synchronized，是Java中用于解決并發(fā)情況下數(shù)據(jù)同步訪問(wèn)的一個(gè)很重要的關(guān)鍵字。當(dāng)我們想要保證一個(gè)共享資源在同一時(shí)間只會(huì)被一個(gè)線程訪問(wèn)到時(shí)，我們可以在代碼中使用synchronized關(guān)鍵字對(duì)類或者對(duì)象加鎖。那么，本文來(lái)介紹一下synchronized關(guān)鍵字的實(shí)現(xiàn)原理是什么。在閱讀本文之間，建議先看下Java虛擬機(jī)是如何執(zhí)行線程同步的 。

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反編譯

眾所周知，在Java中，synchronized有兩種使用形式，同步方法和同步代碼塊。代碼如下：

/** 
 * @author Hollis 17/11/9. 
 */ 
public class SynchronizedTest { 
 
    public synchronized void doSth(){ 
        System.out.println("Hello World"); 
    } 
 
    public void doSth1(){ 
        synchronized (SynchronizedTest.class){ 
            System.out.println("Hello World"); 
        } 
    } 
} 

我們先來(lái)使用Javap來(lái)反編譯以上代碼，結(jié)果如下（部分無(wú)用信息過(guò)濾掉了）：

public synchronized void doSth(); 
    descriptor: ()V 
    flags: ACC_PUBLIC, ACC_SYNCHRONIZED 
    Code: 
      stack=2, locals=1, args_size=1 
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream; 
         3: ldc           #3                  // String Hello World 
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V 
         8: return 
 
  public void doSth1(); 
    descriptor: ()V 
    flags: ACC_PUBLIC 
    Code: 
      stack=2, locals=3, args_size=1 
         0: ldc           #5                  // class com/hollis/SynchronizedTest 
         2: dup 
         3: astore_1 
         4: monitorenter 
         5: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream; 
         8: ldc           #3                  // String Hello World 
        10: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V 
        13: aload_1 
        14: monitorexit 
        15: goto          23 
        18: astore_2 
        19: aload_1 
        20: monitorexit 
        21: aload_2 
        22: athrow 
        23: return 

反編譯后，我們可以看到Java編譯器為我們生成的字節(jié)碼。在對(duì)于doSth和doSth1的處理上稍有不同。也就是說(shuō)。JVM對(duì)于同步方法和同步代碼塊的處理方式不同。

對(duì)于同步方法，JVM采用ACC_SYNCHRONIZED標(biāo)記符來(lái)實(shí)現(xiàn)同步。 對(duì)于同步代碼塊。JVM采用monitorenter、monitorexit兩個(gè)指令來(lái)實(shí)現(xiàn)同步。

關(guān)于這部分內(nèi)容，在JVM規(guī)范中也可以找到相關(guān)的描述。

同步方法

The Java? Virtual Machine Specification中有關(guān)于方法級(jí)同步的介紹：

Method-level synchronization is performed implicitly, as part of method invocation and return. A synchronized method is distinguished in the run-time constant pool's methodinfo structure by the ACCSYNCHRONIZED flag, which is checked by the method invocation instructions. When invoking a method for which ACC_SYNCHRONIZED is set, the executing thread enters a monitor, invokes the method itself, and exits the monitor whether the method invocation completes normally or abruptly. During the time the executing thread owns the monitor, no other thread may enter it. If an exception is thrown during invocation of the synchronized method and the synchronized method does not handle the exception, the monitor for the method is automatically exited before the exception is rethrown out of the synchronized method.

