加不加 synchronized 有什么区别？
synchronized 作为悲观锁，锁住了什么？
synchronized 代码块怎么用？
前面 3 篇文章讲了 「synchronized」 的同步方法和同步代码块两种用法，还有锁实例对象和锁 Class 对象两种锁机制。今天我们来看看同步方法和同步代码块的实现原理。
我们把前 3 篇有涉及到的 synchronized 方法全写在一起，如下面所示。
代码语言：javascript复制public class SynchronizedPrincipleTest {
public void testNoSynchronized() {
System.out.println("hello testNoSynchronized");
}
public synchronized void testSynchronizedMethod() {
System.out.println("hello testSynchronizedMethod");
}
public static synchronized void testSynchronizedStatic() {
System.out.println("hello testSynchronizedStatic");
}
public void testSynchronizedCodethis() {
synchronized (this) {
System.out.println("hello testSynchronizedCode");
}
}
private Object lock = new Object();
public void testSynchronizedCodeObject() {
synchronized (lock) {
System.out.println("hello testSynchronizedCodeObject");
}
}
public void testSynchronizedCodeClass() {
synchronized (SynchronizedPrincipleTest.class) {
System.out.println("hello testSynchronizedCodeClass");
}
}
}
编写好代码之后，我们通过 javac 命令编译代码，使用 javap 命令反编译出汇编代码出来。命令如下所示。
javac SynchronizedPrincipleTest.java
javap -v SynchronizedCodeTest.class
得出我们要汇编代码。
代码语言：javascript复制Classfile /D:/Workspace/finance/test/thread/src/main/java/com/liebrother/study/synchronizeds/SynchronizedPrincipleTest.class
Last modified Apr 26, 2020; size 1363 bytes
MD5 checksum a03ec0b152580bb465b1defe7965a60d
Compiled from "SynchronizedPrincipleTest.java"
public class com.liebrother.study.synchronizeds.SynchronizedPrincipleTest
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #2.#31 // java/lang/Object."<init>":()V
#2 = Class #32 // java/lang/Object
#3 = Fieldref #11.#33 // com/liebrother/study/synchronizeds/SynchronizedPrincipleTest.lock:Ljava/lang/Object;
#4 = Fieldref #34.#35 // java/lang/System.out:Ljava/io/PrintStream;
#5 = String #36 // hello testNoSynchronized
#6 = Methodref #37.#38 // java/io/PrintStream.println:(Ljava/lang/String;)V
#7 = String #39 // hello testSynchronizedMethod
#8 = String #40 // hello testSynchronizedStatic
#9 = String #41 // hello testSynchronizedCode
#10 = String #42 // hello testSynchronizedCodeObject
#11 = Class #43 // com/liebrother/study/synchronizeds/SynchronizedPrincipleTest
#12 = String #44 // hello testSynchronizedCodeClass
#13 = Utf8 lock
#14 = Utf8 Ljava/lang/Object;
#15 = Utf8 <init>
#16 = Utf8 ()V
#17 = Utf8 Code
#18 = Utf8 LineNumberTable
#19 = Utf8 testNoSynchronized
#20 = Utf8 testSynchronizedMethod
#21 = Utf8 testSynchronizedStatic
#22 = Utf8 testSynchronizedCodethis
#23 = Utf8 StackMapTable
#24 = Class #43 // com/liebrother/study/synchronizeds/SynchronizedPrincipleTest
#25 = Class #32 // java/lang/Object
#26 = Class #45 // java/lang/Throwable
#27 = Utf8 testSynchronizedCodeObject
#28 = Utf8 testSynchronizedCodeClass
#29 = Utf8 SourceFile
#30 = Utf8 SynchronizedPrincipleTest.java
#31 = NameAndType #15:#16 // "<init>":()V
#32 = Utf8 java/lang/Object
#33 = NameAndType #13:#14 // lock:Ljava/lang/Object;
#34 = Class #46 // java/lang/System
#35 = NameAndType #47:#48 // out:Ljava/io/PrintStream;
#36 = Utf8 hello testNoSynchronized
#37 = Class #49 // java/io/PrintStream
#38 = NameAndType #50:#51 // println:(Ljava/lang/String;)V
#39 = Utf8 hello testSynchronizedMethod
