libcore/luni/src/main/java/java/lang/InheritableThreadLocal.java

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package java.lang;


/**
 * A thread-local variable whose value is passed from parent to child thread. By
 * default, the value of an inheritable thread-local variable of a child thread
 * is initialized with the value of the parent thread's variable at thread
 * creation time. However, subclasses may override {code #childValue(Object)}
 * to provide an arbitrary function for passing the value of a parent's
 * thread-local variable to the child's thread-local variable.
 * 
 * @see java.lang.Thread
 * @see java.lang.ThreadLocal
 * 
 * @since Android 1.0
 */
public class InheritableThreadLocal<T> extends ThreadLocal<T> {

    /**
     * Creates a new inheritable thread-local variable.
     * 
     * @since Android 1.0
     */
    public InheritableThreadLocal() {}

    /**
     * Computes the initial value of this thread-local variable for the child
     * thread given the parent thread's value. Called from the parent thread
     * when creating a child thread. The default implementation returns the
     * parent thread's value.
     * 
     * @param parentValue
     *            the value of the variable in the parent thread.
     * @return the initial value of the variable for the child thread.
     * @since Android 1.0
     */
    protected T childValue(T parentValue) {
        return parentValue;
    }

    @Override
    Values values(Thread current) {
        return current.inheritableValues;
    }

    @Override
    Values initializeValues(Thread current) {
        return current.inheritableValues = new Values();
    }
}