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<T>视为包含了Map<Thread,T>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-01th1

Thread-02th2

Thread-03th3

Thread-31th1

Thread-11th1

Thread-12th2

Thread-21th1

Thread-13th3

Thread-32th2

Thread-33th3

Thread-22th2

Thread-23th3

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