Java Class文件结构：
u1,u2,u3,u4表示1个，2个，3个，4个字节。
Class文件结构中只有两种数据类型：无符号数和表，表就相当于是结构体了。
//TestClass.java
package com.ejushang.TestClass;
public class TestClass implements Super{
private static final int staticVar = 0;
private int instanceVar=0;
public int instanceMethod(int param){
return param+1;
}
}
interface Super{ }
TestClass.class
00000000h: CA FE BA BE 00 00 00 34 00 18 0A 00 04 00 13 09 ; 漱壕...4........
00000010h: 00 03 00 14 07 00 15 07 00 16 07 00 17 01 00 09 ; ................
00000020h: 73 74 61 74 69 63 56 61 72 01 00 01 49 01 00 0D ; staticVar...I...
00000030h: 43 6F 6E 73 74 61 6E 74 56 61 6C 75 65 03 00 00 ; ConstantValue...
00000040h: 00 00 01 00 0B 69 6E 73 74 61 6E 63 65 56 61 72 ; .....instanceVar
00000050h: 01 00 06 3C 69 6E 69 74 3E 01 00 03 28 29 56 01 ; ...
00000060h: 00 04 43 6F 64 65 01 00 0F 4C 69 6E 65 4E 75 6D ; ..Code...LineNum
00000070h: 62 65 72 54 61 62 6C 65 01 00 0E 69 6E 73 74 61 ; berTable...insta
00000080h: 6E 63 65 4D 65 74 68 6F 64 01 00 04 28 49 29 49 ; nceMethod...(I)I
00000090h: 01 00 0A 53 6F 75 72 63 65 46 69 6C 65 01 00 0E ; ...SourceFile...
000000a0h: 54 65 73 74 43 6C 61 73 73 2E 6A 61 76 61 0C 00 ; TestClass.java..
000000b0h: 0B 00 0C 0C 00 0A 00 07 01 00 20 63 6F 6D 2F 65 ; .......... com/e
000000c0h: 6A 75 73 68 61 6E 67 2F 54 65 73 74 43 6C 61 73 ; jushang/TestClas
000000d0h: 73 2F 54 65 73 74 43 6C 61 73 73 01 00 10 6A 61 ; s/TestClass...ja
000000e0h: 76 61 2F 6C 61 6E 67 2F 4F 62 6A 65 63 74 01 00 ; va/lang/Object..
000000f0h: 1C 63 6F 6D 2F 65 6A 75 73 68 61 6E 67 2F 54 65 ; .com/ejushang/Te
00000100h: 73 74 43 6C 61 73 73 2F 53 75 70 65 72 00 21 00 ; stClass/Super.!.
00000110h: 03 00 04 00 01 00 05 00 02 00 1A 00 06 00 07 00 ; ................
00000120h: 01 00 08 00 00 00 02 00 09 00 02 00 0A 00 07 00 ; ................
00000130h: 00 00 02 00 01 00 0B 00 0C 00 01 00 0D 00 00 00 ; ................
00000140h: 26 00 02 00 01 00 00 00 0A 2A B7 00 01 2A 03 B5 ; &........?..?
00000150h: 00 02 B1 00 00 00 01 00 0E 00 00 00 0A 00 02 00 ; ..?............
00000160h: 00 00 03 00 04 00 07 00 01 00 0F 00 10 00 01 00 ; ................
00000170h: 0D 00 00 00 1C 00 02 00 02 00 00 00 04 1B 04 60 ; ...............`
00000180h: AC 00 00 00 01 00 0E 00 00 00 06 00 01 00 00 00 ; ?..............
00000190h: 0A 00 01 00 11 00 00 00 02 00 12 ; ...........
