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JVM的class文件结构

2014-03-25 21:27 387 查看

请参考《深入java虚拟机第二版》和《java虚拟机规范》。

java虚拟机规范链接：http://docs.oracle.com/javase/specs/jvms/se7/html/

关于class文件结构的介绍：http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.1

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

.
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 as shown in Table 4.1.

Table 4.1. 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.

A class may be marked with the

ACC_SYNTHETIC

flag to indicate that it was generated by a compiler and does not appear in source code.
The

ACC_ENUM

flag indicates that this class or its superclass is declared as an enumerated type.
An interface is distinguished by its

ACC_INTERFACE

flag being set. If its

ACC_INTERFACE

flag is not set, this

class

file
defines a class, not an interface.
If the

ACC_INTERFACE

flag of this

class

file is set, its

ACC_ABSTRACT

flag must also be set
(JLS §9.1.1.1). Such a

class

file must not have its

ACC_FINAL

ACC_SUPER

ACC_ENUM

flags set.
An annotation type must have its

ACC_ANNOTATION

flag set. If the

ACC_ANNOTATION

flag is set, the

ACC_INTERFACE

flag
must be set as well. If the

ACC_INTERFACE

flag of this

class

file is not set, it may have any of the other flags in Table 4.1 set,
except the

ACC_ANNOTATION

flag. However, such a

class

file cannot 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. Compilers
to the instruction set of the Java Virtual Machine should set the

ACC_SUPER

flag.
The

ACC_SUPER

flag exists for backward compatibility with code compiled by older compilers for the Java programming language. In Oracle’s JDK prior to release 1.0.2, the compiler generated

ClassFile

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.
All bits of the

access_flags

item not assigned in Table 4.1 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 (§4.4.1)
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
(§4.4.1) 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 (§4.5) 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

(§4.5)
structure 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

(§4.6)
structure 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