Maven

Apache Maven

Developer(s)
Apache Software Foundation

Stable release
2.2.1 / August 11, 2009; 11 months ago (2009-08-11)

Development status
Active

Written in
Java

Operating system
Cross-platform

Type
Build Tool

License
Apache License 2.0

Website
http://maven.apache.org

 

Maven is a software tool for project management and build automation. While Maven is primarily used for Java programming, it can be used to build and manage projects written in C#, Ruby, Scala, and other languages. It is similar in functionality to the Apache Ant tool, but is based on different concepts. Maven is hosted by the Apache Software Foundation, where it was formerly part of the Jakarta Project.

Maven uses a construct known as a Project Object Model (POM) to describe the software project being built, its dependencies on other external modules and components, and the build order. It comes with pre-defined targets for performing certain well defined tasks such as compilation of code and its packaging.

Maven dynamically downloads Java libraries and Maven plug-ins from one or more repositories. Maven provides built-in support for retrieving files from the Maven 2 Central Repository^[1] and other Maven repositories, and can upload artifacts to specific repositories after a successful build. A local cache of downloaded artifacts acts as the primary means of synchronizing the output of projects on a local system.

Maven is built using a plugin-based architecture that allows it to make use of any application controllable through standard input. Theoretically, this would allow anyone to write plugins to interface with build tools (compilers, unit test tools, etc.) for any other language. In reality, support and use for languages other than Java has been minimal. Currently a plugin for the .Net framework exists and is maintained ^[2], and a C/C++ native plugin was at one time maintained for Maven 1.^[3]

Contents

[hide]

• 1 Example
• 2 Concepts
  • 2.1 Project Object Model
  • 2.2 Plugins
  • 2.3 Build Lifecycles
  • 2.4 Dependencies
• 3 IDE Integration
• 4 History
• 5 Future
• 6 References
• 7 Books
• 8 See also
• 9 External links

[edit] Example

Maven projects are configured using a Project Object Model, which is stored in a pom.xml-file. Here's a minimal example:

<project>
  <!-- model version is always 4.0.0 for Maven 2.x POMs -->
  <modelVersion>4.0.0</modelVersion>
 
  <!-- project coordinates, i.e. a group of values which
       uniquely identify this project -->
 
  <groupId>com.mycompany.app</groupId>
  <artifactId>my-app</artifactId>
  <version>1.0</version>
 
  <!-- library dependencies -->
 
  <dependencies>
    <dependency>
 
      <!-- coordinates of the required library -->
 
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
 
      <!-- this dependency is only used for running and compiling tests -->
 
      <scope>test</scope>
 
    </dependency>
  </dependencies>
</project>

This POM only defines a unique identifier for the project (coordinates) and its dependency on the JUnit framework. However, that is already enough for building the project and running the unit tests associated with the project. Maven accomplishes this by embracing the idea of Convention over Configuration, that is, Maven provides good default values for the project's configuration. The directory structure of a normal idiomatic Maven project has the following directory entries:

Directory name

Purpose

project home

Contains the pom.xml and all subdirectories.

src/main/java

Contains the deliverable Java sourcecode for the project.

src/main/resources

Contains the deliverable resources for the project, such as property files.

src/test/java

Contains the testing classes (JUnit or TestNG test cases, for example) for the project.

src/test/resources

Contains resources necessary for testing.

Then the command

mvn package

will compile all the Java files, run any tests, and package the deliverable code and resources into target/my-app-1.0.jar (assuming the artifactId is my-app and the version is 1.0.)

The main idea of Maven is that the user only provides configuration for the project, while the configurable plug-ins do the actual work of compiling the project, cleaning target directories, running unit tests, generating API documentation and so on. In general, users should not have to write plugins themselves. Contrast this with Ant and make in which one writes imperative procedures for doing the aforementioned tasks.

[edit] Concepts

[edit] Project Object Model

Project Object Model provides all the configuration for a single project. General configuration includes the project's name, its owner and its dependencies on other projects. One can also configure individual phases of the build process, which are implemented as plugins. For example, one can configure the compiler-plugin to use Java version 1.5 for compilation, or specify that project can be packaged even if some unit test fails.

