初始化一個對象時，會自動生成一個與該對象對應(yīng)的對象鎖，被synchronized 修飾的方法就得依靠對象鎖工作。當(dāng)多線程同時訪問某個被synchronized修飾的方法時，一旦某個進(jìn)程搶得對象鎖之后，其他的進(jìn)程只有排隊(duì)對待。

一、synchronized修飾普通方法

初始化一個對象時，會自動生成一個與該對象對應(yīng)的對象鎖，被synchronized 修飾的方法就得依靠對象鎖工作。當(dāng)多線程同時訪問某個被synchronized修飾的方法時，一旦某個進(jìn)程搶得對象鎖之后，其他的進(jìn)程只有排隊(duì)對待。

例如，Person類里有2個被synchronized 修飾的方法，…還是看代碼吧：

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();}});}

運(yùn)行結(jié)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

結(jié)論：因?yàn)閠hread1中的eatApple()方法和thread2中的eatOrange()方法都是通過person對象調(diào)用的，他們用的是同一個對象鎖，互斥。所以，thread2一直等到thread1中的eatApple()方法執(zhí)行完釋放了對象鎖后才開始執(zhí)行其eatOrange()方法。

現(xiàn)在將上面thread2代碼改一下：

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

再運(yùn)行，結(jié)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?yàn)閠hread1中的eatApple()方法是通過person對象調(diào)用的，而thread2中的eatOrange()方法是通過person2對象調(diào)用的，他們用的是兩個不同的對象鎖，并不互斥。所以，thread2中的eatOrange()方法是和thread1中的eatApple()方法同時進(jìn)行的。

結(jié)論：使用“同一個對象”去調(diào)用其被synchronized 修飾的普通方法才互斥。

上面的Person類等效于：

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修飾靜態(tài)方法

直接看代碼：

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();}});}

運(yùn)行結(jié)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

現(xiàn)在將thread1和thread2代碼改為：

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();}});

再運(yùn)行結(jié)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?yàn)閑atApple()和eatOrange()方法都是Person類里的靜態(tài)方法，且被synchronized修飾，所以這兩個方法都是靠唯一的Person類鎖工作，所以無論是通過Person類名調(diào)用還是new不同的person對象調(diào)用，兩個方法都是互斥的。

結(jié)論：被synchronized 修飾的靜態(tài)方法要靠類鎖工作。當(dāng)多線程同時訪問某個被synchronized修飾的靜態(tài)方法時，一旦某個進(jìn)程搶得該類的類鎖之后，其他的進(jìn)程只有排隊(duì)對待。

上面的Person代碼等效于：

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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