如果你想在Foo的任何示例上调用doSomething方法时，Foo返回相同的Bar示例，你应该使用PowerMock。

@RunWith(PowerMockRunner.class)
@PrepareForTest(Foo.class)
public class FooMockAllInstanceTest {

    @Test
    public void testMockInstanceofObjectCreation() throws Exception {
        Bar mockBar = PowerMockito.mock(Bar.class);
        when(mockBar.sayHello()).thenReturn("Hi John!");
        PowerMockito.whenNew(Bar.class)
                .withNoArguments()
                .thenReturn(mockBar);

        Foo myFooOne = new Foo();
        assertEquals(mockBar,  myFooOne.doSomething("Jane"));

        Foo myFooTwo = new Foo();
        assertEquals(mockBar,  myFooTwo.doSomething("Sarah"));

        Baz bazOne = new Baz();
        assertEquals(mockBar, bazOne.doSomething("Sam"));

        Baz bazTwo = new Baz();
        assertEquals(mockBar, bazTwo.doSomething("Nina"));
    }
}

这个例子将返回相同的Bar对象，即使调用了Baz。

public class Baz {

    public Bar doSomething(String name) {
        Foo foo = new Foo();
        return foo.doSomething(name);
    }
}

• • 更新2**

还有一个稍微好一点的方法来测试PowerMock。

@Test
public void testStubbingMethod() throws Exception {
    Bar mockBar = PowerMockito.mock(Bar.class);
    when(mockBar.sayHello()).thenReturn("Hi John!");

    PowerMockito.stub(PowerMockito.method(Foo.class, "doSomething",
            String.class)).toReturn(mockBar);

    Foo myFooOne = new Foo();
    assertEquals(mockBar, myFooOne.doSomething("Jane"));

    Foo myFooTwo = new Foo();
    assertEquals(mockBar, myFooTwo.doSomething("Sarah"));

    Baz bazOne = new Baz();
    assertEquals(mockBar, bazOne.doSomething("Sam"));

    Baz bazTwo = new Baz();
    assertEquals(mockBar, bazTwo.doSomething("Nina"));
}

我发现这个页面告诉"如何模拟新对象的构造"
https://github.com/powermock/powermock/wiki/MockConstructor#example

// Class to be tested and need mock File object

public class PersistenceManager {

    public boolean createDirectoryStructure(String directoryPath) {
        File directory = new File(directoryPath);

        if (directory.exists()) {
            throw new IllegalArgumentException("\"" + directoryPath + "\" already exists.");
        }

        return directory.mkdirs();
    }
}

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

import java.io.File;

import static org.easymock.EasyMock.expect;
import static org.junit.Assert.assertTrue;
import static org.powermock.api.easymock.PowerMock.*;

@RunWith(PowerMockRunner.class)
@PrepareForTest( PersistenceManager.class )
public class PersistenceManagerTest {

    @Test
    public void testCreateDirectoryStructure_ok() throws Exception {
        final String path = "directoryPath";
        File fileMock = createMock(File.class);

        PersistenceManager tested = new PersistenceManager();

        expectNew(File.class, path).andReturn(fileMock);

        expect(fileMock.exists()).andReturn(false);
        expect(fileMock.mkdirs()).andReturn(true);

        replay(fileMock, File.class);

        assertTrue(tested.createDirectoryStructure(path));

        verify(fileMock, File.class);
    }
}