//简单的小例子观察字节码文件的差异:
package com.ydlclass.lock;
public class Test {
public static void main(String[] args) {
synchronized (Test.class){
int a = 1;
}
}
}
下面是字节码文件,发现多了Monitorenter,monitorexit关键字
Classfile /D:/code/MyReviewer/out/production/api/com/ydlclass/lock/Test.class
Last modified 2022-4-13; size 507 bytes
MD5 checksum 26d60269103a6ca368976d152fc706e0
Compiled from "Test.java"
public class com.ydlclass.lock.Test
minor version: 0
major version: 55
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #3.#20 // java/lang/Object."<init>":()V
#2 = Class #21 // com/ydlclass/lock/Test
#3 = Class #22 // java/lang/Object
#4 = Utf8 <init>
#5 = Utf8 ()V
#6 = Utf8 Code
#7 = Utf8 LineNumberTable
#8 = Utf8 LocalVariableTable
#9 = Utf8 this
#10 = Utf8 Lcom/ydlclass/lock/Test;
#11 = Utf8 main
#12 = Utf8 ([Ljava/lang/String;)V
#13 = Utf8 args
#14 = Utf8 [Ljava/lang/String;
#15 = Utf8 StackMapTable
#16 = Class #14 // "[Ljava/lang/String;"
#17 = Class #23 // java/lang/Throwable
#18 = Utf8 SourceFile
#19 = Utf8 Test.java
#20 = NameAndType #4:#5 // "<init>":()V
#21 = Utf8 com/ydlclass/lock/Test
#22 = Utf8 java/lang/Object
#23 = Utf8 java/lang/Throwable
{
public com.ydlclass.lock.Test();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 3: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/ydlclass/lock/Test;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=4, args_size=1
0: ldc #2 // class com/ydlclass/lock/Test
2: dup
3: astore_1
4: monitorenter
5: iconst_1
6: istore_2
7: aload_1
8: monitorexit
9: goto 17
12: astore_3
13: aload_1
14: monitorexit
15: aload_3
16: athrow
17: return
Exception table:
from to target type
5 9 12 any
12 15 12 any
LineNumberTable:
line 5: 0
line 6: 5
line 7: 7
line 8: 17
LocalVariableTable:
Start Length Slot Name Signature
0 18 0 args [Ljava/lang/String;
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 12
locals = [ class "[Ljava/lang/String;", class java/lang/Object ]
stack = [ class java/lang/Throwable ]
frame_type = 250 /* chop */
offset_delta = 4
}
SourceFile: "Test.java"
PS D:\code\MyReviewer\out\production\api\com\ydlclass\lock>
对象头:对象头中包括标记字段Mark Word(锁信息,分代年纪)和类型指针KClass pointer