JVM Constant Pool Entries

Question

Looking at the output of running javap on some classes, I have come across the following oddity:

The Constant Pool of some classes contains entries such as

#33 = Utf8               Code
#34 = Utf8               LocalVariableTable
#35 = Utf8               StackMapTable
#36 = Utf8               MethodParameters

Why do these constants appear in the constant pool?

Answer 1

The following constants

#33 = Utf8               Code
#34 = Utf8               LocalVariableTable
#35 = Utf8               StackMapTable
#36 = Utf8               MethodParameters

are name of attributes.

For all attributes, the attribute_name_index must be a valid unsigned 16-bit index into the constant pool of the class. The constant_pool entry at attribute_name_index must be a CONSTANT_Utf8_info structure (§4.4.7) representing the name of the attribute.

The attributes you mention have the following functions:

• A Code attribute contains the Java Virtual Machine instructions and auxiliary information for a method, including an instance initialization method or a class or interface initialization method.
• The LocalVariableTable attribute [...] may be used by debuggers to determine the value of a given local variable during the execution of a method.
• A StackMapTable attribute is used during the process of verification by type checking
• A MethodParameters attribute records information about the formal parameters of a method, such as their names.