Larger projects should be divided into several modules, or sub-projects, each with its own POM. One can then write a root POM through which one can compile all the modules with a single command. POMs can also inherit configuration from other POMs. All POMs inherit from the Super POM^[4] by default. Super POM provides default configuration, such as default source directories, default plugins and so on.

[edit] Plugins

Most of Maven's functionality is in plugins. A plugin provides a set of goals that can be executed using the following syntax:

mvn [plugin-name]:[goal-name]

For example, a Java project can be compiled with the compiler-plugin's compile-goal^[5] by running mvn compiler:compile.

There are Maven plugins for building, testing, source control management, running a web server, generating Eclipse project files, and much more^[6]. Plugins are introduced and configured in a <plugins>-section of a pom.xml file. Some basic plugins are included in every project by default, and they have sensible default settings.

However, it would be cumbersome if one would have to run several goals manually in order to build, test and package a project:

mvn compiler:compile
mvn surefire:test
mvn jar:jar

Maven's lifecycle-concept handles this issue.

[edit] Build Lifecycles

Build lifecycle is a list of named phases that can be used to give order to goal execution. One of Maven's standard lifecycles is the default lifecycle, which includes the following phases, in this order^[7]:

• process-resources



• compile



• process-test-resources



• test-compile



• test



• package



• install



• deploy

Goals provided by plugins can be associated with different phases of the lifecycle. For example, by default, the goal "compiler:compile" is associated with the compile-phase, while the goal "surefire:test" is associated with the test-phase. When the command

mvn test

is executed, Maven will run all the goals associated with each of the phases up to the test-phase. So it will run the "resources:resources"-goal associated with the process-resources-phase, then "compiler:compile", and so on until it finally runs the "surefire:test"-goal.

Maven also has standard lifecycles for cleaning the project and for generating a project site. If cleaning were part of the default lifecycle, the project would be cleaned every time it was built. This is clearly undesirable, so cleaning has been given its own lifecycle.

Thanks to standard lifecycles, one should be able to build, test and install every Maven-project using the mvn install-command.

[edit] Dependencies

The example-section hinted at Maven's dependency-handling mechanism. A project that needs the Hibernate-library simply has to declare Hibernate's project coordinates in its POM. Maven will automatically download the dependency and all the dependencies that Hibernate itself needs (called transitive dependencies) and store them in the user's local repository. Maven 2 Central Repository^[1] is used by default to search for libraries, but one can configure e.g. company-private repositories in POM.

There are search engines such as mvnrepository, which can be used to find out coordinates for different open-source libraries and frameworks.

Projects developed on a single machine can depend on each other through the local repository. The local repository is a simple folder structure which acts both as a cache for downloaded dependencies and as a centralized storage place for locally built artifacts. The Maven command mvn install builds a project and places its binaries in the local repository. Then other projects can utilize this project by specifying its coordinates in their POMs.

[edit] IDE Integration

Add-ons to several popular IDEs exist to provide integration of Maven with the IDE's build mechanism and source editing tools, allowing Maven to compile projects from within the IDE, and also to set the classpath for code completion, highlighting compiler errors, etc. Examples of popular IDEs supporting development with Maven include:

• Eclipse



• Netbeans



• IntelliJ



• JBuilder

These add-ons also provide the ability to edit the POM or use the POM to determine a project's complete set of dependencies directly within the IDE.

Some built-in features of IDEs are forfeited when the IDE no longer performs compilation. For example, Eclipse's JDT has the ability to recompile a single java source file after it has been edited. Many IDEs work with a flat set of projects instead of the hierarchy of folders preferred by Maven. This complicates the use of SCM systems in IDEs when using Maven. ^{[8] [9] [10]}

[edit] History

Maven, created by Sonatype's Jason van Zyl, began as a subproject of Apache Turbine in 2002. In 2003, it was voted on and accepted as a top level Apache Software Foundation project. In July of 2004, Maven was released as the critical first milestone, v1.0. Maven 2 was declared v1.0 in October of 2005 after about 6 months in beta cycles.

