Mocking ResourceBundle

If you want to unit test a method that depends on java.util.ResourceBundle and you want to isolate bundle access code by mocking ResourceBundle, using, for example, the JMock Dynamic Mocking Library, you will face a restriction. Commonly used methods such as getString(key) are defined as final in the ResourceBundle class, and there is no way to extend and override final method declarations.

Fortunately, there is an abstract subclass called ListResourceBundle which can be used during unit test setups to mock ResourceBundle usage. The following excerpt is from a unit test that involves a JSF backing bean fetching application messages from a ResourceBundle during runtime:

public class ApprovalPageCode {
    private ResourceBundle resourceBundle;
    public ResourceBundle getResourceBundle() {
        if(resourceBundle == null) {
            resourceBundle = ResourceBundle.getBundle("msgResources");
        }
        return resourceBundle;
    }
    public void setResourceBundle(ResourceBundle resourceBundle) {
        this.resourceBundle = resourceBundle;
    }
    public String doSuccessOperation() {
        String outcome = getResourceBundle().getString("msgSuccess");
        return outcome;
    }
}

public class TestApprovalPageCode extends TestCase {
    class MsgResourceBundle extends ListResourceBundle {
        private Object[][] contents = new Object[][]{
            {"msgSuccess","success message"},
            {"msgError","error message"}
        };
        protected Object[][] getContents() {
            return contents;
        }
   };
   public void testSuccess() {
       ApprovalPageCode pageCode = new ApprovalPageCode();
       pageCode.setResourceBundle(new MsgResourceBundle());
       String outcome = pageCode.doSuccessOperation();
       assertEquals(outcome,"success message");
   }
}

As seen above, ApprovalPageCode requires a ResourceBundle and creates one using a property file if no resource bundle is provided with it using its setter method. During unit testing, we created an inner class named MsgResourceBundle, which extends ListResourceBundle to provide the required messages, and set an instance of it into the ApprovalPageCode instance before running the test method. As a result, our test is isolated from accessing the file system to create a resource bundle during unit tests.