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：
- 作用于实例方法，当前实例加锁，进入同步代码前要获得当前实例的锁——对象锁；
- 作用于代码块，对括号里配置的对象加锁——对象锁；
- 作用于静态方法，当前类加锁，进去同步代码前要获得当前类对象的锁——类锁；
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_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: aload_0
5: new #2 // class java/lang/Object
8: dup
9: invokespecial #1 // Method java/lang/Object."":()V
12: putfield #3 // Field object:Ljava/lang/Object;
15: return
public void m1();
Code:
0: aload_0
1: getfield #3 // Field object:Ljava/lang/Object;
4: dup
5: astore_1
// 获得监视器并进入
6: monitorenter
7: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
// here
10: ldc #5 // String 这里是同步代码块
12: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
15: aload_1
// 锁退出
16: monitorexit
17: goto 25
20: astore_2
21: 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: monitorexit
23: aload_2
24: athrow
25: return
Exception table:
from to target type
7 17 20 any
20 23 20 any
public 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.class
Last modified 2023-4-11; size 608 bytes
MD5 checksum ff2328e7a59f847a473b7ed814cc108b
Compiled from "LockSyncTest.java"
public class LockSyncTest
SourceFile: "LockSyncTest.java"
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #6.#21 // java/lang/Object."":()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 // "":()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_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 1: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LLockSyncTest;
public synchronized void m2();
// ACC_SYNCHRONIZED标识
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 普通同步方法
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 6: 0
line 7: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this LLockSyncTest;
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=0, locals=1, args_size=1
0: return
LineNumberTable:
line 11: 0
LocalVariableTable:
Start Length Slot Name Signature
0 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.class
Last modified 2023-4-11; size 590 bytes
MD5 checksum e551241d2333dd7b754d24e72063d547
Compiled from "LockSyncTest.java"
public class LockSyncTest
SourceFile: "LockSyncTest.java"
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #6.#21 // java/lang/Object."":()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 // "":()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_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 1: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LLockSyncTest;
public static synchronized void m3();
// ACC_STATIC, ACC_SYNCHRONIZED标识符
flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED
Code:
stack=2, locals=0, args_size=0
0: 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;)V
8: return
LineNumberTable:
line 6: 0
line 7: 8
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=0, locals=1, args_size=1
0: return
LineNumberTable:
line 11: 0
LocalVariableTable:
Start Length Slot Name Signature
0 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：
