今天我们来分析一下java中synchronized关键字。首先来看一段java代码：(本地编译环境为mac，jdk1.8的环境)
Demo.java
1 package com.example.springcloud.provider.demo;2 3 public class Demo {4 private boolean isOk = true;5 6 public void test(){7 isOk=false;8 }9 } 编译周后的Demo.class二进制文件：
Demo.class
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使用javap -v Demo.class > Demo.txt 文件得到反编译的汇编语言：
Demo.txt
1 Classfile /Users/shiwen/IdeaProjects/springcloud/example-springcloud-provider/target/classes/com/example/springcloud/provider/demo/Demo.class 2 Last modified 2017-10-11; size 424 bytes 3 MD5 checksum 1f57b26a93d1cc466ff58bcec5ff37fb 4 Compiled from "Demo.java" 5 public class com.example.springcloud.provider.demo.Demo 6 minor version: 0 7 major version: 52 8 flags: ACC_PUBLIC, ACC_SUPER 9 Constant pool:10 #1 = Methodref #4.#17 // java/lang/Object."":()V11 #2 = Fieldref #3.#18 // com/example/springcloud/provider/demo/Demo.isOk:Z12 #3 = Class #19 // com/example/springcloud/provider/demo/Demo13 #4 = Class #20 // java/lang/Object14 #5 = Utf8 isOk15 #6 = Utf8 Z16 #7 = Utf8 17 #8 = Utf8 ()V18 #9 = Utf8 Code19 #10 = Utf8 LineNumberTable20 #11 = Utf8 LocalVariableTable21 #12 = Utf8 this22 #13 = Utf8 Lcom/example/springcloud/provider/demo/Demo;23 #14 = Utf8 test24 #15 = Utf8 SourceFile25 #16 = Utf8 Demo.java26 #17 = NameAndType #7:#8 // " ":()V27 #18 = NameAndType #5:#6 // isOk:Z28 #19 = Utf8 com/example/springcloud/provider/demo/Demo29 #20 = Utf8 java/lang/Object30 {31 public com.example.springcloud.provider.demo.Demo();32 descriptor: ()V33 flags: ACC_PUBLIC34 Code:35 stack=2, locals=1, args_size=136 0: aload_037 1: invokespecial #1 // Method java/lang/Object." ":()V38 4: aload_039 5: iconst_140 6: putfield #2 // Field isOk:Z41 9: return42 LineNumberTable:43 line 3: 044 line 4: 445 LocalVariableTable:46 Start Length Slot Name Signature47 0 10 0 this Lcom/example/springcloud/provider/demo/Demo;48 49 public void test();50 descriptor: ()V51 flags: ACC_PUBLIC52 Code:53 stack=2, locals=1, args_size=154 0: aload_055 1: iconst_056 2: putfield #2 // Field isOk:Z57 5: return58 LineNumberTable:59 line 7: 060 line 8: 561 LocalVariableTable:62 Start Length Slot Name Signature63 0 6 0 this Lcom/example/springcloud/provider/demo/Demo;64 }65 SourceFile: "Demo.java"
修改Demo.java文件在方法中新增synchronized如下：
package com.example.springcloud.provider.demo;public class Demo { private boolean isOk = true; private synchronized void test(){ isOk=false;
} }
反编译为汇编后：(不同的地方用黄色标识出来了)
1 Classfile /Users/shiwen/IdeaProjects/springcloud/example-springcloud-provider/target/classes/com/example/springcloud/provider/demo/Demo.class 2 Last modified 2017-10-11; size 424 bytes 3 MD5 checksum 000d5f4eb139b3d12c483f7087c0c970 4 Compiled from "Demo.java" 5 public class com.example.springcloud.provider.demo.Demo 6 minor version: 0 7 major version: 52 8 flags: ACC_PUBLIC, ACC_SUPER 9 Constant pool:10 #1 = Methodref #4.#17 // java/lang/Object."":()V11 #2 = Fieldref #3.