The LocalVariableTypeTable attribute is an optional variable-length attribute of a Code attribute. It may be used by debuggers to determine the value of a given local variable during the execution of a method. If LocalVariableTypeTable attributes are present in the attributes table of a given Code attribute, then they may appear in any order. There may be no more than one LocalVariableTypeTable attribute per local variable in the Code attribute. The LocalVariableTypeTable attribute differs from the LocalVariableTable attribute in that it provides signature information rather than descriptor information. This difference is only significant for variables whose type is a generic reference type. Such variables will appear in both tables, while variables of other types will appear only in LocalVariableTable.
The LocalVariableTypeTable attribute has the following format:
LocalVariableTypeTable_attribute { u2 attribute_name_index; u4 attribute_length; u2 local_variable_type_table_length; { u2 start_pc; u2 length; u2 name_index; u2 signature_index; u2 index; } local_variable_type_table[local_variable_type_table_length]; } The items of the LocalVariableTypeTable_attribute structure are as follows:
- attribute_name_index
- The value of the attribute_name_index item must be a valid index into the constant_pool table a121. The constant_pool entry at that index must be a CONSTANT_Utf8_info (4.5.7) a122 structure representing the string "LocalVariableTypeTable" a123.
- attribute_length
- The value of the attribute_length item indicates the length of the attribute, excluding the initial six bytes.
- local_variable_table_length
- The value of the local_variable_table_length item indicates the number of entries in the local_variable_table array.
- local_variable_table[]
- Each entry in the local_variable_table array indicates a range of code array offsets within which a local variable has a value. It also indicates the index into the local variable array of the current frame at which that local variable can be found. Each entry must contain the following five items:
- start_pc, length
- The given local variable must have a value at indices into the code array in the interval [start_pc, start_pc+length), that is, between start_pc and start_pc+length exclusive. The value of start_pc must be a valid index into the code array of this Code attribute and must be the index of the opcode of an instructiona124. The value of start_pc+length must either be a valid index into the code array of this Code attribute and be the index of the opcode of an instruction, or it must be the first index beyond the end of that code array a125.
- name_index, signature_index
- The value of the name_index item must be a valid index into the constant_pool table a127. The constant_pool entry at that index must contain a CONSTANT_Utf8_info structure a128 representing a valid unqualified name denoting a local variablea128. Careful here - do we want any restrictions at all?
The value of the signature_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must contain a CONSTANT_Utf8_info structure representing a field type signature encoding the type of a local variable in the source program.
- index
- The given local variable must be at index in the local variable array of the current frame. If the local variable at index is of type double or long, it occupies both index and index+1.
@author Eugene Kuleshov