Java – How to identify immutable objects in Java

functional programmingimmutabilityjava

In my code, I am creating a collection of objects which will be accessed by various threads in a fashion that is only safe if the objects are immutable. When an attempt is made to insert a new object into my collection, I want to test to see if it is immutable (if not, I'll throw an exception).

One thing I can do is to check a few well-known immutable types:

private static final Set<Class> knownImmutables = new HashSet<Class>(Arrays.asList(
        String.class, Byte.class, Short.class, Integer.class, Long.class,
        Float.class, Double.class, Boolean.class, BigInteger.class, BigDecimal.class


public static boolean isImmutable(Object o) {
    return knownImmutables.contains(o.getClass());

This actually gets me 90% of the way, but sometimes my users will want to create simple immutable types of their own:

public class ImmutableRectangle {
    private final int width;
    private final int height;
    public ImmutableRectangle(int width, int height) {
        this.width = width;
        this.height = height;
    public int getWidth() { return width; }
    public int getHeight() { return height; }

Is there some way (perhaps using reflection) that I could reliably detect whether a class is immutable? False positives (thinking it's immutable when it isn't) are not acceptable but false negatives (thinking it's mutable when it isn't) are.

Edited to add: Thanks for the insightful and helpful answers. As some of the answers pointed out, I neglected to define my security objectives. The threat here is clueless developers — this is a piece of framework code that will be used by large numbers of people who know next-to-nothing about threading and won't be reading the documentation. I do NOT need to defend against malicious developers — anyone clever enough to mutate a String or perform other shenanigans will also be smart enough to know it's not safe in this case. Static analysis of the codebase IS an option, so long as it is automated, but code reviews cannot be counted on because there is no guarantee every review will have threading-savvy reviewers.

Best Answer

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).