Oracle 官网定义Class 文件： https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html
The ClassFile Structure
A class file consists of a single ClassFile structure:
ClassFile {
u4 magic;
u2 minor_version;
u2 major_version;
u2 constant_pool_count;
cp_info constant_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];
}The items in the ClassFile structure are as follows:
magic
The magic item supplies the magic number identifying the class file format; it has the value 0xCAFEBABE
每个Class文件的头4个字节称为魔数（Magic Number),它唯一作用就是用来确定文件是否能被虚拟机接受。
很多文件存储标准中都用魔数进行身份标识，如图片gif，jpeg都在文件头部中存储着魔数。使用魔数而不是用扩展名来进行识别主要是基于安全考虑，因为扩展名我们可以随意通过重命名等方式改动
minor_version, major_version
The values of the minor_version and major_version items are the minor and major version numbers of this class file. Together, a major and a minor version number determine the version of the class file format. If a class file has major version number M and minor version number m, we denote the version of its class file format as M.m. Thus, class file format versions may be ordered lexicographically, for example, 1.5 < 2.0 < 2.1.
A Java Virtual Machine implementation can support a class file format of version v if and only if v lies in some contiguous range Mi.0 ≤ v ≤ Mj.m. The release level of the Java SE platform to which a Java Virtual Machine implementation conforms is responsible for determining the range.
Oracle's Java Virtual Machine implementation in JDK release 1.0.2 supports class file format versions 45.0 through 45.3 inclusive. JDK releases 1.1.* support class file format versions in the range 45.0 through 45.65535 inclusive. For k ≥ 2, JDK release 1.k supports class file format versions in the range 45.0 through 44+k.0 inclusive.
constant_pool_count
The value of the constant_pool_count item is equal to the number of entries in the constant_pool table plus one. A constant_pool index is considered valid if it is greater than zero and less than constant_pool_count, with the exception for constants of type long and double noted in §4.4.5.
constant_pool[]
The constant_pool is a table of structures (§4.4) representing various string constants, class and interface names, field names, and other constants that are referred to within the ClassFile structure and its substructures. The format of each constant_pool table entry is indicated by its first "tag" byte.
The constant_pool table is indexed from 1 to constant_pool_count - 1.
access_flags
The value of the access_flags item is a mask of flags used to denote access permissions to and properties of this class or interface. The interpretation of each flag, when set, is specified in Table 4.1-A.
Table 4.1-A. Class access and property modifiers
| Flag Name | Value | Interpretation |
|---|---|---|
ACC_PUBLIC |
0x0001 | Declared public; may be accessed from outside its package. |
ACC_FINAL |
0x0010 | Declared final; no subclasses allowed. |
ACC_SUPER |
0x0020 | Treat superclass methods specially when invoked by the invokespecial instruction. |
ACC_INTERFACE |
0x0200 | Is an interface, not a class. |
ACC_ABSTRACT |
0x0400 | Declared abstract; must not be instantiated. |
ACC_SYNTHETIC |
0x1000 | Declared synthetic; not present in the source code. |
ACC_ANNOTATION |
0x2000 | Declared as an annotation type. |
ACC_ENUM |
0x4000 | Declared as an enum type. |
An interface is distinguished by the ACC_INTERFACE flag being set. If the ACC_INTERFACE flag is not set, this class file defines a class, not an interface.
If the ACC_INTERFACE flag is set, the ACC_ABSTRACT flag must also be set, and the ACC_FINAL, ACC_SUPER, and ACC_ENUM flags set must not be set.
If the ACC_INTERFACE flag is not set, any of the other flags in Table 4.1-A may be set except ACC_ANNOTATION. However, such a class file must not have both its ACC_FINAL and ACC_ABSTRACT flags set (JLS §8.1.1.2).
The ACC_SUPER flag indicates which of two alternative semantics is to be expressed by the invokespecial instruction (§invokespecial) if it appears in this class or interface. Compilers to the instruction set of the Java Virtual Machine should set the ACC_SUPER flag. In Java SE 8 and above, the Java Virtual Machine considers the ACC_SUPER flag to be set in every class file, regardless of the actual value of the flag in the class file and the version of the class file.
The ACC_SUPER flag exists for backward compatibility with code compiled by older compilers for the Java programming language. In JDK releases prior to 1.0.2, the compiler generated access_flags in which the flag now representing ACC_SUPER had no assigned meaning, and Oracle's Java Virtual Machine implementation ignored the flag if it was set.
The ACC_SYNTHETIC flag indicates that this class or interface was generated by a compiler and does not appear in source code.
An annotation type must have its ACC_ANNOTATION flag set. If the ACC_ANNOTATION flag is set, the ACC_INTERFACE flag must also be set.
The ACC_ENUM flag indicates that this class or its superclass is declared as an enumerated type.
All bits of the access_flags item not assigned in Table 4.1-A are reserved for future use. They should be set to zero in generated class files and should be ignored by Java Virtual Machine implementations.
this_class
The value of the this_class item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Class_info structure (§4.4.1) representing the class or interface defined by this class file.
super_class
For a class, the value of the super_class item either must be zero or must be a valid index into the constant_pool table. If the value of the super_class item is nonzero, the constant_pool entry at that index must be a CONSTANT_Class_info structure representing the direct superclass of the class defined by this class file. Neither the direct superclass nor any of its superclasses may have the ACC_FINAL flag set in the access_flags item of its ClassFile structure.
If the value of the super_class item is zero, then this class file must represent the class Object, the only class or interface without a direct superclass.
For an interface, the value of the super_class item must always be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Class_info structure representing the class Object.
interfaces_count
The value of the interfaces_count item gives the number of direct superinterfaces of this class or interface type.
interfaces[]
Each value in the interfaces array must be a valid index into the constant_pool table. The constant_pool entry at each value of interfaces[i], where 0 ≤ i < interfaces_count, must be a CONSTANT_Class_info structure representing an interface that is a direct superinterface of this class or interface type, in the left-to-right order given in the source for the type.
fields_count
The value of the fields_count item gives the number of field_info structures in the fields table. The field_info structures represent all fields, both class variables and instance variables, declared by this class or interface type.
fields[]
Each value in the fields table must be a field_info structure (§4.5) giving a complete description of a field in this class or interface. The fields table includes only those fields that are declared by this class or interface. It does not include items representing fields that are inherited from superclasses or superinterfaces.
methods_count
The value of the methods_count item gives the number of method_info structures in the methods table.
methods[]
Each value in the methods table must be a method_info structure (§4.6) giving a complete description of a method in this class or interface. If neither of the ACC_NATIVE and ACC_ABSTRACT flags are set in the access_flags item of a method_info structure, the Java Virtual Machine instructions implementing the method are also supplied.
The method_info structures represent all methods declared by this class or interface type, including instance methods, class methods, instance initialization methods (§2.9), and any class or interface initialization method (§2.9). The methods table does not include items representing methods that are inherited from superclasses or superinterfaces.
attributes_count
The value of the attributes_count item gives the number of attributes in the attributes table of this class.
attributes[]
Each value of the attributes table must be an attribute_info structure (§4.7).
The attributes defined by this specification as appearing in the attributes table of a ClassFile structure are listed in Table 4.7-C.
The rules concerning attributes defined to appear in the attributes table of a ClassFile structure are given in §4.7.
The rules concerning non-predefined attributes in the attributes table of a ClassFile structure are given in §4.7.1.