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.