JUC编程之——synchronized的底层实现与分析
1 synchronized关键字
synchronized 是 Java 中的关键字，是一种同步锁(也是一种悲观锁)。它修饰的对象有以下几种：
注：synchronized不能修饰变量；
2 javap指令
javap是jdk自带的反解析工具。 它的作用就是根据class字节码文件，反解析出当前类对应的code区（汇编指令）、本地变量表、异常表和代码行偏移量映射表、常量池等等信息。
javap -c
- -c代表对代码进行反汇编；
- javap -v ***.class文件反编译
- -v -verbose 输出附加信息，如行号、本地变量表、反汇编等更加详细的信息；
3 从字节码角度分析synchronized实现
3.1 同步代码块
代码示例
public class LockSyncTest {// 一个实例对象Object object = new Object();public void m1(){// 同步代码块synchronized (object) {System.out.println("这里是同步代码块");}}public static void main(String[] args) {}
}
反编译
javap -c LockSyncTest.class
生成内容
Compiled from "LockSyncTest.java"
public class LockSyncTest {java.lang.Object object;public LockSyncTest();Code:0: aload_01: invokespecial #1 // Method java/lang/Object."<init>":()V4: aload_05: new #2 // class java/lang/Object8: dup9: invokespecial #1 // Method java/lang/Object."<init>":()V12: putfield #3 // Field object:Ljava/lang/Object;15: returnpublic void m1();Code:0: aload_01: getfield #3 // Field object:Ljava/lang/Object;4: dup5: astore_1// 获得监视器并进入6: monitorenter7: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream; // here10: ldc #5 // String 这里是同步代码块12: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V15: aload_1// 锁退出16: monitorexit17: goto 2520: astore_221: aload_1// 底层做了异常处理 如果程序出现异常 底层也要保证释放锁22: monitorexit23: aload_224: athrow25: returnException table:from to target type7 17 20 any20 23 20 anypublic static void main(java.lang.String[]);Code:0: return
}两个重要指令
- monitorenter，获得锁；
- monitorexit，释放锁(两个，一个底层实现发生异常时也要释放锁)；
一定是一个enter两个exit么？
——不一定，如果自己在同步代码块throw异常，只有一个enter一个exit；
3.2 普通同步方法
代码示例
public class LockSyncTest {// 普通同步方法public synchronized void m2(){System.out.println("普通同步方法");}public static void main(String[] args) {}
}反编译
javap -v LockSyncTest.class
生成内容
调用指令将会检查方法的ACC_SYNCHRONIZED访问标志是否被设置，如果设置了，执行线程会将先持有monitor然后再执行方法，最后再方法完成时(无论是正常完成还是非正常完成)时释放monitor；
Classfile out/production/ch16/LockSyncTest.classLast modified 2023-4-11; size 608 bytesMD5 checksum ff2328e7a59f847a473b7ed814cc108bCompiled from "LockSyncTest.java"
public class LockSyncTestSourceFile: "LockSyncTest.java"minor version: 0major version: 52flags: ACC_PUBLIC, ACC_SUPER
Constant pool:#1 = Methodref #6.#21 // java/lang/Object."<init>":()V#2 = Fieldref #22.#23 // java/lang/System.out:Ljava/io/PrintStream;#3 = String #24 // 普通同步方法#4 = Methodref #25.#26 // java/io/PrintStream.println:(Ljava/lang/String;)V #5 = Class #27 // LockSyncTest#6 = Class #28 // java/lang/Object#7 = Utf8 <init>#8 = Utf8 ()V#9 = Utf8 Code#10 = Utf8 LineNumberTable#11 = Utf8 LocalVariableTable#12 = Utf8 this#13 = Utf8 LLockSyncTest;#14 = Utf8 m2#15 = Utf8 main#16 = Utf8 ([Ljava/lang/String;)V#17 = Utf8 args#18 = Utf8 [Ljava/lang/String;#19 = Utf8 SourceFile#20 = Utf8 LockSyncTest.java#21 = NameAndType #7:#8 // "<init>":()V#22 = Class #29 // java/lang/System#23 = NameAndType #30:#31 // out:Ljava/io/PrintStream;#24 = Utf8 普通同步方法#25 = Class #32 // java/io/PrintStream#26 = NameAndType #33:#34 // println:(Ljava/lang/String;)V#27 = Utf8 LockSyncTest#28 = Utf8 java/lang/Object#29 = Utf8 java/lang/System#30 = Utf8 out#31 = Utf8 Ljava/io/PrintStream;#32 = Utf8 java/io/PrintStream#33 = Utf8 println#34 = Utf8 (Ljava/lang/String;)V
