Автор
Lars Vogel
Источник
http://www.vogella.com/articles/JavaWebTerminology/article.html
Java Web technologies
This article gives an overview of the standard web technologies for Java. It explains the terms web applications, servlets, JSPs and web container.
1. Web development
1.1. Overview
Java has strong support for web development. The following gives an overview of the available Java technologies starting with an explanation of web applications and then highlighting important standards in Java. After you finished the overview you can create your first Java web application with Servlet and JSP development with Eclipse WTP.
1.2. Web development
If you develop a web application (independent of the programming language your are using) your put your web application on a server (and not your local computer). The web application runs on the server and people can access it there. The server is either a real machine (with CPU, memory, harddisk, etc.) or a virtual server which is basically a machine which is separated by software into smaller machines.
Instead of running your application directly on a dedicated server you could also run it in a cloud environment. This cloud environment provides the necessary server for your application. An example for this is the Google App Engine .
Of course it is possible to use your local computer as a server but usually you want to have a fixed server which runs 24 hours and 7 days so that web clients can always reach your server under a pre-defined address. For example vogella.com blog contains the vogella blog. This blog is a web application powered by Wordpress. Wordpress is a web application written in the programming language "php".
1.3. Container
Java web applications are typically not running directly on the server. Java web applications are running inside a container on the server. This container runs on the server. The container provides a runtime environment for Java web applications. The container is for Java web applications what the JVM (Java Virtual Machine) is for local running Java applications. The container itself runs in the JVM.
In general Java distinguish two containers: the web container and the Java EE container. Typical web container in the Java world are Tomcat or Jetty. A web container supports the execution of Java servlets and JavaServer Pages. A Java EE container supports additional functionality for example distribution of server load.
Most of the modern Java web frameworks are based on servlets and JavaServer Pages. Popular Java web frameworks are JavaServer Faces, Struts, Spring. These web frameworks usually can run in a web container.
1.4. Java hosting at Google
Traditionally is has been difficult to start Java web development as a server was required for hosting the Java web container. Google offers a free of charge (for starters) solution based on Java. For details see Java Development on the Google App Engine
2. Java Web application
A Java web application is a collection of dynamic resources (such as Servlets, JavaServer Pages, Java classes and jars) and static resources (HTML pages and pictures). A Java web application can be deployed as a ".war" file. The ".war" file is a zip file which contains the complete content of the corresponding web application.
3. Java Web Standards
Standard Java technologies are defined via a standard process called the Java Community Process (JSP). The following technologies are defined via the JCP.
3.1. Servlet
A servlet is a Java class which extends "HttpServlet" and answers a HTTP request within a web container. The latest official version is Servlets 3.0 which is also part of Java EE 6. For details see the Java Servlets 3.0 Spec.
3.2. JavaServer Page
JavaServer Pages (JSP) are files which contains HTML and Java code. The web cotainer compiles the JSP into a servlet at the first time of accessing this JSP. The current latest version is 2.1. See Specification for JavaServer Pages 2.1.
3.3. JavaServer Pages Standard Tag Library
The JavaServer Pages Standard Tag Library (JSTL) encapsulates as simple tags the core functionality common to many Web applications. The current version is 1.2 is part of the JavaServer Pages Specification version 2.1.
3.4. Try it out
I hope the above does not sound to scary. Java Web development is actually really easy. You can try it out via the following Servlet and JSP Tutorial.
4. Non standard based Java Web Development
For Java you also find lots of non-standard web development. For example GWT supports the Java development and is compiled into JavaScript. See GWT Tutorial for more information.
7. Links and Literature
7.1. Java Webdevelopment
Servlet and JSP development with Eclipse WTP - Tutorial
7.2. Vogella Resources
Vogella Training – Android and Eclipse Training from the vogella team
Android Tutorial – Introduction to Android Programming
GWT Tutorial – Program in Java and compile to JavaScript and HTML
Eclipse RCP Tutorial – Create native applications in Java
J Unit Tutorial – Test your application
Git Tutorial – Put everything you have under distributed version control system