JVM synchronized关键字所生成的字节码
一、创建测试类
package com.example.jvm.bytecode;
public class MyTest2 {
String str = "Welcome";
private int x = 5;
public static Integer in = 10;
public static void main(String[] args) {
MyTest2 myTest2 = new MyTest2();
myTest2.setX(8);
in = 20;
}
public void setX(int x){
this.x = x;
}
}
反编译MyTest2.class文件
D:\workspace\study\ jvm_demo\build\classes\java\main\com\example\jvm\bytecode>javap -verbose MyTest2.class
Classfile /D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/bytecode/MyTest2.class
Last modified 2019-6-26; size 847 bytes
MD5 checksum d8f82e3e7255d9a738591c52d3a5417b
Compiled from "MyTest2.java"
public class com.example.jvm.bytecode.MyTest2
SourceFile: "MyTest2.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #10.#34 // java/lang/Object."<init>":()V
#2 = String #35 // Welcome
#3 = Fieldref #5.#36 // com/example/jvm/bytecode/MyTest2.str:Ljava/lang/String;
#4 = Fieldref #5.#37 // com/example/jvm/bytecode/MyTest2.x:I
#5 = Class #38 // com/example/jvm/bytecode/MyTest2
#6 = Methodref #5.#34 // com/example/jvm/bytecode/MyTest2."<init>":()V
#7 = Methodref #5.#39 // com/example/jvm/bytecode/MyTest2.setX:(I)V
#8 = Methodref #40.#41 // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
#9 = Fieldref #5.#42 // com/example/jvm/bytecode/MyTest2.in:Ljava/lang/Integer;
#10 = Class #43 // java/lang/Object
#11 = Utf8 str
#12 = Utf8 Ljava/lang/String;
#13 = Utf8 x
#14 = Utf8 I
#15 = Utf8 in
#16 = Utf8 Ljava/lang/Integer;
#17 = Utf8 <init>
#18 = Utf8 ()V
#19 = Utf8 Code
#20 = Utf8 LineNumberTable
#21 = Utf8 LocalVariableTable
#22 = Utf8 this
#23 = Utf8 Lcom/example/jvm/bytecode/MyTest2;
#24 = Utf8 main
#25 = Utf8 ([Ljava/lang/String;)V
#26 = Utf8 args
#27 = Utf8 [Ljava/lang/String;
#28 = Utf8 myTest2
#29 = Utf8 setX
#30 = Utf8 (I)V
#31 = Utf8 <clinit>
#32 = Utf8 SourceFile
#33 = Utf8 MyTest2.java
#34 = NameAndType #17:#18 // "<init>":()V
#35 = Utf8 Welcome
#36 = NameAndType #11:#12 // str:Ljava/lang/String;
#37 = NameAndType #13:#14 // x:I
#38 = Utf8 com/example/jvm/bytecode/MyTest2
#39 = NameAndType #29:#30 // setX:(I)V
#40 = Class #44 // java/lang/Integer
#41 = NameAndType #45:#46 // valueOf:(I)Ljava/lang/Integer;
#42 = NameAndType #15:#16 // in:Ljava/lang/Integer;
#43 = Utf8 java/lang/Object
#44 = Utf8 java/lang/Integer
#45 = Utf8 valueOf
#46 = Utf8 (I)Ljava/lang/Integer;
{
java.lang.String str;
flags: public static java.lang.Integer in;
flags: ACC_PUBLIC, ACC_STATIC public com.example.jvm.bytecode.MyTest2();
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String Welcome
7: putfield #3 // Field str:Ljava/lang/String;
10: aload_0
11: iconst_5
12: putfield #4 // Field x:I
15: return
LineNumberTable:
line 3: 0
line 5: 4
line 7: 10
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 this Lcom/example/jvm/bytecode/MyTest2; public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: new #5 // class com/example/jvm/bytecode/MyTest2
3: dup
4: invokespecial #6 // Method "<init>":()V
7: astore_1
8: aload_1
9: bipush 8
11: invokevirtual #7 // Method setX:(I)V
14: bipush 20
16: invokestatic #8 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
19: putstatic #9 // Field in:Ljava/lang/Integer;
22: return
LineNumberTable:
line 12: 0
line 14: 8
line 16: 14
line 17: 22
LocalVariableTable:
Start Length Slot Name Signature
0 23 0 args [Ljava/lang/String;
8 15 1 myTest2 Lcom/example/jvm/bytecode/MyTest2; public void setX(int);
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: iload_1
2: putfield #4 // Field x:I
5: return
LineNumberTable:
line 20: 0
line 21: 5
LocalVariableTable:
Start Length Slot Name Signature
0 6 0 this Lcom/example/jvm/bytecode/MyTest2;
0 6 1 x I static {};
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: bipush 10
