詳解synchronized鎖的各種用法及注意事項
1 前言
本文主要通過簡單的demo來闡述synchronized鎖的各種用法以及使用synchronized鎖的相關注意事項，記錄下來同時也方便自己記憶。
synchronized鎖是jvm內(nèi)置的鎖，不同于ReentrantLock鎖。synchronized關鍵字可以修飾方法，也可以修飾代碼塊。synchronized關鍵字修飾方法時可以修飾靜態(tài)方法，也可以修飾非靜態(tài)方法；同樣，synchronized關鍵字修飾代碼塊時可以修飾對象，也可以修飾類。當然，synchronized修飾靜態(tài)方法/類和非靜態(tài)方法/對象時的作用范圍是不同的。下面通過各種demo來詳解synchronized的各種用法及注意事項。
2 synchronized類鎖
這里所說的synchronized類鎖的作用范圍是類級別的，不會因為同一個類的不同對象執(zhí)行而失效。
2.1 synchronized修飾同一個類的兩個靜態(tài)方法時互斥
public?class?SynchronizeAndClassLock?{
????public?static?void?main(String[]?args)?throws?Exception?{
????????new?Thread(()?->?{
????????????//?new了一個ClassLock對象
????????????new?ClassLock().test1();
????????}).start();
????????new?Thread(()?->?{
????????????//?new了另一個ClassLock對象
????????????new?ClassLock().test2();
????????}).start();
????}
}
class?ClassLock?{
????public?synchronized?static?void?test1(){
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????try?{
????????????TimeUnit.SECONDS.sleep(1);
????????}?catch?(Exception?e)?{}
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????}
?//?【注意】public static void test2(){?不會互斥，因為此時test2沒有使用類鎖。
????public?synchronized?static?void?test2(){
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????try?{
????????????TimeUnit.SECONDS.sleep(1);
????????}?catch?(Exception?e)?{}
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????}
}
運行結果：
【結論】兩個線程分別同時執(zhí)行同一個類產(chǎn)生的不同對象的兩個不同?synchronized static方法，類鎖生效，雖然是不同對象，因為兩個線程使用的是同一個類鎖。反過來，假如test2方法沒有synchronized修飾的話，只有test1方法有被synchronized修飾，此時兩個方法也不會互斥，一個有鎖，一個沒有鎖，自然不會互斥。
2.2 synchronized分別修飾同一個類的靜態(tài)方法和當前類時互斥
public?class?SynchronizeAndClassLock2?{
????public?static?void?main(String[]?args)?throws?Exception?{
????????new?Thread(()?->?{
????????????// new了一個ClassLock2對象
????????????new?ClassLock2().test1();
????????????//?ClassLock2.test1();
????????}).start();
????????new?Thread(()?->?{
????????????//?new了另一個ClassLock2對象
????????????new?ClassLock2().test2();
????????????//?ClassLock2.test2();
????????}).start();
????}
}
class?ClassLock2?{
????public?synchronized?static?void?test1(){
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????try?{
????????????TimeUnit.SECONDS.sleep(1);
????????}?catch?(Exception?e)?{}
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????}
????public?static?void?test2(){
?????//?【注意】synchronized?(SynchronizeAndClassLock2.class)不會互斥
????????synchronized?(ClassLock2.class)?{
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????????try?{
????????????????TimeUnit.SECONDS.sleep(1);
????????????}?catch?(Exception?e)?{}
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????????}
????}
}
運行結果：
【結論】兩個線程同時分別執(zhí)行一個被synchronized修飾static方法，一個有synchnized(該類)代碼塊的static方法，鎖生效，雖然是不同對象，因為兩個線程使用的同一個類鎖。反過來，如果是修飾的不同類，因為類鎖不同，肯定不會互斥，比如將test2方法的synchronized (ClassLock2.class)這句代碼改成synchronized (SynchronizeAndClassLock2.class),此時不會互斥。
2.3 synchronized分別修飾同一個靜態(tài)對象時互斥
public?class?SynchronizeAndClassLock10?{
????public?static?void?main(String[]?args)?throws?Exception?{
????????new?Thread(()?->?{
????????????new?RunObject1().test1();
????????}).start();
????????new?Thread(()?->?{
????????????new?RunObject2().test2();
????????}).start();
????}
}
class?RunObject1?{
????public?static?void?test1(){
?????//?【1】synchronized?(StaticLock2.staticLock1)?{
????????synchronized?(StaticLock2.staticLock)?{
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????????try?{
????????????????TimeUnit.SECONDS.sleep(1);
????????????}?catch?(Exception?e)?{}
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????????}
????}
}
class?RunObject2?{
????public?static?void?test2()?{
?????//?【2】synchronized?(StaticLock2.staticLock2)?{
????????synchronized?(StaticLock2.staticLock)?{
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????????try?{
