【mockito】Spyで他のクラスのメソッドをテストしたい

Question

Java,junit,Mockitoを使って以下のようなテストをしています。
「ここで落ちる」と書いた箇所でエラーになります。
外部のクラスのメソッドの振る舞いをmock化したいとき、どのように実装すればよいでしょうか？
以下のテストを通したいです。

public class Sample1{

	public Sample2 sample2;


}

public class Sample2{

    public String methodA() {
        return "called method1";
    }
}

import static org.mockito.Mockito.*;

import org.junit.Before;
import org.junit.Test;
import org.mockito.Mockito;
import org.mockito.Spy;


public class MockTest {

	@Spy
	Sample1 sample1 = new Sample1();

	@Before
	public void init() {
	}

	@Test
	public void testcase002() {
		Sample1 sample1 =Mockito.spy(new Sample1());
		Sample2 sample2 = new Sample2();
		sample1.sample2 = sample2;

		when(sample1.sample2.methodA()).thenReturn("when called method1");  //ここで落ちる
		System.out.println(sample1.sample2.methodA());
	}
}

org.mockito.exceptions.misusing.MissingMethodInvocationException: 
when() requires an argument which has to be 'a method call on a mock'.
For example:
    when(mock.getArticles()).thenReturn(articles);

Also, this error might show up because:
1. you stub either of: final/private/equals()/hashCode() methods.
   Those methods *cannot* be stubbed/verified.
2. inside when() you don't call method on mock but on some other object.
3. the parent of the mocked class is not public.
   It is a limitation of the mock engine.

	at MockTest.testcase002(MockTest.java:24)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
	at org.junit.vintage.engine.execution.RunnerExecutor.execute(RunnerExecutor.java:39)
	at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.Iterator.forEachRemaining(Iterator.java:116)
	at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
	at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
	at org.junit.vintage.engine.VintageTestEngine.executeAllChildren(VintageTestEngine.java:79)
	at org.junit.vintage.engine.VintageTestEngine.execute(VintageTestEngine.java:70)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:170)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:154)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:90)
	at org.eclipse.jdt.internal.junit5.runner.JUnit5TestReference.run(JUnit5TestReference.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:538)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)

Answer 1

@Test
	public void testcase002() {
		Sample2 sample2 = Mockito.spy(new Sample2());
		sample1.sample2 = sample2;

		when(sample1.sample2.methodA()).thenReturn("when called method1");
		System.out.println(sample1.sample2.methodA());
	}

Spyすべきはsample2でした。それをsample1に代入しました。そうすればテストできました。