2: invokestatic #8 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
5: putstatic #9 // Field in:Ljava/lang/Integer;
8: return
LineNumberTable:
line 9: 0
}
二、将setX改为private
此时需要使用javap -verbose -p MyTest2.class才能显示私有方法
D:\workspace\study\ jvm_demo\build\classes\java\main\com\example\jvm\bytecode>javap -verbose -p MyTest2.class
Classfile /D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/bytecode/MyTest2.class
Last modified 2019-6-27; size 847 bytes
MD5 checksum f9ed848bb768588b932c395b40c509a8
Compiled from "MyTest2.java"
public class com.example.jvm.bytecode.MyTest2
SourceFile: "MyTest2.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #10.#34 // java/lang/Object."<init>":()V
#2 = String #35 // Welcome
#3 = Fieldref #5.#36 // com/example/jvm/bytecode/MyTest2.str:Ljava/lang/String;
#4 = Fieldref #5.#37 // com/example/jvm/bytecode/MyTest2.x:I
#5 = Class #38 // com/example/jvm/bytecode/MyTest2
#6 = Methodref #5.#34 // com/example/jvm/bytecode/MyTest2."<init>":()V
#7 = Methodref #5.#39 // com/example/jvm/bytecode/MyTest2.setX:(I)V
#8 = Methodref #40.#41 // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
#9 = Fieldref #5.#42 // com/example/jvm/bytecode/MyTest2.in:Ljava/lang/Integer;
#10 = Class #43 // java/lang/Object
#11 = Utf8 str
#12 = Utf8 Ljava/lang/String;
#13 = Utf8 x
#14 = Utf8 I
#15 = Utf8 in
#16 = Utf8 Ljava/lang/Integer;
#17 = Utf8 <init>
#18 = Utf8 ()V
#19 = Utf8 Code
#20 = Utf8 LineNumberTable
#21 = Utf8 LocalVariableTable
#22 = Utf8 this
#23 = Utf8 Lcom/example/jvm/bytecode/MyTest2;
#24 = Utf8 main
#25 = Utf8 ([Ljava/lang/String;)V
#26 = Utf8 args
#27 = Utf8 [Ljava/lang/String;
#28 = Utf8 myTest2
#29 = Utf8 setX
#30 = Utf8 (I)V
#31 = Utf8 <clinit>
#32 = Utf8 SourceFile
#33 = Utf8 MyTest2.java
#34 = NameAndType #17:#18 // "<init>":()V
#35 = Utf8 Welcome
#36 = NameAndType #11:#12 // str:Ljava/lang/String;
#37 = NameAndType #13:#14 // x:I
#38 = Utf8 com/example/jvm/bytecode/MyTest2
#39 = NameAndType #29:#30 // setX:(I)V
#40 = Class #44 // java/lang/Integer
#41 = NameAndType #45:#46 // valueOf:(I)Ljava/lang/Integer;
#42 = NameAndType #15:#16 // in:Ljava/lang/Integer;
#43 = Utf8 java/lang/Object
#44 = Utf8 java/lang/Integer
#45 = Utf8 valueOf
#46 = Utf8 (I)Ljava/lang/Integer;
{
java.lang.String str;
flags: private int x;
flags: ACC_PRIVATE public static java.lang.Integer in;
flags: ACC_PUBLIC, ACC_STATIC public com.example.jvm.bytecode.MyTest2();
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String Welcome
7: putfield #3 // Field str:Ljava/lang/String;
10: aload_0
11: iconst_5
12: putfield #4 // Field x:I
15: return
LineNumberTable:
line 3: 0
line 5: 4
line 7: 10
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 this Lcom/example/jvm/bytecode/MyTest2; public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: new #5 // class com/example/jvm/bytecode/MyTest2
3: dup
4: invokespecial #6 // Method "<init>":()V
7: astore_1
8: aload_1
9: bipush 8
11: invokespecial #7 // Method setX:(I)V
14: bipush 20
16: invokestatic #8 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
19: putstatic #9 // Field in:Ljava/lang/Integer;
22: return
LineNumberTable:
line 12: 0
line 14: 8
line 16: 14
line 17: 22
LocalVariableTable:
Start Length Slot Name Signature
0 23 0 args [Ljava/lang/String;
8 15 1 myTest2 Lcom/example/jvm/bytecode/MyTest2; private void setX(int);
flags: ACC_PRIVATE
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: iload_1
2: putfield #4 // Field x:I
5: return
LineNumberTable:
line 20: 0
line 21: 5
LocalVariableTable:
Start Length Slot Name Signature
0 6 0 this Lcom/example/jvm/bytecode/MyTest2;
0 6 1 x I static {};
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: bipush 10
2: invokestatic #8 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
