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."<init>":()V 11 #2 = Fieldref #3.#18 // com/example/springcloud/provider/demo/Demo.isOk:Z 12 #3 = Class #19 // com/example/springcloud/provider/demo/Demo 13 #4 = Class #20 // java/lang/Object 14 #5 = Utf8 isOk 15 #6 = Utf8 Z 16 #7 = Utf8 <init> 17 #8 = Utf8 ()V 18 #9 = Utf8 Code 19 #10 = Utf8 LineNumberTable 20 #11 = Utf8 LocalVariableTable 21 #12 = Utf8 this 22 #13 = Utf8 Lcom/example/springcloud/provider/demo/Demo; 23 #14 = Utf8 test 24 #15 = Utf8 SourceFile 25 #16 = Utf8 Demo.java 26 #17 = NameAndType #7:#8 // "<init>":()V 27 #18 = NameAndType #5:#6 // isOk:Z 28 #19 = Utf8 com/example/springcloud/provider/demo/Demo 29 #20 = Utf8 java/lang/Object 30 { 31 public com.example.springcloud.provider.demo.Demo(); 32 descriptor: ()V 33 flags: ACC_PUBLIC 34 Code: 35 stack=2, locals=1, args_size=1 36 0: aload_0 37 1: invokespecial #1 // Method java/lang/Object."<init>":()V 38 4: aload_0 39 5: iconst_1 40 6: putfield #2 // Field isOk:Z 41 9: return 42 LineNumberTable: 43 line 3: 0 44 line 4: 4 45 LocalVariableTable: 46 Start Length Slot Name Signature 47 0 10 0 this Lcom/example/springcloud/provider/demo/Demo; 48 49 public void test(); 50 descriptor: ()V 51 flags: ACC_PUBLIC 52 Code: 53 stack=2, locals=1, args_size=1 54 0: aload_0 55 1: iconst_0 56 2: putfield #2 // Field isOk:Z 57 5: return 58 LineNumberTable: 59 line 7: 0 60 line 8: 5 61 LocalVariableTable: 62 Start Length Slot Name Signature 63 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."<init>":()V 11 #2 = Fieldref #3.#18 // com/example/springcloud/provider/demo/Demo.isOk:Z 12 #3 = Class #19 // com/example/springcloud/provider/demo/Demo 13 #4 = Class #20 // java/lang/Object 14 #5 = Utf8 isOk 15 #6 = Utf8 Z 16 #7 = Utf8 <init> 17 #8 = Utf8 ()V 18 #9 = Utf8 Code 19 #10 = Utf8 LineNumberTable 20 #11 = Utf8 LocalVariableTable 21 #12 = Utf8 this 22 #13 = Utf8 Lcom/example/springcloud/provider/demo/Demo; 23 #14 = Utf8 test 24 #15 = Utf8 SourceFile 25 #16 = Utf8 Demo.java 26 #17 = NameAndType #7:#8 // "<init>":()V 27 #18 = NameAndType #5:#6 // isOk:Z 28 #19 = Utf8 com/example/springcloud/provider/demo/Demo 29 #20 = Utf8 java/lang/Object 30 { 31 public com.example.springcloud.provider.demo.Demo(); 32 descriptor: ()V 33 flags: ACC_PUBLIC 34 Code: 35 stack=2, locals=1, args_size=1 36 0: aload_0 37 1: invokespecial #1 // Method java/lang/Object."<init>":()V 38 4: aload_0 39 5: iconst_1 40 6: putfield #2 // Field isOk:Z 41 9: return 42 LineNumberTable: 43 line 3: 0 44 line 4: 4 45 LocalVariableTable: 46 Start Length Slot Name Signature 47 0 10 0 this Lcom/example/springcloud/provider/demo/Demo; 48 49 public synchronized void test(); 50 descriptor: ()V 51 flags: ACC_PUBLIC, ACC_SYNCHRONIZED 52 Code: 53 stack=2, locals=1, args_size=1 54 0: aload_0 55 1: iconst_0 56 2: putfield #2 // Field isOk:Z 57 5: return 58 LineNumberTable: 59 line 7: 0 60 line 8: 5 61 LocalVariableTable: 62 Start Length Slot Name Signature 63 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."<init>":()V 11 #2 = Fieldref #3.#22 // com/example/springcloud/provider/demo/Demo.isOk:Z 12 #3 = Class #23 // com/example/springcloud/provider/demo/Demo 13 #4 = Class #24 // java/lang/Object 14 #5 = Utf8 isOk 15 #6 = Utf8 Z 16 #7 = Utf8 <init> 17 #8 = Utf8 ()V 18 #9 = Utf8 Code 19 #10 = Utf8 LineNumberTable 20 #11 = Utf8 LocalVariableTable 21 #12 = Utf8 this 22 #13 = Utf8 Lcom/example/springcloud/provider/demo/Demo; 23 #14 = Utf8 test 24 #15 = Utf8 StackMapTable 25 #16 = Class #23 // com/example/springcloud/provider/demo/Demo 26 #17 = Class #24 // java/lang/Object 27 #18 = Class #25 // java/lang/Throwable 28 #19 = Utf8 SourceFile 29 #20 = Utf8 Demo.java 30 #21 = NameAndType #7:#8 // "<init>":()V 31 #22 = NameAndType #5:#6 // isOk:Z 32 #23 = Utf8 com/example/springcloud/provider/demo/Demo 33 #24 = Utf8 java/lang/Object 34 #25 = Utf8 java/lang/Throwable 35 { 36 public com.example.springcloud.provider.demo.Demo(); 37 descriptor: ()V 38 flags: ACC_PUBLIC 39 Code: 40 stack=2, locals=1, args_size=1 41 0: aload_0 42 1: invokespecial #1 // Method java/lang/Object."