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.
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.
Other members of this package are ModuleDesc
and PackageDesc
. They represent module and package info structures, suitable for describing modules and their content in bytecode reading and writing APIs.
Class | Description |
---|---|
ClassDesc | A nominal descriptor for a Class constant. |
Constable | Represents a type which is constable. |
ConstantDesc | A nominal descriptor for a loadable constant value, as defined in JVMS 4.4. |
ConstantDescs | Predefined 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. |
DirectMethodHandleDesc | A nominal descriptor for a direct MethodHandle . |
DirectMethodHandleDesc.Kind | Kinds of method handles that can be described with DirectMethodHandleDesc. |
DynamicCallSiteDesc | A nominal descriptor for an invokedynamic call site. |
DynamicConstantDesc<T> | A nominal descriptor for a dynamic constant (one described in the constant pool with Constant_Dynamic_info .) |
MethodHandleDesc | A nominal descriptor for a MethodHandle constant. |
MethodTypeDesc | A nominal descriptor for a MethodType constant. |
ModuleDesc | A nominal descriptor for a Module constant. |
PackageDesc | A nominal descriptor for a Package constant. |
© 1993, 2023, Oracle and/or its affiliates. All rights reserved.
Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/constant/package-summary.html