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 stubTickets 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();}@Overridepublic void run() {// TODO Auto-generated method stubfor(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 blocke.printStackTrace();}}}}

package com.csdn;public class SaleTickets {/** * @param args */public static void main(String[] args) {// TODO Auto-generated method stubSaleTicke 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;@Overridepublic void run() {// TODO Auto-generated method stubwhile(true){if(tickets>0){try {Thread.sleep(20);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.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--;}@Overridepublic 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 blocke.printStackTrace();}}}}

   
    

   package com.csdn;public class SaleTicketsSy {public static void main(String[] args) {// TODO Auto-generated method stubSaleTicketsy 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;@Overridepublic void run() {// TODO Auto-generated method stubObject 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 blocke.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 stubTickets2 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 blocke.printStackTrace();}}}}

   
   

   package com.csdn;public class SaleTicketsSy2 {public static void main(String[] args) {// TODO Auto-generated method stubSaleTicketsy2 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;@Overridepublic void run() {// TODO Auto-generated method stubwhile(true){show();}}public synchronized void show(){//锁方法if(tickets>0){try {Thread.sleep(20);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.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;}@Overridepublic 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；

}

}