Unit Testing a static method

Question

I am trying to write a test case for the method decrypt here.

    private static Codec codec;

    static {
        try {
            codec = new Codec(encryptionType, encryptionKey, false, true, false);
        } catch (CodecException e) {
            throw new RuntimeException("Codec initialisation failed", e);
        }
    }


    public static String decrypt(final String toDecrypt) throws CodecException {
        String decrypted = codec.decryptFromBase64(toDecrypt);
        if (decrypted.endsWith(":")) {
            decrypted = decrypted.substring(0, decrypted.length() - 1);
        }
        return decrypted;
    }

Test Case:

    @Mock
    private Codec codec;
    @Test
    public void test_decrypt_Success() throws CodecException {
        when(codec.decryptFromBase64(TestConstants.toDecrypt)).thenReturn(TestConstants.decrypted);
        assertEquals(DocumentUtils.decrypt(TestConstants.toDecrypt), TestConstants.decrypted);
    }

Since this is a static method, I can't inject an instance of the class in the test suite and mock its codec. The above code throws an error from the codec library at assert as expected.

What is your approach to testing static methods like this? Or should I not be writing tests for this at all?

Answer 1

In Java, static methods are not designed to set dependencies.
So switching the dependency into a mock is really not natural.
You could provide a static setter for the field such as :

private static Codec codec;
public static void setCodec(Codec codec){
   this.codec = codec;
}

And you could set a mock with setCodec(...) but ugh...

But forget, just do things well : refactor the code to remove all static and introduce a constructor that sets the codec.

private Codec codec;
public MyClassUnderTest(Codec codec){
   this.codec codec;
}

IOC could help here to make the class under test a singleton and ease the dependency injections.
If not possible in your case, Java 5 enumeration could help you for at least the singleton concern.