synchronized修饰普通方法，修饰静态方法比较

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/sqf251877543/article/details/78804442

一、synchronized修饰普通方法

初始化一个对象时，会自动生成一个与该对象对应的对象锁，被synchronized 修饰的方法就得依靠对象锁工作。当多线程同时访问某个被synchronized修饰的方法时，一旦某个进程抢得对象锁之后，其他的进程只有排队对待。

例如，Person类里有2个被synchronized 修饰的方法，…还是看代码吧：

public  class Person {

    public synchronized void eatApple() {
        Log.d("TAG", "eatApple begin at:" + System.currentTimeMillis());
        SystemClock.sleep(3000);
        Log.d("TAG", "eatApple end at:" + System.currentTimeMillis());
    }

    public synchronized void eatOrange() {
        for (int i = 0; i < 10; i++) {
            Log.d("TAG", "eatOrange is running");
            SystemClock.sleep(200);
        }
    }
}

public class MainActivity extends AppCompatActivity {

    Person person = new Person();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        thread1.start();
        thread2.start();
    }

    Thread thread1 = new Thread(new Runnable() {
        @Override
        public void run() {
            person.eatApple();
        }
    });

    Thread thread2 = new Thread(new Runnable() {
        @Override
        public void run() {
            person.eatOrange();
        }
    });

}

运行结果：

12-14 16:26:48.243 14867-14920/com.example.shengqf.myapplication D/TAG: eatApple begin at:1513240008242
12-14 16:26:51.244 14867-14920/com.example.shengqf.myapplication D/TAG: eatApple end at:1513240011243
12-14 16:26:51.246 14867-14921/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:26:51.447 14867-14921/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:26:51.648 14867-14921/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:26:51.848 14867-14921/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:26:52.049 14867-14921/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:26:52.250 14867-14921/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:26:52.451 14867-14921/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:26:52.652 14867-14921/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:26:52.853 14867-14921/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:26:53.055 14867-14921/com.example.shengqf.myapplication D/TAG: eatOrange is running

结论：因为thread1中的eatApple()方法和thread2中的eatOrange()方法都是通过person对象调用的，他们用的是同一个对象锁，互斥。所以，thread2一直等到thread1中的eatApple()方法执行完释放了对象锁后才开始执行其eatOrange()方法。

现在将上面thread2代码改一下：

  Thread thread2 = new Thread(new Runnable() {
        @Override
        public void run() {
            Person person2 = new Person();
            person2.eatOrange();
        }
    });

再运行，结果：

12-14 16:43:05.011 16294-16320/com.example.shengqf.myapplication D/TAG: eatApple begin at:1513240985011
12-14 16:43:05.011 16294-16321/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:43:05.211 16294-16321/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:43:05.412 16294-16321/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:43:05.612 16294-16321/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:43:05.813 16294-16321/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:43:06.014 16294-16321/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:43:06.215 16294-16321/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:43:06.416 16294-16321/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:43:06.617 16294-16321/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:43:06.818 16294-16321/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-14 16:43:08.012 16294-16320/com.example.shengqf.myapplication D/TAG: eatApple end at:1513240988011

因为thread1中的eatApple()方法是通过person对象调用的，而thread2中的eatOrange()方法是通过person2对象调用的，他们用的是两个不同的对象锁，并不互斥。所以，thread2中的eatOrange()方法是和thread1中的eatApple()方法同时进行的。

结论：使用“同一个对象”去调用其被synchronized 修饰的普通方法才互斥。

上面的Person类等效于：

public class Person{

    public void eatApple() {
        synchronized(Person.this){
            Log.d("TAG", "eatApple begin at:" + System.currentTimeMillis());
            SystemClock.sleep(3000);
            Log.d("TAG", "eatApple end at:" + System.currentTimeMillis());
        }
    }

    public void eatOrange() {
        synchronized (Person.this) {
            for (int i = 0; i < 10; i++) {
                Log.d("TAG", "eatOrange is running");
                SystemClock.sleep(200);
            }
        }
    }
}

二、synchronized修饰静态方法

直接看代码：

public class Person {

    public synchronized static void eatApple() {
        Log.d("TAG", "eatApple begin at:" + System.currentTimeMillis());
        SystemClock.sleep(3000);
        Log.d("TAG", "eatApple end at:" + System.currentTimeMillis());
    }

    public synchronized static void eatOrange() {
        for (int i = 0; i < 10; i++) {
            Log.d("TAG", "eatOrange is running");
            SystemClock.sleep(200);
        }
    }
}

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        thread1.start();
        thread2.start();
    }

    Thread thread1 = new Thread(new Runnable() {
        @Override
        public void run() {
            Person.eatApple();
        }
    });

    Thread thread2 = new Thread(new Runnable() {
        @Override
        public void run() {
            Person.eatOrange();
        }
    });

}

运行结果：

12-15 09:09:24.378 31625-31912/com.example.shengqf.myapplication D/TAG: eatApple begin at:1513300164378
12-15 09:09:27.379 31625-31912/com.example.shengqf.myapplication D/TAG: eatApple end at:1513300167379
12-15 09:09:27.381 31625-31913/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:09:27.582 31625-31913/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:09:27.783 31625-31913/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:09:27.984 31625-31913/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:09:28.184 31625-31913/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:09:28.385 31625-31913/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:09:28.585 31625-31913/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:09:28.786 31625-31913/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:09:28.986 31625-31913/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:09:29.188 31625-31913/com.example.shengqf.myapplication D/TAG: eatOrange is running

现在将thread1和thread2代码改为：

Thread thread1 = new Thread(new Runnable() {
        @Override
        public void run() {
            Person person1 = new Person();
            person1.eatApple();
        }
    });

    Thread thread2 = new Thread(new Runnable() {
        @Override
        public void run() {
            Person person2 = new Person();
            person2.eatOrange();
        }
    });

再运行结果：

12-15 09:12:38.316 32353-32378/com.example.shengqf.myapplication D/TAG: eatApple begin at:1513300358316
12-15 09:12:41.317 32353-32378/com.example.shengqf.myapplication D/TAG: eatApple end at:1513300361317
12-15 09:12:41.320 32353-32379/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:12:41.521 32353-32379/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:12:41.721 32353-32379/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:12:41.922 32353-32379/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:12:42.123 32353-32379/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:12:42.324 32353-32379/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:12:42.525 32353-32379/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:12:42.727 32353-32379/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:12:42.928 32353-32379/com.example.shengqf.myapplication D/TAG: eatOrange is running
12-15 09:12:43.129 32353-32379/com.example.shengqf.myapplication D/TAG: eatOrange is running

因为eatApple()和eatOrange()方法都是Person类里的静态方法，且被synchronized修饰，所以这两个方法都是靠唯一的Person类锁工作，所以无论是通过Person类名调用还是new不同的person对象调用，两个方法都是互斥的。

结论：被synchronized 修饰的静态方法要靠类锁工作。当多线程同时访问某个被synchronized修饰的静态方法时，一旦某个进程抢得该类的类锁之后，其他的进程只有排队对待。

上面的Person代码等效于：

public class Person {

    public static void eatApple() {
        synchronized(Person.class){
            Log.d("TAG", "eatApple begin at:" + System.currentTimeMillis());
            SystemClock.sleep(3000);
            Log.d("TAG", "eatApple end at:" + System.currentTimeMillis());
        }
    }

    public static void eatOrange() {
        synchronized(Person.class){
            for (int i = 0; i < 10; i++) {
                Log.d("TAG", "eatOrange is running");
                SystemClock.sleep(200);
            }
        }
    }
}