threadlocal

大纲：

1. 用法

2. 源码

一、用法

ThreadLocal是一个容器，顾名思义就是把一个变量存到线程本地。

class Test {public static void main(String[] args)  {new Thread(new TestLocal()).start();new Thread(new TestLocal()).start();new Thread(new TestLocal()).start();    }}class TestLocal implements Runnable {    ThreadLocal<String> localName = new ThreadLocal<>();    @Overridepublic void run() {        localName.set(Thread.currentThread().getName());        System.out.println(localName.get());    }}/**结果： * Thread-0 * Thread-1 * Thread-2 */

二、源码

ThreadLocal是线程本地变量，因此每个Thread对象内部必然存储ThreadLocal，ThreadLocal作为key，存储在ThreadLocalMap中。

class Thread {    ThreadLocal.ThreadLocalMap threadLocals = null; //每个线程对象内部维护了一个ThreadLocal.ThreadLocalMap。...}

ThreadLocal主要方法就是set，get

• set：

public class ThreadLocal<T> {static class ThreadLocalMap {...} //ThreadLocalMap是ThreadLocal的静态内部类...public void set(T value) {        Thread t = Thread.currentThread(); //拿到当前线程ThreadLocalMap map = getMap(t); //取出线程维护的ThreadLocalMapif (map != null)            map.set(this, value); //ThreadLocalMap的key为当前ThreadLocal对象，value就是我们需要存储的变量elsecreateMap(t, value); //该线程第一次使用ThreadLocal.set时创建ThreadLocalMap对象,并赋值。}void createMap(Thread t, T firstValue) {        t.threadLocals = new ThreadLocalMap(this, firstValue);    }    ThreadLocalMap getMap(Thread t) {return t.threadLocals;    }

　　...}

• get：

public class ThreadLocal<T> {public T get() {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t); //从当前线程取出ThreadLocalMapif (map != null) {            ThreadLocalMap.Entry e = map.getEntry(this); //以当前ThreadLocal对象为key取出ThreadLocalMap.Entryif (e != null) {                T result = (T)e.value;return result;            }        }return setInitialValue(); //如果这个ThreadLocal对象没有赋值直接get，会给它赋值为null并返回。}private T setInitialValue() {        T value = initialValue();        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);if (map != null)            map.set(this, value);elsecreateMap(t, value);return value;    }protected T initialValue() {return null;    }

　　...}

ThreadLocal get、set思想小结：

1. 拿到当前线程对象。

2. 拿到线程对象内部维护的ThreadLocalMap对象。

3. 一个线程对象中只有一个ThreadLocalMap对象，所有ThreadLocal对象及这个ThreadLocal对象存储的值都以key-value的形式存在ThreadLocalMap中。（ThreadLocalMap的key是ThreadLocal对象，value是需要存储的变量。）