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/*
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* Copyright (c) 1997, 2007, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package java.lang;
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/**
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* This class provides thread-local variables. These variables differ from
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* their normal counterparts in that each thread that accesses one (via its
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* <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
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* copy of the variable. <tt>ThreadLocal</tt> instances are typically private
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* static fields in classes that wish to associate state with a thread (e.g.,
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* a user ID or Transaction ID).
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*
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* <p>For example, the class below generates unique identifiers local to each
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* thread.
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* A thread's id is assigned the first time it invokes <tt>ThreadId.get()</tt>
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* and remains unchanged on subsequent calls.
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* <pre>
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* import java.util.concurrent.atomic.AtomicInteger;
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*
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* public class ThreadId {
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* // Atomic integer containing the next thread ID to be assigned
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* private static final AtomicInteger nextId = new AtomicInteger(0);
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*
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* // Thread local variable containing each thread's ID
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* private static final ThreadLocal<Integer> threadId =
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* new ThreadLocal<Integer>() {
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* @Override protected Integer initialValue() {
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* return nextId.getAndIncrement();
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* }
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* };
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*
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* // Returns the current thread's unique ID, assigning it if necessary
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* public static int get() {
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* return threadId.get();
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* }
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* }
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* </pre>
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* <p>Each thread holds an implicit reference to its copy of a thread-local
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* variable as long as the thread is alive and the <tt>ThreadLocal</tt>
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* instance is accessible; after a thread goes away, all of its copies of
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* thread-local instances are subject to garbage collection (unless other
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* references to these copies exist).
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*
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* @author Josh Bloch and Doug Lea
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* @since 1.2
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*/
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public class ThreadLocal<T> {
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private static final Object NONE = new Object();
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private Object value = NONE;
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/**
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* Returns the current thread's "initial value" for this
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* thread-local variable. This method will be invoked the first
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* time a thread accesses the variable with the {@link #get}
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* method, unless the thread previously invoked the {@link #set}
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* method, in which case the <tt>initialValue</tt> method will not
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* be invoked for the thread. Normally, this method is invoked at
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* most once per thread, but it may be invoked again in case of
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* subsequent invocations of {@link #remove} followed by {@link #get}.
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*
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* <p>This implementation simply returns <tt>null</tt>; if the
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* programmer desires thread-local variables to have an initial
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* value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be
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* subclassed, and this method overridden. Typically, an
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* anonymous inner class will be used.
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*
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* @return the initial value for this thread-local
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*/
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protected T initialValue() {
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return null;
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}
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/**
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* Creates a thread local variable.
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*/
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public ThreadLocal() {
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}
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/**
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* Returns the value in the current thread's copy of this
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* thread-local variable. If the variable has no value for the
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* current thread, it is first initialized to the value returned
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* by an invocation of the {@link #initialValue} method.
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*
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* @return the current thread's value of this thread-local
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*/
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public T get() {
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if (value == NONE) {
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return setInitialValue();
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} else {
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return (T)value;
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}
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}
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/**
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* Variant of set() to establish initialValue. Used instead
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* of set() in case user has overridden the set() method.
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*
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* @return the initial value
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*/
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private T setInitialValue() {
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T v = initialValue();
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this.value = v;
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return v;
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}
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/**
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* Sets the current thread's copy of this thread-local variable
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* to the specified value. Most subclasses will have no need to
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* override this method, relying solely on the {@link #initialValue}
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* method to set the values of thread-locals.
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*
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* @param value the value to be stored in the current thread's copy of
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* this thread-local.
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*/
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public void set(T value) {
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this.value = value;
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}
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/**
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* Removes the current thread's value for this thread-local
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* variable. If this thread-local variable is subsequently
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* {@linkplain #get read} by the current thread, its value will be
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* reinitialized by invoking its {@link #initialValue} method,
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* unless its value is {@linkplain #set set} by the current thread
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* in the interim. This may result in multiple invocations of the
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* <tt>initialValue</tt> method in the current thread.
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*
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* @since 1.5
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*/
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public void remove() {
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this.value = NONE;
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}
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}
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