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The Java class file lifestyle

An introduction to the basic structure and lifestyle of the Java class file

By Bill Venners, JavaWorld.com, 07/01/96

Page 3 of 4

This class

The next two bytes, the this class component, are an index into the constant pool array. The constant referred to by this class, constant_pool[this_class], has two parts, a one-byte tag and a two-byte name index. The tag will equal CONSTANT_Class, a value that indicates this element contains information about a class or interface. Constant_pool[name_index] is a string constant containing the name of the class or interface.

The this class component provides a glimpse of how the constant pool is used. This class itself is just an index into the constant pool. When a JVM looks up constant_pool[this_class], it finds an element that identifies itself as a CONSTANT_Class with its tag. The JVM knows CONSTANT_Class elements always have a two-byte index into the constant pool, called name index, following their one-byte tag. So it looks up constant_pool[name_index] to get the string containing the name of the class or interface.

Super class

Following the this class component is the super class component, another two-byte index into the constant pool. Constant_pool[super_class] is a CONSTANT_Class element that points to the name of the super class from which this class descends.

Interfaces

The interfaces component starts with a two-byte count of the number of interfaces implemented by the class (or interface) defined in the file. Immediately following is an array that contains one index into the constant pool for each interface implemented by the class. Each interface is represented by a CONSTANT_Class element in the constant pool that points to the name of the interface.

Fields

The fields component starts with a two-byte count of the number of fields in this class or interface. A field is an instance or class variable of the class or interface. Following the count is an array of variable-length structures, one for each field. Each structure reveals information about one field such as the field's name, type, and, if it is a final variable, its constant value. Some information is contained in the structure itself, and some is contained in constant pool locations pointed to by the structure.

The only fields that appear in the list are those that were declared by the class or interface defined in the file; no fields inherited from super classes or superinterfaces appear in the list.

Methods

The methods component starts with a two-byte count of the number of methods in the class or interface. This count includes only those methods that are explicitly defined by this class, not any methods that may be inherited from superclasses. Following the method count are the methods themselves.

The structure for each method contains several pieces of information about the method, including the method descriptor (its return type and argument list), the number of stack words required for the method's local variables, the maximum number of stack words required for the method's operand stack, a table of exceptions caught by the method, the bytecode sequence, and a line number table.