摘要：介紹以下是源碼中對的官方解釋，已經非常精煉了。簡單例子其實源碼里已經給出了使用的樣例，這里就當自我熟悉一下。顧名思義，目的就是讓可以訪問的。而且可以通過重寫方法任意改變的簡單例子年月日主要源碼可以參考這篇文章源碼解讀

以下是JDK1.8源碼中對ThreadLocal的官方解釋，已經非常精煉了。

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).

白話一點就是每個thread在自己的生命周期內維護著一個本地變量，僅供自身上下文使用。

其實JDK源碼里已經給出了使用的樣例，這里就當自我熟悉一下。（這個例子主要是為了用下initialValue()，可以直接看第二個簡單例子）

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終止
            t2.join(); //等待t2終止
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

class MyThread3 implements Runnable {

    @Override
    public void run() {
        // 初始化當前thread的local-value，僅當前thread可見
        ThreadLocal myThreadLocal = new ThreadLocal() {
            @Override protected String initialValue() {
                return UUID.randomUUID().toString();
            }
        };

        String name = Thread.currentThread().getName();
        // 獲取初始值
        String initialValue = myThreadLocal.get();
        
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        // 將當前thread的local-value改為指定值
        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

這個類繼承自ThreadLocal。顧名思義，目的就是讓child-thread可以訪問parent-thread的local-value。而且可以通過重寫childVaule()方法任意改變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 主要源碼可以參考這篇文章：ThreadLocal源碼解讀