????????????????TimeUnit.SECONDS.sleep(1);
????????????}?catch?(Exception?e)?{}
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????????}
????}
}
class?StaticLock2?{
????public?static?Object?staticLock?=?new?Object();
}
運行結果：
【結論】synchronized分別修飾同一個類的靜態(tài)對象時互斥,反過來，如果是修飾不同的靜態(tài)對象，肯定不會互斥，比如將上面代碼中標【1】和【2】的synchronized代碼結合使用。
3 synchronized對象鎖
這里說的synchronized對象鎖的作用范圍是對象級別的即僅僅作用于同一個對象，如果是同一個類的兩個不同的對象是不會互斥的，即沒有效果的。
3.1 synchronized修飾同一個類對象的兩個非靜態(tài)方法時互斥
public?class?SynchronizeAndObjectLock2?{
????public?static?void?main(String[]?args)?throws?Exception?{
????????//?【注意】當且僅當是同一個SynchronizeAndObjectLock2對象
????????SynchronizeAndObjectLock2?synchronizeAndObjectLock2?=?new?SynchronizeAndObjectLock2();
????????new?Thread(()?->?{
????????????synchronizeAndObjectLock2.test1();
????????}).start();
????????new?Thread(()?->?{
????????????synchronizeAndObjectLock2.test2();
????????}).start();
????}
????public?synchronized?void?test1(){
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????try?{
????????????TimeUnit.SECONDS.sleep(1);
????????}?catch?(Exception?e)?{}
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????}
????public?synchronized?void?test2(){
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????try?{
????????????TimeUnit.SECONDS.sleep(1);
????????}?catch?(Exception?e)?{}
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????}
}
運行結果：
【結論】兩個線程同時執(zhí)行被synchronized修飾的相同對象的不同（相同）方法，鎖生效，因為兩個線程使用的是相同的對象鎖
3.2 synchronized分別修飾同一個類對象的非靜態(tài)方法和當前對象時互斥
public?class?SynchronizeAndObjectLock3?{
????public?static?void?main(String[]?args)?throws?Exception?{
????????//?【注意】當且僅當是同一個SynchronizeAndObjectLock3對象
????????SynchronizeAndObjectLock3?synchronizeAndObjectLock3?=?new?SynchronizeAndObjectLock3();
????????new?Thread(()?->?{
????????????synchronizeAndObjectLock3.test1();
????????}).start();
????????new?Thread(()?->?{
????????????synchronizeAndObjectLock3.test2();
????????}).start();
????}
????public?void?test1(){
????????synchronized(this)?{
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????????try?{
????????????????TimeUnit.SECONDS.sleep(1);
????????????}?catch?(Exception?e)?{}
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????????}
????}
????public?synchronized?void?test2(){
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????try?{
????????????TimeUnit.SECONDS.sleep(1);
????????}?catch?(Exception?e)?{}
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????}
}
運行結果：
【結論】snchronized修飾非靜態(tài)方法與synchronized(this)互斥，可見，snchronized修飾非靜態(tài)方法實質(zhì)鎖的是當前對象。
3.3 synchronized修飾不同對象的兩個非靜態(tài)方法時不會互斥
public?class?SynchronizeAndObjectLock?{
????public?static?void?main(String[]?args)?throws?Exception?{
????????new?Thread(()?->?{
????????????//?這里new?了一個SynchronizeAndObjectLock對象
????????????new?SynchronizeAndObjectLock().test1();
????????}).start();
????????new?Thread(()?->?{
????????????//?這里new?了另一個SynchronizeAndObjectLock對象
????????????new?SynchronizeAndObjectLock().test2();
????????}).start();
????}
????public?synchronized?void?test1(){
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????try?{
????????????TimeUnit.SECONDS.sleep(1);
????????}?catch?(Exception?e)?{}
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????}
????public?synchronized?void?test2(){
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????try?{
????????????TimeUnit.SECONDS.sleep(1);
????????}?catch?(Exception?e)?{}
????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????}
}
運行結果：
【結論】兩個線程同時執(zhí)行被synchronized修飾的不同對象的不同（相同）方法，鎖未生效，因為兩個線程使用的是不同的對象鎖。
3.4 synchronized代碼塊修飾同一個對象時互斥
public?class?SynchronizeAndObjectLock5?{
????private?Object?objectLock?=?new?Object();
????public?static?void?main(String[]?args)?throws?Exception?{
????????