分析class文件：
magic: CA FE BA BE
version: 00 00 00 34
cp_count: 00 18
cp[1]:
flag: 0A (CONSTANT_Methodref_info 类中方法的符号引用)
class_index: 00 04
name_and_type_index: 00 13
cp[2]:
flag: 09 (CONSTANT_Fieldref_info 字段的符号引用)
class_index: 00 03
name_and_type_index: 00 14
cp[3]:
flag: 07(CONSTANT_Class_info 类或接口的符号引用)
class_index: 00 15
cp[4]:
flag:07
class_index: 00 16
cp[5]:
flag: 07
class_index: 00 17
cp[6]:
flag: 01(CONSTANT_UTF8_info UTF-8编码的字符串)
len: 00 09
value: 73 74 61 74 69 63 56 61 72(staticVar)
cp[7]:
flag: 01
len: 00 01
value: 49(I)
cp[8]:
flag: 01
len: 00 0D
value: 43 6F 6E 73 74 61 6E 74 56 61 6C 75 65(ConstantValue)
cp[9]:
flag: 03(CONSTANT_Integer_info整形字面量)
value: 00 00 00 00
cp[0x0A]:
flag: 01
len: 00 0B
value: 69 6E 73 74 61 6E 63 65 56 61 72(instanceVar)
cp[0x0B]:
flag: 01
len: 00 06
value: 3C 69 6E 69 74 3E()
cp[0x0C]:
flag: 01
len: 00 03
value: 28 29 56(()I)
cp[0x0D]:
flag: 01
len: 00 04
value: 43 6F 64 65 (Code)
cp[0x0E]:
flag: 01
len: 00 0F
value: 4C 69 6E 65 4E 75 6D 62 65 72 54 61 62 6C 65 (LineNumberTable)
cp[0x0F]:
01 00 0E 69 6E 73 74 61 6E 63 65 4D 65 74 68 6F 64 (instanceMethod)
cp[0x10]:
01 00 04 28 49 29 49 ((I)I)
cp[0x11]:
01 00 0A 53 6F 75 72 63 65 46 69 6C 65 (SourceFile)
cp[0x12]:
01 00 0E 54 65 73 74 43 6C 61 73 73 2E 6A 61 76 61 (TestClass.java)
cp[0x13]:
flag: 0C (CONSTANT_NameAndType_info 字段和方法的名称以及类型的符号引用)
name_index: 00 0B
type_index: 00 0C
cp[0x14]:
0C 00 0A 00 07
cp[0x15]:
01 00 20 63 6F 6D 2F 65 6A 75 73 68 61 6E 67 2F
54 65 73 74 43 6C 61 73 73 2F 54 65 73 74 43 6C
61 73 73 (com/ejushang/TestClass/TestClass)
cp[0x16]:
01 00 10 6A 61 76 61 2F 6C 61 6E 67 2F 4F 62 6A
65 63 74 (java/lang/Object)
cp[0x17]:
01 00 1C
63 6F 6D 2F 65 6A 75 73 68 61 6E 67 2F 54 65 73
74 43 6C 61 73 73 2F 53 75 70 65 72 (com/ejushang/TestClass/Super)
access_flags (访问标志):
00 21 (ACC_PUBLIC ACC_SUPER)
this_class (CONSTANT_class_info 常量池索引):
00 03
super_class:
00 04
interface_count (实现的接口或者父类实现的接口的数量):
00 01
interfaces[interface_count]:
00 05
field_count:
00 02
fields[1]:
access_flags: 00 1A
name_index: 00 06
destriptor_index: 00 07
attribute_count: 00 01
attributes[attribute_count]:
attribute_name_index: 00 08
attribute_len: 00 00 00 02
constantvalue_index: 00 09
fields[2]:
access_flags: 00 02
name_index: 00 0A
destriptor_index: 00 07
attribute_count: 00 00
method_count: 00 02
methods[1]:
access_flags: 00 01
name_index: 00 0B
destriptor_index: 00 0C
attribute_count: 00 01
attribute[1]:
attribute_name_index: 00 0D
attibute_len: 00 00 00 26
max_stack: 00 02
max_locals: 00 01
code_len: 00 00 00 0A
code: 2A B7 00 01 2A 03 B5 00 02 B1
exception_table_len: 00 00
attribute_count: 00 01
attribute[1]:
attribute_name_index: 00 0E
attribute_len: 00 00 00 0A
line_number_table_len: 00 02
line[1]:
start_pc: 00 00
line_number: 00 03
line[2]:
start_pc: 00 04
line_number: 00 07
methods[2]:
access_flags: 00 01
name_index: 00 0F
destriptor_index: 00 10
attribute_count: 00 01
attibute[1]:
attribute_name_index: 00 0D
attribute_len: 00 00 00 1C
max_stack: 00 02
max_locals: 00 02
code_len: 00 00 00 04
code: 1B 04 60 AC
exception_table_len: 00 00
attribute_count: 00 01
attibute[1]:
attribute_name_index: 00 0E
attibute_len: 00 00 00 06
line_number_table_len: 00 01
line[1]:
start_pc: 00 00
line_number: 00 0A
attribute_count: 00 01
attribute[1]:
attribute_name_index: 00 11
attribute_len: 00 00 00 02
sourcefile_index: 00 12
常量池是什么东西？常量池存储字面量和符号引用。字面量包括字符串，final常量的值，以及某个属性的初始值等。符号引用主要
存储类和接口的全限定名称，字段的名称以及描述符，方法的名称以及描述符。
常量池中存储数据类型，字面量和符号引用，字面量就是属性/方法/类名或者描述符这种字符串，符号引用就是第几个常量。
常量池的项目包括：
Class文件结构中有字段表，方法表和属性表，就是变量，方法，属性的结构体
属性表集合：
Java方法中代码和字节码指令怎么转换？有的指令带参数有的不带，带多少参数都是固定的
假如有方法：
public void f(){
a+b;
}
假如这个方法对应的字节码指令是...xxyyzz...，xx是加的指令，xx指令带有两个参数，跟着它的yy和zz指令就是它的参数，而yy和zz又有可能指向常量池中的内容。