一个class文件只能包含一个类或接口。
Java class文件是8位字节的二进制流。
在Java class文件中，可变长度项的大小和长度位于其实际数据之前。
class文件的基本类型
u1 1byte 无符号类型
u2 2byte 无符号类型
u4 4byte 无符号类型
u8 8byte 无符号类型
==========================================
以下是Java源码文件
public class ClassTest {
final static int constantInt=12;
final static String constantString="我很好";
static int variableInt=12;
static String variableString="我很好";
private int variable;
private String variable2;
public static void main(String[] args) {
System.out.println("--这是main()方法---");
}
public void say() {
System.out.println("这是普通方法");
}
public ClassTest(int variable, String variable2) {
super();
this.variable = variable;
this.variable2 = variable2;
}
public ClassTest() {
}
以下是使用uedit 编辑器查看的字节码文件
以下是使用 javap -v ClssTest.class 查看的字节码文件内容
Last modified 2019-7-24; size 924 bytes
MD5 checksum be5b20653620d7d2b495a3f99e3fb417
Compiled from "ClassTest.java"
public class ClassTest
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Fieldref #34.#35 // java/lang/System.out:Ljava/io/PrintStream;
#2 = String #36 // --这是main()方法---
#3 = Methodref #37.#38 // java/io/PrintStream.println:(Ljava/lang/String;)V
#4 = String #39 // 这是普通方法
#5 = Methodref #12.#40 // java/lang/Object."<init>":()V
#6 = Fieldref #11.#41 // ClassTest.variable:I
#7 = Fieldref #11.#42 // ClassTest.variable2:Ljava/lang/String;
#8 = Fieldref #11.#43 // ClassTest.variableInt:I
#9 = String #44 // 我很好
#10 = Fieldref #11.#45 // ClassTest.variableString:Ljava/lang/String;
#11 = Class #46 // ClassTest
#12 = Class #47 // java/lang/Object
#13 = Utf8 constantInt
#14 = Utf8 I
#15 = Utf8 ConstantValue
#16 = Integer 12
#17 = Utf8 constantString
#18 = Utf8 Ljava/lang/String;
#19 = Utf8 variableInt
#20 = Utf8 variableString
#21 = Utf8 variable
#22 = Utf8 variable2
#23 = Utf8 main
#24 = Utf8 ([Ljava/lang/String;)V
#25 = Utf8 Code
#26 = Utf8 LineNumberTable
#27 = Utf8 say
#28 = Utf8 ()V
#29 = Utf8 <init>
#30 = Utf8 (ILjava/lang/String;)V
#31 = Utf8 <clinit>
#32 = Utf8 SourceFile
#33 = Utf8 ClassTest.java
#34 = Class #48 // java/lang/System
#35 = NameAndType #49:#50 // out:Ljava/io/PrintStream;
#36 = Utf8 --这是main()方法---
#37 = Class #51 // java/io/PrintStream
#38 = NameAndType #52:#53 // println:(Ljava/lang/String;)V
#39 = Utf8 这是普通方法
#40 = NameAndType #29:#28 // "<init>":()V
#41 = NameAndType #21:#14 // variable:I
#42 = NameAndType #22:#18 // variable2:Ljava/lang/String;
#43 = NameAndType #19:#14 // variableInt:I
#44 = Utf8 我很好
#45 = NameAndType #20:#18 // variableString:Ljava/lang/String;
#46 = Utf8 ClassTest
#47 = Utf8 java/lang/Object
#48 = Utf8 java/lang/System
#49 = Utf8 out
#50 = Utf8 Ljava/io/PrintStream;
#51 = Utf8 java/io/PrintStream
#52 = Utf8 println
#53 = Utf8 (Ljava/lang/String;)V
{
static final int constantInt;
descriptor: I
flags: ACC_STATIC, ACC_FINAL
ConstantValue: int 12
static final java.lang.String constantString;
descriptor: Ljava/lang/String;
flags: ACC_STATIC, ACC_FINAL
ConstantValue: String 我很好
static int variableInt;
descriptor: I
flags: ACC_STATIC
static java.lang.String variableString;
descriptor: Ljava/lang/String;
flags: ACC_STATIC
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #1 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #2 // String --这是main()方法---
5: invokevirtual #3 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 11: 0
line 13: 8
public void say();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #1 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #4 // String 这是普通方法
5: invokevirtual #3 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 15: 0
line 16: 8
public ClassTest(int, java.lang.String);
descriptor: (ILjava/lang/String;)V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=3
0: aload_0
1: invokespecial #5 // Method java/lang/Object."<init>":()V
4: aload_0
5: iload_1
6: putfield #6 // Field variable:I
9: aload_0
10: aload_2
11: putfield #7 // Field variable2:Ljava/lang/String;
14: return
LineNumberTable:
line 18: 0
line 19: 4
line 20: 9
line 21: 14
public ClassTest();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #5 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 22: 0
line 24: 4
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: bipush 12
2: putstatic #8 // Field variableInt:I
5: ldc #9 // String 我很好
7: putstatic #10 // Field variableString:Ljava/lang/String;
10: return
LineNumberTable:
line 5: 0
line 6: 5
}
SourceFile: "ClassTest.java"
简单版本：
minor version: 0 //---主版本号
major version: 52 //---次版本号
flags: ACC_PUBLIC, ACC_SUPER //---类访问标识
//--- 常量池
Constant pool:
#1 = Fieldref #34.#35 // java/lang/System.out:Ljava/io/PrintStream;
#2 = String #36 // --这是main()方法---
.......
#52 = Utf8 println
#53 = Utf8 (Ljava/lang/String;)V
{
//--- 字段信息
static final int constantInt;
descriptor: I
flags: ACC_STATIC, ACC_FINAL
ConstantValue: int 12
static int variableInt;
descriptor: I
flags: ACC_STATIC
//---方法信息
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #1 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #2 // String --这是main()方法---
5: invokevirtual #3 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 11: 0
line 13: 8
public ClassTest();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #5 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 22: 0
line 24: 4
//-- super()方法
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: bipush 12
2: putstatic #8 // Field variableInt:I
5: ldc #9 // String 我很好
7: putstatic #10 // Field variableString:Ljava/lang/String;
10: return
LineNumberTable:
line 5: 0
line 6: 5
}
class 文件内容项
magic(魔数) : class文件前四位字节,class文件总是以 0xCAFEBABE 开头，作为class文件标识。
minor_version / major_version 主次版本号，JVM 根据版本号决定如何加载class文件
constant_pool_count / constant_pool ：常量池
每一个常量池入口都是从一个标识（长度是一个字节）开始。次标识指明该位置常量的类型。
每一个标志都有一个相对的表，表名就是标志名加上“_info” 即 constant_utf8_info
access_flage : 访问标志，指明class文件定义的是类还是接口，以及类或接口的修饰符。
this_class ：就是指向常量池中constant_class_info的索引。
super_class ：也是一个指向常量池的索引。
interfaces / fields / methods /attribute ：也是指向常量池的索引。
XXX_count :就是对应的数量。