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javajvmjvm-bytecode

JVM Constant Pool Entries

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?

Solution

  • 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.