#40 = Utf8 hello testSynchronizedStatic
#41 = Utf8 hello testSynchronizedCode
#42 = Utf8 hello testSynchronizedCodeObject
#43 = Utf8 com/liebrother/study/synchronizeds/SynchronizedPrincipleTest
#44 = Utf8 hello testSynchronizedCodeClass
#45 = Utf8 java/lang/Throwable
#46 = Utf8 java/lang/System
#47 = Utf8 out
#48 = Utf8 Ljava/io/PrintStream;
#49 = Utf8 java/io/PrintStream
#50 = Utf8 println
#51 = Utf8 (Ljava/lang/String;)V
{
public com.liebrother.study.synchronizeds.SynchronizedPrincipleTest();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=3, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: new #2 // class java/lang/Object
8: dup
9: invokespecial #1 // Method java/lang/Object."<init>":()V
12: putfield #3 // Field lock:Ljava/lang/Object;
15: return
LineNumberTable:
line 7: 0
line 27: 4
/** 无 synchronized 修饰的代码 */
public void testNoSynchronized();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #5 // String hello testNoSynchronized
5: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 10: 0
line 11: 8
/** synchronized 修饰的实例方法 */
public synchronized void testSynchronizedMethod();
descriptor: ()V
flags: ACC_PUBLIC, ACC_SYNCHRONIZED /** 方法标识多了一个 ACC_SYNCHRONIZED */
Code:
stack=2, locals=1, args_size=1
0: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #7 // String hello testSynchronizedMethod
5: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 14: 0
line 15: 8
/** synchronized 修饰的静态方法 */
public static synchronized void testSynchronizedStatic();
descriptor: ()V
flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED /** 方法标识多了 ACC_STATIC 和 ACC_SYNCHRONIZED */
Code:
stack=2, locals=0, args_size=0
0: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #8 // String hello testSynchronizedStatic
5: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 18: 0
line 19: 8
/** synchronized 修饰的 this 代码块 */
public void testSynchronizedCodethis();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=1
0: aload_0
1: dup
2: astore_1
3: monitorenter /** 通过 monitorenter 命令进入监视器锁 */
4: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
7: ldc #9 // String hello testSynchronizedCode
9: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
12: aload_1
13: monitorexit /** 通过 monitorexit 命令退出监视器锁 */
14: goto 22
17: astore_2
18: aload_1
19: monitorexit /** 通过 monitorexit 命令退出监视器锁 */
20: aload_2
21: athrow
22: return
Exception table:
from to target type
4 14 17 any
17 20 17 any
LineNumberTable:
line 22: 0
line 23: 4
line 24: 12
line 25: 22
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 17
locals = [ class com/liebrother/study/synchronizeds/SynchronizedPrincipleTest, class java/lang/Object ]
stack = [ class java/lang/Throwable ]
frame_type = 250 /* chop */
offset_delta = 4
/** synchronized 修饰的 object 代码块 */
public void testSynchronizedCodeObject();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=1
0: aload_0
1: getfield #3 // Field lock:Ljava/lang/Object;
4: dup
5: astore_1
6: monitorenter /** 通过 monitorenter 命令进入监视器锁 */
7: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
10: ldc #10 // String hello testSynchronizedCodeObject
12: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
15: aload_1
16: monitorexit /** 通过 monitorexit 命令退出监视器锁 */
17: goto 25
20: astore_2
