Java多线程--synchronized修饰普通方法和修饰静态方法的区别

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。

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();@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);thread1.start();thread2.start();}Thread thread1 = new Thread(new Runnable() {@Overridepublic void run() {person.eatApple();}});Thread thread2 = new Thread(new Runnable() {@Overridepublic 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() {@Overridepublic 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 {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);thread1.start();thread2.start();}Thread thread1 = new Thread(new Runnable() {@Overridepublic void run() {Person.eatApple();}});Thread thread2 = new Thread(new Runnable() {@Overridepublic 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() {@Overridepublic void run() {Person person1 = new Person();person1.eatApple();}});Thread thread2 = new Thread(new Runnable() {@Overridepublic 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);}}}
}

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Java多线程--synchronized修饰普通方法和修饰静态方法的区别

一、synchronized修饰普通方法

二、synchronized修饰静态方法

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