赵雅智_java 多线程（2）之线程同步

线程同步

package com.csdn;

class Tickets{
	public int tickets;
	public Tickets(){
		tickets = 10;
	}
}
public class SaleTicket {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Tickets t = new Tickets();
		Ticket st1 = new Ticket(t,"小王");
		Ticket st2 = new Ticket(t,"小张");
		
	}

}
class Ticket extends Thread {
	Tickets t;
	String name;
	public Ticket(Tickets t,String name){
		this.t = t;
		this.name = name;
		start();
	}
	
	@Override
	public void run() {
		// TODO Auto-generated method stub
		for(int i=0; i<5; i++){
			System.out.println(name+"抢到了第"+t.tickets+"票号");
			t.tickets--;
			try {
				Thread.sleep(20);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
	}
	
}

package com.csdn;

public class SaleTickets {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		SaleTicke st = new SaleTicke();
		Thread t1 = new Thread(st);
		Thread t2 = new Thread(st);
		t1.start();
		t2.start();
	}

}
class SaleTicke implements Runnable{
	private int tickets = 20;
	@Override
	public void run() {
		// TODO Auto-generated method stub
		while(true){
			if(tickets>0){
				try {
					Thread.sleep(20);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				System.out.println(Thread.currentThread().getName()+"......"+tickets--);
			}
		}
	}
	
}

为什么需要“线程同步” ？？

线程间共享代码和数据可以节省系统开销，提高程序运行效率，但同时也导致了数据的“访问冲突”问题

如何实现线程间的有机交互、并确保共享资源在某些关键时段只能被一个线程访问，即所谓的“线程同步”(Synchronization)就变得至关重要。

临界资源： 多个线程间共享的数据称为临界资源(Critical Resource)，

由于是线程调度器负责线程的调度，程序员无法精确控制多线程的交替顺序。因此，多线程对临界资源的访问有时会导致数据的不一致行。

互斥锁

每个对象都对应于一个可称为“互斥锁”的标记，这个标记用来保证在任一时刻，只能有一个线程访问该对象。

Java对象默认是可以被多个线程共用的，只是在需要时才启动“互斥锁”机制，成为专用对象。

关键字synchronized用来与对象的互斥锁联系

当某个对象用synchronized修饰时，表明该对象已启动“互斥锁”机制，在任一时刻只能由一个线程访问，即使该线程出现堵塞，该对象的被锁定状态也不会解除，其他线程任不能访问该对象。

synchronized关键字的使用方式有两种：
1.同步代码块
synchronized（对象）{

需要同步的代码
}

2.同步函数： 使用的锁是this

public synchronized void show（）{

}

 

1. 用在对象前面限制一段代码的执行（同步代码块）

   package com.csdn;
   
   class Tickets1{
   	public int tickets;
   	public Tickets1(){
   		tickets = 10;
   	}
   	
   }
   public class TestMulThread2{
        
   	/**
   	 * @param args
   	 */
   	public static void main(String[] args) {
   	
   		Tickets1 t = new Tickets1();
   		Ticket1 st1 = new Ticket1(t,"小王");
   		Ticket1 st2 = new Ticket1(t,"小张");
   		
   	}
   
   }
   class Ticket1 extends Thread {
   	static String st1 ="aaa";
   	Tickets1 t;
   	String name;
   	public Ticket1(Tickets1 t,String name){
   		this.t = t;
   		this.name = name;
   		start();
   	}
   	public synchronized void show(){//同步代码块
   		System.out.println(name+"抢到了第"+t.tickets+"票号");
   		t.tickets--;
   	}
   	@Override
   	public void run() {
   			for(int i=0; i<5; i++){
   			synchronized(st1){
   			System.out.println(name+"抢到了第"+t.tickets+"票号");
   			t.tickets--;
   			}
   				//show();
   			try {
   				Thread.sleep(1000);
   			} catch (InterruptedException e) {
   				// TODO Auto-generated catch block
   				e.printStackTrace();
   			}
   		
   		
   		}
   	}
   	
   }

   
    

   package com.csdn;
   
   public class SaleTicketsSy {
   		public static void main(String[] args) {
   			// TODO Auto-generated method stub
   			SaleTicketsy st = new SaleTicketsy();
   			Thread t1 = new Thread(st);
   			Thread t2 = new Thread(st);
   			t1.start();
   			t2.start();
   		}
   
   	}
   	class SaleTicketsy implements Runnable{
   		private int tickets = 100;
   		
   		@Override
   		public void run() {
   			// TODO Auto-generated method stub
   			Object obj = new Object();
   			while(true){
   				synchronized(obj){//同步代码块
   				if(tickets>0){
   					try {
   						Thread.sleep(20);
   					} catch (InterruptedException e) {
   						// TODO Auto-generated catch block
   						e.printStackTrace();
   					}
   					System.out.println(Thread.currentThread().getName()+"......"+tickets--);
   				}
   				}
   			}
   		}
   	}
   

