Module java.base

Class LambdaMetafactory


public final class LambdaMetafactory extends Object

Methods to facilitate the creation of simple "function objects" that implement one or more interfaces by delegation to a provided MethodHandle, possibly after type adaptation and partial evaluation of arguments. These methods are typically used as bootstrap methods for invokedynamic call sites, to support the lambda expression and method reference expression features of the Java Programming Language.

Indirect access to the behavior specified by the provided MethodHandle proceeds in order through three phases:

  • Linkage occurs when the methods in this class are invoked. They take as arguments an interface to be implemented (typically a functional interface, one with a single abstract method), a name and signature of a method from that interface to be implemented, a direct method handle describing the desired implementation behavior for that method, and possibly other additional metadata, and produce a CallSite whose target can be used to create suitable function objects.

    Linkage may involve dynamically loading a new class that implements the target interface, or re-using a suitable existing class.

    The CallSite can be considered a "factory" for function objects and so these linkage methods are referred to as "metafactories".

  • Capture occurs when the CallSite's target is invoked, typically through an invokedynamic call site, producing a function object. This may occur many times for a single factory CallSite.

    If the behavior MethodHandle has additional parameters beyond those of the specified interface method, these are referred to as captured parameters, which must be provided as arguments to the CallSite target. The expected number and types of captured parameters are determined during linkage.

    Capture may involve allocation of a new function object, or may return a suitable existing function object. The identity of a function object produced by capture is unpredictable, and therefore identity-sensitive operations (such as reference equality, object locking, and System.identityHashCode()) may produce different results in different implementations, or even upon different invocations in the same implementation.

  • Invocation occurs when an implemented interface method is invoked on a function object. This may occur many times for a single function object. The method referenced by the implementation MethodHandle is invoked, passing to it the captured arguments and the invocation arguments. The result of the method is returned.

It is sometimes useful to restrict the set of inputs or results permitted at invocation. For example, when the generic interface Predicate<T> is parameterized as Predicate<String>, the input must be a String, even though the method to implement allows any Object. At linkage time, an additional MethodType parameter describes the "dynamic" method type; on invocation, the arguments and eventual result are checked against this MethodType.

This class provides two forms of linkage methods: a standard version (metafactory(MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)) using an optimized protocol, and an alternate version altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)). The alternate version is a generalization of the standard version, providing additional control over the behavior of the generated function objects via flags and additional arguments. The alternate version adds the ability to manage the following attributes of function objects:

  • Multiple methods. It is sometimes useful to implement multiple variations of the method signature, involving argument or return type adaptation. This occurs when multiple distinct VM signatures for a method are logically considered to be the same method by the language. The flag FLAG_BRIDGES indicates that a list of additional MethodTypes will be provided, each of which will be implemented by the resulting function object. These methods will share the same name and instantiated type.
  • Multiple interfaces. If needed, more than one interface can be implemented by the function object. (These additional interfaces are typically marker interfaces with no methods.) The flag FLAG_MARKERS indicates that a list of additional interfaces will be provided, each of which should be implemented by the resulting function object.
  • Serializability. The generated function objects do not generally support serialization. If desired, FLAG_SERIALIZABLE can be used to indicate that the function objects should be serializable. Serializable function objects will use, as their serialized form, instances of the class SerializedLambda, which requires additional assistance from the capturing class (the class described by the MethodHandles.Lookup parameter caller); see SerializedLambda for details.

Assume the linkage arguments are as follows:

  • factoryType (describing the CallSite signature) has K parameters of types (D1..Dk) and return type Rd;
  • interfaceMethodType (describing the implemented method type) has N parameters, of types (U1..Un) and return type Ru;
  • implementation (the MethodHandle providing the implementation) has M parameters, of types (A1..Am) and return type Ra (if the method describes an instance method, the method type of this method handle already includes an extra first argument corresponding to the receiver);
  • dynamicMethodType (allowing restrictions on invocation) has N parameters, of types (T1..Tn) and return type Rt.

Then the following linkage invariants must hold:

  • interfaceMethodType and dynamicMethodType have the same arity N, and for i=1..N, Ti and Ui are the same type, or Ti and Ui are both reference types and Ti is a subtype of Ui
  • Either Rt and Ru are the same type, or both are reference types and Rt is a subtype of Ru
  • K + N = M
  • For i=1..K, Di = Ai
  • For i=1..N, Ti is adaptable to Aj, where j=i+k
  • The return type Rt is void, or the return type Ra is not void and is adaptable to Rt

Further, at capture time, if implementation corresponds to an instance method, and there are any capture arguments (K > 0), then the first capture argument (corresponding to the receiver) must be non-null.

A type Q is considered adaptable to S as follows:

adaptable types
QSLink-time checksInvocation-time checks
PrimitivePrimitive Q can be converted to S via a primitive widening conversion None
PrimitiveReference S is a supertype of the Wrapper(Q) Cast from Wrapper(Q) to S
ReferencePrimitive for parameter types: Q is a primitive wrapper and Primitive(Q) can be widened to S
for return types: If Q is a primitive wrapper, check that Primitive(Q) can be widened to S
If Q is not a primitive wrapper, cast Q to the base Wrapper(S); for example Number for numeric types
ReferenceReference for parameter types: S is a supertype of Q
for return types: none
Cast from Q to S

API Note:
These linkage methods are designed to support the evaluation of lambda expressions and method references in the Java Language. For every lambda expressions or method reference in the source code, there is a target type which is a functional interface. Evaluating a lambda expression produces an object of its target type. The recommended mechanism for evaluating lambda expressions is to desugar the lambda body to a method, invoke an invokedynamic call site whose static argument list describes the sole method of the functional interface and the desugared implementation method, and returns an object (the lambda object) that implements the target type. (For method references, the implementation method is simply the referenced method; no desugaring is needed.)

The argument list of the implementation method and the argument list of the interface method(s) may differ in several ways. The implementation methods may have additional arguments to accommodate arguments captured by the lambda expression; there may also be differences resulting from permitted adaptations of arguments, such as casting, boxing, unboxing, and primitive widening. (Varargs adaptations are not handled by the metafactories; these are expected to be handled by the caller.)

Invokedynamic call sites have two argument lists: a static argument list and a dynamic argument list. The static argument list is stored in the constant pool; the dynamic argument is pushed on the operand stack at capture time. The bootstrap method has access to the entire static argument list (which in this case, includes information describing the implementation method, the target interface, and the target interface method(s)), as well as a method signature describing the number and static types (but not the values) of the dynamic arguments and the static return type of the invokedynamic site.

The implementation method is described with a direct method handle referencing a method or constructor. In theory, any method handle could be used, but this is not compatible with some implementation techniques and would complicate the work implementations must do.