How to mock ResourceBundle.getString()?

Instead of mocking you can create a dummy ResourceBundle implementation, and then pass it as an argument to .thenReturn(resourceBundle):

    import java.util.ResourceBundle;

    ResourceBundle dummyResourceBundle = new ResourceBundle() {
        @Override
        protected Object handleGetObject(String key) {
            return "fake_translated_value";
        }

        @Override
        public Enumeration<String> getKeys() {
            return Collections.emptyEnumeration();
        }
    };

    // Example usage
    when(request.getResourceBundle(any(Locale.class))).thenReturn(dummyResourceBundle)

If you need the actual keys and values, then you'll need to provide an implementation for getKeys(), e.g. a hashmap for storage and key lookup.

@Powermockito did not worked as ResourceBundle.class have static final methods which were not easy to mock.

I tried.

In the Main Class extract your method inside another public method, and then overide it with implementaion.

Here ReviewEChannelApplicationMBean is my Controller, where i overrriden the getBundle.

ReviewEChannelApplicationMBean = spy(new ReviewEChannelApplicationMBean(){
            @Override
            public ResourceBundle getBundle(FacesContext fcContext) {
                return TestDataBuilder.getResourceBundle();
            }
        });

//This Class i my TestDataBuilder using ListResourceBundle

public class TestDataBuilder {
    public static ResourceBundle getResourceBundle() {
            return new ListResourceBundle(){

                @Override
                protected Object[][] getContents() {
                    return contents;
                }

                private Object[][] contents = {
                        {"test1","01"},
                        {"test2","01"},
                        {"test3","01"},
                        {"test4","01"}
                };
            };

        }
}

You'll find an example of solution below :

@RunWith(PowerMockRunner.class)
@PrepareForTest({ ResourceBundle.class })
public class ResourceBundleTest {

    @Test
    public void getStringByPowerMock() throws Exception {   
        ResourceBundle resourceBundle = PowerMockito.mock(ResourceBundle.class);
        Mockito.when(resourceBundle.getString(Mockito.anyString())).thenReturn("Hello world....");
        System.out.println(resourceBundle.getString("keyword"));
    }

}

I figured out a way to mock ResourceBundle by subclassing ResourceBundle.Control. My answer is here:

https://stackoverflow.com/a/28640458/290254

I prefer to avoid the dynamic bytecode rewriting (to remove final) of PowerMock, JMockit and friends, since Jacoco and other things seem to hate it.