Tutorials : Managing Data with the ThreadLocal Class :

Managing Data with the ThreadLocal Class

by Keld H. Hansen

Recently one of my colleagues introduced me to a class from the JDK with which I was not familiar: ThreadLocal. This class allows you to put local data on a thread, so that every module running in the thread can access it. ThreadLocal has been around since JDK 1.2, but hasn't been used much, maybe because of a first, rather poor implementation, performance-wise.

The ThreadLocal Class

An example may help clarify this concept. A servlet is executed in a thread, but since many users may use the same servlet at the same time, many threads will be running the same servlet code concurrently. If the servlet uses a ThreadLocal object, it can hold data local to each thread. The user ID is a good example of what could be stored in the ThreadLocal object. I like to think of this object as a hash map where a kind of thread ID is used as the key.

ThreadLocal contains these methods:

Method	Purpose
Object get()	Returns the value for the current thread
set(Object)	Sets a new value for the current thread
Object initialValue()	Used to return an initial value (if ThreadLocal is subclassed)
remove()	In JDK 5 only - used to delete the current thread's value (for clean-up only)

The simplest way to use a ThreadLocal object is to implement it as a singleton. Here's an example in which the value stored in the ThreadLocal is a List:

public class MyThreadLocal {

  private static ThreadLocal tLocal = new ThreadLocal();

  public static void set(List list) {
    tLocal.set(list);
  }

  public static List get() {
    return (List) tLocal.get();
  }
. . .

MyThreadLocal.set(list);
. . .
list = MyThreadLocal.get();

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