WAR (file format)
|Developed by||Sun Microsystems|
|Container for||JSP, Java Servlet|
In software engineering, a WAR file (Web Application Resource or Web application ARchive) is a file used to distribute a collection of JAR-files, JavaServer Pages, Java Servlets, Java classes, XML files, tag libraries, static web pages (HTML and related files) and other resources that together constitute a web application.
Content and structure
There are special files and directories within a WAR file:
- The /WEB-INF directory in the WAR file contains a file named web.xml which defines the structure of the web application. If the web application is only serving JSP files, the web.xml file is not strictly necessary. If the web application uses servlets, then the servlet container uses web.xml to ascertain to which servlet a URL request will be routed. The web.xml file is also used to define context variables which can be referenced within the servlets and it is used to define environmental dependencies which the deployer is expected to set up. An example of this is a dependency on a mail session used to send email. The servlet container is responsible for providing this service.
Advantages of WAR files
- Easy testing and deployment of web applications
- Easy identification of the version of the deployed application
- All Java EE containers support WAR files
- MVC structure supports WAR files.
Assuming production environments do not promote a fix without sufficient testing prior to deployment, a WAR file has a distinct advantage when properties files are used to identify environment specific variables. For example, an LDAP server in a TEST environment may be something like ldaps://testauth.company.com:636. The LDAP server in a production environment is ldaps://auth.company.com:636. An external properties file would define the link with some thing like:
The source code reads the property file to determine the target LDAP server. In this way, developers can be certain that the WAR file tested and verified is exactly the same as that which is being promoted to production.
Disadvantages of WAR files
Some consider web deployment using WAR files to be disadvantageous when minor changes to source code are required for dynamic environments. Each change to source code must be repackaged and deployed in development.  This does not require stopping the web server if configured for runtime deployment. 
The following sample web.xml file demonstrates the declaration and association of a servlet:
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN" "http://java.sun.com/j2ee/dtds/web-app_2_2.dtd"> <web-app> <servlet> <servlet-name>HelloServlet</servlet-name> <servlet-class>mypackage.HelloServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>HelloServlet</servlet-name> <url-pattern>/HelloServlet</url-pattern> </servlet-mapping> <resource-ref> <description> Resource reference to a factory for javax.mail.Session instances that may be used for sending electronic mail messages, preconfigured to connect to the appropriate SMTP server. </description> <res-ref-name>mail/Session</res-ref-name> <res-type>javax.mail.Session</res-type> <res-auth>Container</res-auth> </resource-ref> </web-app>
The /WEB-INF/classes directory is on the ClassLoader's classpath. (The classpath consists of a list of locations from which .class files can be loaded and executed by the JVM.) The /WEB-INF/classes directory contains the classes associated with the web application itself.
Any JAR files placed in the /WEB-INF/lib directory will also be placed on the ClassLoader's classpath.
- Related file formats:
- EXE (file format)
- ELF file format ("Executable and Linkable Format")
- Crossley, Allistair. "Apache Tomcat 8 (8.0.44) - Tomcat Web Application Deployment". tomcat.apache.org. Retrieved 2017-06-27.
- "What's New in Java Servlet API 2.2?". JavaWorld (Reprinted by Oracle Corporation). October 1999. Retrieved 2014-08-26.
- "Web Application Lifecycle". Oracle.
- "Deploying on a running Tomcat server". Apache.
- Oracle Java EE 7 Tutorial: Packaging Web Archives
- Oracle Java EE 6 Tutorial: Web Modules
- Oracle Java EE 5 Tutorial: Web Modules
- Sun Microsystems: XML Schema for the Servlet 2.5 Web ARchive (WAR) File
- Sun Microsystems: XML Schema for the Servlet 2.4 Web ARchive (WAR) File
- JSR 154: Java Servlet 2.4 Specification