
        return new FeatureMatcher<String, Integer>(matcher, "a String of length that", "length") {
                @Override
                protected Integer featureValueOf(String actual) {
                  return actual.length();
                }
        };
}

5. Exercise: Writing your custom Hamcrest matcher using TypeSafeMatcher

It is possible to write your custom Hamcrest matcher by extending TypeSafeMatcher. In contrast to BaseMatcher the TypeSafeMatcher class automatically checks for null values, checks the type and casts appropriately before delegating to matchesSafely(). It provides type safety by default. The following is an example for defining a matcher which allows testing if a String matches a regular expression.

import org.hamcrest.Description;
import org.hamcrest.TypeSafeMatcher;

public class RegexMatcher extends TypeSafeMatcher<String> {

        private final String regex;

        public RegexMatcher(final String regex) {
                this.regex = regex;
        }

        @Override
        public void describeTo(final Description description) {
                description.appendText("matches regular expression=`" + regex + "`");
        }

        @Override
        public boolean matchesSafely(final String string) {
                return string.matches(regex);
        }


         // matcher method you can call on this matcher class
    public static RegexMatcher matchesRegex(final String regex) {
        return new RegexMatcher(regex);
    }
}

The following snippet gives an example how to use it.

package com.vogella.android.testing.applicationtest;


import org.junit.Test;

import static org.hamcrest.MatcherAssert.assertThat;

public class TestCustomMatcher {

    @Test
    public void testRegularExpressionMatcher() throws Exception {
        String s ="aaabbbaaaa";
        assertThat(s, RegexMatcher.matchesRegex("a*b*a*"));
    }

}

6. Exercise: Combining matchers

Combining matchers is supported by Hamcrest out of the box but it has the limitation that the error is hard to read:

@Test
public void () {
    List<Integer> list = new ArrayList<>();
    assertThat(list, both(hasSize(1)).and(contains(42)));
}
Expected: (a collection with size <1> and iterable containing [<42>])
     but: a collection with size <1> collection size was <0>.

This not very readable.

6.1. Target

We want to write our own MatcherCombiner that provides us with a readable error message, even when multiple matchers fail.

6.2. Create MatchCombiner

We do this by inheriting from BaseMatch and by providing a starting method that let’s us chain matchers together. The matchers get saved in a list that we iterate over during the matching phase.

public class MatcherCombinator<T> extends BaseMatcher<T> {
    private final List<Matcher<? super T>> matchers = new ArrayList<>();
    private final List<Matcher<? super T>> failedMatchers = new ArrayList<>();

    private MatcherCombinator(final Matcher<? super T> matcher) {
        matchers.add(matcher);
    }

    public MatcherCombinator<T> and(final Matcher<? super T> matcher) {
        matchers.add(matcher);
        return this;
    }

    @Override
    public boolean matches(final Object item) {
        boolean matchesAllMatchers = true;
        for (final Matcher<? super T> matcher : matchers) {
            if (!matcher.matches(item)) {
                failedMatchers.add(matcher);
                matchesAllMatchers = false;
            }
        }
        return matchesAllMatchers;
    }

    @Override
    public void describeTo(final Description description) {
        description.appendValueList("\n", " " + "and" + "\n", "", matchers);
    }

    @Override
    public void describeMismatch(final Object item, final Description description) {
        description.appendText("\n");
        for (Iterator<Matcher<? super T>> iterator = failedMatchers.iterator(); iterator.hasNext();) {
            final Matcher<? super T> matcher = iterator.next();
            description.appendText("Expected: <");
            description.appendDescriptionOf(matcher).appendText(" but ");
            matcher.describeMismatch(item, description);
            if (iterator.hasNext()) {
                description.appendText(">\n");
            }
        }
    }

    public static <LHS> MatcherCombinator<LHS> matches(final Matcher<? super LHS> matcher) {
        return new MatcherCombinator<LHS>(matcher);
    }
}

To validate the implementation we write a new test.

@Test
public void test() {
    List<Integer> list = new ArrayList<>();
    assertThat(list, matches(hasSize(1)).and(contains(42)));
}
java.lang.AssertionError:
Expected:
<a collection with size <1>> and
<iterable containing [<42>]>
     but:
Expected: <a collection with size <1> but collection size was <0>>
Expected: <iterable containing [<42>] but No item matched: <42>.

You can adjust this output in the describeMismatch method.

7. Grouping your matchers for import

If you define many custom matchers it might become tedious to import them one by one into your test files. By grouping them into a single class you can import them with one statement. You can also group them together with Hamcrest matchers.

package com.vogella.hamcrest;
import com.vogella.hamcrest.matchers.RegexMatcher;
import org.hamcrest.Matcher;
import org.hamcrest.Matchers;

public class MyMatchers
{
    public static <T> Matcher<T> instanceOf(Class<T> target) {
        return Matchers.instanceOf(target);
    }

   public static Matcher<String> matchesRegex(String target) {
          return RegexMatcher.matchesRegex(target);
   }
}

In your test file:

import static com.vogella.hamcrest.MyMatchers.*;

8. About this website

9. Hamcrest resources

9.1. vogella GmbH training and consulting support

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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.