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## java.lang
Class ThreadLocal

    java.lang.Object
      java.lang.ThreadLocal

Direct Known Subclasses:
  InheritableThreadLocal

public class ThreadLocal
extends Object

This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its `get` or `set` method) has its own, independently initialized copy of the variable. `ThreadLocal` instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or Transaction ID).

For example, in the class below, the private static `ThreadLocal` instance (`serialNum`) maintains a "serial number" for each thread that invokes the class's static `SerialNum.get()` method, which returns the current thread's serial number. (A thread's serial number is assigned the first time it invokes `SerialNum.get()`, and remains unchanged on subsequent calls.)

     public class SerialNum {
         // The next serial number to be assigned
         private static int nextSerialNum = 0;

         private static ThreadLocal serialNum = new ThreadLocal() {
             protected synchronized Object initialValue() {
                 return new Integer(nextSerialNum++);
             }
         };

         public static int get() {
             return ((Integer) (serialNum.get())).intValue();
         }
     }

Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the `ThreadLocal` instance is accessible; after a thread goes away, all of its copies of thread-local instances are subject to garbage collection (unless other references to these copies exist).

Since:
  1.2

Constructor Summary
`ThreadLocal()`

Method Summary
`Object` | `get()` Returns the value in the current thread's copy of this thread-local variable.
`protected Object` | `initialValue()` Returns the current thread's initial value for this thread-local variable.
`void` | `set(Object value)` Sets the current thread's copy of this thread-local variable to the specified value.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### ThreadLocal

    public ThreadLocal()

Method Detail

### initialValue

    protected Object initialValue()

  Returns the current thread's initial value for this thread-local variable. This method will be invoked at most once per accessing thread for each thread-local, the first time the thread accesses the variable with the `get()` method. The `initialValue` method will not be invoked in a thread if the thread invokes the `set(Object)` method prior to the `get` method.

  This implementation simply returns `null`; if the programmer desires thread-local variables to be initialized to some value other than `null`, `ThreadLocal` must be subclassed, and this method overridden. Typically, an anonymous inner class will be used. Typical implementations of `initialValue` will invoke an appropriate constructor and return the newly constructed object.

  Returns:
    the initial value for this thread-local

### get

    public Object get()

  Returns the value in the current thread's copy of this thread-local variable. Creates and initializes the copy if this is the first time the thread has called this method.

  Returns:
    the current thread's value of this thread-local

### set

    public void set(Object value)

  Sets the current thread's copy of this thread-local variable to the specified value. Many applications will have no need for this functionality, relying solely on the `initialValue()` method to set the values of thread-locals.

  Parameters:
    `value` - the value to be stored in the current threads' copy of this thread-local.

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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

Copyright 2003 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.