[edit] Future

Maven 3.0 information began trickling out in 2008. After eight alpha releases, the first beta version of Maven 3.0 was released in April 2010. Maven 3.0 has reworked the core Project Builder infrastructure such that the POMs file-based representation is now decoupled from its in-memory object representation. This has expanded the possibility for Maven 3.0 add-ons to leverage non-XML based project definition files. Languages suggested include Ruby (already in private prototype by Jason van Zyl), YAML, and Groovy. Experimental work for a YAML-based POM definition file (requires an external conversion script to be executed) has been piloted by Don Brown of Atlassian.

Special attention has been paid to ensuring compatibility between Maven 2 and 3. For most projects, an upgrade to Maven 3 won't require any adjustments of their project structure. The first beta of Maven 3 saw the introduction of a parallel build feature which leverages a configurable amount of cores on a multi-core machine and is especially suited for large multi-module projects.

[edit] References

1. ^ ^{a b} Maven 2 Central Repository



2. ^ .NET Maven Plugin



3. ^ Maven Native Plug-in



4. ^ Super POM



5. ^ Maven Compiler Plugin



6. ^ Maven - Available Plugins



7. ^ Maven Build Lifecycle Reference



8. ^ Eclipse plugins for Maven



9. ^ IntelliJ IDEA - Ant and Maven support



10. ^ Best Practices for Apache Maven in NetBeans 6.x

[edit] Books

• Van Zyl, Jason (2008-10-01), Maven: Definitive Guide (first ed.), O'Reilly Media, pp. 468, ISBN 0596517335, http://www.sonatype.com/products/maven/documentation/book-defguide

Available for free as PDF download or online reading

[edit] See also

• Apache Continuum, a continuous integration server which integrates tightly with Maven



• Apache Ant



• Apache Jelly, a tool for turning XML into executable code



• Apache Ivy, alternative dependency management tool for Java



• Sonatype Nexus, a repository manager commonly used with Maven



• Hudson



• List of build automation software

[edit] External links

• Official Maven project website



• The Maven 2 tutorial: A practical guide for Maven 2 users - tutorial at Codehaus.org



• Maven: The Definitive Guide, a printed and free online reading book.



• Building Web Applications with Maven 2



• The Maven 2 POM demystified - article at JavaWorld



• Verify the dependencies in your POM



• Maven history

Apache Maven Project (http://maven.apache.org)

Introduction

Maven, a Yiddish word meaning accumulator of knowledge, was originally started as an attempt to simplify the build processes in the Jakarta Turbine project. There were several projects each with their own Ant build files that were all slightly different and JARs were checked into CVS. We wanted a standard way to build the projects, a clear definition of what the project consisted of, an easy way to publish project information and a way to share JARs across several projects.

The result is a tool that can now be used for building and managing any Java-based project. We hope that we have created something that will make the day-to-day work of Java developers easier and generally help with the comprehension of any Java-based project.

Maven's Objectives

Maven's primary goal is to allow a developer to comprehend the complete state of a development effort in the shortest period of time. In order to attain this goal there are several areas of concern that Maven attempts to deal with:

• Making the build process easy
• Providing a uniform build system
• Providing quality project information
• Providing guidelines for best practices development
• Allowing transparent migration to new features

Format a String (JDK1.5)

JDK1.5 simplifies the operation of formatting a String based on parameters.

The String class now provides a new method called format(). The parameter substitution mechanism is heavily inspired by C's printf.

 String s = String.format
   ("Welcome %s at %s", "Real's HowTo", "http://www.rgagnon.com");
 System.out.println(s);
 // output : Welcome Real's HowTo at http://www.rgagnon.com

A printf method has been added to System.out !

System.out.printf
  ("Welcome %s at %s", "Real's HowTo", "http://www.rgagnon.com");

As you can see, it is now possible to call a method with a variable number of parameters. But it is also possible to use an array (with the new String.format()).

