ClassValue in Java 7 | 易学教程

问题

While browsing the Java 7 API documentation I stumbled upon the new class java.lang.ClassValue with the following rather minimal documentation:

Lazily associate a computed value with (potentially) every type. For example, if a dynamic language needs to construct a message dispatch table for each class encountered at a message send call site, it can use a ClassValue to cache information needed to perform the message send quickly, for each class encountered.

Can anyone give a better explanation of what problem this class solves and perhaps some sample code or open source project that already uses this class?

Update: I'm still interested in some actual source code or examples using this new class.

I also found this mail on the mlvm-dev mailing list concerning some implementation improvements. It was apparently changed from using a WeakHashMap to a new private field on java.lang.Class to make it more scalable.

回答1:

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.

回答2:

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.

回答3:

Well, it is an abstract class. I've found a copy. Have a look at it.

回答4:

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;
}
};

来源：https://stackoverflow.com/questions/7444420/classvalue-in-java-7

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