5: putstatic #9 // Field in:Ljava/lang/Integer;
8: return
LineNumberTable:
line 9: 0
}
三、现在将setX方法增加synchronized 修饰符
private synchronized void setX(int x)
反编译后无synchronized和有synchronized的区别如下图：
四、在类中增加test方法，方法里面使用了synchronize
private void test(String str){
synchronized (obj){
System.out.println("hello world");
}
}
反编译后如下：
private void test(java.lang.String);
flags: ACC_PRIVATE
Code:
stack=2, locals=4, args_size=2
0: aload_0
1: getfield #6 // Field obj:Ljava/lang/Object;
4: dup
5: astore_2
6: monitorenter
7: getstatic #12 // Field java/lang/System.out:Ljava/io/PrintStream;
10: ldc #13 // String hello world
12: invokevirtual #14 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
15: aload_2
16: monitorexit
17: goto 25
20: astore_3
21: aload_2
22: monitorexit
23: aload_3
24: athrow
25: return
Exception table:
from to target type
7 17 20 any
20 23 20 any
LineNumberTable:
line 26: 0
line 27: 7
line 28: 15
line 29: 25
LocalVariableTable:
Start Length Slot Name Signature
0 26 0 this Lcom/example/jvm/bytecode/MyTest2;
0 26 1 str Ljava/lang/String;
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 20
locals = [ class com/example/jvm/bytecode/MyTest2, class java/lang/String, class java/lang/Object ]
stack = [ class java/lang/Throwable ]
frame_type = 250 /* chop */
offset_delta = 4
出现了6: monitorenter和22: monitorexit
给当前MyTest2类所对应的class对象上锁
private synchronized static void test2(){
}
给当前对象上锁
private synchronized void setX(int x){
this.x = x;
}
五、字节码文件分析
最终需要分析的java文件
package com.example.jvm.bytecode;
public class MyTest2 {
String str = "Welcome";
private int x = 5;
public static Integer in = 10;
public static void main(String[] args) {
MyTest2 myTest2 = new MyTest2();
myTest2.setX(8);
in = 20;
}
private synchronized void setX(int x){
this.x = x;
}
private void test(String str){
synchronized (str){
System.out.println("hello world");
}
}
private synchronized static void test2(){
}
}
然后反编译MyTest2.class
Classfile /D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/bytecode/MyTest2.class
Last modified 2019-6-27; size 1265 bytes
MD5 checksum 1ca3fedabb74af431cc850f3f87b7763
Compiled from "MyTest2.java"
public class com.example.jvm.bytecode.MyTest2
SourceFile: "MyTest2.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #13.#45 // java/lang/Object."<init>":()V
#2 = String #46 // Welcome
#3 = Fieldref #5.#47 // com/example/jvm/bytecode/MyTest2.str:Ljava/lang/String;
#4 = Fieldref #5.#48 // com/example/jvm/bytecode/MyTest2.x:I
#5 = Class #49 // com/example/jvm/bytecode/MyTest2
#6 = Methodref #5.#45 // com/example/jvm/bytecode/MyTest2."<init>":()V
#7 = Methodref #5.#50 // com/example/jvm/bytecode/MyTest2.setX:(I)V
#8 = Methodref #51.#52 // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
#9 = Fieldref #5.#53 // com/example/jvm/bytecode/MyTest2.in:Ljava/lang/Integer;
#10 = Fieldref #54.#55 // java/lang/System.out:Ljava/io/PrintStream;
#11 = String #56 // hello world
#12 = Methodref #57.#58 // java/io/PrintStream.println:(Ljava/lang/String;)V
#13 = Class #59 // java/lang/Object
#14 = Utf8 str
#15 = Utf8 Ljava/lang/String;
#16 = Utf8 x
#17 = Utf8 I
#18 = Utf8 in
#19 = Utf8 Ljava/lang/Integer;
#20 = Utf8 <init>
#21 = Utf8 ()V
#22 = Utf8 Code
#23 = Utf8 LineNumberTable
#24 = Utf8 LocalVariableTable
#25 = Utf8 this
#26 = Utf8 Lcom/example/jvm/bytecode/MyTest2;
#27 = Utf8 main
#28 = Utf8 ([Ljava/lang/String;)V
#29 = Utf8 args
#30 = Utf8 [Ljava/lang/String;
#31 = Utf8 myTest2
#32 = Utf8 setX
#33 = Utf8 (I)V
#34 = Utf8 test
#35 = Utf8 (Ljava/lang/String;)V
#36 = Utf8 StackMapTable
#37 = Class #49 // com/example/jvm/bytecode/MyTest2
#38 = Class #60 // java/lang/String
#39 = Class #59 // java/lang/Object
#40 = Class #61 // java/lang/Throwable
#41 = Utf8 test2
#42 = Utf8 <clinit>
#43 = Utf8 SourceFile