String a[] = { "Real's HowTo", "http://www.rgagnon.com" };

String s = String.format("Welcome %s at %s", a);
System.out.println(s);

Object a[] = { "Real's HowTo", "http://www.rgagnon.com" ,
        java.util.Calendar.getInstance()};

String s = String.format("Welcome %1$s at %2$s ( %3$tY %3$tm %3$te )", a);
System.out.println(s);
//  output : Welcome Real's HowTo at http://www.rgagnon.com (2010 06 26)

You can use this new feature to quickly format strings into table :

public class Divers {
  public static void main(String args[]){
      String format = "|%1$-10s|%2$-10s|%3$-20s|\n";
      System.out.format(format, "FirstName", "Init.", "LastName");
      System.out.format(format, "Real", "", "Gagnon");
      System.out.format(format, "John", "D", "Doe");

      String ex[] = { "John", "F.", "Kennedy" };

      System.out.format(String.format(format, (Object[])ex));
  }
}

Output:

|FirstName |Init.     |LastName            |
|Real      |          |Gagnon              |
|John      |D         |Doe                 |
|John      |F.        |Kennedy             |

To align numbers :

  String format = "%10.2f\n"; // width == 10 and 2 digits after the dot
  float [] floats = {123.45f, 99.0f, 23.2f, 45.0f};
  for(int i=0; i<floats.length; i++) {
    float value = floats[i];
    System.out.format(format, value);
  }

Output :

    123.45
     99.00
     23.20
     45.00

MVC1 Vs MVC2

I was reading on internet about MVC 1 and MVC 2. I got this difference for quick learners I hope this will be helpful.

MVC is a design pattern. It contains two models. MVC Model 1 MVC Model 2.Struts framework implements MVC Design Pattern. Struts can implement Model 1 and Model 2.

Model 2 most properly describes the application of MVC in a Web-Application context.

• Following are the important feature of MVC1 architecture:
  • HTML or JSP files are used to code the presentation. JSP files use java beans to retrieve data if required.
  • MVC1 architecture is page-centric design all the business and processing logic means any JSP page can either present in the JSP or may be called directly from the JSP page.
  • Data access is usually done using Custom tag or through java bean call.Therefore we can say that in MVC1 there is tight coupling between page and model.

• Following are the important feature of MVC2 architecture:
  • This architecture removes the page-centric property of MVC1 architecture by separating Presentation Control logic and Application state
  • In MVC2 architecture there is one Controller which receive all request for the application and is responsible for taking appropriate action in response to each request. Second one is Model which is represented by JavaBeans business object and database. Third one is View or is JSP page it takes the information provided by Controller and Module and presents it to user.

Figure 1: JSP Model 1 architecture

Figure 2: JSP Model 2 architecture

For more Information you can read this article on JavaWorld

Can a Java application have memory leak?

Yes, there could be memory leak in Java applications. Wait a minute, doesn't Java virtual machine have a garbage collector that will collect and free all unreferenced memory automatically?

Let's find out in general what memory leaks are, and how they occur in applications. If an application fails to return the not-in-use memory back to the heap, the "lost" memory is called memory leak.

Memory leaks occur when the application doesn't free the memory allocated, usually are the objects no longer in use, but the object references are lost. If an object is no longer accessible, there is no way to free its memory. Each time such a leak is re-created, additional memory is used and not freed. Eventually, the process that runs the application will run out of memory and crash.

It's true that for other programming languages, such as C/C++, there is not such a thing called garbage collector. The programmer is responsible for freeing the memory when the object is no longer in use.

In Java, all unreferenced objects are indeed automatically freed by the garbage collector. The garbage collector looks for objects that are no longer needed and to remove them when they can no longer be accessed or referenced. The garbage collector starts at the root nodes, classes that persist throughout the life of a Java application, and sweeps though all of the nodes that are referenced. As it traverses the nodes, it keeps track of which objects are actively being referenced. Any classes that are no longer being referenced are then eligible to be garbage collected. The memory resources used by these objects can be returned to the Java virtual machine (JVM) when the objects are deleted.

But in some situations, when the object is no longer in use, but some references to that object has not been removed. This kind of objects will not be collected by the garbage collector. That means there is a memory leak. Sometimes memory leaks in Java is also referred to as "dangling references".

What are the symptoms of a memory leak?

When the application has a memory leak, basically, you will notice:

1. Memory usage consistently increases during the application life span. Sooner or later the application will crash because out of memory.
2. Performance consistently decreases. This is because more and more un collectable objects are in the heap, which will trigger the garbage collector to work more frequently and work longer, on the other hand, the application will run slower.

