线程安全学习笔记（一）

Java面试的话,基本肯定会问到的一个问题就是关于volatile与syncronized的区别. Syncronized强同步,volatile弱;volat不好用,想我这样的菜鸟用不来.这是我对于这两个的认识,其他更高深的就说不上来了.
先来看下syncronized的作用,这里以StringBuilder和StringBuffer为例来说明。
线程类实现：

package com.maximilian.www;

public class MyTestThread extends Thread
{
	private int _loopTimes=50;
	private int _threadId;
	private static StringBuffer sb1 =new StringBuffer();
	private static StringBuffer sb3 =new StringBuffer();
	private static StringBuilder sb2 =new StringBuilder();
	public MyTestThread(int threadId ,int loopTimes)
	{
		_threadId = threadId;
		_loopTimes = loopTimes;
	}
	@Override
    public void run()
    {
	    while(_loopTimes > 0)
	    {
	    	sb1.append("thread").append(_threadId).append(":").append(_loopTimes).append("\r\n");
	    	sb2.append("thread").append(_threadId).append(":").append(_loopTimes).append("\r\n");
	    	sb3.append("thread"+_threadId+":"+_loopTimes+"\r\n");
	    	_loopTimes--;
	    }
	 }
	public static void printSb1()
	{
		System.out.println(sb1);
	}
	public static void printSb2()
	{
		System.out.println(sb2);
	}
	public static void printSb3()
	{
		System.out.println(sb3);
	}
}

测试代码：

package test;

import java.util.Vector;

import com.maximilian.www.MyTestThread;



public class MyTest
{
	private static int loopTimes=10;
	private static int threadNum = 2;
	private static int threadId;
	private static Vector<Thread> threads = new Vector<Thread>();
	public static void main (String [] args)
	{
	    for(threadId=0;threadId < threadNum ;threadId++)
	    {
		    Thread t = new Thread(new MyTestThread(threadId, loopTimes),"mytestthread");
		    threads.add(t);
	    }
	    for(Thread t:threads)
	    {
	    	t.start();
	    }
	    for(Thread t:threads)
	    {
	    	try
            {
	            t.join();
            } catch (InterruptedException e)
            {
	            // TODO Auto-generated catch block
	            e.printStackTrace();
            }
	    }
	    System.out.println("+++++++++++++++++++++++++sb1++++++++++++++++++++++");
	    MyTestThread.printSb1();
	    System.out.println("+++++++++++++++++++++++++sb2++++++++++++++++++++++");
	    MyTestThread.printSb2();
	    System.out.println("+++++++++++++++++++++++++sb3++++++++++++++++++++++");
	    MyTestThread.printSb3();
	}
}

上面的代码主要是实现字符串的连接。先看结果如何：
+++++++++++++++++++++++++sb1++++++++++++++++++++++
thread0:10
thread0:9
thread1:10
thread0:8
thread1:9
thread0:7
thread1:8
thread0:6
thread1:7
thread0:5
thread1:6
thread0:4
thread1:5
thread0:thread31
:4
thread1thread:03:
2
thread1:2
thread0:thread11
:1
+++++++++++++++++++++++++sb2++++++++++++++++++++++
thread0:10
thread0:9
thread1:10
thread0:8
thread1:9
thread0:7
thread1:8
thread0:6
thread1:7
thread0:5
thread1:6
thread0:4
thread1:5t
read
0:3
thread1:4hread 0:1
thread1:2
thread1:1
+++++++++++++++++++++++++sb3++++++++++++++++++++++
thread0:10
thread0:9
thread1:10
thread0:8
thread1:9
thread0:7
thread1:8
thread0:6
thread1:7
thread0:5
thread1:6
thread0:4
thread1:5
thread1:4
thread0:3
thread1:3
thread0:2
thread1:2
thread1:1
thread0:1

看一下StringBuilder和StringBuffer的代码可以发现，他们的代码除了StringBuffer在方法前面加了syncronized其他完全一样，这也是导致sb2会出现即使是在一个append中的内容都会有其他的加入的情况（红色部分），而sb1出现红色部分的情况是因为虽然append这个方法是线程安全，但是append操作之间是没有同步的，这个在要求线程一次完全连接操作时可以用sb3的方法，或者个人认为更好的对整个连接过程加syncronized。
    另外，注意join的调用不能跟start一起遍历时操作，不然会导致线程是一个个顺序线性执行。