Chris Chris - 7 months ago 47
Java Question

How to share JUnit BeforeClass logic among multiple test classes

Currently, all of my JUnit tests extend from a common base class that provides methods tagged with

@BeforeClass
and
@AfterClass
annotations - all these really do is setup a bunch of static resources/services for the tests to use.

This seems a awkward to me for a few reasons:


  1. Part of the point of JUnit4 (from my understanding) is that we shouldn't need this classical test inheritance anymore.

  2. When I run these tests as part of a suite instead of individually (which we often do), the
    @BeforeClass
    and
    @AfterClass
    get invoked multiple times, slowing down the tests - we really should only be calling these once



What I'd like to do is somehow move the current BeforeClass/AfterClass logic out of the inheritance chain and into something that can be shared by individual tests and the suite as a whole.

Can this be done? If so, how? (If it matters, I'm using JUnit 4.7, and it could be a hard sell to update to a different version)

Answer

A solution to the first issue is to move the logic into an extension of org.junit.rules.ExternalResource hooked up to the test via a @ClassRule, introduced in JUnit 4.9:

public class MyTest {
    @ClassRule
    public static final TestResources res = new TestResources();
    @Test
    public void testFoo() {
        // test logic here
    }
}

public class TestResources extends ExternalResource {
    protected void before() {
        // Setup logic that used to be in @BeforeClass
    }
    protected void after() {
        // Setup logic that used to be in @AfterClass
    }
}

In this way, the resources previously managed by the base class are moved out of the test class hierarchy and into more modular/consumable "resources" that can be created before a class runs and destroyed after a class runs.

As for solving both issues at the same time though - ie: having the same high level setup/teardown run as both part of an individual test and as part of a suite - there doesn't seem to be any specific built in support for this. However..., you could implement it yourself:

Simply change the @ClassRule resource creation into a factory pattern that does reference counting internally to determine whether or not to create/destroy the resource.

For example (please note this is rough and might need some tweaks/error handling for robustness):

public class TestResources extends ExternalResource {
    private static int refCount = 0;

    private static TestResources currentInstance;

    public static TestResources getTestResources () {
        if (refCount == 0) {
            // currentInstance either hasn't been created yet, or after was called on it - create a new one
            currentInstance = new TestResources();
        }
        return currentInstance;
    }

    private TestResources() {
        System.out.println("TestResources construction");
        // setup any instance vars
    }

    protected void before() {
        System.out.println("TestResources before");
        try {
            if (refCount == 0) {
                System.out.println("Do actual TestResources init");
            }
        }
        finally {
            refCount++;
        }
    }

    protected void after() {
        System.out.println("TestResources after");
        refCount--;
        if (refCount == 0) {
            System.out.println("Do actual TestResources destroy");
        }
    }
}

Both your suite / test classes would just use the resource as a @ClassResource through the factory method:

@RunWith(Suite.class)
@SuiteClasses({FooTest.class, BarTest.class})
public class MySuite {
    @ClassRule
    public static TestResources res = TestResources.getTestResources();
    @BeforeClass
    public static void suiteSetup() {
        System.out.println("Suite setup");
    }
    @AfterClass
    public static void suiteTeardown() {
        System.out.println("Suite teardown");
    }
}
public class FooTest {
    @ClassRule
    public static TestResources res = TestResources.getTestResources();

    @Test
    public void testFoo() {
        System.out.println("testFoo");
    }
}
public class BarTest {
    @ClassRule
    public static TestResources res = TestResources.getTestResources();

    @Test
    public void testBar() {
        System.out.println("testBar");
    }
}

When running an individual test, the refcounting won't have any effect - the "actual init" and "actual teardown" will only happen once. When running through the suite, the suite will create the TestResource, and the individual tests will just reuse the already instantated one (the refcounting keeps it from being actually destroyed and recreated between tests in the suite).

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