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JAR Files, WAR Files, and EAR Files

Oracle Application Server Tips by Burleson Consulting

To deploy EJBs and other components in the J2EE application, you must package them. In keeping with J2EE modular programming methodology, all parts of a module are packaged together. This includes JSP files, images, utility classes, and whatever is required to make that component a self-contained package. JAR files, or Java Archives, are really just zipped archives of files ending in the .jar extension. In fact, you can pack and unpack a JAR file with any zip utility. Java provides a utility for creating archives, called jar, that has the advantage of creating a Manifest automatically. By creating an archive of the required class file and other supporting files (like images, bitmaps, etc.), you package everything into one neat file. An entire application can be packaged into a JAR file that the Java Virtual Machine will execute. This allows you to deploy applications in one file. The JAR file maintains the file?s subdirectories there by maintaining Java package integrity. Standard JavaBeans and Enterprise JavaBeans, mentioned earlier, are packaged into JAR files.

WAR files are Web Application Archives. WAR files are JAR files that end in .war and are used to package and deploy web components into web containers. They can contain HTML documents, servlets, JSPs, or even applet class files. WAR archives also contain an XML file called a deployment descriptor that describes the components. This file is normally called web.xml.

An EAR file is an Enterprise Archive and is used to package modules of a J2EE application. An EAR file is a JAR file that ends in .ear and can contain JAR and WAR files in addition to any other file required by the module. EAR archives also contain a file describing their contents?called an application descriptor, typically, application.xml.

As with all zip files, Java archives maintain subdirectories and thus package scope. To create these files manually, you use the Java packager tool called jar.

jar ?cfv archive_name.jar
jar ?cvf archive_name.war

Almost all IDEs, such as JDeveloper, automate the creation of these archives and their deployment into containers.

J2EE Components

      As we?ve established, J2EE is a set of Java technologies designed to assist in the implementation of large, distributed, multitiered applications from the client back to the database connection?in other words, enterprise applications. J2EE includes a wide range of components that support security, messaging, transactional integrity, sending e-mail, processing XML, and more. The Java 2 Platform, Standard Edition Software Development Kit (J2SE SDK) is the basic Java language and is required to run J2EE. The J2EE SDK provides the specifications and APIs (Application Programming Interface) to build J2EE applications. In addition to defining the EJB specification, J2EE provides APIs to support a number of technologies, discussed next.

Java Database Connectivity

The Java Database Connectivity (JDBC) API defines methods for connecting and accessing a database within the Java language. Most major databases, such as Oracle?s, come with JDBC drivers. The JDBC API allows complete access to the database, including data definition and data manipulation. The JDBC also supports bulk inserts, bind variables, and prepare statements. Entity EJBs (discussed later in the chapter) have built-in connectivity with the database provided by the container; however, if you override the container-maintained persistence, you must use the JDBC API. Also, any bean that is not an entity EJB must use the JDBC API to connect to a database.

Java Message Service

The Java Message Service (JMS) allows J2EE components to create, send, receive, and read messages. By implementing JMS, a distributed component can process messages asynchronously, thereby improving the reliability of the distributed action. JMS allows for loose coupling of components. JMS is used by message-driver EJBs and provides a publish and subscribe (topic-based) model and also a point-to-point (queue-based) model for messaging flexibility.

Java Naming and Directory Interface

The Java Naming and Directory Interface (JNDI) provides methods for finding objects by using attributes. This is how EJBs and other registered objects are located inside a container. A container can list the EJBs that it holds with JNDI so that other components can locate them. When you deploy an archive into a container, you may need to identify what JNDI resource it uses. The JNDI can be used to interface with other naming directories such as LDAP (Lightweight Directory Access Protocol). Using JNDI, a J2EE application can coexist with legacy systems.

Java Transaction API

Database administrators understand transactions, but many developers do not. When the application accesses the database, it will return only committed data. For simple updates, inserts, or deletes, committing or rolling back data can be handled easily by the application either manually or using autocommit. If the application conducts multiple database operations that are interdependent, you will need to use the JTA  to demarcate the transaction limits so that the entire transaction is committed or rolled back together.

JavaMail API

Implementing the JavaMail API allows the application to send e-mail notifications. Using JavaMail, an application can notify an administrator when problems or errors are encountered. JavaMail can also be used to provide standard performance statistics at periodic intervals and to integrate e-mail into an application.

Java API for XML Processing

XML (Extensible Markup Language) is an effective way to write out data in a structured hierarchy. Almost all of the configuration files used by the Application Server are XML documents. (Some components that were brought into the Application Server do not use XML, such as the http.conf file for OHS, which came from the Apache web server.) An example of an XML configuration file is the web.xml deployment descriptor stored in a WAR file. As you work with XML parameter files, you need to understand two important requirements of XML: it is case sensitive, and it must have an ending tag.

Here is an example of an XML file that defines three servlets?intro, timer, and recTime:


    <param-value>60000</param-value>      <!-- every minute -->


Each servlet has a servlet-name tag and a servlet-class tag defining the name and class file for each servlet. The timer servlet also has a few parameters defined. The time parameter defines a temp file to store information as /tmp/ The other parameter defines the number of Timer ticks to count, in this case 60,000, or every minute. Notice that each tag has a closing tag.

Because XML produces ?well-formed? text documents, it is a great way to pass information between different platforms or systems. J2EE provides native support to both create and read XML documents. Both J2EE and Oracle Application Server 10g use XML extensively, so as an administrator, you will need to have a basic understanding of XML.

Java Authentication and Authorization Service

The Java Authentication and Authorization Service (JAAS) API supports user (or groups of users) authorization and privileges. JAAS implements a standard pluggable authentication module (PAM) in Java that extends Java?s security framework in order to support user authorizations using standard authentication providers such a LDAP, JNDI, or the operating system. Security is discussed in detail in Chapter 12.

That?s a lot of information, but you will run into these terms throughout the remainder of this book and need to have a basic understanding of their meaning.

This is an excerpt from "Oracle 10g Application Server Administration Handbook" by Don Burleson and John Garmany.

If you like Oracle tuning, you may enjoy the new book "Oracle Tuning: The Definitive Reference", over 900 pages of BC's favorite tuning tips & scripts. 

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