How do I identify immutable objects in Java

There is no reliable way to detect if a class is immutable. This is because there are so many ways a property of a class might be altered and you can’t detect all of them via reflection.

The only way to get close to this is:

• Only allow final properties of types that are immutable (primitive types and classes you know are immutable),
• Require the class to be final itself
• Require that they inherit from a base class you provide (which is guaranteed to be immutable)

Then you can check with the following code if the object you have is immutable:

static boolean isImmutable(Object obj) {
    Class<?> objClass = obj.getClass();

    // Class of the object must be a direct child class of the required class
    Class<?> superClass = objClass.getSuperclass();
    if (!Immutable.class.equals(superClass)) {
        return false;
    }

    // Class must be final
    if (!Modifier.isFinal(objClass.getModifiers())) {
        return false;
    }

    // Check all fields defined in the class for type and if they are final
    Field[] objFields = objClass.getDeclaredFields();
    for (int i = 0; i < objFields.length; i++) {
        if (!Modifier.isFinal(objFields[i].getModifiers())
                || !isValidFieldType(objFields[i].getType())) {
            return false;
        }
    }

    // Lets hope we didn't forget something
    return true;
}

static boolean isValidFieldType(Class<?> type) {
    // Check for all allowed property types...
    return type.isPrimitive() || String.class.equals(type);
}

Update: As suggested in the comments, it could be extended to recurse on the superclass instead of checking for a certain class. It was also suggested to recursively use isImmutable in the isValidFieldType Method. This could probably work and I have also done some testing. But this is not trivial. You can’t just check all field types with a call to isImmutable, because String already fails this test (its field hash is not final!). Also you are easily running into endless recursions, causing StackOverflowErrors 😉 Other problems might be caused by generics, where you also have to check their types for immutablity.

I think with some work, these potential problems might be solved somehow. But then, you have to ask yourself first if it really is worth it (also performance wise).