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Java Class File Format
The class File Format | ||||||||||||||||||
| This chapter describes the Java virtual machine class file format. Each class file contains the definition of a single class or interface. Although a class or interface need not have an external representation literally contained in a file (for instance, because the class is generated by a class loader), we will colloquially refer to any valid representation of a class or interface as being in the class file format. A class file consists of a stream of 8-bit bytes. All 16-bit, 32-bit, and 64-bit quantities are constructed by reading in two, four, and eight consecutive 8-bit bytes, respectively. Multibyte data items are always stored in big-endian order, where the high bytes come first. In the Java and Java 2 platforms, this format is supported by interfaces java.io.DataInput and java.io.DataOutput and classes such as java.io.DataInputStream and java.io.DataOutputStream. This chapter defines its own set of data types representing class file data: The types u1, u2, and u4 represent an unsigned one-, two-, or four-byte quantity, respectively. In the Java and Java 2 platforms, these types may be read by methods such as readUnsignedByte, readUnsignedShort, and readInt of the interface java.io.DataInput. This chapter presents the class file format using pseudostructures written in a C-like structure notation. To avoid confusion with the fields of classes and class instances, etc., the contents of the structures describing the class file format are referred to as items. Successive items are stored in the class file sequentially, without padding or alignment. Tables, consisting of zero or more variable-sized items, are used in several class file structures. Although we use C-like array syntax to refer to table items, the fact that tables are streams of varying-sized structures means that it is not possible to translate a table index directly to a byte offset into the table. Where we refer to a data structure as an array, it consists of zero or more contiguous fixed-sized items and can be indexed like an array. | ||||||||||||||||||
| 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. 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. Only Sun can specify what range of versions a Java virtual machine implementation conforming to a certain release level of the Java platform may support. 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.
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 (§4.4.1) structure 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 (§4.4.1) structure representing the direct superclass of the class defined by this class file. Neither the direct superclass nor any of its superclasses may be a final class. 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 (§4.4.1) 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 (§4.5) structures represent all fields, both class variables and instance variables, declared by this lass 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 the method is not native or abstract, 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 (static) methods, instance initialization methods (§3.9), and any class or interface initialization method (§3.9). 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 (§4.7) in the attributes table of this class. attributes[] Each value of the attributes table must be an attribute structure (§4.7). The only attributes defined by this specification as appearing in the attributes table of a ClassFile structure are the SourceFile attribute (§4.7.7) and the Deprecated (§4.7.10) attribute. A Java virtual machine implementation is required to silently ignore any or all attributes in the attributes table of a ClassFile structure that it does not recognize. Attributes not defined in this specification are not allowed to affect the semantics of the class file, but only to provide additional descriptive information (§4.7.1). |
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Latest page update: made by rahulrastogi
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