????????SynchronizeAndObjectLock5?synchronizeAndObjectLock5?=?new?SynchronizeAndObjectLock5();
????????new?Thread(()?->?{
????????????synchronizeAndObjectLock5.test1();
????????}).start();
????????new?Thread(()?->?{
????????????synchronizeAndObjectLock5.test2();
????????}).start();
????}
????public?void?test1(){
????????synchronized(objectLock)?{
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????????try?{
????????????????TimeUnit.SECONDS.sleep(1);
????????????}?catch?(Exception?e)?{}
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????????}
????}
????public?void?test2(){
????????synchronized(objectLock)?{
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????????try?{
????????????????TimeUnit.SECONDS.sleep(1);
????????????}?catch?(Exception?e)?{}
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????????}
????}
}
運行結果：
【結論】synchronized代碼塊修飾同一個對象時互斥，若synchronized代碼塊修飾的是不同對象，那么不會互斥。
4 synchronized修飾當前類和當前對象時不會互斥
public?class?ClassAndObjectLock?{
????public?static?void?main(String[]?args)?throws?Exception?{
????????new?Thread(()?->?{
????????????ClassAndObjectLock.test1();
????????}).start();
????????new?Thread(()?->?{
????????????new?ClassAndObjectLock().test2();
????????}).start();
????}
????public?static?void?test1(){
????????synchronized?(ClassAndObjectLock.class)?{
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????????try?{
????????????????TimeUnit.SECONDS.sleep(1);
????????????}?catch?(Exception?e)?{}
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????????}
????}
????public?void?test2(){
????????synchronized?(this)?{
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?begin...");
????????????try?{
????????????????TimeUnit.SECONDS.sleep(1);
????????????}?catch?(Exception?e)?{}
????????????System.out.println(new?Date()?+?"?"?+?Thread.currentThread().getName()?+?"?end...");
????????}
????}
}
運行結果：
【結論】可見，類鎖和對象鎖是相互獨立的，互不相斥。
5 synchronized鎖注意事項
5.1 synchronized鎖不能被中斷
為了模擬synchronized鎖不可中斷，下面先讓兩個線程進入死鎖，然后再用main線程去中斷其中一個線程，看被中斷的線程能否釋放鎖并被喚醒。
public?class?DeadLockCannotInterruptDemo?{
????private?static?Object?lock1?=?new?Object();
????private?static?Object?lock2?=?new?Object();
????public?static?void?main(String[]?args)?throws?Exception?{
????????Thread?threadA?=?new?Thread(new?Runnable()?{
????????????@Override
????????????public?void?run()?{
????????????????synchronized?(lock1)?{
????????????????????System.out.println(Thread.currentThread().getName()?+?"?get?lock1");
????????????????????try?{
????????????????????????Thread.sleep(10);
????????????????????????synchronized?(lock2)?{
????????????????????????????System.out.println(Thread.currentThread().getName()?+?"?get?lock2");
????????????????????????}
????????????????????}?catch?(InterruptedException?e)?{
????????????????????????e.printStackTrace();
????????????????????}
????????????????}
????????????}
????????});
????????Thread?threadB?=?new?Thread(new?Runnable()?{
????????????@Override
????????????public?void?run()?{
????????????????synchronized?(lock2)?{
