ClassValue in Java 7

Its purpose it to allow adding runtime information to arbitrary target classes (reference).

I think its targeted more towards dynamic language programmers. I am not sure how it will be useful for general application developers though.

Initially the class was there in the package java.dyn. This bug shows it moving to java.lang.

ClassValue cache something about the class.

Here is a part of code (at Lucene 5.0 AttributeSource.java):

/** a cache that stores all interfaces for known implementation classes for performance (slow reflection) */
private static final ClassValue<Class<? extends Attribute>[]> implInterfaces = new ClassValue<Class<? extends Attribute>[]>() {
    @Override
    protected Class<? extends Attribute>[] computeValue(Class<?> clazz) {
      final Set<Class<? extends Attribute>> intfSet = new LinkedHashSet<>();
      // find all interfaces that this attribute instance implements
      // and that extend the Attribute interface
      do {
        for (Class<?> curInterface : clazz.getInterfaces()) {
          if (curInterface != Attribute.class && Attribute.class.isAssignableFrom(curInterface)) {
            intfSet.add(curInterface.asSubclass(Attribute.class));
          }
        }
        clazz = clazz.getSuperclass();
      } while (clazz != null);
      @SuppressWarnings({"unchecked", "rawtypes"}) final Class<? extends Attribute>[] a =
          intfSet.toArray(new Class[intfSet.size()]);
      return a;
    }
};

The best explanation of the purpose of this class is that it solves Java Bug 6389107

There are many use cases where one wants to essentially have a Map<Class<?>, T> for some reason, but this causes all sorts of trouble since Class objects will then not be GC-able until the Map is. WeakHashMap<Class<?>, T> doesn't solve the problem because very frequently, T references the class.

The bug above goes into a much more detailed explanation and contains example projects/code that face this problem.

ClassValue is the answer to this problem. A thread-safe, classloader loading/unloading safe way to associate data with a Class.