上一篇笔记的内容大部分没有实际动手操作，因此决定完成这个完整的练习并记录下来。
源代码如下：
package com.learn.jvm; /** * @author * @Description * @date 2019/09/05 16:31 */ public class Test { String str = "Welcome"; private int x = 5; public static Integer num = 10; public static void main(String[] args) { Test test = new Test(); test.setX(8); num = 20; } public void setX(int x) { this.x = x; } }
编译后在Windows平台下用HexEditor打开，得到结果如下，由于该软件似乎不支持将内容复制出来，因此截图
javap -verbose的输出结果如下
D:\workspace-learn\common-learn\learn-classloader\target\classes\com\learn\jvm>javap -verbose Test
警告: 二进制文件Test包含com.learn.jvm.Test
Classfile /D:/workspace-learn/common-learn/learn-classloader/target/classes/com/learn/jvm/Test.class
Last modified 2019-9-5; size 814 bytes
MD5 checksum d9651b05ecd94c86c354ab2551983cea
Compiled from "Test.java"
public class com.learn.jvm.Test
SourceFile: "Test.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/learn/jvm/Test.str:Ljava/lang/String;
#4 = Fieldref #5.#37 // com/learn/jvm/Test.x:I
#5 = Class #38 // com/learn/jvm/Test
#6 = Methodref #5.#34 // com/learn/jvm/Test."<init>":()V
#7 = Methodref #5.#39 // com/learn/jvm/Test.setX:(I)V
#8 = Methodref #40.#41 // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
#9 = Fieldref #5.#42 // com/learn/jvm/Test.num: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 num
#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/learn/jvm/Test;
#24 = Utf8 main
#25 = Utf8 ([Ljava/lang/String;)V
#26 = Utf8 args
#27 = Utf8 [Ljava/lang/String;
#28 = Utf8 test
#29 = Utf8 setX
#30 = Utf8 (I)V
#31 = Utf8 <clinit>
#32 = Utf8 SourceFile
#33 = Utf8 Test.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/learn/jvm/Test
#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 // num: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 num;
flags: ACC_PUBLIC, ACC_STATIC
public com.learn.jvm.Test();
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 8: 0
line 9: 4
line 10: 10
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 this Lcom/learn/jvm/Test;
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/learn/jvm/Test
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 num:Ljava/lang/Integer;
22: return
LineNumberTable:
line 14: 0
line 15: 8
line 16: 14
line 17: 22
LocalVariableTable:
Start Length Slot Name Signature
0 23 0 args [Ljava/lang/String;
8 15 1 test Lcom/learn/jvm/Test;
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/learn/jvm/Test;
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 num:Ljava/lang/Integer;
8: return
LineNumberTable:
line 11: 0
}
同时贴出Java字节码整体结构图便于对比查看
以下开始逐个字节进行分析
第1~4个字节：ca fe ba be 为魔数
第5~6个字节：00 00 代表 minor version ，值为0
第7~8个字节：00 33 代表 major version ，值为51
第9~10个字节：00 2f ，换算成十进制为 32+15 = 47 ，代表常量池中共有46个常量（除去一个为null保留的常量），我们通过查看上面 avap -verbose打印出来的信息可以验证，确实是这样的
接下来是具体的常量分析，所以将常量池中的数据类型结构贴出来
（以下为第1个变量）
第11~12个字节：0a 00