vmspec: Format

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2.1 Format

The following pseudo-structure gives a top-level description of the format of a class file:

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[attribute_count];
}

magic

This field must have the value 0xCAFEBABE.

minor_version, major_version

These fields contain the version number of the Java compiler that produced this class file. An implementation of the virtual machine will normally support some range of minor version numbers 0-n of a particular major version number. If the minor version number is incremented the new code won't run on the old virtual machines, but it is possible to make a new virtual machine which can run versions up to n+1.

A change of the major version number indicates a major incompatible change, one that requires a different virtual machine that may not support the old major version in any way.

The current major version number is 45; the current minor version number is 3.

constant_pool_count

This field indicates the number of entries in the constant pool in the class file.

constant_pool

The constant pool is an table of values. These values are the various string constants, class names, field names, and others that are referred to by the class structure or by the code.

constant_pool[0] is always unused by the compiler, and may be used by an implementation for any purpose.

Each of the constant_pool entries 1 through constant_pool_count-1 is a variable-length entry, whose format is given by the first ''tag'' byte, as described in section 2.3.

access_flags

This field contains a mask of up to sixteen modifiers used with class, method, and field declarations. The same encoding is used on similar fields in field_info and method_info as described below. Here is the encoding:


Flag Name	Value	Meaning	Used By
ACC_PUBLIC	0x0001	Visible to everyone	Class, Method, Variable
ACC_PRIVATE	0x0002	Visible only to the defining class	Method, Variable
ACC_PROTECTED	0x0004	Visible to subclasses	Method, Variable
ACC_STATIC	0x0008	Variable or method is static	Method, Variable
ACC_FINAL	0x0010	No further subclassing, overriding, or assignment after initialization	Class, Method, Variable
ACC_SYNCHRONIZED	0x0020	Wrap use in monitor lock	Method
ACC_VOLATILE	0x0040	Can't cache	Variable
ACC_TRANSIENT	0x0080	Not to be written or read by a persistent object manager	Variable
ACC_NATIVE	0x0100	Implemented in a language other than Java	Method
ACC_INTERFACE	0x0200	Is an interface	Class
ACC_ABSTRACT	0x0400	No body provided	Class, Method

this_class

This field is an index into the constant pool; constant_pool[this_class] must be a CONSTANT_class.

super_class

This field is an index into the constant pool. If the value of super_class is nonzero, then constant_pool[super_class] must be a class, and gives the index of this class's superclass in the constant pool.

If the value of super_class is zero, then the class being defined must be java.lang.Object, and it has no superclass.

interfaces_count

This field gives the number of interfaces that this class implements.

interfaces

Each value in this table is an index into the constant pool. If an table value is nonzero (interfaces[i] != 0, where 0 <= i < interfaces_count), then constant_pool[interfaces[i]] must be an interface that this class implements.

Question: How could one of these entries ever be 0?

fields_count

This field gives the number of instance variables, both static and dynamic, defined by this class. The fields table includes only those variables that are defined explicitly by this class. It does not include those instance variables that are accessible from this class but are inherited from superclasses.

fields

Each value in this table is a more complete description of a field in the class. See section 2.4 for more information on the field_info structure.

methods_count

This field indicates the number of methods, both static and dynamic, defined by this class. This table only includes those methods that are explicitly defined by this class. It does not include inherited methods.

methods

Each value in this table is a more complete description of a method in the class. See section 2.5 for more information on the method_info structure.

attributes_count

This field indicates the number of additional attributes about this class.

attributes

A class can have any number of optional attributes associated with it. Currently, the only class attribute recognized is the "SourceFile" attribute, which indicates the name of the source file from which this class file was compiled. See section 2.6 for more information on the attribute_info structure.

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