Handler

A handler is a person or thing that handles something. In general usage, a handler is someone responsible for managing, processing, or controlling a task, object, or event. The term is common in logistics (baggage handler) and in governance where an operator or administrator is described as a handler of a program or operation. In computing, a handler often denotes a software component that processes certain events or requests.

In software, an event handler is a routine or function registered to respond to a particular event, such as a user action or system signal. Handlers are usually invoked by the runtime when the event occurs; they may update state, perform I/O, or dispatch further work. In low-level programming an interrupt handler (ISR) services hardware interrupts and then returns control.

In web and application frameworks a request handler processes an incoming request and returns a response. In GUI toolkits and mobile platforms, handlers enable asynchronous and event-driven architectures. Some languages provide a dedicated handler type that posts tasks to an execution thread, such as Android’s Handler class that schedules runnable objects.

Design considerations include keeping handlers small and focused, avoiding long-running work, ensuring thread-safety, and minimizing side effects that hinder testability. Proper lifecycle management, such as registering and unregistering listeners, helps prevent resource leaks. Clear documentation and predictable error handling are also important.

The term also appears in other domains with specialized meanings, such as baggage handlers and dog show handlers, illustrating its core sense as a person or mechanism that takes care of a defined task.