7d0a5c0edb
* Adding support for maven assembly plugin to generate executable jar with all dependencies in built * Merge branch 'master' into issue-989 # Conflicts: # abstract-document/pom.xml # pom.xml * Adding maven assemly plugin for projects with name A * Update in format as per checkstyle, i.e. Spcae in place of tab with size of 2 * batch set - 2 having all project with B and C * issue-989 d-e-f * fixing eip pom and adding g-h-i-l-m-n Skipping naked object as it seems it doesn't have main method, will consider this at end * Adding for O and P projects Skipping Object-Mother as we don't have main method for same.
layout, title, folder, permalink, categories, tags
| layout | title | folder | permalink | categories | tags | |
|---|---|---|---|---|---|---|
| pattern | Command | command | /patterns/command/ | Behavioral |
|
Also known as
Action, Transaction
Intent
Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
Class diagram
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, that is, a function that's 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 around high-level operations build 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
Typical Use Case
- to keep a history of requests
- implement callback functionality
- implement the undo functionality