主要說(shuō)的是： 方法級(jí)的同步是隱式的。同步方法的常量池中會(huì)有一個(gè)ACC_SYNCHRONIZED標(biāo)志。當(dāng)某個(gè)線程要訪問(wèn)某個(gè)方法的時(shí)候，會(huì)檢查是否有ACC_SYNCHRONIZED，如果有設(shè)置，則需要先獲得監(jiān)視器鎖，然后開始執(zhí)行方法，方法執(zhí)行之后再釋放監(jiān)視器鎖。這時(shí)如果其他線程來(lái)請(qǐng)求執(zhí)行方法，會(huì)因?yàn)闊o(wú)法獲得監(jiān)視器鎖而被阻斷住。值得注意的是，如果在方法執(zhí)行過(guò)程中，發(fā)生了異常，并且方法內(nèi)部并沒有處理該異常，那么在異常被拋到方法外面之前監(jiān)視器鎖會(huì)被自動(dòng)釋放。

同步代碼塊

同步代碼塊使用monitorenter和monitorexit兩個(gè)指令實(shí)現(xiàn)。 The Java? Virtual Machine Specification 中有關(guān)于這兩個(gè)指令的介紹：

monitorenter

Each object is associated with a monitor. A monitor is locked if and only if it has an owner. The thread that executes monitorenter attempts to gain ownership of the monitor associated with objectref, as follows:

If the entry count of the monitor associated with objectref is zero, the thread enters the monitor and sets its entry count to one. The thread is then the owner of the monitor.

If the thread already owns the monitor associated with objectref, it reenters the monitor, incrementing its entry count.

If another thread already owns the monitor associated with objectref, the thread blocks until the monitor's entry count is zero, then tries again to gain ownership.

monitorexit

The thread that executes monitorexit must be the owner of the monitor associated with the instance referenced by objectref.

The thread decrements the entry count of the monitor associated with objectref. If as a result the value of the entry count is zero, the thread exits the monitor and is no longer its owner. Other threads that are blocking to enter the monitor are allowed to attempt to do so.

大致內(nèi)容如下： 可以把執(zhí)行monitorenter指令理解為加鎖，執(zhí)行monitorexit理解為釋放鎖。 每個(gè)對(duì)象維護(hù)著一個(gè)記錄著被鎖次數(shù)的計(jì)數(shù)器。未被鎖定的對(duì)象的該計(jì)數(shù)器為0，當(dāng)一個(gè)線程獲得鎖（執(zhí)行monitorenter）后，該計(jì)數(shù)器自增變?yōu)?1 ，當(dāng)同一個(gè)線程再次獲得該對(duì)象的鎖的時(shí)候，計(jì)數(shù)器再次自增。當(dāng)同一個(gè)線程釋放鎖（執(zhí)行monitorexit指令）的時(shí)候，計(jì)數(shù)器再自減。當(dāng)計(jì)數(shù)器為0的時(shí)候。鎖將被釋放，其他線程便可以獲得鎖。

總結(jié)

同步方法通過(guò)ACC_SYNCHRONIZED關(guān)鍵字隱式的對(duì)方法進(jìn)行加鎖。當(dāng)線程要執(zhí)行的方法被標(biāo)注上ACC_SYNCHRONIZED時(shí)，需要先獲得鎖才能執(zhí)行該方法。

同步代碼塊通過(guò)monitorenter和monitorexit執(zhí)行來(lái)進(jìn)行加鎖。當(dāng)線程執(zhí)行到monitorenter的時(shí)候要先獲得所鎖，才能執(zhí)行后面的方法。當(dāng)線程執(zhí)行到monitorexit的時(shí)候則要釋放鎖。

每個(gè)對(duì)象自身維護(hù)這一個(gè)被加鎖次數(shù)的計(jì)數(shù)器，當(dāng)計(jì)數(shù)器數(shù)字為0時(shí)表示可以被任意線程獲得鎖。當(dāng)計(jì)數(shù)器不為0時(shí)，只有獲得鎖的線程才能再次獲得鎖。即可重入鎖。

至此，我們大致了解了Synchronized的原理。但是還有幾個(gè)問(wèn)題并沒有介紹清楚，比如，Monitor到底是什么？對(duì)象的鎖的狀態(tài)保存在哪里？ 別急，后面會(huì)再介紹。

【本文是51CTO專欄作者Hollis的原創(chuàng)文章，作者微信公眾號(hào)Hollis(ID：hollischuang)】

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