多线程-ThreadLocal，InheritableThreadLocal

ThreadLocal

变量值得共享可以使用public static变量的形式，所有的线程都使用同一个public static变量。如果想实现每一个线程都有自己的共享变量该如何解决呢？JDK中提供的ThreadLocal正是解决这样的问题。 
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，盒子中可以存储每个线程的私有数据。ThreadLocal解决的是变量在不同线程键的隔离性，也就是不同 线程拥有自己的值，不同线程中的值是可以放入ThreadLocal类中进行保存的。

package org.github.lujiango;

public class Test03 {
    public static ThreadLocal<Object> tl = new ThreadLocal<Object>();

    static class ThreadA extends Thread {
        @Override
        public void run() {
            try {
                tl.set("ThreadA");
                Thread.sleep(200);
                System.out.println("ThreadA get value = " + tl.get());

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    static class ThreadB extends Thread {
        @Override
        public void run() {
            try {
                tl.set("ThreadB");
                Thread.sleep(200);
                System.out.println("ThreadB get value = " + tl.get());

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        try {
            ThreadA a = new ThreadA();
            ThreadB b = new ThreadB();
            a.start();
            b.start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

　　第一次调用ThreadLocal类的get()方法返回值是null，可以自定义protected T initialValue()方法，设置默认值。

package org.github.lujiango;

class ThreadLocalExt extends ThreadLocal<String> {
    @Override
    protected String initialValue() {
        return "defalut";
    }
}

public class Test03 {
    public static ThreadLocalExt tl = new ThreadLocalExt();

    static class ThreadA extends Thread {
        @Override
        public void run() {
            try {
                System.out.println("ThreadA get value = " + tl.get());
                tl.set("ThreadA");
                Thread.sleep(200);
                System.out.println("ThreadA get value = " + tl.get());

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    static class ThreadB extends Thread {
        @Override
        public void run() {
            try {
                System.out.println("ThreadB get value = " + tl.get());
                tl.set("ThreadB");
                Thread.sleep(200);
                System.out.println("ThreadB get value = " + tl.get());

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        try {
            ThreadA a = new ThreadA();
            ThreadB b = new ThreadB();
            a.start();
            b.start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

InheritableThreadLocal

InheritableThreadLocal可以在子线程中取得父线程继承下来的值，即可以让子线程从父进程中取得值。

package org.github.lujiango;

public class Test04 {
    static InheritableThreadLocal<String> itl = new InheritableThreadLocal<String>();

    static class ThreadA extends Thread {
        @Override
        public void run() {
            try {
                System.out.println("ThreadA get: " + itl.get());
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {

        try {
            itl.set("Main");
            Thread.sleep(1000);
            ThreadA a = new ThreadA();
            a.start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

InheritableThreadLocal即可以设置默认值，也可以在继承父线程值的同时修改。

package org.github.lujiango;

class InheritableThreadLocalExt extends InheritableThreadLocal<String> {
    @Override
    protected String initialValue() {
        return "defalut";
    }

    @Override
    protected String childValue(String parentValue) {
        return parentValue + " child";
    }
}

public class Test04 {
    static InheritableThreadLocalExt itl = new InheritableThreadLocalExt();

    static class ThreadA extends Thread {
        @Override
        public void run() {
            try {
                Thread.sleep(1000);
                System.out.println("ThreadA get: " + itl.get());
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {

        try {
            System.out.println(itl.get());
            itl.set("Main1");
            Thread.sleep(1000);
            ThreadA a = new ThreadA();
            a.start();
            itl.set("Main2");
            System.out.println(itl.get());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

注：如果子线程在取得值得同时，主线程将InheritableThreadLocal中的值进行更改，那么子线程取到的值还是旧值。