SynchronizedMonitor总结

同步锁

Monitor

monitor指与synchronized关联的同步资源所关联的锁
monitor有一个计数器，初始化为0，如果monitor的计数器为0，则意味着该monitor的lock还没有被获得，某个线程获得之后将立即对该计数器加一，从此该线程就是这个monitor的所有者了
如果一个已经拥有该monitor所有权的线程重入(重新调用该资源)，monitor的计数器会再次累加
如果monitor已经被其他线程所拥有，则其他线程尝试获取该monitor的所有权时，会被陷入阻塞状态知道monitor计数器变为0，才能再次尝试获取对monitor的拥有权

使用synchronized需要注意的问题

与monitor关联的对象不能为空

private final Object mutex=null;
public void synMethod(){synchronized(mutex){//TODO}
}

synchronized关键字作用域不应该太大
```
public static class Task implements Runnable{public synchronized void run(){//TODO}
}
```
上面的代码对整个线程的执行域也就是run方法都进行了synchronized同步，从而丧失了并发的能力，synchronized关键字应该尽可能的只作用于共享资源的读写作用域

不同的minitor企图锁相同的方法

//模拟排队取号
package online.hengtian.Thread;import java.util.Arrays;
import java.util.List;
class Run implements Runnable{private int index=1;private final static int MAX=500;private final static Object obj=new Object();@Overridepublic void run() {synchronized (obj) {while (index <= MAX) {System.out.println(Thread.currentThread() + " 的号码是：" + index++);}}}
}
public class TicketWindow {public static void main(String[] args){List<String> windows= Arrays.asList("一号窗口","二号窗口","三号窗口","四号窗口");
/*****************错误示例开始*********************************/windows.stream().map(t->new Thread(new Run(),t)).forEach(Thread::start);
/*****************错误示例结束*********************************/windows= Arrays.asList("一号窗口","二号窗口","三号窗口","四号窗口");Runnable r=new Run();windows.stream().map(t->new Thread(r,t)).forEach(Thread::start);}
}

在上面所示的错误示例中，构造了四个Runnable实例，从而让每个线程都拥有了自己的monitor，起不到互斥的作用，在错误实例的下面也给出了正确的做法

多个锁交叉导致死锁

private final Object READ=new Object();
private final Object WRITE=new Object();
public void read(){synchronized(READ){synchronized(WRITED){//TDDO}}
}
public void write(){synchronized(WRITE){synchronized(READ){//TDDO}}
}

This Monitor和Class Monitor的详细介绍

This Monitor

synchronized关键字修饰的同一个实例对象的不同方法时，线程争抢的是同一个Monitor的lock，也就是this的Monitor，通过代码二可以验证

//代码一
package online.hengtian.Thread;import java.util.concurrent.TimeUnit;public class MonitorDemo {public synchronized void Method1(){System.out.println("Now here is method1");try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Method1 Termined");}public synchronized void Method2(){System.out.println("Now here is method2");try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Method2 Termined");}public static void main(String[] args){MonitorDemo demo=new MonitorDemo();new Thread(demo::Method1,"t1").start();new Thread(demo::Method2,"t2").start();}
}

//代码二
package online.hengtian.Thread;import java.util.concurrent.TimeUnit;public class MonitorDemo {public synchronized void Method1(){System.out.println("Now here is method1");try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Method1 Termined");}public void Method2(){synchronized (this) {System.out.println("Now here is method2");try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Method2 Termined");}}public static void main(String[] args){MonitorDemo demo=new MonitorDemo();new Thread(demo::Method1,"t1").start();new Thread(demo::Method2,"t2").start();}
}

代码一是采用的同步方法的方式，代码而是用this的monitor，运行后效果一样，都是运行完方法一之后再运行方法二

Class Monitor

//代码一
package online.hengtian.Thread;import java.util.concurrent.TimeUnit;public class ClassMonitor {public static synchronized void Method1(){System.out.println("Now here is method1");try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Method1 Termined");}public static synchronized void Method2(){System.out.println("Now here is method2");try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Method2 Termined");}public static void main(String[] args){new Thread(ClassMonitor::Method1,"t1").start();new Thread(ClassMonitor::Method2,"t2").start();}
}

//代码二
package online.hengtian.Thread;import java.util.concurrent.TimeUnit;public class ClassMonitor {public static synchronized void Method1(){System.out.println("Now here is method1");try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Method1 Termined");}public static void Method2(){synchronized(ClassMonitor.class){System.out.println("Now here is method2");try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Method2 Termined");}}public static void main(String[] args){new Thread(ClassMonitor::Method1,"t1").start();new Thread(ClassMonitor::Method2,"t2").start();}
}

代码一和二的运行结果一致，都是先运行方法一之后执行方法二，从而可以说明，在同一个类中的静态方法同步化时，竞争的是class的monitor

SynchronizedMonitor总结

同步锁

Monitor

使用synchronized需要注意的问题

This Monitor和Class Monitor的详细介绍

This Monitor

Class Monitor

SynchronizedMonitor总结相关推荐

最新文章

热门文章