Typical Leaks

Now that we know it is indeed possible to create memory leaks in Java, let's have a look at some typical leaks and what causes them.

Global collections

It is quite common in larger applications to have some kind of global data repository, a JNDI-tree for example, or a session table. In these cases care has to be taken to manage the size of the repository. There has to be some mechanism in place to remove data that is no longer needed from the repository.

Caches

A cache is a data structure used for fast lookup of results for already-executed operations. Therefore, if an operation is slow to execute, you can cache the result of the operation for common input data and use that cached data the next time the operation is invoked. Usually, the application keeps adding new data that was not in the cache, but not controlling the size of the cache. Depends on what data is kept in the cache, the cache will potentially increase to too big for the application to handle. When designing the cache, the program has to make sure the cache has an upper bound on the amount of memory it will use.

Types of Class Loader: JVM Architecture..

basic function of class loader is to read bytecodes into array and create namespace in namespace. there are two types of class loaders: promodia class loader and class loader objects

1. promodia class loader

it loads all necessary classes required for VM, it is bootstrap class loader.

2. class loader objects

there are 3 class loaders AppletClassLoader, RemoteClassLoader and SecurityClassloader.

Enterprise Java Beans [EJB]

Enterprise JavaBean

From Wikipedia, the free encyclopedia

Jump to: navigation, search

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (September 2007)

Simple EJB2 Architecture

Enterprise JavaBeans (EJB) is a managed, server-side component architecture for modular construction of enterprise applications.

The EJB specification is one of several Java APIs in the Java Platform, Enterprise Edition. EJB is a server-side model that encapsulates the business logic of an application. The EJB specification was originally developed in 1997 by IBM and later adopted by Sun Microsystems (EJB 1.0 and 1.1) in 1999^[1] and enhanced under the Java Community Process as JSR 19 (EJB 2.0), JSR 153 (EJB 2.1) and JSR 220 (EJB 3.0).

The EJB specification intends to provide a standard way to implement the back-end 'business' code typically found in enterprise applications (as opposed to 'front-end' interface code). Such code was frequently found to address the same types of problems, and it was found that solutions to these problems are often repeatedly re-implemented by programmers. Enterprise JavaBeans were intended to handle such common concerns as persistence, transactional integrity, and security in a standard way, leaving programmers free to concentrate on the particular problem at hand.

Accordingly, the EJB specification details how an application server provides:

• Persistence via the Java Persistence API (JPA)
• Transaction processing
• Concurrency control
• Events using Java Message Service
• Naming and directory services (JNDI)
• Security ( Java Cryptography Extension (JCE) and JAAS )
• Deployment of software components in an application server
• Remote procedure calls using RMI-IIOP.
• Exposing business methods as Web Services.

Additionally, the Enterprise JavaBean specification defines the roles played by the EJB container and the EJBs as well as how to deploy the EJBs in a container.

Contents [hide] 1 History 1.1 Rapid adoption followed by criticism 1.2 Reinventing EJBs 2 Types 3 Execution 3.1 Remote communication 3.2 Transactions 3.3 Events 3.4 Naming and directory services 3.5 Security 3.6 Deployment 4 Version history 4.1 EJB 3.1, in development 4.2 EJB 3.0, final release (2006) 4.3 EJB 2.1, final release (2003-11-24) 4.4 EJB 2.0, final release (2001-08-22) 4.5 EJB 1.1, final release (1999-12-17) 4.6 EJB 1.0 (1998-03-24) 5 External links 6 References

[edit] History

[edit] Rapid adoption followed by criticism

The neutrality of this section is disputed. Please see the discussion on the talk page. Please do not remove this message until the dispute is resolved. (December 2007)

This vision was persuasively presented by EJB advocates such as IBM and Sun Microsystems, and Enterprise JavaBeans were quickly adopted by large companies. Problems were quick to appear, however, and the reputation of EJBs began to suffer as a result. Some developers felt that the APIs of the EJB standard were far more complex than those developers were used to. An abundance of checked exceptions, required interfaces, and the implementation of the bean class as an abstract class were all unusual and counter-intuitive for many programmers. Granted, the problems that the EJB standard was attempting to address, such as object-relational mapping and transactional integrity, are complex. However many programmers found the APIs to be just as difficult if not more so, leading to a widespread perception that EJBs introduced complexity without delivering real benefits.

