Relationship between .iml file and pom.xml file - maven

Question

Each time I create a Maven project, there is a .iml file along with a pom.xml file.
What exactly is their relationship?

Answer 1

The pom.xml is used by maven to resolve your project's dependencies, which plugins to execute and a lot of other things. From the maven website:
A Project Object Model or POM is the fundamental unit of work in Maven. It is an XML file that contains information about the project and configuration details used by Maven to build the project.
The .iml file on the other hand is part of IntelliJ's own project structure. The short version is that it declares the libraries (e.g. jars) that are visible only to the module, and not the rest of the project or other projects. It's an xml file containing a library entry for each artifact declared in your pom.xml, along with its scope (e.g. TEST or COMPILE). For example:
<?xml version="1.0" encoding="UTF-8"?>
<module org.jetbrains.idea.maven.project.MavenProjectsManager.isMavenModule="true" type="JAVA_MODULE" version="4">
<component name="NewModuleRootManager" LANGUAGE_LEVEL="JDK_1_8">
<output url="file://$MODULE_DIR$/target/classes" />
<output-test url="file://$MODULE_DIR$/target/test-classes" />
<content url="file://$MODULE_DIR$">
<sourceFolder url="file://$MODULE_DIR$/src/main/java" isTestSource="false" />
<sourceFolder url="file://$MODULE_DIR$/src/test/java" isTestSource="true" />
<excludeFolder url="file://$MODULE_DIR$/target" />
</content>
<orderEntry type="inheritedJdk" />
<orderEntry type="sourceFolder" forTests="false" />
<orderEntry type="library" scope="COMPILE" name="Maven: com.google.guava:guava:18.0" level="project" />
<orderEntry type="library" scope="TEST" name="Maven: org.hamcrest:hamcrest-library:1.3" level="project" />
<orderEntry type="library" scope="TEST" name="Maven: org.hamcrest:hamcrest-core:1.3" level="project" />
<orderEntry type="library" scope="TEST" name="Maven: junit:junit:4.12" level="project" />
</component>
</module>
I assume IntelliJ keeps its own file format so it can read the project faster, regardless of which build system the project is using (e.g. maven vs. gradle).

Answer 2

IntelliJ idea doesn't understand the maven project model (POM.xml) itself. When creating or importing maven project in idea. It also creating its own project structure, maven dependencies , module details etc etc.. basically required project metadata in the format it can understand and use it internally for it actions. These metadata stored in .iml file and .idea project directory.
Main advantage of keeping it's own structure would provide capacity to run faster and manage project efficiently. I hope this give some explanation on the relationship between these two files.
Sample pom.xml
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>test</groupId>
<artifactId>test</artifactId>
<version>1.0-SNAPSHOT</version>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</build>
<dependencies>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>${log4j.version}</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
<version>${org.slf4j.version}</version>
</dependency>
<dependency>
<groupId>org.easymock</groupId>
<artifactId>easymock</artifactId>
<version>2.5.2</version>
<optional>true</optional>
</dependency>
<dependency>
<groupId>org.jmock</groupId>
<artifactId>jmock</artifactId>
<version>${jmock.version}</version>
<optional>true</optional>
</dependency>
<dependency>
<groupId>org.jmock</groupId>
<artifactId>jmock-junit4</artifactId>
<version>${jmock.version}</version>
<optional>true</optional>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>${junit.version}</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>junitperf</groupId>
<artifactId>junitperf</artifactId>
<version>1.8</version>
</dependency>
</dependencies>
<properties>
<jmock.version>2.5.1</jmock.version>
<junit.version>4.6</junit.version>
<log4j.version>1.2.14</log4j.version>
<org.slf4j.version>1.5.2</org.slf4j.version>
</properties>
</project>
Relavent IDEA .iml file
<?xml version="1.0" encoding="UTF-8"?>
<module org.jetbrains.idea.maven.project.MavenProjectsManager.isMavenModule="true" type="JAVA_MODULE" version="4">
<component name="NewModuleRootManager" LANGUAGE_LEVEL="JDK_1_8">
<output url="file://$MODULE_DIR$/target/classes" />
<output-test url="file://$MODULE_DIR$/target/test-classes" />
<content url="file://$MODULE_DIR$">
<sourceFolder url="file://$MODULE_DIR$/src/main/java" isTestSource="false" />
<sourceFolder url="file://$MODULE_DIR$/src/main/resources" type="java-resource" />
<sourceFolder url="file://$MODULE_DIR$/src/test/java" isTestSource="true" />
<excludeFolder url="file://$MODULE_DIR$/target" />
</content>
<orderEntry type="inheritedJdk" />
<orderEntry type="sourceFolder" forTests="false" />
<orderEntry type="library" name="Maven: log4j:log4j:1.2.14" level="project" />
<orderEntry type="library" name="Maven: org.slf4j:slf4j-log4j12:1.5.2" level="project" />
<orderEntry type="library" name="Maven: org.slf4j:slf4j-api:1.5.2" level="project" />
<orderEntry type="library" name="Maven: org.easymock:easymock:2.5.2" level="project" />
<orderEntry type="library" name="Maven: org.jmock:jmock:2.5.1" level="project" />
<orderEntry type="library" name="Maven: org.hamcrest:hamcrest-core:1.1" level="project" />
<orderEntry type="library" name="Maven: org.hamcrest:hamcrest-library:1.1" level="project" />
<orderEntry type="library" name="Maven: org.jmock:jmock-junit4:2.5.1" level="project" />
<orderEntry type="library" name="Maven: junit:junit-dep:4.4" level="project" />
<orderEntry type="library" scope="TEST" name="Maven: junit:junit:4.6" level="project" />
<orderEntry type="library" name="Maven: junitperf:junitperf:1.8" level="project" />
</component>
</module>

Answer 3

The .iml file is generated using the pom.xml.
When you open a project in IntelliJ for the first time, IntelliJ downloads all the required dependencies using the pom.xml as reference. There is a one-way sync between the pom.xml and the .iml file.
When you try to run the project, the .iml file and .idea directory are created. These files and directories allow IntelliJ to run faster.