synchronized关键字的使用

在日常开发中我们经常看到synchronized关键字,常见的几种应用是synchronized(this)、synchronized(class)与synchronized(Object)，和synchronize方法，静态同步synchronized方法，这几种方式究竟有什么区别呢，当初学习java同步锁那块的时候好像就没怎么注意过，现在来通过实例比较下：

对于synchronized(this):

  //核心业务代码
  public class HotProductService {
      public void serviceMethodA(){//业务A
          try {
              synchronized (this) {
                  Log.d("HotProductService","A begin time="+System.currentTimeMillis());
                  Thread.sleep(2000);
                  Log.d("HotProductService","A end   time="+System.currentTimeMillis());
              }
          } catch (InterruptedException e) {
              e.printStackTrace();
          }
      }
      public void serviceMethodB(){
          synchronized (this) {
              Log.d("HotProductService","B begin time="+System.currentTimeMillis());
              Log.d("HotProductService","B end   time="+System.currentTimeMillis());
          }
      }
  }

  //线程1
  public class ThreadA extends Thread{
      private HotProductService objectService;
      public ThreadA(HotProductService objectService){
          super();
          this.objectService=objectService;
      }
      @Override
      public void run() {
          super.run();//调用业务a
          objectService.serviceMethodA();
      }
  }
  //线程2
  public class ThreadB extends Thread {
      private HotProductService objectService;
      public ThreadB(HotProductService objectService){
          super();
          this.objectService=objectService;
      }
      @Override
      public void run() {
          super.run()；//调用业务b
          objectService.serviceMethodB();
      }
  }
  //测试代码
  private void test1() {
          HotProductService service=new HotProductService();
          ThreadA a=new ThreadA(service);
          a.start();
          ThreadB b=new ThreadB(service);
          b.start();
      }

运行结果:

    02-24 10:05:12.135 6434-7477/com.example.refreshdemo D/HotProductService: A begin time=1519437912141
    02-24 10:05:14.135 6434-7477/com.example.refreshdemo D/HotProductService: A end   time=1519437914142
    02-24 10:05:14.135 6434-7478/com.example.refreshdemo D/HotProductService: B begin time=1519437914145
    02-24 10:05:14.135 6434-7478/com.example.refreshdemo D/HotProductService: B end   time=1519437914146

上面的结果我们能看到即使A业务耗时较长，B业务也并不会获得执行的机会，如果我们将核心业务代码中B业务或者A业务的同步锁去掉，其他照旧会是怎么样

    ...
    比如去掉B业务的同步锁
    public void serviceMethodB(){
    //synchronized (this) 
        {
        Log.d("HotProductService","B begin time="+System.currentTimeMillis());
        Log.d("HotProductService","B end   time="+System.currentTimeMillis());
        }
    }
    ...

运行结果:

    02-24 02:27:19.248 3562-3629/com.example.refreshdemo D/HotProductService: A begin time=1519439239248
    02-24 02:27:19.248 3562-3630/com.example.refreshdemo D/HotProductService: B begin time=1519439239248
    02-24 02:27:19.248 3562-3630/com.example.refreshdemo D/HotProductService: B end   time=1519439239248
    02-24 02:27:21.248 3562-3629/com.example.refreshdemo D/HotProductService: A end   time=1519439241248

结论：synchronized (this)使用的对象监视器是一个,即是该对象自身，当一个线程访问HotProductService的一个synchronized (this)同步代码块时，其它线程对同一个HotProductService中其它的synchronized (this)同步代码块的访问将是堵塞，实现了代码顺序的同步执行。

对于synchronized修饰的方法:
上面的结论对于synchronized修饰的方法是否同样有效呢，通过实例看下:

  //核心业务代码
  public class HotProductService {
      //业务A
      public synchronized void objectMethodA(){
          Log.d("HotProductService","run----objectMethodA2");
      }
      //业务B
      public void objectMethodB(){
          synchronized (this) {
              try {
                  for (int i = 1; i <= 10; i++) {
                      Log.d("HotProductService","synchronized thread name:"+Thread.currentThread().getName()+"-->i="+i);
                      Thread.sleep(1000);
                  }
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
          }
      }
  }
  
