2.2 Notations for Class File Attributes

For descriptions of the class file attributes we follow the rules described in Chapter 4 of The Java Virtual Machine Specification [Lindholm-Yellin]. We summarise here its most important features.

The format of attributes is a binary format with big-endian representation of multibyte quantities. It is stored in the class file sequentially without padding or alignment. Java's standard library contains routines to properly store data in this binary layout. The descriptions of the format use a C-like structure notation. The subsequent entries in these structures are by convention called items. In order to describe the multibyte quantities, special identifiers are used: u1, u2, and u4, which unambiguously describe the binary size of the values.

All the attributes adhere to the following general format:

attribute_info {
  u2 attribute_name_index;
  u4 attribute_length;
  u1 info[attribute_length];
}

The field attribute_name_index is an unsigned 16-bit index into the constant pool of the class. The entry pointed to by the attribute_name_index is a CONSTANT_Utf8_info (see Section 4.4.7 of The Java Virtual Machine Specification [Lindholm-Yellin]) structure that represents the name of the attribute. The following item attribute_length is the length of the essential attribute information stored in the info array. The value of the length does not cover the items attribute_name_index and attribute_length.

Here is an example of an attribute description:

ConstantValue_attribute {
  u2 attribute_name_index;
  u4 attribute_length;
  u2 constantvalue_index;
}

For the exact format of BML bytecode attributes see Encoding of BML in Class File Format. The next chapter recapitulates the standard classfile format.