ConstantDesc (Java SE 13 & JDK 13 )
Description: declaration: module: java.base, package: java.lang.constant, interface: ConstantDesc
Keywords: java.lang.constant.ConstantDesc interface

Module java.base
Package java.lang.constant

# Interface ConstantDesc

All Known Subinterfaces:
  `ClassDesc`, `DirectMethodHandleDesc`, `MethodHandleDesc`, `MethodTypeDesc`

    public interface ConstantDesc

A nominal descriptor for a loadable constant value, as defined in JVMS 4.4. Such a descriptor can be resolved via `resolveConstantDesc(MethodHandles.Lookup)` to yield the constant value itself.

Class names in a nominal descriptor, like class names in the constant pool of a classfile, must be interpreted with respect to a particular class loader, which is not part of the nominal descriptor.

Static constants that are expressible natively in the constant pool (`String`, `Integer`, `Long`, `Float`, and `Double`) implement `ConstantDesc`, and serve as nominal descriptors for themselves. Native linkable constants (`Class`, `MethodType`, and `MethodHandle`) have counterpart ConstantDesc types: `ClassDesc`, `MethodTypeDesc`, and `MethodHandleDesc`. Other constants are represented by subtypes of `DynamicConstantDesc`.

APIs that perform generation or parsing of bytecode are encouraged to use ConstantDesc to describe the operand of an `ldc` instruction (including dynamic constants), the static bootstrap arguments of dynamic constants and `invokedynamic` instructions, and other bytecodes or classfile structures that make use of the constant pool.

Constants describing various common constants (such as `ClassDesc` instances for platform types) can be found in `ConstantDescs`.

Implementations of ConstantDesc must be value-based classes.

Non-platform classes should not implement ConstantDesc directly. Instead, they should extend `DynamicConstantDesc` (as `Enum.EnumDesc` and `VarHandle.VarHandleDesc` do.)

Nominal descriptors should be compared using the `Object.equals(Object)` method. There is no guarantee that any particular entity will always be represented by the same descriptor instance.

API Note:
  In the future, if the Java language permits, ConstantDesc may become a `sealed` interface, which would prohibit subclassing except by explicitly permitted types. Clients can assume that the following set of subtypes is exhaustive: `String`, `Integer`, `Long`, `Float`, `Double`, `ClassDesc`, `MethodTypeDesc`, `MethodHandleDesc`, and `DynamicConstantDesc`; this list may be extended to reflect future changes to the constant pool format as defined in JVMS 4.4.
See Java Virtual Machine Specification:
  4.4 The Constant Pool
Since:
  12
See Also:
  `Constable`, `ConstantDescs`

- ## Method Summary

  Modifier and Type | Method | Description
  --- | --- | ---
  `Object` | `resolveConstantDesc​(MethodHandles.Lookup lookup)` | Resolves this descriptor reflectively, emulating the resolution behavior of JVMS 5.4.3 and the access control behavior of JVMS 5.4.4.

- ## Method Details

  - ### resolveConstantDesc

    Object resolveConstantDesc​(MethodHandles.Lookup lookup) throws ReflectiveOperationException
    Resolves this descriptor reflectively, emulating the resolution behavior of JVMS 5.4.3 and the access control behavior of JVMS 5.4.4. The resolution and access control context is provided by the `MethodHandles.Lookup` parameter. No caching of the resulting value is performed.
    API Note:
      MethodTypeDesc can represent method type descriptors that are not representable by MethodType, such as methods with more than 255 parameter slots, so attempts to resolve these may result in errors.
    Parameters:
      `lookup` - The `MethodHandles.Lookup` to provide name resolution and access control context
    Returns:
      the resolved constant value
    Throws:
      `ReflectiveOperationException` - if a class, method, or field could not be reflectively resolved in the course of resolution
      `LinkageError` - if a linkage error occurs
    See Java Virtual Machine Specification:
      5.4.3 Resolution
      5.4.4 Access Control