MethodHandles

MethodHandles is a Java API introduced in Java 7 that provides a way to invoke methods indirectly. It offers a more flexible and powerful alternative to traditional reflection, allowing for dynamic method invocation with better performance and type safety. The core concept revolves around a lookup object and a method handle. A lookup object is used to find a method handle corresponding to a specific method. Once a method handle is obtained, it can be invoked directly, similar to calling a regular method.

Unlike reflection, which provides access to method metadata, method handles provide a direct reference to executable code. This distinction allows method handles to be composed, filtered, and adapted, enabling sophisticated dynamic programming techniques. For instance, method handles can be used to create combinators that transform method calls, such as creating a method handle that binds arguments, invokes another method handle, or performs type conversions.

The API is organized around the java.lang.invoke package, featuring classes like MethodHandles, MethodHandle, MethodType, and Lookup. MethodType defines the signature of a method handle, specifying its return type and argument types. Lookup is used to create method handles, and it has different flavors depending on the access privileges required. MethodHandles provides static factory methods for creating and manipulating method handles.

MethodHandles are particularly useful in scenarios requiring advanced dynamic behavior, such as implementing dynamically typed languages on the JVM, creating framework abstractions, or optimizing reflection-based operations. They offer a balance between the flexibility of dynamic dispatch and the performance benefits of static invocation.