JVM class 文件结构

class 文件的结构如下：

ClassFile {
    u4          magic;
    u2          minor_version;
    u2          major_version;
    u2          constant_pool_count;
    cp_info     contant_pool[constant_pool_count – 1];
    u2          access_flags;
    u2          this_class;
    u2          super_class;
    u2          interfaces_count;
    u2          interfaces[interfaces_count];
    u2          fields_count;
    field_info      fields[fields_count];
    u2          methods_count;
    method_info     methods[methods_count];
    u2          attributes_count;
    attribute_info  attributes[attributes_count];
}

Constant Pool

每个Constant Pool Entry由cp_info结构来表示

cp_info {
    u1 tag;
    u1 info[];
}

tag 表示constant type：

Constant Type	Value
CONSTANT_Class	7
CONSTANT_Fieldref	9
CONSTANT_Methodref	10
CONSTANT_InterfaceMethodref	11
CONSTANT_String	8
CONSTANT_Integer	3
CONSTANT_Float	4
CONSTANT_Long	5
CONSTANT_Double	6
CONSTANT_NameAndType	12
CONSTANT_Utf8	1
CONSTANT_MethodHandle	15
CONSTANT_MethodType	16
CONSTANT_InvokeDynamic	18

不同tag对应的info[]不同。

类的属性

access_flags ：类的访问属性
this_class：index to constant pool，指向一个CONSTANT_Class_info。当前类
super_class：index to constant pool（0或者valid index），指向一个CONSTANT_Class_info。当前类的父类

interface

interfaces：index to constant pool，指向一个CONSTANT_Class_info，当前类实现的接口。

fields

field_info的数组：

field_info {
    u2             access_flags;
    u2             name_index;    //index to constant pool, field的name
    u2             descriptor_index; //index to constant pool, field的type
    u2             attributes_count;
    attribute_info attributes[attributes_count];
}

methods

method_info {
    u2             access_flags;
    u2             name_index;
    u2             descriptor_index;
    u2             attributes_count;
    attribute_info attributes[attributes_count];
}

method_info中的attribute_info包括：Code , Exceptions , Synthetic , Signature , Deprecated , RuntimeVisibleAnnotations , RuntimeInvisibleAnnotations , RuntimeVisibleParameterAnnotations , RuntimeInvisibleParameterAnnotations , AnnotationDefault

其中Code attribute_info包含了method的字节码：

Code_attribute {
    u2 attribute_name_index;
    u4 attribute_length;
    u2 max_stack;
    u2 max_locals;
    u4 code_length;
    u1 code[code_length];    //字节码保存在这里
    u2 exception_table_length;
    {   u2 start_pc;
        u2 end_pc;
        u2 handler_pc;
        u2 catch_type;
    } exception_table[exception_table_length];
    u2 attributes_count;
    attribute_info attributes[attributes_count];
}

attributes

包含了当前class的一些attribute_info

Reference:

https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html