invokedynamiccall sites. These classes are suitable for use in bytecode reading and writing APIs,
invokedynamicbootstraps, 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.
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
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
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
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
Interface Summary Interface Description ClassDesc ConstableRepresents a type which is constable. ConstantDescA nominal descriptor for a loadable constant value, as defined in JVMS 4.4. DirectMethodHandleDesc MethodHandleDesc MethodTypeDesc
Class Summary Class Description ConstantDescsPredefined values of nominal descriptor for common constants, including descriptors for primitive class types and other common platform types, and descriptors for method handles for standard bootstrap methods. DynamicCallSiteDescA nominal descriptor for an
DynamicConstantDesc<T>A nominal descriptor for a dynamic constant (one described in the constant pool with
Enum Summary Enum Description DirectMethodHandleDesc.KindKinds of method handles that can be described with DirectMethodHandleDesc.