What is the difference in maven between dependency and plugin tags in pom xml?

I'm new to the maven tool, I have made a project with Spring and Hibernate and they are configured in pom.xml as plugins, but JUnit is tagged under dependency. My question is what is the logic behind one as a plugin and one as dependency ?

Both plugins and dependencies are Jar files.

But the difference between them is, most of the work in maven is done using plugins; whereas dependency is just a Jar file which will be added to the classpath while executing the tasks.

For example, you use a compiler-plugin to compile the java files. You can't use compiler-plugin as a dependency since that will only add the plugin to the classpath, and will not trigger any compilation. The Jar files to be added to the classpath while compiling the file, will be specified as a dependency.

Same goes with your scenario. You have to use spring-plugin to execute some spring executables [ I'm not sure what spring-plugins are used for. I'm just taking a guess here ]. But you need dependencies to execute those executables. And Junit is tagged under dependency since it is used by surefire-plugin for executing unit-tests.

So, we can say, plugin is a Jar file which executes the task, and dependency is a Jar which provides the class files to execute the task.

Hope that answers your question!

Maven itself can be described as food processor which has many different units that can be used to accomplish different tasks. Those units are called plugins. For example, to compile your project maven uses maven-compiler-plugin, to run tests - maven-surefire-plugin and so on.

Dependency in terms of maven is a packaged piece of classes that your project depends on. It can be jar, war etc. For example, if you want to be able to write JUnit test, you'll have to use JUnit annotations and classes thus you have to declare that your project depends on JUnit.

Plugins and dependencies are very different things and these are complementary.

What plugins are ?

Plugins perform tasks for a Maven build. These are not packaged in the application.

These are the heart of Maven.
Any task executed by Maven is performed by plugins.
There are two categories of plugins : the build and the reporting plugins :

• Build plugins will be executed during the build and they should be configured in the <build/> element from the POM.
• Reporting plugins will be executed during the site generation and they should be configured in the <reporting/> element from the POM.

According to the maven goal specified in the command line (for example mvn clean, mvn clean package or mvn site) , a specific lifecyle will be used and a specific set of plugins goals will be executed.
There are three built-in build lifecycles: default, clean and site. The default lifecycle handles your project deployment, the clean lifecycle handles project cleaning, while the site lifecycle handles the creation of your project's site documentation.

A plugin goal may be bound to a specific phase of a specific lifecyle.
For example the maven-compiler-plugin binds by default the compile goal to the lifecycle phase: compile.
Most of maven plugins (both core plugins and third party plugins) favor convention over configuration. So these generally bound a plugin goal to a specific phase to make their usage simpler.

That is neater and less error prone :

<plugin>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.7.0</version>
</plugin>

than :

<plugin>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.7.0</version>
  <executions>
	<execution>
		<phase>compile</phase>
		<goals>
			<goal>compile</goal>
		</goals>
	</execution>
  </executions>
</plugin>

What dependencies are ?

Dependencies are Maven artifacts/components required for the project.

Concretely most of dependencies are jar (that is libraries) but these may also be other kinds of archives : war, ear, test-jar, ejb-client ... or still POM or BOM.

In a pom.xml, dependencies may be specified at multiple places : the <build><dependencies> part , the dependencies management part or still in a plugin declaration ! Indeed some plugins may need to have some dependencies in the classpath during their execution. That is not common but that may happen.
Here is an example from the documentation that shows that plugin and dependency may work together :

> For instance, the Maven Antrun Plugin version 1.2 uses Ant version > 1.6.5, if you want to use the latest Ant version when running this plugin, you need to add <dependencies> element like the following:

<project>
  ...
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-antrun-plugin</artifactId>
        <version>1.2</version>
        ...
        <dependencies>
          <dependency>
            <groupId>org.apache.ant</groupId>
            <artifactId>ant</artifactId>
            <version>1.7.1</version>
          </dependency>
          <dependency>
            <groupId>org.apache.ant</groupId>
            <artifactId>ant-launcher</artifactId>
            <version>1.7.1</version>
          </dependency>
         </dependencies>
      </plugin>
    </plugins>
  </build>
  ...
</project>

In Maven, dependencies are referenced in a specific format :
groupId:artifactId:packaging:classifier:version.
The classifier (that is optional) and the packaging (JAR by default) are not commonly specified. So the common format in the dependency declaration is rather : groupId:artifactId:version.
Here is an example of dependency declared in the <build><dependencies> part :

<build>
   <dependencies>
      <dependency>
         <groupId>org.hibernate</groupId>
         <artifactId>hibernate-core</artifactId>
         <version>5.2.14.Final</version>
      </dependency>
   <dependencies>
</build>

Dependency doesn't have a phase binding as plugins to address the "when" question.
But it has a counterpart : the scope.
Indeed declared dependencies are usable by the application at a specific time according to the scope we defined for these.
The scope is a central concept about how a dependency will be visible for the project.

The default scope is compile. That is the most commonly needed scope (convention over configuration again).
The compile scope means that the dependency is available in all classpaths of a project.

The scope defines in which classpaths the dependency should be added. For example do we need it at compile and runtime, or only for tests compilation and execution ?

For example we previously defined Hibernate as a compile dependency as we need it everywhere : source compilation, test compilation, runtime and so for....
But we don't want that testing libraries may be packaged in the application or referenced in the source code. So we specify the test scope for them :

<build>
   <dependencies>
     <dependency>
    	<groupId>org.junit.jupiter</groupId>
    	<artifactId>junit-jupiter-engine</artifactId>
    	<version>5.1.0</version>
    	<scope>test</scope>
     </dependency>
   <dependencies>
</build>

If you're coming from a front-end background like me, and are familiar with Grunt and npm, think of it like this:

First you would run, say, npm install grunt-contrib-copy --save-dev. This is like maven's <dependency></dependency>. It downloads the files needed to execute a build task.

Then you would configure the task in Gruntfile.js

copy: {
  main: {
    src: 'src/*',
    dest: 'dest/',
  },
}

This is like maven's <plugin>/<plugin>. You are telling the build tool what to do with the code downloaded by npm/<dependency></dependency>.

Of course this is not an exact analogy, but close enough to help wrap your head around it.

One line answer - basic understanding

Plugin is a tool you use at the execution of your maven build

Dependency means kind of any library which you will use in your code

Plug-ins are used for adding functionalities to Maven itself (like adding eclipse support or SpringBoot support to Maven etc.). Dependencies are needed by your source code to pass any Maven phase (compile or test for example). In case of JUnit since the test code is basically part of your code base and you call JUnit specific commands inside test suites and those commands are not provided by Java SDK therefore JUnit must be present at the time Maven is in the test phase and this is handled by mentioning JUnit as a dependency in your pom.xml file.

Maven at its heart is a plugin execution framework -- as per formal and standard compact definition. To make it more clear, the commands you use like maven-install/clean/compile/build etc for creating/executing jars, which we sometimes manually run too. So, the things which you want to run (or configure or execute) you basically put them in dependency tag of mavens pom and the answer so as to who will run these dependencies (required for environment setup) be the plugins.

        javac (compiler) dependency.java (dependency) 

A plugin is an extension to Maven, something used to produce your artifact (maven-jar-plugin for an example, is used to, you guess it, make a jar out of your compiled classes and resources).

A dependency is a library that is needed by the application you are building, at compile and/or test and/or runtime time.