In addition, businesses found that using EJBs to encapsulate business logic brought a performance penalty. This is because the original specification only allowed for remote method invocation through CORBA (and optionally other protocols), even though the large majority of business applications actually do not require this distributed computing functionality. The EJB 2.0 specification addressed this concern by adding the concept of local interfaces which could be called directly without performance penalties by applications that were not distributed over multiple servers. It was introduced largely to address the performance problems that existed with EJB 1.0.^[2]

The complexity issue, however, continued to hinder EJB's acceptance. Although high-quality developer tools made it easy to create and use EJBs by automating most of the repetitive tasks, these tools did not make it any easier to learn how to use the technology. Moreover, a counter-movement had grown up on the grass-roots level among programmers. The main products of this movement were the so-called 'lightweight' (i. e. in comparison to EJB) technologies of Hibernate (for persistence and object-relational mapping) and Spring Framework (which provided an alternate and far less verbose way to encode business logic). Despite their lacking the support of big businesses that EJBs had, these technologies grew in popularity and were adopted more and more by businesses who had become disillusioned with EJBs.

EJBs were promoted by Sun's Java Pet Store demo Java BluePrints. The use of EJBs was controversial and influential J2EE programmers such as Rod Johnson took positions in response to Java Pet Store that sought to deemphasize EJB use. Sun itself produced an alternative called Java Data Objects. Later, EJBs, Java Data Forms, and many of the ideas underlying Hibernate were combined to form EJB 3.0 which included the Java Persistence API and Plain Old Java Objects (POJOs). EJB 3.0 was less heavy weight than EJB 2.0 and provided more choices to developers.

[edit] Reinventing EJBs

Gradually an industry consensus emerged that the original EJB specification's primary virtue — enabling transactional integrity over distributed applications — was of limited use to most enterprise applications, and the functionality delivered by simpler frameworks like Spring and Hibernate was more useful. Accordingly, the EJB 3.0 specification (JSR 220) was a radical departure from its predecessors, following this new paradigm. It shows a clear influence from Spring in its use of POJOs, and its support for dependency injection to simplify configuration and integration of heterogeneous systems. Gavin King, the creator of Hibernate, participated in the EJB 3.0 process and is an outspoken advocate of the technology. Many features originally in Hibernate were incorporated in the Java Persistence API, the replacement for entity beans in EJB 3.0. The EJB 3.0 specification relies heavily on the use of annotations, a feature added to the Java language with its 5.0 release, to enable a much less verbose coding style.

Accordingly, in practical terms EJB 3.0 is very nearly a completely new API, bearing little resemblance to the previous EJB specifications.

[edit] Types

An EJB container holds three major types of beans:

• Session Beans that can be either "Stateful" or "Stateless"
• Entity Beans that can be either "CMP" (Container managed persistence) or "BMP" (Bean managed persistence)
• Message Driven Beans (also known as MDBs or Message Beans)

Stateful Session Beans are distributed objects having state: that is, they keep track of which calling program they are dealing with throughout a session. For example, checking out in a web store might be handled by a stateful session bean that would use its state to keep track of where the customer is in the checkout process. Stateful session beans' state may be persisted, but access to the bean instance is limited to only one client.

Stateless Session Beans are distributed objects that do not have state associated with them thus allowing concurrent access to the bean. The contents of instance variables are not guaranteed to be preserved across method calls. The lack of overhead to maintain a conversation with the calling program makes them less resource-intensive than stateful beans. Sending an e-mail to customer support might be handled by a stateless bean, since this is a one-off operation and not part of a multi-step process.

