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
knows how to resolve itself -- compute the value that it describes --
This allows an API which accepts
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
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(DirectMethodHandleDesc, java.lang.String, ClassDesc, ConstantDesc),
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
Constable is a type whose values
can describe themselves in nominal form as a
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
This package also includes
which represents a (non-loadable)
pool entry. It describes the bootstrap method, invocation name and type,
and bootstrap arguments associated with an
It is also suitable for describing
invokedynamic call sites in bytecode
reading and writing APIs.
Represents a type which is constable.
A nominal descriptor for a loadable constant value, as defined in JVMS 4.4.
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.
A nominal descriptor for an
A nominal descriptor for a dynamic constant (one described in the constant pool with
Kinds of method handles that can be described with DirectMethodHandleDesc.