Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, a WAR file (or Web application ARchive) is a JAR
JAR (file format)
In software, JAR is an archive file format typically used to aggregate many Java class files and associated metadata and resources into one file to distribute application software or libraries on the Java platform.JAR files are built on the ZIP file format and have the .jar file extension...
JavaServer Pages is a Java technology that helps software developers serve dynamically generated web pages based on HTML, XML, or other document types...
A servlet is a Java programming language class used to extend the capabilities of servers that host applications accessed via a request-response programming model. Although servlets can respond to any type of request, they are commonly used to extend the applications hosted by Web servers...
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
In the Java programming language, source files are compiled into machine-readable class files which have a .class extension. Since Java is a platform-independent language, source code is compiled into an output file known as bytecode, which it stores in a .class file. If a source file has more...
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
and related files) that together constitute a Web application
Web application
A web application is an application that is accessed over a network such as the Internet or an intranet. The term may also mean a computer software application that is coded in a browser-supported language and reliant on a common web browser to render the application executable.Web applications are...
A digital signature or digital signature scheme is a mathematical scheme for demonstrating the authenticity of a digital message or document. A valid digital signature gives a recipient reason to believe that the message was created by a known sender, and that it was not altered in transit...
in the same way as a JAR file in order to assert that the code is trusted.
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
Java Servlet
A servlet is a Java programming language class used to extend the capabilities of servers that host applications accessed via a request-response programming model. Although servlets can respond to any type of request, they are commonly used to extend the applications hosted by Web servers...
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
request is to be routed. web.xml 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:
security - no one can modify the deployment, once the war file is signed and deployed
easy development, testing and deployment
the version of the deployed application is easily identified
all J2EE containers support .WAR files
One disadvantage of web deployment using WAR files in very dynamic environments is that minor changes cannot be made during runtime. Any change whatsoever requires regenerating and redeploying the entire WAR file.
The following sample web.xml file demonstrates how a servlet
Java Servlet
A servlet is a Java programming language class used to extend the capabilities of servers that host applications accessed via a request-response programming model. Although servlets can respond to any type of request, they are commonly used to extend the applications hosted by Web servers...
is declared and associated.
PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"
"http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">
HelloServlet mypackage.HelloServlet
HelloServlet /HelloServlet
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.
mail/Session javax.mail.Session Container
The /WEB-INF/classes directory is on the ClassLoader
Java Classloader
The Java Classloader is a part of the Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine. Usually classes are only loaded on demand. The Java run time system does not need to know about files and file systems because of class loaders...
Classpath is a parameter—set either on the command-line, or through an environment variable—that tells the Java Virtual Machine or the compiler where to look for user-defined classes and packages.-Overview and architecture:...
. This is where .class files are loaded from when the web application is executing.
Any JAR files placed in the /WEB-INF/lib directory will also be placed on the ClassLoader's classpath.
