被问到傻傻不懂synchronized底层原理
0x01：synchronized的基本语法
修饰实例方法，作用于当前实例加锁，进入同步代码前要获得当前实例的锁静态方法，作用于当前类对象加锁，进入同步代码前要获得当前类对象的锁修饰代码块，指定加锁对象，对给定对象加锁，进入同步代码库前要获得给定对象的锁。
0x02：代码分析synchronized
package com.lesson8;
public class SynchronizedDemo {
public Object lock = new Object();
public synchronized void syncCommonMethod(){
System.out.println("==syncCommonMethod==");
}
public static synchronized void syncStaticMethod(){
System.out.println("==syncStaticMethod==");
}
public void syncBlockCode(){
synchronized (lock) {
System.out.println("==syncBlockCode==");
}
}
}代码中包三个方法，分别是synchronized修饰实例方法、synchronized静态方法和synchronized修饰代码块。使用命令：
javap -v SynchronizedDemo.class
得到如下汇编指令：
Classfile /D:/jmeterws/xml/com-lesson8/target/classes/com/lesson8/SynchronizedDemo.class
Last modified 2020-4-5; size 926 bytes
MD5 checksum f23cff99c70b8a401db1cc3bd74538ec
Compiled from "SynchronizedDemo.java"
public class com.lesson8.SynchronizedDemo
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Class #2 // com/lesson8/SynchronizedDemo
#2 = Utf8 com/lesson8/SynchronizedDemo
#3 = Class #4 // java/lang/Object
#4 = Utf8 java/lang/Object
#5 = Utf8 lock
#6 = Utf8 Ljava/lang/Object;
#7 = Utf8 <init>
#8 = Utf8 ()V
#9 = Utf8 Code
#10 = Methodref #3.#11 // java/lang/Object."<init>":()V
#11 = NameAndType #7:#8 // "<init>":()V
#12 = Fieldref #1.#13 // com/lesson8/SynchronizedDemo.lock:Ljava/lang/Object;
#13 = NameAndType #5:#6 // lock:Ljava/lang/Object;
#14 = Utf8 LineNumberTable
#15 = Utf8 LocalVariableTable
#16 = Utf8 this
#17 = Utf8 Lcom/lesson8/SynchronizedDemo;
#18 = Utf8 syncCommonMethod
#19 = Fieldref #20.#22 // java/lang/System.out:Ljava/io/PrintStream;
#20 = Class #21 // java/lang/System
#21 = Utf8 java/lang/System
#22 = NameAndType #23:#24 // out:Ljava/io/PrintStream;
#23 = Utf8 out
#24 = Utf8 Ljava/io/PrintStream;
#25 = String #26 // ==syncCommonMethod==
#26 = Utf8 ==syncCommonMethod==
#27 = Methodref #28.#30 // java/io/PrintStream.println:(Ljava/lang/String;)V
#28 = Class #29 // java/io/PrintStream
#29 = Utf8 java/io/PrintStream
#30 = NameAndType #31:#32 // println:(Ljava/lang/String;)V
#31 = Utf8 println
#32 = Utf8 (Ljava/lang/String;)V
#33 = Utf8 syncStaticMethod
#34 = String #35 // ==syncStaticMethod==
#35 = Utf8 ==syncStaticMethod==
#36 = Utf8 syncBlockCode
#37 = String #38 // ==syncBlockCode==
#38 = Utf8 ==syncBlockCode==
#39 = Utf8 StackMapTable
#40 = Class #41 // java/lang/Throwable
#41 = Utf8 java/lang/Throwable
#42 = Utf8 SourceFile
#43 = Utf8 SynchronizedDemo.java
{
public java.lang.Object lock;
descriptor: Ljava/lang/Object;
flags: ACC_PUBLIC
public com.lesson8.SynchronizedDemo();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=3, locals=1, args_size=1
0: aload_0
1: invokespecial #10 // Method java/lang/Object."<init>":()V
4: aload_0
5: new #3 // class java/lang/Object
8: dup
9: invokespecial #10 // Method java/lang/Object."<init>":()V
12: putfield #12 // Field lock:Ljava/lang/Object;
15: return
