Java并发编程之ThreadLocal类详解

ThreadLocal类可以理解为 ThreadLocalVariable (线程局部变量) ， 提供了get与set等访问接口或方法，这些方法为每个使用该变量的线程都存有一份独立的副本，因此get总是返回当前执行线程在调用set时设置的最新值。可以将ThreadLocal视为 包含了Map对象，保存了特定于该线程的值。

概括起来说，对于多线程资源共享的问题，同步机制采用了“以时间换空间”的方式，而ThreadLocal采用了“以空间换时间”的方式。前者仅提供一份变量，让不同的线程排队访问，而后者为每一个线程都提供了一份变量，因此可以同时访问而互不影响。

模拟ThreadLocal

import java.util.Collections;

import java.util.HashMap;

import java.util.Map;

 

public class SimpleThreadLocal<T> {

private Map<Thread, T> valueMap = Collections

.synchronizedMap(new HashMap<Thread, T>());

 

public void set(T newValue) {

valueMap.put(Thread.currentThread(), newValue); // ①键为线程对象，值为本线程的变量副本

}

 

public T get() {

Thread currentThread = Thread.currentThread();

T o = valueMap.get(currentThread); // ②返回本线程对应的变量

if (o == null && !valueMap.containsKey(currentThread)) { // ③如果在Map中不存在，放到Map中保存起来。

o = initialValue();

valueMap.put(currentThread, o);

}

return o;

}

 

public void remove() {

valueMap.remove(Thread.currentThread());

}

 

protected T initialValue() {

return null;

}

}

实用ThreadLocal

class Count {

private SimpleThreadLocal<Integer> count = new SimpleThreadLocal<Integer>() {

@Override

protected Integer initialValue() {

return 0;

}

};

 

public Integer increase() {

count.set(count.get() + 1);

return count.get();

}

 

}

 

class TestThread implements Runnable {

private Count count;

 

public TestThread(Count count) {

this.count = count;

}

 

@Override

public void run() {

// TODO Auto-generated method stub

for (int i = 1; i <= 3; i++) {

System.out.println(Thread.currentThread().getName() + "\t" + i

+ "th\t" + count.increase());

}

}

}

 

public class TestThreadLocal {

public static void main(String[] args) {

Count count = new Count();

Thread t1 = new Thread(new TestThread(count));

Thread t2 = new Thread(new TestThread(count));

Thread t3 = new Thread(new TestThread(count));

Thread t4 = new Thread(new TestThread(count));

t1.start();

t2.start();

t3.start();

t4.start();

}

}

输出

Thread-0    1th    1

Thread-0    2th    2

Thread-0    3th    3

Thread-3    1th    1

Thread-1    1th    1

Thread-1    2th    2

Thread-2    1th    1

Thread-1    3th    3

Thread-3    2th    2

Thread-3    3th    3

Thread-2    2th    2

Thread-2    3th    3