Module java.base

Package java.lang.constant

Classes and interfaces to represent nominal descriptors for run-time entities such as classes or method handles, and classfile entities such as constant pool entries or invokedynamic call sites. These classes are suitable for use in bytecode reading and writing APIs, invokedynamic bootstraps, bytecode intrinsic APIs, and compile-time or link-time program analysis tools.

Every API that reads and writes bytecode instructions needs to model the operands to these instructions and other classfile structures (such as entries in the bootstrap methods table or stack maps, which frequently reference entries in the classfile constant pool.) Such entries can denote values of fundamental types, such as strings or integers; parts of a program, such as classes or method handles; or values of arbitrary user-defined types. The ConstantDesc hierarchy provides a representation of constant pool entries in nominal form that is convenient for APIs to model operands of bytecode instructions.

Nominal Descriptors

A ConstantDesc is a description of a constant value. Such a description is the nominal form of the constant value; it is not the value itself, but rather a "recipe" for describing the value, storing the value in a constant pool entry, or reconstituting the value given a class loading context. Every ConstantDesc knows how to resolve itself -- compute the value that it describes -- via ConstantDesc.resolveConstantDesc. This allows an API which accepts ConstantDesc objects to evaluate them reflectively, provided that the classes and methods referenced in their nominal description are present and accessible.

The subtypes of ConstantDesc describe various kinds of constant values. For each type of loadable constant pool entry defined in JVMS 4.4, there is a corresponding subtype of ConstantDesc: ClassDesc, MethodTypeDesc, DirectMethodHandleDesc, String, Integer, Long, Float, Double, and DynamicConstantDesc. These classes provide type-specific accessor methods to extract the nominal information for that kind of constant. When a bytecode-writing API encounters a ConstantDesc, it should examine it to see which of these types it is, cast it, extract its nominal information, and generate the corresponding entry to the constant pool. When a bytecode-reading API encounters a constant pool entry, it can convert it to the appropriate type of nominal descriptor. For dynamic constants, bytecode-reading APIs may wish to use the factory DynamicConstantDesc.ofCanonical, which will inspect the bootstrap and, for well-known bootstraps, return a more specific subtype of DynamicConstantDesc, such as Enum.EnumDesc.

Another way to obtain the nominal description of a value is to ask the value itself. A Constable is a type whose values can describe themselves in nominal form as a ConstantDesc. Fundamental types such as String and Class implement Constable, as can user-defined classes. Entities that generate classfiles (such as compilers) can introspect over constable objects to obtain a more efficient way to represent their values in classfiles.

This package also includes DynamicCallSiteDesc, which represents a (non-loadable) Constant_InvokeDynamic_info constant pool entry. It describes the bootstrap method, invocation name and type, and bootstrap arguments associated with an invokedynamic instruction. It is also suitable for describing invokedynamic call sites in bytecode reading and writing APIs.

See The Java™ Virtual Machine Specification:
4.4 The Constant Pool