Mocking Exception Throwing using Mockito

1. Overview

In this quick tutorial, we'll focus on how to configure a method call to throw an exception with Mockito.

For more information on the library, also check out our Mockito series.

Here's the simple dictionary class that we'll use:

class MyDictionary {
    private Map<String, String> wordMap = new HashMap<>();

    public void add(String word, String meaning) {
        wordMap.put(word, meaning);
    }

    public String getMeaning(String word) {
        return wordMap.get(word);
    }
}

2. Non-Void Return Type

First, if our method return type is not void, we can use when().thenThrow():

@Test
void whenConfigNonVoidRetunMethodToThrowEx_thenExIsThrown() {
    MyDictionary dictMock = mock(MyDictionary.class);
    when(dictMock.getMeaning(anyString())).thenThrow(NullPointerException.class);
    assertThrows(NullPointerException.class, () -> {
        dictMock.getMeaning("word");
    });
}

Notice that we configured the getMeaning() method — which returns a value of type String — to throw a NullPointerException when called.

3. Void Return Type

Now, if our method returns void, we'll use doThrow():

@Test
void whenConfigVoidRetunMethodToThrowEx_thenExIsThrown() {
    MyDictionary dictMock = mock(MyDictionary.class);
    doThrow(IllegalStateException.class).when(dictMock)
        .add(anyString(), anyString());
    assertThrows(IllegalStateException.class, () -> {
        dictMock.add("word", "meaning");
    });
}

Here, we configured an add() method — which returns void — to throw IllegalStateException when called.

We can't use when().thenThrow() with void return type, as the compiler doesn't allow void methods inside brackets.

4. Exception as an Object

To configure the exception itself, we can pass the exception's class as in our previous examples or as an object:

@Test
void whenConfigNonVoidRetunMethodToThrowExWithNewExObj_thenExIsThrown() {
    MyDictionary dictMock = mock(MyDictionary.class);
    when(dictMock.getMeaning(anyString())).thenThrow(new NullPointerException("Error occurred"));
    assertThrows(NullPointerException.class, () -> {
        dictMock.getMeaning("word");
    });
}

And we can do the same with doThrow():

@Test
void whenConfigVoidRetunMethodToThrowExWithNewExObj_thenExIsThrown() {
    MyDictionary dictMock = mock(MyDictionary.class);
    doThrow(new IllegalStateException("Error occurred")).when(dictMock)
        .add(anyString(), anyString());
    assertThrows(IllegalStateException.class, () -> {
        dictMock.add("word", "meaning");
    });
}

5. Spy

We can also configure Spy to throw an exception the same way we did with the mock:

@Test
void givenSpy_whenConfigNonVoidRetunMethodToThrowEx_thenExIsThrown() {
    MyDictionary dict = new MyDictionary();
    MyDictionary spy = Mockito.spy(dict);
    when(spy.getMeaning(anyString())).thenThrow(NullPointerException.class);
    assertThrows(NullPointerException.class, () -> {
        spy.getMeaning("word");
    });
}

6. Conclusion

In this article, we explored how to configure method calls to throw an exception in Mockito.

As always, the full source code can be found over on GitHub.

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