#44 = Utf8 MyTest2.java
#45 = NameAndType #20:#21 // "<init>":()V
#46 = Utf8 Welcome
#47 = NameAndType #14:#15 // str:Ljava/lang/String;
#48 = NameAndType #16:#17 // x:I
#49 = Utf8 com/example/jvm/bytecode/MyTest2
#50 = NameAndType #32:#33 // setX:(I)V
#51 = Class #62 // java/lang/Integer
#52 = NameAndType #63:#64 // valueOf:(I)Ljava/lang/Integer;
#53 = NameAndType #18:#19 // in:Ljava/lang/Integer;
#54 = Class #65 // java/lang/System
#55 = NameAndType #66:#67 // out:Ljava/io/PrintStream;
#56 = Utf8 hello world
#57 = Class #68 // java/io/PrintStream
#58 = NameAndType #69:#35 // println:(Ljava/lang/String;)V
#59 = Utf8 java/lang/Object
#60 = Utf8 java/lang/String
#61 = Utf8 java/lang/Throwable
#62 = Utf8 java/lang/Integer
#63 = Utf8 valueOf
#64 = Utf8 (I)Ljava/lang/Integer;
#65 = Utf8 java/lang/System
#66 = Utf8 out
#67 = Utf8 Ljava/io/PrintStream;
#68 = Utf8 java/io/PrintStream
#69 = Utf8 println
{
java.lang.String str;
flags: private int x;
flags: ACC_PRIVATE public static java.lang.Integer in;
flags: ACC_PUBLIC, ACC_STATIC public com.example.jvm.bytecode.MyTest2();
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String Welcome
7: putfield #3 // Field str:Ljava/lang/String;
10: aload_0
11: iconst_5
12: putfield #4 // Field x:I
15: return
LineNumberTable:
line 3: 0
line 5: 4
line 7: 10
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 this Lcom/example/jvm/bytecode/MyTest2; public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: new #5 // class com/example/jvm/bytecode/MyTest2
3: dup
4: invokespecial #6 // Method "<init>":()V
7: astore_1
8: aload_1
9: bipush 8
11: invokespecial #7 // Method setX:(I)V
14: bipush 20
16: invokestatic #8 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
19: putstatic #9 // Field in:Ljava/lang/Integer;
22: return
LineNumberTable:
line 12: 0
line 14: 8
line 16: 14
line 17: 22
LocalVariableTable:
Start Length Slot Name Signature
0 23 0 args [Ljava/lang/String;
8 15 1 myTest2 Lcom/example/jvm/bytecode/MyTest2; private synchronized void setX(int);
flags: ACC_PRIVATE, ACC_SYNCHRONIZED
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: iload_1
2: putfield #4 // Field x:I
5: return
LineNumberTable:
line 20: 0
line 21: 5
LocalVariableTable:
Start Length Slot Name Signature
0 6 0 this Lcom/example/jvm/bytecode/MyTest2;
0 6 1 x I private void test(java.lang.String);
flags: ACC_PRIVATE
Code:
stack=2, locals=4, args_size=2
0: aload_1
1: dup
2: astore_2
3: monitorenter
4: getstatic #10 // Field java/lang/System.out:Ljava/io/PrintStream;
7: ldc #11 // String hello world
9: invokevirtual #12 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
12: aload_2
13: monitorexit
14: goto 22
17: astore_3
18: aload_2
19: monitorexit
20: aload_3
21: athrow
22: return
Exception table:
from to target type
4 14 17 any
17 20 17 any
LineNumberTable:
line 24: 0
line 25: 4
line 26: 12
line 27: 22
LocalVariableTable:
Start Length Slot Name Signature
0 23 0 this Lcom/example/jvm/bytecode/MyTest2;
0 23 1 str Ljava/lang/String;
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 17
locals = [ class com/example/jvm/bytecode/MyTest2, class java/lang/String, class java/lang/Object ]
stack = [ class java/lang/Throwable ]
frame_type = 250 /* chop */
offset_delta = 4 private static synchronized void test2();
flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNCHRONIZED
Code:
stack=0, locals=0, args_size=0
0: return
LineNumberTable:
line 31: 0 static {};
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: bipush 10
2: invokestatic #8 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
5: putstatic #9 // Field in:Ljava/lang/Integer;
8: return
LineNumberTable:
line 9: 0
}
然后打开WinHex打开MyTest2.class 文件
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