java.lang

Class ThreadLocal<T>

Known Direct Subclasses:
InheritableThreadLocal<T>

public class ThreadLocal<T>
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

Constructor Summary

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

Method Summary

protected void
finalize()
Called on an object by the Virtual Machine at most once, at some point after the Object is determined unreachable but before it is destroyed.
T
get()
Gets the value associated with the ThreadLocal object for the currently executing Thread.
protected T
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(T value)
Sets the value associated with the ThreadLocal object for the currently executing Thread.

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

ThreadLocal

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

Method Details

finalize

protected void finalize()
            throws Throwable
Called on an object by the Virtual Machine at most once, at some point after the Object is determined unreachable but before it is destroyed. You would think that this means it eventually is called on every Object, but this is not necessarily the case. If execution terminates abnormally, garbage collection does not always happen. Thus you cannot rely on this method to always work. For finer control over garbage collection, use references from the java.lang.ref package.

Virtual Machines are free to not call this method if they can determine that it does nothing important; for example, if your class extends Object and overrides finalize to do simply super.finalize().

finalize() will be called by a Thread that has no locks on any Objects, and may be called concurrently. There are no guarantees on the order in which multiple objects are finalized. This means that finalize() is usually unsuited for performing actions that must be thread-safe, and that your implementation must be use defensive programming if it is to always work.

If an Exception is thrown from finalize() during garbage collection, it will be patently ignored and the Object will still be destroyed.

It is allowed, although not typical, for user code to call finalize() directly. User invocation does not affect whether automatic invocation will occur. It is also permitted, although not recommended, for a finalize() method to "revive" an object by making it reachable from normal code again.

Unlike constructors, finalize() does not get called for an object's superclass unless the implementation specifically calls super.finalize().

The default implementation does nothing.

Overrides:
finalize in interface Object
Throws:
Throwable - permits a subclass to throw anything in an overridden version; but the default throws nothing

get

public T 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().
Returns:
the value of the variable in this thread

initialValue

protected T 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.
Returns:
the initial value of the variable in this thread

remove

public void remove()
Removes the value associated with the ThreadLocal object for the currently executing Thread.
Since:
1.5

set

public void set(T 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.
Parameters:
value - the value to set this thread's view of the variable to

ThreadLocal -- a variable with a unique value per thread Copyright (C) 2000, 2002, 2003, 2006 Free Software Foundation, Inc. This file is part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU Classpath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.