Reflection Exception in parameterized JUnit test using array parameter

Question

I tried to setup a parameterized test in JUnit 4.11 which generates a single parameter. This parameter, however, needs to be an array itself.

The expected behavior for the following code snippet would be that test() is run twice while arguments holds an array {"test1", "test2"} in the first run and an array {"test3", "test4"} in the second run.

@RunWith(Parameterized.class)
public class Tester {

    @Parameter
    private String[] arguments;

    @Parameters
    public static Iterable<Object[]> data() {
        return Arrays.asList(
                new Object[][]{
                    {new String[] {"test1","test2"}},
                    {new String[] {"test3","test4"}}
                }
        );  
    }

    @Test
    public void test() {
        fail();
    }
}

When I execute the test, however, a SlowCheckMemberAccess Exception is thrown. Google delivered no results for that kind of problem.

Can anyone explain what is going wrong here?

The unfiltered JUnit trace is:

java.lang.IllegalAccessException: Class org.junit.runners.Parameterized$TestClassRunnerForParameters can not access a member of class org.mafagafogigante.dungeon.entity.creatures.Tester with modifiers "private"
at sun.reflect.Reflection.ensureMemberAccess(Unknown Source)
at java.lang.reflect.AccessibleObject.slowCheckMemberAccess(Unknown Source)
at java.lang.reflect.AccessibleObject.checkAccess(Unknown Source)
at java.lang.reflect.Field.set(Unknown Source)
at org.junit.runners.Parameterized$TestClassRunnerForParameters.createTestUsingFieldInjection(Parameterized.java:201)
at org.junit.runners.Parameterized$TestClassRunnerForParameters.createTest(Parameterized.java:179)
at org.junit.runners.BlockJUnit4ClassRunner$1.runReflectiveCall(BlockJUnit4ClassRunner.java:244)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.BlockJUnit4ClassRunner.methodBlock(BlockJUnit4ClassRunner.java:241)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.junit.runners.Suite.runChild(Suite.java:127)
at org.junit.runners.Suite.runChild(Suite.java:26)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

Answer 1

The field that is annotated with @Parameter has to be public .

JUnit 4.13 will have an improved error message :

Cannot set parameter 'arguments'. Ensure that the field 'arguments' is public.

Answer 2

I do not think you need Arrays.asList() as arrays are Iterable , too.

Also Arrays.asList() can be fooled if you specify a single array argument to it. If you want to use it, add explicit cast.