Maven dependency mechanism, how it works

Maven’s dependency mechanism help to download all the necessary dependency libraries automatically, and maintain the version upgrade as well.

Case study

Let see a case study to understand how it works. Assume you want to use Log4J as your project logging mechanism. Here is what you do…

1. In traditional way

  1. Visit http://logging.apache.org/log4j/
  2. Download the Log4j jar library
  3. Copy jar to project classpath
  4. Include it into your project dependency manually
  5. All manages by yourself, you need to do everything

If there is a Log4j version upgrade, you need to repeat above steps again.

2. In Maven way

  1. You need to know the log4j Maven coordinates, for example
    
    	<groupId>log4j</groupId>
    	<artifactId>log4j</artifactId>
    	<version>1.2.14</version>
    

    It will download the log4j version 1.2.14 library automatically. If the “version” tag is ignored, it will upgrade the library automatically when there is a newer version.

  2. Declares Maven coordinates into pom.xml file.
    
    <dependencies>
        <dependency>
    	<groupId>log4j</groupId>
    	<artifactId>log4j</artifactId>
    	<version>1.2.14</version>
        </dependency>
    </dependencies>
    
  3. When Maven is compiling or building, the log4j jar will be downloaded automatically and put it into your Maven local repository.
  4. All manages by Maven.

Explanation

See the different? So what just happened in Maven? When you build a Maven’s project, the pom.xml file will be parsed, if it see the log4j Maven coordinate, then Maven search the log4j library in this order :

  1. Search log4j in Maven local repository.
  2. Search log4j in Maven central repository.
  3. Search log4j in Maven remote repository (if defined in pom.xml).

This Maven dependency library management is a very nice tool, and save you a lot of work.

How to find the Maven coordinates?
Visit this Maven center repository, search the jar you want to download.

Reference

  1. Introduction to the Dependency Mechanism

About the Author

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mkyong
Founder of Mkyong.com, love Java and open source stuff. Follow him on Twitter. If you like my tutorials, consider make a donation to these charities.

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Maven Tutorials | Tutorials

[…] Maven library dependency explanation Article about the different of dependency library in tradition way and Maven way, and explains where Maven will search those library. […]

marzapower
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marzapower

Hi, I recently released a log4j meta-library, called “Loggable”.
It will ease development time and reduce the common log4j overhead while defining your application architecture.

Please, check it out and let me know what you think about it:
http://www.marzapower.com/loggable/

Thank you!

Siddhanta Kumar Pattnaik
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Siddhanta Kumar Pattnaik

Hi,and value are log4j it is any logical name or its name must same with jar file name…….

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Why need to use Maven in Java project?

[…] Central Repository Search Engine) in your company, by using archiva or Nexus. Read this similar Maven dependency article i wrote previously. Ok, back to your question 1. IDE is run and debug your project. Maven […]

Guru
Guest
Guru

Thanks a million. Saved me lot of time.

Luis
Guest
Luis

Hi Mykong,

Nice post again, thank you very much!

One question, imaging this classic scenario, application that uses hibernate. Would it be possible that once you declared the hibernate dependency Maven download for you the rest of the “dependency tree” (i.e the commons.logging).

Thanks in advance,

Luis

omkar
Guest
omkar

Dear sir, I am currently a learning Java Developer. I had been working in a workless small company from 2 years 6 months. for better opportunity i have decided to learn complete j2ee and using 3 years of experience i need to look for other company. I know only j2SE, servlets, jsp, “Struts & Hibernate(currently learning)”. I heard and realized that not only these there are so many development tools for development. Recently i heared about ant, “maven & log4j, SLF4j from you.” could you please give me the guidense to learn these… if there any other development tools which… Read more »

SteveBlack.Co
Guest
SteveBlack.Co

Does Maven work for PHP projects?

Nanuk
Guest
Nanuk

if jar version will be upgraded maven will
download new version but what happens with old version jar which is
already in my local repository?

Nandha
Guest
Nandha

Hi, Thanks for all your work.Really has helped me a lot several time.
One question, Lets say i have a dependency test.jar version 1.0, how to make sure that, this jar gets downloaded everytime i build the project. The problem I faced was, this jar is uploaded to nexus from another project, but they dont change the version. So only the creation date and time changes. Any pointer please.
Thanks

balaguru
Guest
balaguru

Failed to execute goal org.apache.maven.plugins:maven-archetype-plugin:3.0.1:generate (default-cli) on project standalone-pom: Error merging velocity templates -> [Help 1]