Tomcat-based Web Applications
- web application
- end-user software accessed via a web client
- J2EE
- Java 2 Enterprise Edition. Provides framework necessary for building and running distributed, multitier applications that can scale from small to large situations
J2EE based on 3-tiered model
- client; e.g., web browser, stand-alone application, mobile device
- middle (or logic); e.g., J2EE platform with Tomcat Web Container running Web applications and EJB container with business objects
- EIS (Enterprise Information Systems); e.g., databases, data file, other back-end services, legacy systems
- J2EE service
- Implementation of a server that conforms to the J2EE specification. Two most basic are Web server (or Web container) and business object server or EJB container
J2EE Interfaces
- JDBC
- Java Database Connection. Provides SQL interface to external databases
- JTA
- Java Transaction API. Provides interface to transaction services
- JNDI
- Java Naming and Directory Interface. Provides interface to naming and directory services like LDAP, DNS, and NIS (Network Information Name Service)
- J2EE Connector Architecture
- API for interfacing to external EIS systems, including legacy systems
- JAXP
- Java API for XML processing
- JavaMail
- API for accessing mail servers
- JAF
- API for Java Beans for constructing applications and manipulating component functionality
- JMS
- API for acessing messaging services
- JAAS
- Java Authentication and Authorization Service
- RMI-IIOP
- API for acessing remote services over a common protocol
Current J2EE spec is version 1.4 (Nov 2003)
- reference implementation
- an actual implementation of a spec so developers know how it is supposed to work.
- web container
- J2EE service responsible for running Web applications. Tomcat is the most widely used.
- web application
- Comprised of JSP's and servlets (and JSP's get turned into servlets by the container), and static files. It is packaged as a single archive file or directory that is described as a unit by the web descriptor file
web.xml. So from the web container's perspective, any directory that contains a root subdirectoryWEB-INFwith theweb.xmldescriptor file is considered a web application. The web container creates an object that represents the web application. In Tomcat these are contexts and are represented by the<Context>tag. - servlet
- a unit of code that takes an HTTP request and returns an HTTP response. The servlet can handle all its processing internally or can rely on external services that are accessed by the defined J2EE interface.
- JSP
- a servlet skeleton that the Web container converts to a servlet at runtime
- scope
- The layer on which an operation is taking place.
Basic Objects
The documentation for the full list of J2EE objects is available here.
- HttpServlet
- The basic sevlet object extended by any servlet you write. There are five stages in its lifecycle:
| Stage | Meaning |
|---|---|
| Instantiation | When the web container calls new on its class |
| Initialization | When the init method is called. Startup functions can be performed |
| Service | Ready for use. The doPost, doGet, doDelete, etc. methods are called |
| Destruction | The destroy method is called |
| Finalization | The object is garbage-collected by the JVM |
There are 7 methods used to access a servlet during its Service stage:
- doDelete
- doGet
- doHead
- doOptions
- doPost
- doPut
- doTrace
These methods map to various HTTP request types. Each takes two parameters, an HttpServletRequest for the request and an HttpServletResponse for the response.
You can implement the service method, which takes the same parameters, and it will be called by the framework. The init (which takes a ServletConfig argument) can be used, for example, to connect to legacy services and the destroy method can free up that connection.
- ServletConfig
- represents the configuration of the servlet and can be accessed through the
getServletConfig()method. - ServletContext
- represents the Web container's configuration and can be used by the servlet to access the
logmethod of the Web application. - HttpServletRequest
- wrapper for an HTTP request
- HttpServletResponse
- Provides and OutputStream to send info back to client, along with headers, cookies, encoding, etc.
- HttpSession
- Encapsulates state. Can be accessed through the
getSessionmethod of theHttpServletRequestobject. - enterprise Web application
- relies on services such as LDAP authentication, database servers, legacy systems, etc.