21: aload_1
22: monitorexit /** 通过 monitorexit 命令退出监视器锁 */
23: aload_2
24: athrow
25: return
Exception table:
from to target type
7 17 20 any
20 23 20 any
LineNumberTable:
line 29: 0
line 30: 7
line 31: 15
line 32: 25
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 20
locals = [ class com/liebrother/study/synchronizeds/SynchronizedPrincipleTest, class java/lang/Object ]
stack = [ class java/lang/Throwable ]
frame_type = 250 /* chop */
offset_delta = 4
/** synchronized 修饰的 xxx.Class 代码块 */
public void testSynchronizedCodeClass();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=1
0: ldc #11 // class com/liebrother/study/synchronizeds/SynchronizedPrincipleTest
2: dup
3: astore_1
4: monitorenter /** 通过 monitorenter 命令进入监视器锁 */
5: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
8: ldc #12 // String hello testSynchronizedCodeClass
10: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
13: aload_1
14: monitorexit /** 通过 monitorexit 命令退出监视器锁 */
15: goto 23
18: astore_2
19: aload_1
20: monitorexit /** 通过 monitorexit 命令退出监视器锁 */
21: aload_2
22: athrow
23: return
Exception table:
from to target type
5 15 18 any
18 21 18 any
LineNumberTable:
line 35: 0
line 36: 5
line 37: 13
line 38: 23
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 18
locals = [ class com/liebrother/study/synchronizeds/SynchronizedPrincipleTest, class java/lang/Object ]
stack = [ class java/lang/Throwable ]
frame_type = 250 /* chop */
offset_delta = 4
}
SourceFile: "SynchronizedPrincipleTest.java"
这段代码有点多，加了些注释方便大家看，这里我抽一些重要的点讲一下。
- 我们可以看到同步方法和同步代码块的同步实现不太一样。
「同步方法」的实现是在方法标识 flags 中加了 ACC_SYNCHRONIZED 标识，是一种隐式实现，具体是 JVM 在执行方法的时候，检查是否有 ACC_SYNCHRONIZED 同步标识，有的话会等待获取监控器 monitor，然后在方法执行结束时释放监控器 monitor。
「同步代码块」的实现是在加同步代码块前加上 monitorenter 指令，在同步代码块后加上 monitorexit 指令，每个对象都有一个 monitor 监视器，当 monitor 被某线程占用了，该线程就锁定了该 monitor。每个 monitor 都维护一个自己的计数器，当执行 monitorenter 时，该计数器 1，当执行 monitorexit 时候释放锁,计数器变为 0。其他线程才可以尝试获得 monitor，对共享资源进行操作。
- 同步实例方法
testSynchronizedMethod()和同步静态方法testSynchronizedStatic()差别只是在于 flags 有没有ACC_STATIC标识，其实锁实例对象还是锁 Class 对象，也是 JVM 底层实现根据这个标识去做判断，对我们来说是透明的。 - 同步代码块锁什么对象
thisVSobjectVSxxx.class，在这个汇编代码可以看出来的。
锁 this 的代码如下。在进入 monitor 监听器前，先获取 this 对象，也就是进入 this 对象的 monitor 锁。
0: aload_0 /** 加载当前 this 对象 */
1: dup /** 将 this 对象压入栈顶 */
2: astore_1 /** 从栈顶取出 this 对象 */
3: monitorenter /** 获取 this 对象的 monitor 锁 */
锁 object 的代码如下。在进入 monitor 监听器前，先获取 lock 对象，也就是进入 lock 对象的 monitor 锁。
0: aload_0 /** 加载当前 this 对象 */
1: getfield #3 /** 获取 this 对象的实例变量 lock */ // Field lock:Ljava/lang/Object;
4: dup /** 将实例变量 lock 压入栈顶 */
5: astore_1 /** 从栈顶取出 lock 对象 */
6: monitorenter /** 获取 lock 对象的 monitor 锁 */
锁 xxx.class 的代码如下。在进入 monitor 监听器前，先获取 Class 对象，也就是进入 Class 对象的 monitor 锁。
0: ldc #11 /** 从常量池中获取 SynchronizedPrincipleTest 类对象 */ // class com/liebrother/study/synchronizeds/SynchronizedPrincipleTest
2: dup /** 将 Class 对象压入栈顶 */
3: astore_1 /** 从栈顶取出 Class 对象 */
4: monitorenter /** 获取 Class 对象的 monitor 锁 */
今天从 Java 的汇编代码来分析同步方法和同步代码块的底层实现，其实这块还不算是真正的底层实现，只是站在 Java 层面上来说，这已经是最底层了。站在 JVM 这是最高层，接下来会从 JVM 角度来分析为什么同步方法加上 ACC_SYNCHRONIZED 和 同步代码块加上 monitorenter & monitorexit 就可以实现多线程同步？
悄悄打个预防针，接下来的文章会有些晦涩难懂，但是我觉得很有必要弄懂它，弄懂了最底层原理，那么多线程就不怕了，弄懂了，后面会给大家讲的 AQS 就很容易懂，它是把 JVM 底层的实现搬到 Java 源库。
