2.2.9静态同步synchronized方法与synchronized(class)代码块

关键字synchronized还可以应用在static静态方法上，这样写那是对当前的*.java文件对应的class类进行持锁，

测试如下

package com.cky.bean;

/**
 * Created by edison on 2017/12/8.
 */
public class Service {

   synchronized public static  void printA() {
       try {
           System.out.println("printA ThreadName"+Thread.currentThread().getName()+ "begin");
           Thread.sleep(2000);
           System.out.println("printA ThreadName"+Thread.currentThread().getName()+ "end");
       } catch (InterruptedException e) {
           e.printStackTrace();
       }
   }

   synchronized public static void printB(){
       System.out.println("printb ThreadName"+Thread.currentThread().getName()+ "begin");
       System.out.println("printb ThreadName"+Thread.currentThread().getName()+ "end");

   }

}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadA extends  Thread{

    @Override
    public void run() {
        super.run();
        Service.printA();
    }
}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadB extends  Thread{

    @Override
    public void run() {
        super.run();
        Service.printB();
    }
}

package com.cky.test;

import com.cky.bean.Service;
import com.cky.thread.ThreadA;
import com.cky.thread.ThreadB;

/**
 * Created by edison on 2017/12/8.
 */
public class Test {
    public static void main(String[] args) {
        Service service = new Service();
        ThreadA threadA = new ThreadA();
        threadA.setName("a");
        threadA.start();
        ThreadB threadB = new ThreadB();
        threadB.setName("b");
        threadB.start();
    }
}

printA ThreadNameabegin
printA ThreadNameaend
printb ThreadNamebbegin
printb ThreadNamebend

Process finished with exit code 0

程序运行结果：

同步的效果，和将synchronized关键字加到非static方法上使用的效果一致，但还是存在本质上的区别，synchronized加到static静态方法上时给Class类上锁，而加到非static静态方法上是给对象上锁。

为了验证不是同一个锁，测试如下。

package com.cky.bean;

/**
 * Created by edison on 2017/12/8.
 */
public class Service {

   synchronized public static  void printA() {
       try {
           System.out.println("printA ThreadName"+Thread.currentThread().getName()+ "begin");
           Thread.sleep(2000);
           System.out.println("printA ThreadName"+Thread.currentThread().getName()+ "end");
       } catch (InterruptedException e) {
           e.printStackTrace();
       }
   }

   synchronized public static void printB(){
       System.out.println("printb ThreadName"+Thread.currentThread().getName()+ "begin");
       System.out.println("printb ThreadName"+Thread.currentThread().getName()+ "end");

   }

    synchronized public  void printC(){
        System.out.println("printc ThreadName"+Thread.currentThread().getName()+ "begin");
        System.out.println("printc ThreadName"+Thread.currentThread().getName()+ "end");

    }

}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadA extends  Thread{

    @Override
    public void run() {
        super.run();
        Service.printA();
    }
}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadB extends  Thread{

    @Override
    public void run() {
        super.run();
        Service.printB();
    }
}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadC extends Thread{

    private Service service;

    public ThreadC(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.printC();
    }
}

package com.cky.test;

import com.cky.bean.Service;
import com.cky.thread.ThreadA;
import com.cky.thread.ThreadB;
import com.cky.thread.ThreadC;

/**
 * Created by edison on 2017/12/8.
 */
public class Test {
    public static void main(String[] args) {
        Service service = new Service();
        ThreadA threadA = new ThreadA();
        threadA.setName("a");
        threadA.start();
        ThreadB threadB = new ThreadB();
        threadB.setName("b");
        threadB.start();
        ThreadC threadC = new ThreadC(service);
        threadC.setName("c");
        threadC.start();
    }
}

