Class类文件的结构
作者：bin我们来看这样一个类
public class Test extends TestB{
public static Integer myData = TestB.data;
public String normalMyData = "123";
public String normalMyDataEmpty;
public static void main(String[] args) {
System.out.println("333333");
}
public String doTest(int date){
date = date + 1;
System.out.println(date);
return date + "";
}
}
我们使用如下命令查看class文件的字节码内容
javap -verbose Test.class > Test.verbose
Test.verbose内容过长，我贴在文章末尾，我们先逐个来看
minor为次版本号，major为主版本号，版本号说明了当前的文件要用多少JDK版本以上的虚拟机才能执行
minor version: 0 major version: 52
Constant pool为常量池
Constant pool: ...
访问标识(access_flags)
标识接口、属性的访问权限
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
父类索引(super_class)
这里我门可以看到指向常量池的#11，#11是一个class，他指向一个utf-8的#47，包含类类的全名
super_class: #11 ... Constant pool: ... #11 = Class #47 ... #47 = Utf8 com/xiaoying/core/api/controller/TestB
字段表集合(field_info)，其中字段集合会用到flags标识别权限，用descriptor标识类型
public static java.lang.Integer myData;
descriptor: Ljava/lang/Integer;
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
public java.lang.String normalMyData;
descriptor: Ljava/lang/String;
flags: (0x0001) ACC_PUBLIC
public java.lang.String normalMyDataEmpty;
descriptor: Ljava/lang/String;
flags: (0x0001) ACC_PUBLIC
方法表集合(methods)
descriptor表示入参和反参类型，(I)表示入参Integer类型，括号后面表示返回类型
public java.lang.String doTest(int);
descriptor: (I)Ljava/lang/String;
flags: (0x0001) ACC_PUBLIC
属性表集合(attribute_info)
Class文件、字段表、方法表都可以携带自己的属性表集合，以描述某些场景专有的信息。
例如方法的代码，放在Code里面：
Code:
...
代码关联的行号放在LineNumberTable中：
LineNumberTable:
line 14: 0
line 15: 4
line 16: 11
Test.verbose内容
Classfile /Users/zengbin/IdeaProjects/credit.guarantor_platform.core/core-portal/out/production/classes/Test.class
Last modified 2021年4月1日; size 1179 bytes
MD5 checksum 23ca2f79d1f7b785efa19f45ad0d7dd4
Compiled from "Test.java"
public class com.zengbingo.core.api.controller.Test extends com.zengbingo.core.api.controller.TestB
minor version: 0
major version: 52
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #16 // com/zenbgingo/Test
super_class: #17 // com/zenbgingo/TestB
interfaces: 0, fields: 3, methods: 4, attributes: 1
Constant pool:
#1 = Methodref #17.#41 // com/zenbgingo/TestB."<init>":()V
#2 = String #42 // 123
#3 = Fieldref #16.#43 // com/zenbgingo/Test.normalMyData:Ljava/lang/String;
#4 = Fieldref #44.#45 // java/lang/System.out:Ljava/io/PrintStream;
#5 = String #46 // 333333
#6 = Methodref #47.#48 // java/io/PrintStream.println:(Ljava/lang/String;)V
#7 = Methodref #47.#49 // java/io/PrintStream.println:(I)V
#8 = Class #50 // java/lang/StringBuilder
#9 = Methodref #8.#41 // java/lang/StringBuilder."<init>":()V
#10 = Methodref #8.#51 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#11 = String #52 //
#12 = Methodref #8.#53 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#13 = Methodref #8.#54 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#14 = Fieldref #17.#55 // com/zenbgingo/TestB.data:Ljava/lang/Integer;
#15 = Fieldref #16.#56 // com/zenbgingo/Test.myData:Ljava/lang/Integer;
#16 = Class #57 // com/zenbgingo/Test
#17 = Class #58 // com/zenbgingo/TestB
#18 = Utf8 myData
#19 = Utf8 Ljava/lang/Integer;
#20 = Utf8 normalMyData
#21 = Utf8 Ljava/lang/String;
#22 = Utf8 normalMyDataEmpty
#23 = Utf8 <init>
#24 = Utf8 ()V
#25 = Utf8 Code
#26 = Utf8 LineNumberTable
#27 = Utf8 LocalVariableTable
#28 = Utf8 this
#29 = Utf8 Lcom/zenbgingo/Test;
#30 = Utf8 main