Message Driven Beans were introduced in the EJB 2.0 specification that is supported by J2EE 1.3 or higher. The message bean represents the integration of JMS (Java Message Service) with EJB to create an entirely new type of bean designed to handle asynchronous JMS messages. Message Driven Beans are distributed objects that behave asynchronously. That is, they handle operations that do not require an immediate response. For example, a user of a website clicking on a "keep me informed of future updates" box may trigger a call to a Message Driven Bean to add the user to a list in the company's database. (This call is asynchronous because the user does not need to wait to be informed of its success or failure.) These beans subscribe to JMS (Java Message Service) message queues or message topics. They were added in the EJB 2.0 specification to allow event-driven processing inside EJB Container. Unlike other types of beans, MDB does not have a client view (Remote/Home interfaces), i. e. clients can not look-up an MDB instance. It just listens for any incoming message on a JMS queue (or topic) and processes them automatically.

Previous versions of EJB also used a type of bean known as an Entity Bean. These were distributed objects having persistent state. Beans in which their container managed the persistent state were said to be using Container-Managed Persistence (CMP), whereas beans that managed their own state were said to be using Bean-Managed Persistence (BMP). Entity Beans were replaced by the Java Persistence API in EJB 3.0, though as of 2007, CMP 2.x style Entity beans are still available for backward compatibility.

Other types of Enterprise Beans have been proposed. For instance, Enterprise Media Beans (JSR 86) address the integration of multimedia objects in Java EE applications.

[edit] Execution

EJBs are deployed in an EJB container within the application server. The specification describes how an EJB interacts with its container and how client code interacts with the container/EJB combination. The EJB classes used by applications are included in the javax.ejb package. (The javax.ejb.spi package is a service provider interface used only by EJB container implementations.)

With EJB 2.1 and earlier, each EJB had to provide a Java implementation class and two Java interfaces. The EJB container created instances of the Java implementation class to provide the EJB implementation. The Java interfaces were used by client code of the EJB.

The two interfaces, referred to as the Home and the Component interface, specified the signatures of the EJB's remote methods. The methods were split into two groups:

Class methods 

Not tied to a specific instance, such as those used to create an EJB instance (factory method) or to find an existing entity EJB (see EJB Types, above). These were declared by the Home interface.

Instance methods 

I. e. methods tied to a specific instance. These are placed in the Component interface.

Because these are merely Java interfaces and not concrete classes, the EJB container must generate classes for these interfaces that will act as a proxy in the client. Client code invokes a method on the generated proxies that in turn places the method arguments into a message and sends the message to the EJB server.

[edit] Remote communication

The EJB specification requires that EJB containers support accessing the EJBs using RMI-IIOP. EJBs may be accessed from any CORBA application or provide Web Services.

[edit] Transactions

EJB containers must support both container managed ACID transactions and bean managed transactions. Container-managed transactions use a declarative syntax for specifying transactions in the deployment descriptor.

[edit] Events

JMS is used to send messages from the beans to client objects, to let clients receive asynchronous messages from these beans. MDB can be used to receive messages from client applications asynchronously using either a JMS Queue or a Topic.

[edit] Naming and directory services

Clients of the EJB locate the Home Interface implementation object using JNDI. The Home interface may also be found using the CORBA name service. From the home interface, client code can find entity beans, as well as create and delete existing EJBs.

[edit] Security

The EJB Container is responsible for ensuring the client code has sufficient access rights to an EJB.

[edit] Deployment

The EJB specification defines a mechanism that allows EJBs to be deployed in a consistent manner regardless of the specific EJB platform that is chosen. Information about how the bean should be deployed (such as the name of the home or remote interfaces, whether and how to store the bean in a database, etc.) are specified in the deployment descriptor.

The deployment descriptor is an XML document having an entry for each EJB to be deployed. This XML document specifies the following information for each EJB:

• Name of the Home interface
• Java class for the Bean (business object)
• Java interface for the Home interface
• Java interface for the business object
• Persistent store (only for Entity Beans)
• Security roles and permissions
• Stateful or Stateless (for Session Beans)

EJB containers from many vendors require more deployment information than that in the EJB specification. They will require the additional information as separate XML files, or some other configuration file format. An EJB platform vendor generally provides their own tools that will read this deployment descriptor, and possibly generate a set of classes that will implement the Home and Remote interfaces.

Since EJB3.0 (JSR 220), the XML descriptor is replaced by Java annotations set in the Enterprise Bean implementation (at source level), although it is still possible to use an XML descriptor instead of (or in addition to) the annotations. If an XML descriptor and annotations are both applied to the same attribute within an Enterprise Bean, the XML definition overrides the corresponding source-level annotation.

