ThreadLocal、InheritableThreadLocal

ThreadLoal介绍

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。

ThreadLocal 简单例子

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<String> myThreadLocal = new ThreadLocal<String>() {
            @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

InheritableThreadLocal

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.

InheritableThreadLocal 简单例子

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<String> localValue = new ThreadLocal<>();
    private static final InheritableThreadLocal<String> 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