java多线程学习笔记(五) ——死锁与饿汉式懒汉式

本文探讨了死锁的概念及其在多线程环境中的常见情景，通过具体代码实例展示了同步嵌套导致死锁的过程，并提供了解决策略。

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死锁：常见情景之一：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();
	}
}

死锁举例：

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();
	}
}

多线程下的单例：



//饿汉式
class Single
{
	private static final Single s = new Single();
	private Single(){}
	public static Single getInstance()
	{
		return s;
	}
}



//懒汉式

加入同步为了解决多线程安全问题。

加入双重判断是为了解决效率问题。




class Single
{
	private static Single s = null;

	private Single(){}

	public static Single getInstance()
	{
		if(s==null)
		{
			synchronized(Single.class)		
			{
				if(s==null)
		//				-->0 -->1
					s = new Single();
			}
		}
		return s;
	}
}
class  SingleDemo
{
	public static void main(String[] args) 
	{
		System.out.println("Hello World!");
	}
}