<init>":()V 43 4: aload_0 44 5: iconst_1 45 6: putfield #2 // Field isOk:Z 46 9: return 47 LineNumberTable: 48 line 3: 0 49 line 4: 4 50 LocalVariableTable: 51 Start Length Slot Name Signature 52 0 10 0 this Lcom/example/springcloud/provider/demo/Demo; 53 54 public void test(); 55 descriptor: ()V 56 flags: ACC_PUBLIC 57 Code: 58 stack=2, locals=3, args_size=1 59 0: aload_0 60 1: dup 61 2: astore_1 62 3: monitorenter 63 4: aload_0 64 5: iconst_0 65 6: putfield #2 // Field isOk:Z 66 9: aload_1 67 10: monitorexit 68 11: goto 19 69 14: astore_2 70 15: aload_1 71 16: monitorexit 72 17: aload_2 73 18: athrow 74 19: return 75 Exception table: 76 from to target type 77 4 11 14 any 78 14 17 14 any 79 LineNumberTable: 80 line 7: 0 81 line 8: 4 82 line 9: 9 83 line 10: 19 84 LocalVariableTable: 85 Start Length Slot Name Signature 86 0 20 0 this Lcom/example/springcloud/provider/demo/Demo; 87 StackMapTable: number_of_entries = 2 88 frame_type = 255 /* full_frame */ 89 offset_delta = 14 90 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 = 4 94 } 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."<init>":()V 11 #2 = Fieldref #3.#22 // com/example/springcloud/provider/demo/Demo.isOk:Z 12 #3 = Class #23 // com/example/springcloud/provider/demo/Demo 13 #4 = Class #24 // java/lang/Object 14 #5 = Utf8 isOk 15 #6 = Utf8 Z 16 #7 = Utf8 <init> 17 #8 = Utf8 ()V 18 #9 = Utf8 Code 19 #10 = Utf8 LineNumberTable 20 #11 = Utf8 LocalVariableTable 21 #12 = Utf8 this 22 #13 = Utf8 Lcom/example/springcloud/provider/demo/Demo; 23 #14 = Utf8 test 24 #15 = Utf8 StackMapTable 25 #16 = Class #23 // com/example/springcloud/provider/demo/Demo 26 #17 = Class #24 // java/lang/Object 27 #18 = Class #25 // java/lang/Throwable 28 #19 = Utf8 SourceFile 29 #20 = Utf8 Demo.java 30 #21 = NameAndType #7:#8 // "<init>":()V 31 #22 = NameAndType #5:#6 // isOk:Z 32 #23 = Utf8 com/example/springcloud/provider/demo/Demo 33 #24 = Utf8 java/lang/Object 34 #25 = Utf8 java/lang/Throwable 35 { 36 public volatile boolean isOk; 37 descriptor: Z 38 flags: ACC_PUBLIC, ACC_VOLATILE 39 40 public com.example.springcloud.provider.demo.Demo(); 41 descriptor: ()V 42 flags: ACC_PUBLIC 43 Code: 44 stack=2, locals=1, args_size=1 45 0: aload_0 46 1: invokespecial #1 // Method java/lang/Object."<init>":()V 47 4: aload_0 48 5: iconst_1 49 6: putfield #2 // Field isOk:Z 50 9: return 51 LineNumberTable: 52 line 3: 0 53 line 4: 4 54 LocalVariableTable: 55 Start Length Slot Name Signature 56 0 10 0 this Lcom/example/springcloud/provider/demo/Demo; 57 58 public void test(); 59 descriptor: ()V 60 flags: ACC_PUBLIC 61 Code: 62 stack=2, locals=3, args_size=1 63 0: aload_0 64 1: dup 65 2: astore_1 66 3: monitorenter 67 4: aload_0 68 5: iconst_0 69 6: putfield #2 // Field isOk:Z 70 9: aload_1 71 10: monitorexit 72 11: goto 19 73 14: astore_2 74 15: aload_1 75 16: monitorexit 76 17: aload_2 77 18: athrow 78 19: return 79 Exception table: 80 from to target type 81 4 11 14 any 82 14 17 14 any 83 LineNumberTable: 84 line 7: 0 85 line 8: 4 86 line 9: 9 87 line 10: 19 88 LocalVariableTable: 89 Start Length Slot Name Signature 90 0 20 0 this Lcom/example/springcloud/provider/demo/Demo; 91 StackMapTable: number_of_entries = 2 92 frame_type = 255 /* full_frame */ 93 offset_delta = 14 94 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 = 4 98 } 99 SourceFile: "Demo.java"