Generated Code Testing Incorporating PowerMock

Question

I ran into a snag recently. I am writing some unit tests to exercise some portions of my library responsible for code generation. The code takes an input of a configuration object, writes code using CodeModel, dumps these classes to String, then uses the Java 6 compiler api to compile them on the fly into usable classes.

The snag I am hitting is one of the methods I am testing in the generated code uses a class with final methods (Android Bundle) and throws exceptions when used ("java.lang.RuntimeException: Stub!"). So, to get around this Ive used PowerMock to mock the final methods. In this case, however, the compiler API throws a NPE. I assume this is because of the facilities PowerMock is using behind the scenes, but I am not sure.

Here's the exception:

at org.androidtransfuse.gen.classloader.MemoryFileManager.<init>(MemoryFileManager.java:11)
at org.androidtransfuse.gen.classloader.MemoryClassLoader.<init>(MemoryClassLoader.java:16)
at org.androidtransfuse.gen.ParcelableGeneratorTest.setup(ParcelableGeneratorTest.java:67)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at org.junit.internal.runners.MethodRoadie.runBefores(MethodRoadie.java:129)
at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:93)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.executeTest(PowerMockJUnit44RunnerDelegateImpl.java:296)

MemoryFileManager.java:

class MemoryFileManager extends ForwardingJavaFileManager<JavaFileManager> {
    public final Map<String, Output> map = new HashMap<String, Output>();

    MemoryFileManager(JavaCompiler compiler) {
        super(compiler.getStandardFileManager(null, null, null));
    }

    @Override
    public Output getJavaFileForOutput
            (Location location, String name, JavaFileObject.Kind kind, FileObject source) {
        Output mc = new Output(name, kind);
        this.map.put(name, mc);
        return mc;
    }
}

MemoryClassLoader.java (which uses MemoryFileManager):

public class MemoryClassLoader extends ClassLoader {
    private final JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
    private final MemoryFileManager manager = new MemoryFileManager(this.compiler);

    public void add(String classname, String filecontent) {
        add(Collections.singletonMap(classname, filecontent));
    }

    public void add(Map<String, String> map) {
        List<Source> list = new ArrayList<Source>();
        for (Map.Entry<String, String> entry : map.entrySet()) {
            list.add(new Source(entry.getKey(), JavaFileObject.Kind.SOURCE, entry.getValue()));
        }
        this.compiler.getTask(null, this.manager, null, null, null, list).call();
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        synchronized (this.manager) {
            Output mc = this.manager.map.remove(name);
            if (mc != null) {
                byte[] array = mc.toByteArray();
                return defineClass(name, array, 0, array.length);
            }
        }
        return super.findClass(name);
    }
}

And a very basic test demonstrating this problem:

@RunWith(PowerMockRunner.class)
@PrepareForTest(Parcel.class)
public class ExampleTest {

    private static final String TEST_VALUE = "test value";

    String classToCompile =
            "import android.os.Parcel;\n" +
            "\n" +
            "public class ClassToCompile\n" +
            "{\n" +
            "\n" +
            "    private Parcel parcel;\n" +
            "}";

    @Test
    public void test() throws ClassNotFoundException, IOException, NoSuchMethodException, InvocationTargetException, IllegalAccessException, InstantiationException {
        MemoryClassLoader classLoader = new MemoryClassLoader();
        classLoader.add("ClassToCompile", classToCompile);
        classLoader.loadClass("ClassToCompile");
    }
}

Answer 1

Johan on the Powermock Google Group answered this for me, but I thought I would add the results here as well.

What worked for me is changing from the @RunWith(PowerMockRunner.class) approach to the Java Agent approach described here.

This required me to remove the @RunWith annotation and add the following to my Maven POM:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <configuration>
        <argLine>
            -javaagent:${settings.localRepository}/org/powermock/powermock-module-javaagent/1.4.12/powermock-module-javaagent-1.4.12.jar
        </argLine>
    </configuration>
</plugin>

And when running unit tests in the IDE I had to add

-javaagent: <jarpath>/powermock-module-javaagent-1.4.12.jar

To the run configuration.