MethodHandle

MethodHandle is a fundamental concept in the Java programming language, introduced in Java 7 as part of Project Coin. It provides a more flexible and powerful mechanism for invoking methods compared to traditional reflection. A MethodHandle represents a reference to an executable method, constructor, or even a field accessor. Unlike Java reflection's Method objects, MethodHandles are designed to be more efficient and to allow for method-like operations, such as adapting the arguments passed to a method or changing the receiver of a method invocation.

The core idea behind MethodHandles is that they are a "handle" to an invocation. This handle can be transformed and combined to create new MethodHandles with different behaviors. This transformation process, known as "inlining," allows for significant optimizations. MethodHandles are typically obtained through a MethodHandles.Lookup object, which is used to find and create handles to specific members. This lookup process involves specifying the desired method type, which describes the signature of the method the handle will represent.

Key advantages of MethodHandles over reflection include better performance, as they can be inlined by the JVM, and the ability to perform advanced operations like currying (pre-setting arguments), uncurrying (converting a method that takes multiple arguments into one that takes a single array or list of arguments), and converting method signatures. This makes them suitable for implementing dynamic languages on the JVM and for various metaprogramming scenarios. While more powerful, MethodHandles are also considered more complex to use than reflection, requiring a deeper understanding of invocation semantics and type conversions.