併發編程基礎之ThreadLocal

一：概念

在多線程併發訪問的狀況下，爲了解決線程安全，通常咱們會使用synchronized關鍵字，若是併發訪問量不是很大，能夠使用synchronized，

可是若是數據量比較大，咱們能夠考慮使用ThreadLocal，顧名思義，就是線程的本地存儲，對於類中的成員變量，若是多個線程同時訪問

就會存在線程安全問題，ThreadLocal提供給咱們不一樣於synchronized的另一種思路，就是把變量值在每一個線程存儲副本

以下示例：

/**
 * 
 */
package com.hlcui.main;

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

/**
 * @author Administrator
 *
 */
public class ThreadLocalDemo {
	
	//保留變量在線程中的副本
	private ThreadLocal<String> threadLocal = new ThreadLocal<String>();
	
	private void set(String name) {
		threadLocal.set(name);
	}
	
	private String get() {
		return threadLocal.get();
	}
	
	public static void main(String[] args) {
		final ThreadLocalDemo demo = new ThreadLocalDemo();
		
		ExecutorService executors = Executors.newFixedThreadPool(2);
		executors.execute(new Runnable() {
			@Override
			public void run() {
				demo.set("tom");
				String threadName = Thread.currentThread().getName();
				System.out.println(threadName+":::"+demo.get());
			}
		});
		
		executors.execute(new Runnable() {
			@Override
			public void run() {
				demo.set("jack");
				String threadName = Thread.currentThread().getName();
				System.out.println(threadName+":::"+demo.get());
			}
		});
		
		executors.shutdown();
	}
}

　　

運行結果：

pool-1-thread-1:::tom
pool-1-thread-2:::jack

　　

經過結果能夠看出同一個實例的同一個方法，不一樣的線程獲取的結果是不同的。

 

示例二：

測試5個線程計數：

/**
 * 
 */
package com.hlcui.main;

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

/**
 * @author Administrator
 *
 */
public class ThreadDemo2 {
	public static void main(String[] args) {
		ExecutorService executors = Executors.newFixedThreadPool(5);
		for (int i = 0; i < 5; i++) {
			executors.execute(new ThreadHolder(i));
		}
		try {
			TimeUnit.SECONDS.sleep(3);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		executors.shutdown();
	}

}

class ThreadHolder implements Runnable {

	private final int id;

	ThreadHolder(int i) {
		this.id = i;
	}

	private static ThreadLocal<Integer> tl = new ThreadLocal<Integer>() {
		Random r = new Random(47);
		public synchronized Integer initialValue() {
			return r.nextInt(100);
		}
	};

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

	@Override
	public void run() {
		System.out.println(this);
		ThreadHolder.increment();
		System.out.println(this);
	}
	
	public String toString() {
		return "id:::"+id+", "+tl.get();
	}

}

　　

運行結果：

id:::0, 58
id:::1, 55
id:::2, 93
id:::0, 59
id:::2, 94
id:::1, 56
id:::3, 61
id:::3, 62
id:::4, 61
id:::4, 62