# Interface MissingMemberHandlerFactory

Functional Interface:
  This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

@FunctionalInterface public interface MissingMemberHandlerFactory
A factory for creating method handles for linking missing member behavior in `BeansLinker`. BeansLinker links these method handles into guarded invocations for link requests specifying `GET_*` and `SET_*` `StandardOperation`s when it is either certain or possible that the requested member (property, method, or element) is missing. They will be linked both for `named` and unnamed operations. The implementer must ensure that the parameter types of the returned method handle match the parameter types of the call site described in the link request. The return types can differ, though, to allow `DynamicLinkerFactory.setPrelinkTransformer(jdk.dynalink.linker.GuardedInvocationTransformer)` late return type transformations}. It is allowed to return `null` for a method handle if the default behavior is sufficient.

## Default missing member behavior

When a `BeansLinker` is configured without a missing member handler factory, or the factory returns `null` for a particular handler creation invocation, the default behavior is used. The default behavior is to return `null` from `BeansLinker.getGuardedInvocation(LinkRequest, LinkerServices)` when it can be determined at link time that the linked operation will never address an existing member. This lets the `DynamicLinker` attempt the next linker if there is one, or ultimately fail the link request with `NoSuchDynamicMethodException`. For other cases (typically all unnamed member operations as well as most named operations on collection elements) `BeansLinker` will produce a conditional linkage that will return `null` when invoked at runtime with a name that does not match any member for getters and silently ignore the passed values for setters.

## Implementing exception-throwing behavior

Note that if the language-specific behavior for an operation on a missing member is to throw an exception then the factory should produce a method handle that throws the exception when invoked, and must not throw an exception itself, as the linkage for the missing member is often conditional.

Since:
  9
See Also:
  - `BeansLinker(MissingMemberHandlerFactory)`

- ## Method Summary

  Modifier and Type
  Method
  Description
  `MethodHandle`
  `createMissingMemberHandler(LinkRequest linkRequest, LinkerServices linkerServices)`
  Returns a method handle suitable for implementing missing member behavior for a particular link request.

- ## Method Details

  - ### createMissingMemberHandler

    MethodHandle createMissingMemberHandler(LinkRequest linkRequest, LinkerServices linkerServices) throws Exception
    Returns a method handle suitable for implementing missing member behavior for a particular link request. See the class description for details.
    Parameters:
      `linkRequest` - the current link request
      `linkerServices` - the current link services
    Returns:
      a method handle that can be invoked if the property, element, or method being addressed by an operation is missing. The return value can be null.
    Throws:
      `Exception` - if the operation fails for any reason. Please observe the class documentation notes for implementing exception-throwing missing member behavior.