ThreadLocal、InheritableThreadLocal

摘要：介绍以下是源码中对的官方解释，已经非常精炼了。简单例子其实源码里已经给出了使用的样例，这里就当自我熟悉一下。顾名思义，目的就是让可以访问的。而且可以通过重写方法任意改变的简单例子年月日主要源码可以参考这篇文章源码解读

以下是JDK1.8源码中对ThreadLocal的官方解释，已经非常精炼了。

Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the ThreadLocal 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).

白话一点就是每个thread在自己的生命周期内维护着一个本地变量，仅供自身上下文使用。

其实JDK源码里已经给出了使用的样例，这里就当自我熟悉一下。（这个例子主要是为了用下initialValue()，可以直接看第二个简单例子）

package cn.lw.thread;

import java.util.UUID;

/**
 * @author wanglei 2018年5月3日
 */
public class ThreadLocalTest3 {

    public static void main(String[] args) {
        MyThread3 myThread = new MyThread3();
        
        Thread t1 = new Thread(myThread, "t1");
        Thread t2 = new Thread(myThread, "t2");
        t1.start();
        t2.start();
        
        try {
            t1.join(); //等待t1终止
            t2.join(); //等待t2终止
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

class MyThread3 implements Runnable {

    @Override
    public void run() {
        // 初始化当前thread的local-value，仅当前thread可见
        ThreadLocal myThreadLocal = new ThreadLocal() {
            @Override protected String initialValue() {
                return UUID.randomUUID().toString();
            }
        };

        String name = Thread.currentThread().getName();
        // 获取初始值
        String initialValue = myThreadLocal.get();
        
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        // 将当前thread的local-value改为指定值
        myThreadLocal.set(UUID.randomUUID().toString());
        System.out.println("Thread name: " + name + ", init: " + initialValue + ", next: " + myThreadLocal.get());
    }
    
}

Output：

Thread name: t2, init: 26e62cc8-1457-43a2-956c-6e53f5ccadd3, next: 7d32cd8d-7143-4879-a65b-a0841d2b6e9d
Thread name: t1, init: 9ab664fc-0af2-4a1d-bcd8-e4f056b6748f, next: 7a6f44dc-bd20-438e-85be-ff42ba19ad6e

这个类继承自ThreadLocal。顾名思义，目的就是让child-thread可以访问parent-thread的local-value。而且可以通过重写childVaule()方法任意改变parent-thread的local-value.

package cn.lw.thread;

import java.util.UUID;

/**
 * @author wanglei 2018年5月3日
 */
public class ThreadLocalDemo {

    public static void main(String[] args) {
        new Thread(new ParentClass("p1")).start();
        new Thread(new ParentClass("p2")).start();
        
    }

}

class ThreadLocalManager {
//    private static final ThreadLocal localValue = new ThreadLocal<>();
    private static final InheritableThreadLocal localValue = new InheritableThreadLocal<>();
    
    public static void setLocalValue() {
        localValue.set(UUID.randomUUID().toString());
    }
    
    public static String getLocalValue() {
        return localValue.get();
    }
    
    public static void removeLocalValue() {
        localValue.remove();
    }
    
}

class ParentClass implements Runnable {
    private String name;

    public ParentClass(String name) {
        this.name = name;
    }

    @Override
    public void run() {
        ThreadLocalManager.setLocalValue();
        System.out.println("parent name: "+name+", loval value: "+ThreadLocalManager.getLocalValue());
        
        String childName = name + "_child";
        ChildClass childClass = new ChildClass(childName);
        Thread childThread = new Thread(childClass);
        
        childThread.start();
        try {
            childThread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    
}

class ChildClass implements Runnable {
    private String name;

    public ChildClass(String name) {
        this.name = name;
    }

    @Override
    public void run() {
        System.out.println("child name: "+name+", loval value: "+ThreadLocalManager.getLocalValue());
    }
    
}

Output：

parent name: p2, loval value: 3a623559-9457-4cb0-8ba1-b958336a70cd
parent name: p1, loval value: 48594e57-a114-43c1-980e-034074387d0a
child name: p2_child, loval value: 3a623559-9457-4cb0-8ba1-b958336a70cd
child name: p1_child, loval value: 48594e57-a114-43c1-980e-034074387d0a

ThreadLocal 主要源码可以参考这篇文章：ThreadLocal源码解读