[edit] Version history

[edit] EJB 3.1, in development

JSR 318. The purpose of the Enterprise JavaBeans 3.1 specification is to further simplify the EJB architecture by reducing its complexity from the developer's point of view, while also adding new functionality in response to the needs of the community:

Topics under consideration:

• Local view without interface
• .war packaging of EJB components
• EJB Lite: definition of a subset of EJB
• Portable EJB Global JNDI Names
• Singletons
• Application Initialization and Shutdown Events
• EJB Timer Service Enhancements
• Simple Asynchrony

[edit] EJB 3.0, final release (2006)

JSR 220 - Major changes: This release made it much easier to write EJBs, using 'annotations' rather than the complex 'deployment descriptors' used in version 2.x. The use of home and remote interfaces and the ejb-jar.xml file were also no longer required in this release, having been replaced with a business interface and a bean that implements the interface.

[edit] EJB 2.1, final release (2003-11-24)

JSR 153 - Major changes:

• Web service support (new): stateless session beans can be invoked over SOAP/HTTP. Also, an EJB can easily access a Web service using the new service reference.
• EJB timer service (new): Event-based mechanism for invoking EJBs at specific times.
• Message-driven beans accepts messages from sources other than JMS.
• Message destinations (the same idea as EJB references, resource references, etc.) has been added.
• EJB query language (EJB-QL) additions: ORDER BY, AVG, MIN, MAX, SUM, COUNT, and MOD.
• XML schema is used to specify deployment descriptors, replaces DTDs

[edit] EJB 2.0, final release (2001-08-22)

JSR 19 - Major changes: Overall goals:

• The standard component architecture for building distributed object-oriented business applications in Java.
• Make it possible to build distributed applications by combining components developed using tools from different vendors.
• Make it easy to write (enterprise) applications: Application developers will not have to understand low-level transaction and state management details, multi-threading, connection pooling, and other complex low-level APIs.
• Will follow the Write Once, Run Anywhere - philosophy of Java. An enterprise Bean can be developed once, and then deployed on multiple platforms without recompilation or source code modification.
• Address the development, deployment, and runtime aspects of an enterprise application’s life cycle.
• Define the contracts that enable tools from multiple vendors to develop and deploy components that can interoperate at runtime.
• Be compatible with existing server platforms. Vendors will be able to extend their existing products to support EJBs.
• Be compatible with other Java APIs.
• Provide interoperability between enterprise Beans and Java 2 Platform Enterprise Edition (J2EE) components as well as non-Java programming language applications.
• Be compatible with the CORBA protocols (RMI-IIOP).

[edit] EJB 1.1, final release (1999-12-17)

Major changes:

• XML deployment descriptors
• Default JNDI contexts
• RMI over IIOP
• Security - role driven, not method driven
• Entity Bean support - mandatory, not optional

Goals for Release 1.1:

• Provide better support for application assembly and deployment.
• Specify in greater detail the responsibilities of the individual EJB roles.

[edit] EJB 1.0 (1998-03-24)

Announced at JavaOne 1998, Sun's third Java developers conference (March 24 through 27) Goals for Release 1.0:

• Defined the distinct “EJB Roles” that are assumed by the component architecture.
• Defined the client view of enterprise Beans.
• Defined the enterprise Bean developer’s view.
• Defined the responsibilities of an EJB Container provider and server provider; together these make up a system that supports the deployment and execution of enterprise Beans.

[edit] External links

• Sun's EJB Product main page
• Java EE 5 API Javadocs
• EJB 3.0 API Javadocs
• The EJB 3.0 Specification
• Sun's EJB 3.0 Tutorial
• EJB (3.0) Glossary
• JSR 318 (EJB 3.1)
• JSR 220 (EJB 3.0)
• JSR 153 (EJB 2.1)
• JSR 19 (EJB 2.0)
• EJB 2 example based tutorial issa

[edit] References

1. ^ J2EE Design and Development, © 2002 Wrox Press Ltd., p. 5.
2. ^ J2EE Design and Development, © 2002 Wrox Press Ltd., p. 19.

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