#18 // com/example/springcloud/provider/demo/Demo.isOk:Z12 #3 = Class #19 // com/example/springcloud/provider/demo/Demo13 #4 = Class #20 // java/lang/Object14 #5 = Utf8 isOk15 #6 = Utf8 Z16 #7 = Utf8 17 #8 = Utf8 ()V18 #9 = Utf8 Code19 #10 = Utf8 LineNumberTable20 #11 = Utf8 LocalVariableTable21 #12 = Utf8 this22 #13 = Utf8 Lcom/example/springcloud/provider/demo/Demo;23 #14 = Utf8 test24 #15 = Utf8 SourceFile25 #16 = Utf8 Demo.java26 #17 = NameAndType #7:#8 // " ":()V27 #18 = NameAndType #5:#6 // isOk:Z28 #19 = Utf8 com/example/springcloud/provider/demo/Demo29 #20 = Utf8 java/lang/Object30 {31 public com.example.springcloud.provider.demo.Demo();32 descriptor: ()V33 flags: ACC_PUBLIC34 Code:35 stack=2, locals=1, args_size=136 0: aload_037 1: invokespecial #1 // Method java/lang/Object." ":()V38 4: aload_039 5: iconst_140 6: putfield #2 // Field isOk:Z41 9: return42 LineNumberTable:43 line 3: 044 line 4: 445 LocalVariableTable:46 Start Length Slot Name Signature47 0 10 0 this Lcom/example/springcloud/provider/demo/Demo;48 49 public synchronized void test();50 descriptor: ()V51 flags: ACC_PUBLIC, ACC_SYNCHRONIZED52 Code:53 stack=2, locals=1, args_size=154 0: aload_055 1: iconst_056 2: putfield #2 // Field isOk:Z57 5: return58 LineNumberTable:59 line 7: 060 line 8: 561 LocalVariableTable:62 Start Length Slot Name Signature63 0 6 0 this Lcom/example/springcloud/provider/demo/Demo;64 }65 SourceFile: "Demo.java"
改为synchronized代码块：
1 package com.example.springcloud.provider.demo; 2 3 public class Demo { 4 private boolean isOk = true; 5 6 public void test() { 7 synchronized (this) { 8 isOk = false; 9 }10 }11 } 反编译汇编为：
1 Classfile /Users/shiwen/IdeaProjects/springcloud/example-springcloud-provider/target/classes/com/example/springcloud/provider/demo/Demo.class 2 Last modified 2017-10-11; size 536 bytes 3 MD5 checksum 64305bd51f24f439444d9240efd483fd 4 Compiled from "Demo.java" 5 public class com.example.springcloud.provider.demo.Demo 6 minor version: 0 7 major version: 52 8 flags: ACC_PUBLIC, ACC_SUPER 9 Constant pool:10 #1 = Methodref #4.#21 // java/lang/Object."":()V11 #2 = Fieldref #3.#22 // com/example/springcloud/provider/demo/Demo.isOk:Z12 #3 = Class #23 // com/example/springcloud/provider/demo/Demo13 #4 = Class #24 // java/lang/Object14 #5 = Utf8 isOk15 #6 = Utf8 Z16 #7 = Utf8 17 #8 = Utf8 ()V18 #9 = Utf8 Code19 #10 = Utf8 LineNumberTable20 #11 = Utf8 LocalVariableTable21 #12 = Utf8 this22 #13 = Utf8 Lcom/example/springcloud/provider/demo/Demo;23 #14 = Utf8 test24 #15 = Utf8 StackMapTable25 #16 = Class #23 // com/example/springcloud/provider/demo/Demo26 #17 = Class #24 // java/lang/Object27 #18 = Class #25 // java/lang/Throwable28 #19 = Utf8 SourceFile29 #20 = Utf8 Demo.java30 #21 = NameAndType #7:#8 // " ":()V31 #22 = NameAndType #5:#6 // isOk:Z32 #23 = Utf8 com/example/springcloud/provider/demo/Demo33 #24 = Utf8 java/lang/Object34 #25 = Utf8 java/lang/Throwable35 {36 public com.example.springcloud.provider.demo.Demo();37 descriptor: ()V38 flags: ACC_PUBLIC39 Code:40 stack=2, locals=1, args_size=141 0: aload_042 1: invokespecial #1 // Method java/lang/Object." ":()V43 4: aload_044 5: iconst_145 6: putfield #2 // Field isOk:Z46 9: return47 LineNumberTable:48 line 3: 049 line 4: 450 LocalVariableTable:51 Start Length Slot Name Signature52 0 10 0 this Lcom/example/springcloud/provider/demo/Demo;53 54 public void test();55 descriptor: ()V56 flags: ACC_PUBLIC57 Code:58 stack=2, locals=3, args_size=159 0: aload_060 1: dup61 2: astore_162 3: monitorenter63 4: aload_064 5: iconst_065 6: putfield #2 // Field isOk:Z66 9: aload_167 10: monitorexit68 11: goto 1969 14: astore_270 15: aload_171 16: monitorexit72 17: aload_273 18: athrow74 19: