java多线程为什么加同步锁

package com.robingdo.test;
public class ThreadWithOutLockTest implements Runnable {
	private int  bit=100;
	public void run() {
		System.out.println("pre"+this.bit+":threadname:"+Thread.currentThread().getName());
		bit--;
		System.out.println("aft"+this.bit+":threadname:"+Thread.currentThread().getName());

	}
	public static void main(String s[]) {
		ThreadWithOutLockTest test=new ThreadWithOutLockTest();
		Thread thread1=new Thread(test,"thread1");
		Thread thread2=new Thread(test,"thread2");
		Thread thread3=new Thread(test,"thread3");
		Thread thread4=new Thread(test,"thread4");
		thread1.start();
		thread2.start();
		thread3.start();
		thread4.start();
	}
}

 两次执行后，后台的打印结果是：

 

pre100:threadname:thread2
pre100:threadname:thread4
pre100:threadname:thread3
aft97:threadname:thread3
pre100:threadname:thread1
aft98:threadname:thread4
aft99:threadname:thread2
aft96:threadname:thread1

 

pre100:threadname:thread1
aft99:threadname:thread1
pre99:threadname:thread2
pre99:threadname:thread3
aft97:threadname:thread3
aft98:threadname:thread2
pre97:threadname:thread4
aft96:threadname:thread4

 

第二次的运行结果明显看出有问题，原因就在于多线程是并发运行的，4个线程几乎同一时间进入成员方法run()，然后对bit减一，执行system打印的时候也是并发的，所以四个线程打印出来的结果都可能是100,99,98,97,96其中任何一个，所以综上，多线程对于成员对象和成员方法的调用，如果不控制并发机制，那么几乎是乱套的，所以现在来看下怎么做才能使线程有序执行，其实乱套的根本原因就是4个线程并发进入成员方法run()，所以我们确保进入run()方法的线程只能有一个，而且必须在执行完了该方法，另外一个线程才能进入该方法，这就是多线程里面的锁的概念，现在对以上程序做下修改

package com.robingdo.test;
public class ThreadWithLockTest implements Runnable {
	private int  bit=100;
	public void run() {
		String lock="lock";
		synchronized (lock) {
			System.out.println("pre"+this.bit+":threadname:"+Thread.currentThread().getName());
			bit--;
			System.out.println("aft"+this.bit+":threadname:"+Thread.currentThread().getName());	
		}
		

	}
	public static void main(String s[]) {
		ThreadWithLockTest test=new ThreadWithLockTest();
		Thread thread1=new Thread(test,"thread1");
		Thread thread2=new Thread(test,"thread2");
		Thread thread3=new Thread(test,"thread3");
		Thread thread4=new Thread(test,"thread4");
		thread1.start();
		thread2.start();
		thread3.start();
		thread4.start();
	}
}

 

在run方法加入锁对象，把方法体用同步块上锁，这样进入方法的线程只有一个，并且其他线程只能在释放资源后才能获得资源，执行两次程序，打印出来的结果是

pre100:threadname:thread1
aft99:threadname:thread1
pre99:threadname:thread4
aft98:threadname:thread4
pre98:threadname:thread3
aft97:threadname:thread3
pre97:threadname:thread2
aft96:threadname:thread2

 

pre100:threadname:thread1
aft99:threadname:thread1
pre99:threadname:thread4
aft98:threadname:thread4
pre98:threadname:thread3
aft97:threadname:thread3
pre97:threadname:thread2
aft96:threadname:thread2
这样打印结果就正常了