   
   
2. 用在方法声明中，表示整个方法为同步方法

   package com.csdn;
   
   public class TestMulThread {
   
   	/**
   	 * @param args
   	 */
   	public static void main(String[] args) {
   		// TODO Auto-generated method stub
   		Tickets2 t = new Tickets2();
   		TicketsThread d1 = new TicketsThread(t,"Jam");
   		TicketsThread d2 = new TicketsThread(t,"Jack");	
   	}
   }
   
   class Tickets2{
   	public int tickets;
   	public Tickets2(){
   		tickets = 10;
   	}
   	public synchronized void action(String name){//锁方法
   		System.out.println(name+"抢到了第"+tickets+"票号");
   		tickets--;
   	}
   }
   
   class TicketsThread extends Thread{
   	Tickets2 t;
   	String name;
   	public TicketsThread(Tickets2 t, String name){
   		this.t = t;
   		this.name = name;
   		start();
   	}
   	public void run(){
   		for(int i=0;i<5;i++){
   			t.action(name);
   			try {
   				Thread.sleep(20);
   			} catch (InterruptedException e) {
   				// TODO Auto-generated catch block
   				e.printStackTrace();
   			}
   		}
   	}
   }
   

   
   

   package com.csdn;
   
   public class SaleTicketsSy2 {
   	public static void main(String[] args) {
   		// TODO Auto-generated method stub
   		SaleTicketsy2 st = new SaleTicketsy2();
   		Thread t1 = new Thread(st);
   		Thread t2 = new Thread(st);
   		t1.start();
   		t2.start();
   	}
   
   }
   class SaleTicketsy2 implements Runnable{
   	private int tickets = 100;
   	
   	@Override
   	public void run() {
   		// TODO Auto-generated method stub
   		while(true){
   			show();
   		}
   	}
   		public synchronized void show(){//锁方法
   			if(tickets>0){
   				try {
   					Thread.sleep(20);
   				} catch (InterruptedException e) {
   					// TODO Auto-generated catch block
   					e.printStackTrace();
   				}
   				System.out.println(Thread.currentThread().getName()+"......"+tickets--);
   			}
   		}
   	}
   
   

   
   

同步好处：决了线程安全问题

同步弊端：降低了运行效率（判断锁是较为消耗资源的）

死锁：同步嵌套，容易出现死锁

死锁 两个线程A、B用到同一个对象s(s为共享资源)，且线程A在执行中要用到B运行后所创造的条件。在这种前提下A先开始运行，进入同步块后，对象s被锁定，接着线程A因等待B运行结束而进入阻塞状态，于是B开始运行，但因无法访问对象s，线程B也进入阻塞状态，等待s被线程A解锁。最终的结果：两个线程互相等待，都无法运行。

package com.csdn;

public class DeadLock {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		Demo6 d1=new Demo6(true);
		Demo6 d2=new Demo6(false);
		Thread t1=new Thread(d1);
		Thread t2=new Thread(d2);
		t1.start();
		t2.start();

	}

}

class MyLock{
	static MyLock lock1=new MyLock();
	static MyLock lock2=new MyLock();
	
}

class Demo6 implements Runnable{
	//String str1=new String("aaa");
	//String str2=new String("bbb");
	
	private boolean flag;
	public Demo6(boolean flag){
		this.flag=flag;
	}

	@Override
	public void run() {
		if(flag){
			synchronized(MyLock.lock1){
				System.out.println(Thread.currentThread().getName()+"...if...str1");	
				
				synchronized(MyLock.lock2){
					System.out.println(Thread.currentThread().getName()+"...if...str2");
					
				}
			}
			
			
		}else{
	         synchronized(MyLock.lock2){
	        	 System.out.println(Thread.currentThread().getName()+"...else...str2");
	        	
				 synchronized(MyLock.lock1){
					System.out.println(Thread.currentThread().getName()+"...else...str1");
				}
			}
		}
		
	}
	
}

分析：这是线程死锁的典型表现，两个以上线程并发运行，他们均因其他线程锁定了自己运行所需资源而陷入阻塞状态，同时自己也锁定了其他线程所需资源。

 

单例类

懒汉式

class Single{
   private static Single s=null;
   private Single(){}
   public static Single getInstance(){
       if（s==null）
          synchronized(Singel。class){
             if（s==null）
               s=new Single();
         }

         return s;

}

class Single{
   private static Single s=null；
   private Single(){}
   public static synchronized Single getInstance(){
       
       
          if（s==null）
              s=new Single();
       

         return s;}
Single.getInstance();

饿汉式

class Single{
   private static Single s=new Single（）；
   private Single（）{}
   public static Single getInstance（）{
         return s；

}

}