{public LockSyncTest();flags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1 // Method java/lang/Object."<init>":()V4: returnLineNumberTable:line 1: 0LocalVariableTable:Start Length Slot Name Signature0 5 0 this LLockSyncTest;public synchronized void m2();// ACC_SYNCHRONIZED标识flags: ACC_PUBLIC, ACC_SYNCHRONIZEDCode:stack=2, locals=1, args_size=10: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #3 // String 普通同步方法5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V8: returnLineNumberTable:line 6: 0line 7: 8LocalVariableTable:Start Length Slot Name Signature0 9 0 this LLockSyncTest;public static void main(java.lang.String[]);flags: ACC_PUBLIC, ACC_STATICCode:stack=0, locals=1, args_size=10: returnLineNumberTable:line 11: 0LocalVariableTable:Start Length Slot Name Signature0 1 0 args [Ljava/lang/String;
}
3.3 静态同步方法
代码示例
public class LockSyncTest {// 静态同步方法public static synchronized void m3(){System.out.println("静态同步方法");}public static void main(String[] args) {}
}反编译
javap -v LockSyncTest.class
生成内容
ACC_STATIC, ACC_SYNCHRONIZED标识符，区分方法是否为静态同步方法；
Classfile out/production/ch16/LockSyncTest.classLast modified 2023-4-11; size 590 bytesMD5 checksum e551241d2333dd7b754d24e72063d547Compiled from "LockSyncTest.java"
public class LockSyncTestSourceFile: "LockSyncTest.java"minor version: 0major version: 52flags: ACC_PUBLIC, ACC_SUPER
Constant pool:#1 = Methodref #6.#21 // java/lang/Object."<init>":()V#2 = Fieldref #22.#23 // java/lang/System.out:Ljava/io/PrintStream;#3 = String #24 // 静态同步方法#4 = Methodref #25.#26 // java/io/PrintStream.println:(Ljava/lang/String;)V #5 = Class #27 // LockSyncTest#6 = Class #28 // java/lang/Object#7 = Utf8 <init>#8 = Utf8 ()V#9 = Utf8 Code#10 = Utf8 LineNumberTable#11 = Utf8 LocalVariableTable#12 = Utf8 this#13 = Utf8 LLockSyncTest;#14 = Utf8 m3#15 = Utf8 main#16 = Utf8 ([Ljava/lang/String;)V#17 = Utf8 args#18 = Utf8 [Ljava/lang/String;#19 = Utf8 SourceFile#20 = Utf8 LockSyncTest.java#21 = NameAndType #7:#8 // "<init>":()V#22 = Class #29 // java/lang/System#23 = NameAndType #30:#31 // out:Ljava/io/PrintStream;#24 = Utf8 静态同步方法#25 = Class #32 // java/io/PrintStream#26 = NameAndType #33:#34 // println:(Ljava/lang/String;)V#27 = Utf8 LockSyncTest#28 = Utf8 java/lang/Object#29 = Utf8 java/lang/System#30 = Utf8 out#31 = Utf8 Ljava/io/PrintStream;#32 = Utf8 java/io/PrintStream#33 = Utf8 println#34 = Utf8 (Ljava/lang/String;)V
{public LockSyncTest();flags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1 // Method java/lang/Object."<init>":()V4: returnLineNumberTable:line 1: 0LocalVariableTable:Start Length Slot Name Signature0 5 0 this LLockSyncTest;public static synchronized void m3();// ACC_STATIC, ACC_SYNCHRONIZED标识符flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZEDCode:stack=2, locals=0, args_size=00: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #3 // String 静态同步方法5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V8: returnLineNumberTable:line 6: 0line 7: 8public static void main(java.lang.String[]);flags: ACC_PUBLIC, ACC_STATICCode:stack=0, locals=1, args_size=10: returnLineNumberTable:line 11: 0LocalVariableTable:Start Length Slot Name Signature0 1 0 args [Ljava/lang/String;
}
4 synchronized锁的是什么
Q：为什么任何一个对象都可以成为一个锁？
A：在HotSpot虚拟机中，monitor采用ObjectMonitor实现；
4.1 什么是管程？
管程也叫监视器(Monitors)，是一种程序结构，结构内的多个子程序（对象或模块）形成的多个工作线程互斥访问共享资源。
这些共享资源一般是硬件设备或一群变量。对共享变量能够进行的所有操作集中在一个模块中。（把信号量及其操作原语“封装”在一个对象内部）管程实现了在一个时间点，最多只有一个线程在执行管程的某个子程序。管程提供一种机制，管程可以看作一个软件模块，它是将共享的变量和对于这些共享变量的操作封装起来，形成一个具有一定接口的功能模块，进程可以调用管程来实现进程级别的并发控制。
4.2 底层C++代码解读
ObjectMonitor.java -> ObjectMonitor.cpp ->ObjectMonitor.hpp
ObjectMonitor.hpp部分源码：
ObjectMonitor() {_header = NULL;_count = 0;_waiters = 0,_recursions = 0;_object = NULL;_owner = NULL;_WaitSet = NULL;_WaitSetLock = 0 ;_Responsible = NULL ;_succ = NULL ;_cxq = NULL ;FreeNext = NULL ;_EntryList = NULL ;_SpinFreq = 0 ;_SpinClock = 0 ;OwnerIsThread = 0 ;_previous_owner_tid = 0;}ObjectMonitor中几个关键的属性：
