JIRA is very flexible, and has a number of extension points where JIRA's data can be queried or its functionality extended. You can also follow further instructions on Developing your Plugin using the Atlassian Plugin SDK.
JIRA Plugins: For information on installing and/or enabling existing plugins, please read the Managing JIRA's Plugins documentation.
Custom Field Types
JIRA comes with various custom field types defined. New types can be written and plugged into JIRA. See the How to create a new Custom Field Type tutorial for more information.
JIRA comes with many options to change the look and feel of features in the system. User formats are a feature that can be customised by plugins. You can write your own User Format plugin to change the display of user details in JIRA, e.g. display a profile picture. See the User Format Plugin Module for more information.
New gadgets can be created by writing an XML descriptor file, packaged as an Atlassian plugin. See Writing an Atlassian Gadget for more information.
JIRA comes with various reports built-in. Using the plugin system, new reports can be written, providing new ways of viewing and summarising JIRA's data.
Workflow functions and conditions
JIRA's issue workflow (states and state transitions an issue can go through) can be customized through the web interface (see the workflow documentation. The workflow engine (OSWorkflow) provides hooks where you can plug in your own behaviour:
Issue and Project Tabs
One the 'View Issue' page, some issue information (comments, change history) is displayed in tabs:
Likewise, the 'Browse Project' page contains tab panels displaying project information:
By writing a plugin, you can add new issue or project tab panels to JIRA. For instance, you may wish to display project/issue data pulled in from an external source. This is how the JIRA Subversion plugin works. See the plugin guide for more information on writing these plugin types.
JIRA has a complete event subsystem which fires events whenever anything happens. For example an
ISSUE_CREATED event is fired whenever an issue is created. A listener is just a class which implements a
JiraListener interface and is called whenever events occur in JIRA. Using those events, you can then perform any action you want. For example the email sent by JIRA is driven by the
MailListener. This is useful when you want to drive or affect external systems from events which occur within JIRA - usually used to push data into outside systems. For more information, read the listeners documentation.
Services are classes which implement the
JiraService interface. When installed, you specify an update period and JIRA will call the
run() method of your service periodically. A sample service is provided called
POPCommentService. This service checks a particular POP mailbox periodically and if it finds messages, tries to extract an issue key from the subject. If the subject contains a key, the body of the mail is added as a comment to the message. Services are useful when you want to periodically pull data into JIRA from outside systems. For more information, see the services guide.
SOAP and XML-RPC remote interfaces
JIRA has a growing SOAP and XML-RPC interface. This enables you to drive JIRA automatically from external systems. For example you can have a Java program, Perl script or C# client add issues to JIRA. See the JIRA RPC Services overview for general information. For building RPC clients, check out the Creating a JIRA SOAP Client and Creating an XML-RPC Client tutorials. New RPC endpoints can also be added to JIRA as plugins - see the RPC Endpoint Plugin Module.
JIRA has a full set of Java APIs that can be used to update information with in JIRA.
You can view the API here. JIRA commercial customers get full access to the JIRA source (see bottom of the downloads page), so you can modify JIRA itself if necessary. See the Building JIRA from Source page for more information.