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 m

Answer 1

To my knowledge, there is no way to identify immutable objects that is 100% correct. However, I have written a library to get you closer. It performs analysis of bytecode of a class to determine if it is immutable or not, and can execute at runtime. It is on the strict side, so it also allows whitelisting known immutable classes.

You can check it out at: www.mutabilitydetector.org

It allows you to write code like this in your application:

/*
* Request an analysis of the runtime class, to discover if this
* instance will be immutable or not.
*/
AnalysisResult result = analysisSession.resultFor(dottedClassName);

if (result.isImmutable.equals(IMMUTABLE)) {
    /*
    * rest safe in the knowledge the class is
    * immutable, share across threads with joyful abandon
    */
} else if (result.isImmutable.equals(NOT_IMMUTABLE)) {
    /*
    * be careful here: make defensive copies,
    * don't publish the reference,
    * read Java Concurrency In Practice right away!
    */
}

It is free and open source under the Apache 2.0 license.

Answer 2

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 3

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.