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java.lang
Class ThreadLocal  

java.lang.Object
  java.lang.ThreadLocal

Direct Known Subclasses:

InheritableThreadLocal

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public class ThreadLocal

extends Object

ThreadLocal objects have a different state associated with every Thread that accesses them. Every access to the ThreadLocal object (through the get() and set() methods) only affects the state of the object as seen by the currently executing Thread.

The first time a ThreadLocal object is accessed on a particular Thread, the state for that Thread's copy of the local variable is set by executing the method initialValue().

An example how you can use this:

 class Connection
 {
   private static ThreadLocal owner = new ThreadLocal()
     {
       public Object initialValue()
       {
         return("nobody");
       }
     };
 ...
 }
 

Now all instances of connection can see who the owner of the currently executing Thread is by calling owner.get(). By default any Thread would be associated with 'nobody'. But the Connection object could offer a method that changes the owner associated with the Thread on which the method was called by calling owner.put("somebody"). (Such an owner changing method should then be guarded by security checks.)

When a Thread is garbage collected all references to values of the ThreadLocal objects associated with that Thread are removed.

Since:

1.2

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Field Summary
(package private) static Object	NULL           Placeholder to distinguish between uninitialized and null set by the user.

 

Constructor Summary

ThreadLocal()           Creates a ThreadLocal object without associating any value to it yet.

 

Method Summary
Object	get()           Gets the value associated with the ThreadLocal object for the currently executing Thread.
protected  Object	initialValue()           Called once per thread on the first invocation of get(), if set() was not already called.
void	remove()           Removes the value associated with the ThreadLocal object for the currently executing Thread.
void	set(Object value)           Sets the value associated with the ThreadLocal object for the currently executing Thread.

 

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

 

Field Detail

NULL

static final Object NULL

Placeholder to distinguish between uninitialized and null set by the user. Do not expose this to the public. Package visible for use by InheritableThreadLocal

Constructor Detail

ThreadLocal

public ThreadLocal()

Creates a ThreadLocal object without associating any value to it yet.

Method Detail

initialValue

protected Object initialValue()

Called once per thread on the first invocation of get(), if set() was not already called. The default implementation returns null. Often, this method is overridden to create the appropriate initial object for the current thread's view of the ThreadLocal.

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get

public Object get()

Gets the value associated with the ThreadLocal object for the currently executing Thread. If this is the first time the current thread has called get(), and it has not already called set(), the value is obtained by initialValue().

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set

public void set(Object value)

Sets the value associated with the ThreadLocal object for the currently executing Thread. This overrides any existing value associated with the current Thread and prevents initialValue() from being called if this is the first access to this ThreadLocal in this Thread.

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remove

public void remove()

Removes the value associated with the ThreadLocal object for the currently executing Thread.

Since:

1.5

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