Command Pattern

Overview

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Participants

Command

declares an interface for executing an operation.

Concrete Command

defines a binding between a Receiver object and an action.
implements Execute by invoking the corresponding operation(s) on Receiver.

Macro Command (optional)

a subclass of the Concrete Command, has no receivers, because the commands it sequences defines their own receiver.

Parameters

Use Macro Command

create the class defined in the Macro Command field.

Create Pattern Links

when selected, will create special JavaDoc links in the code between pattern participants, describing their relationships.

Applicability

Use the Command pattern when you want to

parameterize objects by an action to perform. You can express such parameterization in a procedural language with a callback function, in other words. a function that is registered somewhere to be called at a later point. Commands are an object-oriented replacement for callbacks.
specify, queue, and execute requests at different times. A Command object can have a lifetime independent of the original request. If the receiver of a request can be represented in an address space-independent way, then you can transfer a command object for the request to a different process and fulfill the request there.
support undo. The Command's Execute operation can store state for reversing its effects in the command itself. The Command interface must have an added Unexecute operation that reverses the effects of a previous call to Execute. Executed commands are stored in a history list. Unlimited-level undo and redo is achieved by traversing this list backwards and forwards calling Unexecute and Execute, respectively.
support logging changes so that they can be reapplied in case of a system crash. By augmenting the Command interface with load and store operations, you can keep a persistent log of changes. Recovering from a crash involves reloading logged commands from disk and re-executing them with the execute operation.
structure a system command around high-level operations built on primitive operations. Such a structure is common in information systems that support transactions. A transaction encapsulates a set of changes to data. The Command pattern offers a way to model transactions. Commands have a common interface, letting you invoke all transactions the same way. The pattern also makes it easy to extend the system with new transactions.