return75 Exception table:76 from to target type77 4 11 14 any78 14 17 14 any79 LineNumberTable:80 line 7: 081 line 8: 482 line 9: 983 line 10: 1984 LocalVariableTable:85 Start Length Slot Name Signature86 0 20 0 this Lcom/example/springcloud/provider/demo/Demo;87 StackMapTable: number_of_entries = 288 frame_type = 255 /* full_frame */89 offset_delta = 1490 locals = [ class com/example/springcloud/provider/demo/Demo, class java/lang/Object ]91 stack = [ class java/lang/Throwable ]92 frame_type = 250 /* chop */93 offset_delta = 494 }95 SourceFile: "Demo.java"
和synchronized方法对比如下：
给isOk增加volatile关键字后
1 Classfile /Users/shiwen/IdeaProjects/springcloud/example-springcloud-provider/target/classes/com/example/springcloud/provider/demo/Demo.class 2 Last modified 2017-10-11; size 536 bytes 3 MD5 checksum 8dc910b015c7d5af2feac29d19f519a1 4 Compiled from "Demo.java" 5 public class com.example.springcloud.provider.demo.Demo 6 minor version: 0 7 major version: 52 8 flags: ACC_PUBLIC, ACC_SUPER 9 Constant pool:10 #1 = Methodref #4.#21 // java/lang/Object."":()V11 #2 = Fieldref #3.#22 // com/example/springcloud/provider/demo/Demo.isOk:Z12 #3 = Class #23 // com/example/springcloud/provider/demo/Demo13 #4 = Class #24 // java/lang/Object14 #5 = Utf8 isOk15 #6 = Utf8 Z16 #7 = Utf8 17 #8 = Utf8 ()V18 #9 = Utf8 Code19 #10 = Utf8 LineNumberTable20 #11 = Utf8 LocalVariableTable21 #12 = Utf8 this22 #13 = Utf8 Lcom/example/springcloud/provider/demo/Demo;23 #14 = Utf8 test24 #15 = Utf8 StackMapTable25 #16 = Class #23 // com/example/springcloud/provider/demo/Demo26 #17 = Class #24 // java/lang/Object27 #18 = Class #25 // java/lang/Throwable28 #19 = Utf8 SourceFile29 #20 = Utf8 Demo.java30 #21 = NameAndType #7:#8 // " ":()V31 #22 = NameAndType #5:#6 // isOk:Z32 #23 = Utf8 com/example/springcloud/provider/demo/Demo33 #24 = Utf8 java/lang/Object34 #25 = Utf8 java/lang/Throwable35 {36 public volatile boolean isOk;37 descriptor: Z38 flags: ACC_PUBLIC, ACC_VOLATILE 39 40 public com.example.springcloud.provider.demo.Demo();41 descriptor: ()V42 flags: ACC_PUBLIC43 Code:44 stack=2, locals=1, args_size=145 0: aload_046 1: invokespecial #1 // Method java/lang/Object." ":()V47 4: aload_048 5: iconst_149 6: putfield #2 // Field isOk:Z50 9: return51 LineNumberTable:52 line 3: 053 line 4: 454 LocalVariableTable:55 Start Length Slot Name Signature56 0 10 0 this Lcom/example/springcloud/provider/demo/Demo;57 58 public void test();59 descriptor: ()V60 flags: ACC_PUBLIC61 Code:62 stack=2, locals=3, args_size=163 0: aload_064 1: dup65 2: astore_166 3: monitorenter67 4: aload_068 5: iconst_069 6: putfield #2 // Field isOk:Z70 9: aload_171 10: monitorexit72 11: goto 1973 14: astore_274 15: aload_175 16: monitorexit76 17: aload_277 18: athrow78 19: return79 Exception table:80 from to target type81 4 11 14 any82 14 17 14 any83 LineNumberTable:84 line 7: 085 line 8: 486 line 9: 987 line 10: 1988 LocalVariableTable:89 Start Length Slot Name Signature90 0 20 0 this Lcom/example/springcloud/provider/demo/Demo;91 StackMapTable: number_of_entries = 292 frame_type = 255 /* full_frame */93 offset_delta = 1494 locals = [ class com/example/springcloud/provider/demo/Demo, class java/lang/Object ]95 stack = [ class java/lang/Throwable ]96 frame_type = 250 /* chop */97 offset_delta = 498 }99 SourceFile: "Demo.java"