  //A线程
  public class ThreadA2 extends Thread{
      private HotProductService objectService;
      public ThreadA2(HotProductService objectService){
          super();
          this.objectService=objectService;
      }
      @Override
      public void run() {
          super.run();
          objectService.objectMethodA();
      }
  }

  //B线程
  public class ThreadB2 extends Thread {
      private HotProductService objectService;
      public ThreadB2(HotProductService objectService){
          super();
          this.objectService=objectService;
      }
      @Override
      public void run() {
          super.run();
          objectService.objectMethodB();
      }
  }
  
  private void test2() {
          HotProductService service=new HotProductService();
          ThreadA2 a=new ThreadA2(service);
          a.start();
          ThreadB2 b=new ThreadB2(service);
          b.start();
      }

运行结果：

    02-24 02:48:32.212 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=1
    02-24 02:48:33.213 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=2
    02-24 02:48:34.214 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=3
    02-24 02:48:35.215 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=4
    02-24 02:48:36.216 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=5
    02-24 02:48:37.217 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=6
    02-24 02:48:38.218 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=7
    02-24 02:48:39.219 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=8
    02-24 02:48:40.220 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=9
    02-24 02:48:41.221 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=10
    02-24 02:48:42.223 3562-3764/com.example.refreshdemo D/HotProductService: run----objectMethodA2

同样验证了只能同时被一个线程所访问synchronized代码块。其他线程遇到synchronized代码块将会堵塞。

那么synchronized(Object)呢

  //核心业务代码
      public class HotProductService {
          private String uname;
          private String pwd;
          String lock=new String();
          public void setUserNamePassWord(String userName,String passWord){
              try {
                  synchronized (lock) {
                      Log.d("HotProductService","thread name=" + Thread.currentThread().getName()
                              + " 进入代码快:" + System.currentTimeMillis());
                      uname=userName;
                      Thread.sleep(3000);
                      pwd=passWord;
                      Log.d("HotProductService","thread name="+Thread.currentThread().getName()
                              +" 进入代码快:"+System.currentTimeMillis()+"入参uname:"+uname+"入参pwd:"+pwd);
                  }
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
          }
      }

  //A线程
  public class ThreadA3 extends Thread{
      private HotProductService objectService;
      public ThreadA3(HotProductService objectService){
          super();
          this.objectService=objectService;
      }
      @Override
      public void run() {
          super.run();
          objectService.setUserNamePassWord("a","aa");
      }
  }

  //B线程
  public class ThreadB3 extends Thread {
      private HotProductService objectService;
      public ThreadB3(HotProductService objectService){
          super();
          this.objectService=objectService;
      }
      @Override
      public void run() {
          super.run();
          objectService.setUserNamePassWord("b","bb");
      }
  }

  HotProductService service=new HotProductService();
  ThreadA2 a=new ThreadA2(service);
  a.start();
  ThreadB2 b=new ThreadB2(service);
  b.start();

运行结果：

    02-24 02:57:41.380 3975-4016/? D/HotProductService: thread name=A 进入代码快:1519441061380
    02-24 02:57:44.382 3975-4016/com.example.refreshdemo D/HotProductService: thread name=A 进入代码快:1519441064382入参uname:a入参pwd:aa
    02-24 02:57:44.382 3975-4017/com.example.refreshdemo D/HotProductService: thread name=B 进入代码快:1519441064382
    02-24 02:57:47.383 3975-4017/com.example.refreshdemo D/HotProductService: thread name=B 进入代码快:1519441067383入参uname:b入参pwd:bb

