Synchronized和ThreadLocal的区别

偏执的太偏执、 2022-07-19 02:23 97阅读 0赞

关于理论知识，以下这两个文章讲的很好：

[Spring单例与线程安全小结][Spring]  
  
[ThreadLocal的原理和在框架中的应用][ThreadLocal]

不多说，上代码，自己运行一下就知道区别了

package com.spri.test;
    
    public class SequenceNumber {
    	
    	public Object obj = new Object();
    	// ①通过匿名内部类覆盖ThreadLocal的initialValue()方法，指定初始值
    	private static ThreadLocal<Integer> seqNum = new ThreadLocal<Integer>() {
    		public Integer initialValue() {
    			return 0;
    		}
    	};
    	
    	public int getCount(){
    		synchronized(obj){
    			return count++;
    		}
    		
    	}
    	public int getAno(){
    		ano++;
    		return ano;
    	}
    	public int ano = 0;
    	public int count =0;
    	//②获取下一个序列值
    	public int getNextNum() {
    		seqNum.set(seqNum.get() + 1);
    		return seqNum.get();
    	}
    
    	public static void main(String[] args) {
    		SequenceNumber sn = new SequenceNumber();
    		//③3个线程共享sn，各自产生序列号
    		TestClient t1 = new TestClient(sn);
    		TestClient t2 = new TestClient(sn);
    		TestClient t3 = new TestClient(sn);
    		t1.start();
    		t2.start();
    		t3.start();
    	}
    
    	private static class TestClient extends Thread {
    		
    		private int num = 0;
    		private SequenceNumber sn;
    
    		public TestClient(SequenceNumber sn) {
    			this.sn = sn;
    		}
    
    		public void run() {
    			for (int i = 0; i < 3; i++) {// ④每个线程打出3个序列值
    				System.out.println("thread[" + Thread.currentThread().getName() + "] sn[" + sn.getNextNum() + "]");
    				System.out.println(sn.getCount());
    				System.out.println(sn.getAno());
    			}
    		}
    	}
    }

结果：

thread\[Thread-0\] sn\[1\]  
thread\[Thread-2\] sn\[1\]  
1  
1  
thread\[Thread-1\] sn\[1\]  
2  
2  
thread\[Thread-2\] sn\[2\]  
3  
3  
0  
4  
thread\[Thread-2\] sn\[3\]  
4  
5  
thread\[Thread-1\] sn\[2\]  
5  
6  
thread\[Thread-0\] sn\[2\]  
6  
7  
thread\[Thread-1\] sn\[3\]  
7  
8  
thread\[Thread-0\] sn\[3\]  
8  
9

readLocal不是为了解决并发同步用的，是为了隔离变量用的。同步是为了让多个线程共同操作一个对象而不乱掉，而ThreadLocal直接就把某个对象在各自线程中重新实例化一个了，各个线程都有自己的该对象，所以就不用管同步不同步了

[Spring]: http://www.cnblogs.com/doit8791/p/4093808.html
[ThreadLocal]: http://zhxing.iteye.com/blog/306070