C:\itsoft\jdk\bin\java -Didea.launcher.port=7532 "-Didea.launcher.bin.path=C:\itsoft\idea\IntelliJ IDEA 2016.3.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\itsoft\jdk\jre\lib\charsets.jar;C:\itsoft\jdk\jre\lib\deploy.jar;C:\itsoft\jdk\jre\lib\ext\access-bridge-32.jar;C:\itsoft\jdk\jre\lib\ext\cldrdata.jar;C:\itsoft\jdk\jre\lib\ext\dnsns.jar;C:\itsoft\jdk\jre\lib\ext\jaccess.jar;C:\itsoft\jdk\jre\lib\ext\jfxrt.jar;C:\itsoft\jdk\jre\lib\ext\localedata.jar;C:\itsoft\jdk\jre\lib\ext\nashorn.jar;C:\itsoft\jdk\jre\lib\ext\sunec.jar;C:\itsoft\jdk\jre\lib\ext\sunjce_provider.jar;C:\itsoft\jdk\jre\lib\ext\sunmscapi.jar;C:\itsoft\jdk\jre\lib\ext\sunpkcs11.jar;C:\itsoft\jdk\jre\lib\ext\zipfs.jar;C:\itsoft\jdk\jre\lib\javaws.jar;C:\itsoft\jdk\jre\lib\jce.jar;C:\itsoft\jdk\jre\lib\jfr.jar;C:\itsoft\jdk\jre\lib\jfxswt.jar;C:\itsoft\jdk\jre\lib\jsse.jar;C:\itsoft\jdk\jre\lib\management-agent.jar;C:\itsoft\jdk\jre\lib\plugin.jar;C:\itsoft\jdk\jre\lib\resources.jar;C:\itsoft\jdk\jre\lib\rt.jar;C:\多线程核心技术\第一章\out\production\第一章;C:\itsoft\idea\IntelliJ IDEA 2016.3.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.cky.test.Test
printA ThreadNameabegin
printc ThreadNamecbegin
printc ThreadNamecend
printA ThreadNameaend
printb ThreadNamebbegin
printb ThreadNamebend

结果可见：

a和b和c是异步执行的，而不是按顺序执行的，因为一个是对象锁，一个是class锁，而class锁可以对所有的实例起作用

测试如下

package com.cky.bean;

/**
 * Created by edison on 2017/12/8.
 */
public class Service {

   synchronized public static  void printA() {
       try {
           System.out.println("printA ThreadName"+Thread.currentThread().getName()+ "begin");
           Thread.sleep(2000);
           System.out.println("printA ThreadName"+Thread.currentThread().getName()+ "end");
       } catch (InterruptedException e) {
           e.printStackTrace();
       }
   }

   synchronized public static void printB(){
       System.out.println("printb ThreadName"+Thread.currentThread().getName()+ "begin");
       System.out.println("printb ThreadName"+Thread.currentThread().getName()+ "end");

   }

}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadA extends  Thread{
    private Service service;

    public ThreadA(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.printA();
    }
}

package com.cky.thread;

import com.cky.bean.Service;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadB extends  Thread{

    private Service service;

    public ThreadB(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.printB();
    }
}

package com.cky.test;

import com.cky.bean.Service;
import com.cky.thread.ThreadA;
import com.cky.thread.ThreadB;

/**
 * Created by edison on 2017/12/8.
 */
public class Test {
    public static void main(String[] args) {
        Service service1 = new Service();
        Service service2 = new Service();
        ThreadA threadA = new ThreadA(service1);
        threadA.setName("a");
        threadA.start();
        ThreadB threadB = new ThreadB(service2);
        threadB.setName("b");
        threadB.start();

    }
}

printA ThreadNameabegin
printA ThreadNameaend
printb ThreadNamebbegin
printb ThreadNamebend

同步synchronized（class）代码块的作用和synchronized static 方法一样，

测试如下修改Service类如下

package com.cky.bean;

/**
 * Created by edison on 2017/12/8.
 */
public class Service {

     public static  void printA() {
       try {
           synchronized (Service.class) {
               System.out.println("printA ThreadName"+Thread.currentThread().getName()+ "begin");
               Thread.sleep(2000);
               System.out.println("printA ThreadName"+Thread.currentThread().getName()+ "end");
           }

       } catch (InterruptedException e) {
           e.printStackTrace();
       }
   }

   public static void printB(){
       synchronized(Service.class) {
           System.out.println("printb ThreadName"+Thread.currentThread().getName()+ "begin");
           System.out.println("printb ThreadName"+Thread.currentThread().getName()+ "end");
       }

   }

}

printA ThreadNameabegin
printA ThreadNameaend
printb ThreadNamebbegin
printb ThreadNamebend