Module java.base

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 should be immutable and their behavior should not rely on object identity.

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:
  • Method Summary

    Modifier and Type
    Method
    Description
    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