线程本地存储-ThreadLocal

线程本地存储（线程本地变量）可以使相同变量的每个不同的线程都创建不同的存储。因此，如果有5个线程都使用同一个变量V所表示的对象，那线程本地存储就会生成5个用于V的不同的存储块，它们使得你可以将状态与线程关联起来。通过查看源码可以发现，其内部实现是通过一个叫ThrealLocalMap来做的，每个线程都有一个ThreadLocalMap，每个线程的ThreadLocalMap对象可以用来保存每个线程运行时的线程本地变量值。


【示例代码】

package test.demo.threadlocal;

import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class ThreadLocalHolder {
	private static ThreadLocal value = new ThreadLocal() {
		private Random rand = new Random(47);

		protected synchronized Integer initialValue() {
			return rand.nextInt(10000);
		}
	};

	public static void increment() {
		value.set(value.get() + 1);
	}

	public static int get() {
		return value.get();
	}

	public static void main(String[] args) throws InterruptedException {
		System.out.println("*****************************************");
		ExecutorService exec = Executors.newCachedThreadPool();
		for (int i = 0; i < 3; i++) {
			exec.execute(new Accessor(i));
		}
		TimeUnit.MILLISECONDS.sleep(50);
		exec.shutdownNow();
	}
}

 
 

package test.demo.threadlocal;

public class Accessor implements Runnable {
	private final int id;

	public Accessor(int id) {
		this.id = id;
	}

	@Override
	public void run() {
		while (!Thread.currentThread().isInterrupted()) {
			ThreadLocalHolder.increment();
			System.out.println(this);
			Thread.yield();
		}
	}

	public String toString() {
		return "#Thread[" + id + "] [ThreadLocal] value:" + ThreadLocalHolder.get();
	}
}

 

 【输出结果】

*****************************************
#Thread[0] [ThreadLocal] value:9259
#Thread[0] [ThreadLocal] value:9260
#Thread[0] [ThreadLocal] value:9261
#Thread[2] [ThreadLocal] value:556
#Thread[0] [ThreadLocal] value:9262
#Thread[2] [ThreadLocal] value:557
#Thread[0] [ThreadLocal] value:9263
#Thread[2] [ThreadLocal] value:558
#Thread[0] [ThreadLocal] value:9264