/*

 * Copyright (c) 1997, 2007, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code 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

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package java.lang;

/**

 * This class provides thread-local variables.  These variables differ from

 * their normal counterparts in that each thread that accesses one (via its

 * <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized

 * copy of the variable.  <tt>ThreadLocal</tt> instances are typically private

 * static fields in classes that wish to associate state with a thread (e.g.,

 * a user ID or Transaction ID).

 *

 * <p>For example, the class below generates unique identifiers local to each

 * thread.

 * A thread's id is assigned the first time it invokes <tt>ThreadId.get()</tt>

 * and remains unchanged on subsequent calls.

 * <pre>

 * import java.util.concurrent.atomic.AtomicInteger;

 *

 * public class ThreadId {

 *     // Atomic integer containing the next thread ID to be assigned

 *     private static final AtomicInteger nextId = new AtomicInteger(0);

 *

 *     // Thread local variable containing each thread's ID

 *     private static final ThreadLocal<Integer> threadId =

 *         new ThreadLocal<Integer>() {

 *             @Override protected Integer initialValue() {

 *                 return nextId.getAndIncrement();

 *         }

 *     };

 *

 *     // Returns the current thread's unique ID, assigning it if necessary

 *     public static int get() {

 *         return threadId.get();

 *     }

 * }

 * </pre>

 * <p>Each thread holds an implicit reference to its copy of a thread-local

 * variable as long as the thread is alive and the <tt>ThreadLocal</tt>

 * 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).

 *

 * @author  Josh Bloch and Doug Lea

 * @since   1.2

 */

public class ThreadLocal<T> {

    private static final Object NONE = new Object();

    private Object value = NONE;

    /**

     * Returns the current thread's "initial value" for this

     * thread-local variable.  This method will be invoked the first

     * time a thread accesses the variable with the {@link #get}

     * method, unless the thread previously invoked the {@link #set}

     * method, in which case the <tt>initialValue</tt> method will not

     * be invoked for the thread.  Normally, this method is invoked at

     * most once per thread, but it may be invoked again in case of

     * subsequent invocations of {@link #remove} followed by {@link #get}.

     *

     * <p>This implementation simply returns <tt>null</tt>; if the

     * programmer desires thread-local variables to have an initial

     * value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be

     * subclassed, and this method overridden.  Typically, an

     * anonymous inner class will be used.

     *

     * @return the initial value for this thread-local

     */

    protected T initialValue() {

        return null;

    }

    /**

     * Creates a thread local variable.

     */

    public ThreadLocal() {

    }

    /**

     * Returns the value in the current thread's copy of this

     * thread-local variable.  If the variable has no value for the

     * current thread, it is first initialized to the value returned

     * by an invocation of the {@link #initialValue} method.

     *

     * @return the current thread's value of this thread-local

     */

    public T get() {

        if (value == NONE) {

            return setInitialValue();

        } else {

            return (T)value;

        }

    }

    /**

     * Variant of set() to establish initialValue. Used instead

     * of set() in case user has overridden the set() method.

     *

     * @return the initial value

     */

    private T setInitialValue() {

        T v = initialValue();

        this.value = v;

        return v;

    }

    /**

     * Sets the current thread's copy of this thread-local variable

     * to the specified value.  Most subclasses will have no need to

     * override this method, relying solely on the {@link #initialValue}

     * method to set the values of thread-locals.

     *

     * @param value the value to be stored in the current thread's copy of

     *        this thread-local.

     */

    public void set(T value) {

        this.value = value;

    }

    /**

     * Removes the current thread's value for this thread-local

     * variable.  If this thread-local variable is subsequently

     * {@linkplain #get read} by the current thread, its value will be

     * reinitialized by invoking its {@link #initialValue} method,

     * unless its value is {@linkplain #set set} by the current thread

     * in the interim.  This may result in multiple invocations of the

     * <tt>initialValue</tt> method in the current thread.

     *

     * @since 1.5

     */

     public void remove() {

         this.value = NONE;

     }

}