大纲:
- 用法
- 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<>(); @Override public void run() { localName.set(Thread.currentThread().getName()); System.out.println(localName.get()); } } /**结果: * Thread-0 * Thread-1 * Thread-2 */
上例中有三个线程每个线程将自己的线程名称放到各自的ThreadLocal中再打印出来,每个线程取出的都是各自的名字。
二、源码
ThreadLocal是线程本地变量,因此每个线程对象内部必然有存储ThreadLocal的地方
class Thread { ThreadLocal.ThreadLocalMap threadLocals = null; //每个线程对象内部维护了一个ThreadLocal.ThreadLocalMap,存储键值对。 ... }
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ThreadLocalMap if (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 else createMap(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ThreadLocalMap if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this); //以当前ThreadLocal对象为key取出ThreadLocalMap.Entry if (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); else createMap(t, value); return value; } protected T initialValue() { return null; } }