多线程—— (4) 死锁

死锁：举个栗子，两个人面对面过独木桥，甲和乙都已经在桥上走了一段距离，即占用了桥的资源，甲如果想通过独木桥的话，乙必须退出桥面让出桥的资源，让甲通过，但是乙不服，为什么让我先退出去，我还想先过去呢，于是就僵持不下，导致谁也过不了桥，这就是死锁。

死锁：最常见的情景之一 ：同步的嵌套

class Ticket implements Runnable
{
	private  int num = 100;
	Object obj = new Object();
	boolean flag = true;
	public void run()
	{


		if(flag)
			while(true)
			{
				synchronized(obj)
				{
					show();
				}
			}
		else
			while(true)
				this.show();
	}

	public synchronized void show()
	{

		synchronized(obj)
		{
			if(num>0)
			{
				try{Thread.sleep(10);}catch (InterruptedException e){}
				
				System.out.println(Thread.currentThread().getName()+".....sale...."+num--);
			}
		}
	}
}


class DeadLockDemo 
{
	public static void main(String[] args) 
	{
		Ticket t = new Ticket();
//		System.out.println("t:"+t);

		Thread t1 = new Thread(t);
		Thread t2 = new Thread(t);

		t1.start();
		try{Thread.sleep(10);}catch(InterruptedException e){}
		t.flag = false;
		t2.start();
	}
}

简单的死锁示例：

class Test implements Runnable
{
	private boolean flag;
	Test(boolean flag)
	{
		this.flag = flag;
	}

	public void run()
	{
		
		if(flag)
		{
			while(true)
				synchronized(MyLock.locka)
				{
					System.out.println(Thread.currentThread().getName()+"..if   locka....");
					synchronized(MyLock.lockb)				{
						
						System.out.println(Thread.currentThread().getName()+"..if   lockb....");
					}
				}
		}
		else
		{
			while(true)			
				synchronized(MyLock.lockb)
				{
					System.out.println(Thread.currentThread().getName()+"..else  lockb....");
					synchronized(MyLock.locka)
					{
						System.out.println(Thread.currentThread().getName()+"..else   locka....");
					}
				}
		}

	}

}

class MyLock
{
	public static final Object locka = new Object();
	public static final Object lockb = new Object();
}




class DeadLockTest 
{
	public static void main(String[] args) 
	{
		Test a = new Test(true);
		Test b = new Test(false);

		Thread t1 = new Thread(a);
		Thread t2 = new Thread(b);
		t1.start();
		t2.start();
	}
}