How do I identify immutable objects in Java

Question

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.

Answer 1

Basically no.

You could build a giant white-list of accepted classes but I think the less crazy way would be to just write in the documentation for the collection that everything that goes is this collection must be immutable.

Edit: Other people have suggested having an immutable annotation. This is fine, but you need the documentation as well. Otherwise people will just think "if I put this annotation on my class I can store it in the collection" and will just chuck it on anything, immutable and mutable classes alike. In fact, I would be wary of having an immutable annotation just in case people think that annotation makes their class immutable.

Answer 2

At my company we've defined an Attribute called @Immutable. If you choose to attach that to a class, it means you promise you're immutable.

It works for documentation, and in your case it would work as a filter.

Of course you're still depending on the author keeping his word about being immutable, but since the author explicitly added the annotation it's a reasonable assumption.

Answer 3

You Can Ask your clients to add metadata (annotations) and check them at runtime with reflection, like this:

Metadata:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.CLASS)
public @interface Immutable{ }

Client Code:

@Immutable
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; }
}

Then by using reflection on the class, check if it has the annotation (I would paste the code but its boilerplate and can be found easily online)

Answer 4

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).

Answer 5

This could be another hint:

If the class has no setters then it cannot be mutated, granted the parameters it was created with are either "primitive" types or not mutable themselves.

Also no methods could be overriden, all fields are final and private,

I'll try to code something tomorrow for you, but Simon's code using reflection looks pretty good.

In the mean time try to grab a copy of the "Effective Java" book by Josh Block , it has an Item related to this topic. While is does not for sure say how to detect an inmmutable class, it shows how to create a good one.

The item is called: "Favor immutability"

link: http://java.sun.com/docs/books/effective/

Answer 6

Like the other answerers already said, IMHO there is no reliable way to find out if an object is really immutable.

I would just introduce an interface "Immutable" to check against when appending. This works as a hint that only immutable objects should be inserted for whatever reason you're doing it.

interface Immutable {}

class MyImmutable implements Immutable{...}

public void add(Object o) {
  if (!(o instanceof Immutable) && !checkIsImmutableBasePrimitive(o))
    throw new IllegalArgumentException("o is not immutable!");
  ...
}

Answer 7

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.

Not a direct answer to your question, but keep in mind that objects that are immutable are not automatically guaranteed to be thread safe (sadly). Code needs to be side-effect free to be thread safe, and that's quite a bit more difficult.

Suppose you have this class:

class Foo {
  final String x;
  final Integer y;
  ...

  public bar() {
    Singleton.getInstance().foolAround();
  }
}

Then the foolAround() method might include some non-thread safe operations, which will blow up your app. And it's not possible to test for this using reflection, as the actual reference can only be found in the method body, not in the fields or exposed interface.

Other than that, the others are correct: you can scan for all declared fields of the class, check if every one of them is final and also an immutable class, and you're done. I don't think methods being final is a requirement.

Also, be careful about recursively checking dependent fields for immutability, you might end up with circles:

class A {
  final B b; // might be immutable...
}

class B {
  final A a; // same so here.
}

Classes A and B are perfectly immutable (and possibly even usable through some reflection hacks), but naive recursive code will go into an endless loop checking A, then B, then A again, onwards to B, ...

You can fix that with a 'seen' map that disallows cycles, or with some really clever code that decides classes are immutable if all their dependees are immutable only depending on themselves, but that's going to be really complicated...

Answer 8

Use the Immutable annotation from Java Concurrency in Practice. The tool FindBugs can then help in detecting classes which are mutable but shouldn't be.

Answer 9

why do all the recommendations require the class to be final? if you are using reflection to check the class of each object, and you can determine programmatically that that class is immutable (immutable, final fields), then you don't need to require that the class itself is final.

Answer 10

Check out joe-e, a implementation of capabilities for java.

Answer 11

Something which works for a high percentage of builtin classes is test for instanceof Comparable. For the classes which are not immutable like Date, they are often treated as immutable in most cases.