LineNumberTable:
line 3: 0
line 5: 4
line 3: 15
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 this Lcom/lesson8/SynchronizedDemo;
public synchronized void syncCommonMethod();
descriptor: ()V
flags: ACC_PUBLIC, ACC_SYNCHRONIZED
Code:
stack=2, locals=1, args_size=1
0: getstatic #19 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #25 // String ==syncCommonMethod==
5: invokevirtual #27 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 8: 0
line 9: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this Lcom/lesson8/SynchronizedDemo;
public static synchronized void syncStaticMethod();
descriptor: ()V
flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED
Code:
stack=2, locals=0, args_size=0
0: getstatic #19 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #34 // String ==syncStaticMethod==
5: invokevirtual #27 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 12: 0
line 13: 8
LocalVariableTable:
Start Length Slot Name Signature
public void syncBlockCode();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=1
0: aload_0
1: getfield #12 // Field lock:Ljava/lang/Object;
4: dup
5: astore_1
6: monitorenter
7: getstatic #19 // Field java/lang/System.out:Ljava/io/PrintStream;
10: ldc #37 // String ==syncBlockCode==
12: invokevirtual #27 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
15: aload_1
16: monitorexit
17: goto 23
20: aload_1
21: monitorexit
22: athrow
23: return
Exception table:
from to target type
7 17 20 any
20 22 20 any
LineNumberTable:
line 16: 0
line 17: 7
line 16: 15
line 19: 23
LocalVariableTable:
Start Length Slot Name Signature
0 24 0 this Lcom/lesson8/SynchronizedDemo;
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 20
locals = [ class com/lesson8/SynchronizedDemo, class java/lang/Object ]
stack = [ class java/lang/Throwable ]
frame_type = 250 /* chop */
offset_delta = 2
}
SourceFile: "SynchronizedDemo.java"
分析如上代码发现synchronized修饰实例方法、synchronized静态方法与synchronized修饰代码块不一样，synchronized修饰实例方法和synchronized静态方法一样
synchronized修饰代码块：
涉及两条指令：
- monitorenter：每个对象有一个监视器锁（monitor）。当monitor被占用时就会处于锁定状态，线程执行monitorenter指令时尝试获取monitor的所有权，过程如下：
如果monitor的进入数为0，则该线程进入monitor，然后将进入数设置为1，该线程即为monitor的所有者。
如果线程已经占有该monitor，只是重新进入，则进入monitor的进入数加1。
如果其他线程已经占用了monitor，则该线程进入阻塞状态，直到monitor的进入数为0，再重新尝试获取monitor的所有权。
- monitorexit：执行monitorexit的线程必须是objectref所对应的monitor的所有者。
指令执行时，monitor的进入数减1，如果减1后进入数为0，那线程退出monitor，不再是这个monitor的所有者。其他被这个monitor阻塞的线程可以尝试去获取这个monitor 的所有权。
synchronized修饰实例方法：
synchronized静态方法：
从反编译的结果来看，方法的同步并没有通过指令monitorenter和monitorexit来完成（理论上其实也可以通过这两条指令来实现）。相对于普通方法，其常量池中多了ACC_SYNCHRONIZED标示符。
JVM就是根据该标示符来实现方法的同步的：当方法被调用时，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，执行线程将先获取monitor，获取成功之后才能执行方法体，方法执行完后再释放monitor。在方法执行期间，其他任何线程都无法再获得同一个monitor对象。其实本质上没有区别，只是方法的同步是一种隐式的方式来实现，无需通过字节码来完成。
本文参与 腾讯云自媒体同步曝光计划，分享自微信公众号。
原始发表：2020-04-05，如有侵权请联系 cloudcommunity@tencent.com 删除
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