Description: InvocationHandler is an interface in Java used to handle method invocations on proxy instances. This interface allows developers to define the behavior of a dynamic proxy, which is an object that acts as an intermediary for another object. By implementing the InvocationHandler interface, one can intercept method calls on the proxy and provide custom logic for their execution. This is particularly useful in situations where additional functionalities such as logging, transactions, or access control are required without modifying the original object’s code. The InvocationHandler interface has a single method, invoke, which takes three parameters: the proxy invoking the method, the method being invoked, and the method’s arguments. This flexibility allows developers to create proxies that can adapt to various needs, facilitating aspect-oriented programming and the implementation of design patterns like the Proxy pattern.
History: The InvocationHandler interface was introduced in Java 1.3, released in 2000. This development was part of Java’s evolution towards dynamic programming and improved reflection, allowing developers to create dynamic proxies more easily and effectively. The inclusion of this interface facilitated the implementation of design patterns that previously required more complex solutions.
Uses: InvocationHandler is primarily used in the creation of dynamic proxies, which are useful in aspect-oriented programming, such as implementing logging, transaction handling, and access control. It is also used in frameworks for Java applications to intercept method calls and apply additional logic without modifying business code.
Examples: A practical example of InvocationHandler is in creating a proxy that logs method calls of an object. By implementing the interface, one can intercept each invocation and log information such as the method name and parameters before delegating the call to the real object. Another common use is in implementing a transaction-handling proxy, where one can start and commit a transaction around the call to the original object’s method.
