Java多线程-ThreadLocal和InheritableThreadLocal的使用

Posted 2020-11-26 Alan Lau

ThreadLocal和InheritableThreadLocal的使用

ThreadLocal类

变量值的共享可以通过使用public static变量的形式。如果想每个线程都有自己的共享变量，可以通过JDK提供的ThreadLocal来实现。

类 ThreadLocal 主要解决的就是每个线程绑定自己的值，可以将 ThreadLocal 类比喻成全局存放数据的盒子，盒子中可以存放每个线程的私有变量。

示例：

public class Test {
    public static void main(String[] args) {
        try {
            for (int i = 0; i < 5; i++) {
                System.out.println("   在 Main 线程中取值 = " + ThreadLocalHolder.getInstance().get());
                Thread.sleep(500);
            }
            Thread.sleep(2500);
            ThreadA a = new ThreadA();
            a.start();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    static class ThreadA extends Thread {
        @Override
        public void run() {
            try {
                for (int i = 0; i < 5; i++) {
                    System.out.println("在 ThreadA 线程中取值 = " + ThreadLocalHolder.getInstance().get());
                    Thread.sleep(100);
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    static class ThreadLocalHolder {
        private ThreadLocalHolder() {
        }

        public static ThreadLocal getInstance() {
            return ThreadLocalHolder.Holder.INSTANCE;
        }

        private static class Holder {
            static final ThreadLocalHolder.MyThreadLocal INSTANCE = new ThreadLocalHolder.MyThreadLocal();
        }

        /**
         * 集成 ThreadLocal，给每个线程的共享变量一个初始值
         */
        private static class MyThreadLocal extends ThreadLocal {
            @Override
            protected Object initialValue() {
                return System.currentTimeMillis();
            }
        }
    }
}

运行结果如图：

 

 

类InheritableThreaddLocal 的使用

使用类 InheritableThreadLocal 可以在子线程中取得父线程继承下来的值。

示例：

public class Test {
    public static void main(String[] args) {
        try {
            for (int i = 0; i < 5; i++) {
                System.out.println("   在 Main 线程中取值 = " + ThreadLocalHolder.getInstance().get());
                Thread.sleep(500);
            }
            Thread.sleep(2500);
            ThreadA a = new ThreadA();
            a.start();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    static class ThreadA extends Thread {
        @Override
        public void run() {
            try {
                for (int i = 0; i < 5; i++) {
                    System.out.println("在 ThreadA 线程中取值 = " + ThreadLocalHolder.getInstance().get());
                    Thread.sleep(100);
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    static class ThreadLocalHolder {
        private ThreadLocalHolder() {
        }

        public static InheritableThreadLocal getInstance() {
            return ThreadLocalHolder.Holder.INSTANCE;
        }

        private static class Holder {
            static final ThreadLocalHolder.MyInheritableThreadLocal INSTANCE = new ThreadLocalHolder.MyInheritableThreadLocal();
        }

        /**
         * 集成 InheritableThreadLocal，给每个线程的共享变量一个初始值
         */
        private static class MyInheritableThreadLocal extends InheritableThreadLocal {
            @Override
            protected Object initialValue() {
                return System.currentTimeMillis();
            }
        }
    }
}

运行结果如下：