How to make gradle generate a valid pom.xml file at the root of a project for maven users?

You can use the gradle maven plugin. This adds the pom convention method to your project, which you can use in a task to generate a pom.xml file, like

task writeNewPom {
    doLast {
        pom {
            project {
                groupId 'org.example'
                artifactId 'test'
                version '1.0.0'
                inceptionYear '2008'
                licenses {
                    license {
                        name 'The Apache Software License, Version 2.0'
                        url 'http://www.apache.org/licenses/LICENSE-2.0.txt'
                        distribution 'repo'
                    }
                }
            }
        }.writeTo("pom.xml")
    }
}

Then you call gradle createPom to generate the pom.xml in the project root. Of all the things in the pom definition, you should really provide groupId, artifactId and version, other thins like licenses are not that important.

You can also look at this example for a project definition with some dependencies, and try running it to see what it produces.

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Here is my build.gradle

apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'idea'
apply plugin: 'maven'

repositories { 
mavenLocal() 
mavenCentral()
}

dependencies {

    compile 'org.springframework:spring-core:4.0.5.RELEASE'
    compile 'org.springframework:spring-webmvc:4.0.5.RELEASE'

    compile 'org.slf4j:slf4j-api:1.7.5'
    runtime 'org.slf4j:slf4j-log4j12:1.7.5'

    testCompile 'org.springframework:spring-test:4.0.5.RELEASE'

    testCompile 'junit:junit:4.11'
    testCompile "org.mockito:mockito-core:1.9.5"
    testCompile "org.hamcrest:hamcrest-library:1.3"

    testCompile 'javax.servlet:javax.servlet-api:3.0.1'
}
test {
  testLogging {
    // Show that tests are run in the command-line output
    events 'started', 'passed'
  }
}
task wrapper(type: Wrapper) { gradleVersion = '1.12' }
task createPom  {
    pom {
        project {
            groupId 'sg.test.spring.web.guide'
            artifactId 'sg-web-initial'
            version '1.0.0-SNAPSHOT'

            inceptionYear '2008'
            licenses {
                license {
                    name 'The Apache Software License, Version 2.0'
                    url 'http://www.apache.org/licenses/LICENSE-2.0.txt'
                    distribution 'repo'
                }
            }
        }
    }.writeTo("pom.xml")
}

You can name the task createPom to anyTaskName as you like. Then just run gradle clean or grale build or simply gradle createPom.

This will generate it as pom.xml in the root of the project. Although you can replace writeTo("pom.xml") with writeTo("<anyDir>/newpom.xml").

The resulting pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"
    xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <modelVersion>4.0.0</modelVersion>
    <groupId>sg.test.spring.web.guide</groupId>
    <artifactId>sg-web-initial</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <inceptionYear>2008</inceptionYear>
    <licenses>
        <license>
            <name>The Apache Software License, Version 2.0</name>
            <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
            <distribution>repo</distribution>
        </license>
    </licenses>
    <dependencies>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>3.0.1</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>4.0.5.RELEASE</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>4.0.5.RELEASE</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-log4j12</artifactId>
            <version>1.7.5</version>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-core</artifactId>
            <version>1.9.5</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>1.7.5</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-test</artifactId>
            <version>4.0.5.RELEASE</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.hamcrest</groupId>
            <artifactId>hamcrest-library</artifactId>
            <version>1.3</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.11</version>
            <scope>test</scope>
        </dependency>
    </dependencies>
</project>