????????????????????System.out.println(Thread.currentThread().getName()?+?"?get?lock2");
????????????????????try?{
????????????????????????Thread.sleep(10);
????????????????????????synchronized?(lock1)?{
????????????????????????????System.out.println(Thread.currentThread().getName()?+?"?get?lock1");
????????????????????????}
????????????????????}?catch?(InterruptedException?e)?{
????????????????????????e.printStackTrace();
????????????????????}
????????????????}
????????????}
????????});
????????threadA.start();
????????threadB.start();
????????TimeUnit.SECONDS.sleep(3);
????????System.out.println("main?thread?begin?to?interrupt?"?+?threadA.getName()?+?"?and?"?+?threadA.getName()?+?"?will?release?lock1...");
????????threadA.interrupt();
????}
}
運行結果：
【結論】如上圖，main線程中斷Thread-0后，Thread-0并不會釋放鎖并醒過來。同樣的，ReentrantLock的tryLock或lockInterruptibly是可以被中斷的。
5.2 synchronized鎖可重入
5.2.1 不同方法，synchronized是可重入的
public?class?SynchronizeAndReentrant?{
????public?static?void?main(String[]?args)?throws?Exception?{
????????SynchronizeAndReentrant?synchronizeAndReentrant?=?new?SynchronizeAndReentrant();
????????synchronizeAndReentrant.test1();
????}
????public?synchronized?void?test1(){
????????System.out.println("?test1?method?is?called...");
????????test2();
????}
????public?synchronized?void?test2(){
????????System.out.println("?test2?method?is?called...");
????}
}
運行結果：
5.2.2 相同方法，synchronized是可重入的
public?class?SynchronizeAndReentrant2?{
????int?i?=?1;
????public?static?void?main(String[]?args)?throws?Exception?{
????????SynchronizeAndReentrant2?synchronizeAndReentrant?=?new?SynchronizeAndReentrant2();
????????synchronizeAndReentrant.test1();
????}
????public?synchronized?void?test1(){
????????System.out.println("?test1?method?is?called?"?+?i++?+?"st?time..."?);
????????while(i?5)?{
????????????test1();
????????}
????}
}
運行結果：
5.3 synchronized鎖不帶超時功能
synchronized鎖不帶超時功能,而ReentrantLock的tryLock是具備帶超時功能的，在指定時間沒獲取到鎖，該線程會蘇醒，有助于預防死鎖的產(chǎn)生。
5.4 喚醒/等待需要synchronized鎖
public?class?NotifyNeedSynchronized?{
????public?static?Object?lock?=?new?Object();
????public?static?void?main(String[]?args)?throws?Exception{
????????//?拋出IllegalMonitorStateException
????????//lock.notify();
????????lock.wait();
????}
}
運行結果：
【結論】使用Object的notify和wait等方法時，必須要使用synchronized鎖，否則會拋出IllegalMonitorStateException。
5.5 使用synchronized鎖時盡量縮小范圍以保證性能
使用synchronized鎖時，為了盡可能提高性能，我們應該盡量縮小鎖的范圍。能不鎖方法就不鎖方法，推薦盡量使用synchronized代碼塊來降低鎖的范圍。以下面的一段netty源碼為例：
//?ServerBootstrap.java
public??ServerBootstrap?childOption(ChannelOption?childOption,?T?value) ?{
????if?(childOption?==?null)?{
????????throw?new?NullPointerException("childOption");
????}
????if?(value?==?null)?{
????????synchronized?(childOptions)?{
????????????childOptions.remove(childOption);
????????}
????}?else?{
????????synchronized?(childOptions)?{
????????????childOptions.put(childOption,?value);
????????}
????}
????return?this;
}
可見，找到并發(fā)訪問代碼的臨界區(qū)，并不用synchronized鎖全部代碼，盡量避免使用synchronized來修飾方法。
6 總結
本文對synchronized的各種用法及注意事項通過demo簡單梳理了下，后面有時間會探討下synchronized的原理。
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