#31 = Utf8 ([Ljava/lang/String;)V
#32 = Utf8 args
#33 = Utf8 [Ljava/lang/String;
#34 = Utf8 doTest
#35 = Utf8 (I)Ljava/lang/String;
#36 = Utf8 date
#37 = Utf8 I
#38 = Utf8 <clinit>
#39 = Utf8 SourceFile
#40 = Utf8 Test.java
#41 = NameAndType #23:#24 // "<init>":()V
#42 = Utf8 123
#43 = NameAndType #20:#21 // normalMyData:Ljava/lang/String;
#44 = Class #59 // java/lang/System
#45 = NameAndType #60:#61 // out:Ljava/io/PrintStream;
#46 = Utf8 333333
#47 = Class #62 // java/io/PrintStream
#48 = NameAndType #63:#64 // println:(Ljava/lang/String;)V
#49 = NameAndType #63:#65 // println:(I)V
#50 = Utf8 java/lang/StringBuilder
#51 = NameAndType #66:#67 // append:(I)Ljava/lang/StringBuilder;
#52 = Utf8
#53 = NameAndType #66:#68 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#54 = NameAndType #69:#70 // toString:()Ljava/lang/String;
#55 = NameAndType #71:#19 // data:Ljava/lang/Integer;
#56 = NameAndType #18:#19 // myData:Ljava/lang/Integer;
#57 = Utf8 com/zenbgingo/Test
#58 = Utf8 com/zenbgingo/TestB
#59 = Utf8 java/lang/System
#60 = Utf8 out
#61 = Utf8 Ljava/io/PrintStream;
#62 = Utf8 java/io/PrintStream
#63 = Utf8 println
#64 = Utf8 (Ljava/lang/String;)V
#65 = Utf8 (I)V
#66 = Utf8 append
#67 = Utf8 (I)Ljava/lang/StringBuilder;
#68 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#69 = Utf8 toString
#70 = Utf8 ()Ljava/lang/String;
#71 = Utf8 data
{
public static java.lang.Integer myData;
descriptor: Ljava/lang/Integer;
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
public java.lang.String normalMyData;
descriptor: Ljava/lang/String;
flags: (0x0001) ACC_PUBLIC
public java.lang.String normalMyDataEmpty;
descriptor: Ljava/lang/String;
flags: (0x0001) ACC_PUBLIC
public com.zengbingo.core.api.controller.Test();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method com/zenbgingo/TestB."<init>":()V
4: aload_0
5: ldc #2 // String 123
7: putfield #3 // Field normalMyData:Ljava/lang/String;
10: return
LineNumberTable:
line 3: 0
line 6: 4
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this Lcom/zenbgingo/Test;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #5 // String 333333
5: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 10: 0
line 11: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 args [Ljava/lang/String;
public java.lang.String doTest(int);
descriptor: (I)Ljava/lang/String;
flags: (0x0001) ACC_PUBLIC
Code:
stack=2, locals=2, args_size=2
0: iload_1
1: iconst_1
2: iadd
3: istore_1
4: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
7: iload_1
8: invokevirtual #7 // Method java/io/PrintStream.println:(I)V
11: new #8 // class java/lang/StringBuilder
14: dup
15: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
18: iload_1
19: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
22: ldc #11 // String
24: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
27: invokevirtual #13 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
30: areturn
LineNumberTable:
line 14: 0
line 15: 4
line 16: 11
LocalVariableTable:
Start Length Slot Name Signature
0 31 0 this Lcom/zenbgingo/Test;
0 31 1 date I
static {};
descriptor: ()V
flags: (0x0008) ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: getstatic #14 // Field com/zenbgingo/TestB.data:Ljava/lang/Integer;
3: putstatic #15 // Field myData:Ljava/lang/Integer;
6: return
LineNumberTable:
line 5: 0
}
SourceFile: "Test.java"