如果我们将

    //核心业务代码
        public class HotProductService {
            private String uname;
            private String pwd;
           //String lock=new String();移入方法会得到什么样的结果呢
            public void setUserNamePassWord(String userName,String passWord){
                try {
                    String lock=new String();
                    synchronized (lock) {
                        Log.d("HotProductService","thread name=" + Thread.currentThread().getName()
                                + " 进入代码快:" + System.currentTimeMillis());
                        uname=userName;
                        Thread.sleep(3000);
                        pwd=passWord;
                        Log.d("HotProductService","thread name="+Thread.currentThread().getName()
                                +" 进入代码快:"+System.currentTimeMillis()+"入参uname:"+uname+"入参pwd:"+pwd);
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }

其他代码照旧得到
运行结果：

    02-24 03:04:16.757 4148-4187/? D/HotProductService: thread name=A 进入代码快:1519441456757
    02-24 03:04:16.758 4148-4188/? D/HotProductService: thread name=B 进入代码快:1519441456758
    02-24 03:04:19.758 4148-4187/com.example.refreshdemo D/HotProductService: thread name=A 进入代码快:1519441459758入参uname:b入参pwd:aa
    02-24 03:04:19.759 4148-4188/com.example.refreshdemo D/HotProductService: thread name=B 进入代码快:1519441459759入参uname:b入参pwd:bb

是不是很奇怪出错了？为什么呢移入方法内部就不对了，比较前后改变，发现修改后每次调用方法时（哪个线程调用方法时）都会重新new一个对象作为对象锁，但是修改前不管哪次进入方法时（哪个线程调用方法时）用的都是同一个对象，所以我们得到结论:

使用自定义任意对象进行同步锁不同线程必须为同一对象，否则仍旧是异步运行的

synchronized(.class)与静态同步synchronized方法

可以直接给出的结论是这两种方式的同步是一样的，使用的同步锁都是对应的类作为对象锁，在jvm中类是唯一的，那也就是说对任何对象都是同步的，因为具有唯一的对象锁。下面来验证下：

    //核心业务代码
    public class HotProductService {
        //业务A
        public  void methodA(){
            try {
                synchronized (HotProductService.class){
                    Log.d("HotProductService","static methodA begin Name:" + Thread.currentThread().getName() + " times:" + System.currentTimeMillis());
                    Thread.sleep(2000);
                    Log.d("HotProductService","static methodA end   Name:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis());
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        //业务B
        public  void methodB(){
            synchronized (HotProductService.class){
                Log.d("HotProductService","static methodB begin Name:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis());
                Log.d("HotProductService","static methodB end   Name:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis());
            }
        }
    }

    //A线程
    public class ThreadA5 extends Thread{
    
        private  HotProductService mHotProductService;
    
        public ThreadA5(HotProductService hotProductService){
            mHotProductService = hotProductService;
        }
        @Override
        public void run() {
            super.run();
            mHotProductService.methodA();
        }
    }
    
    //B线程
    public class ThreadB5 extends Thread {
    
        private  HotProductService mHotProductService;
    
        public ThreadB5(HotProductService hotProductService){
            mHotProductService = hotProductService;
        }
        @Override
        public void run() {
            super.run();
            mHotProductService.methodB();
        }
    }

    private void test5() {
        HotProductService service= new HotProductService();
        ThreadA5 a=new ThreadA5(service);
        a.start();
        ThreadB5 b=new ThreadB5(service);
        b.start();
    }

运行结果：

    02-24 03:24:11.250 4148-4282/com.example.refreshdemo D/HotProductService: static methodA begin Name:Thread-174 times:1519442651250
    02-24 03:24:13.252 4148-4282/com.example.refreshdemo D/HotProductService: static methodA end   Name:Thread-174 times:1519442653252
    02-24 03:24:13.252 4148-4283/com.example.refreshdemo D/HotProductService: static methodB begin Name:Thread-175 times:1519442653252
    02-24 03:24:13.252 4148-4283/com.example.refreshdemo D/HotProductService: static methodB end   Name:Thread-175 times:1519442653252

使用.class作为对象锁保证了不能线程的同步。
引用网上常见的说法就是:

synchronize修饰的方法和 synchronize(this) 都是锁住自己本身的对象而synchronize(class) synchronize(object) 都是锁别的对象

synchronized(this/.class/Object),synchronize方法区别

synchronized关键字的使用

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