Building Java projects with Maven.

In last post we saw what is maven and how do we install it. Now lets create a java project and use maven to build and then  test by running it.

What you will need?

1. Maven installed
2. JDK(6 or higher)
3. IDE(Eclipse/Intellij IDEA) or a text editor(gedit/vim)

Setting up the project

Projects that are to be built by maven must follow a particular directory pattern. For detailed directory structure you can visit their documentation on directory structure.

For our example we are going to create a HelloWorld sample inside a package called hello.

So our directory structure will be something like

So go ahead and create such structure. You can use following command

mkdir -p src/main/java/hello

Now inside hello directory create two java files - HelloWorld.java  and Greeter.java.

Code for them is provided below. For src/main/java/hello/HelloWorld.java

package hello;

public class HelloWorld {
    public static void main(String[] args) {
        Greeter greeter = new Greeter();
        System.out.println(greeter.sayHello());
    }
}

and for src/main/java/hello/Greeter.java

package hello;

public class Greeter {
    public String sayHello() {
        return "Hello world!";
    }
}

Now that Maven is installed, you need to create a Maven project definition. Maven projects are defined with an XML file named pom.xml. Among other things, this file gives the project’s name, version, and dependencies that it has on external libraries.

Note :  This file must be in the project root directory which means in the folder which has src folder.

Create a file named pom.xml at the root of the project and give it the following contents:

<?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/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>HelloWorld</groupId>
    <artifactId>gs-maven-initial</artifactId>
    <version>0.1.0</version>
    <packaging>jar</packaging>
</project>

Now you can build your java project using 

mvn compile

This will download your dependencies and compile you java files. Corresponding .class files will be created in a target folder in the projects root folder.

To make the jar file of your project you can use

mvn package

This will create a jar file in the target folder. Now you can run your code with

java -jar PathToYourJarFile

Maven Build lifecycle

compile and package were some of the initial phases of Maven build lifecycle. All of them are listed in the following diagram

For more details on the maven build life cycle phases you can visit their official site.

I am getting Can't execute jar- file: “no main manifest attribute”. What should i do?

That is because you have not told maven while packaging how do you built the project. You need to provide build information withing <build> and </build> tags in the build file. You can add following tag between <project> and </project>.

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <version>2.1</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                            <transformers>
                                <transformer
                                    implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                                    <mainClass>hello.HelloWorld</mainClass>
                                </transformer>
                            </transformers>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

and you are done. By this you essentially say what is your main class so that while executing java knows the entry point from that jar file.

I want to use IDE instead of text editor. How do I import maven projects to my IDE?

Even this is very simple. You can run the following commands in the projects root directory -

• mvn eclipse:eclipse  (For Eclispe IDE)
• mvn idea:idea  (For intellij IDEA IDE)

For example idea command will generate all the required files like .ipr(project file), .iws(workspace file) etc. You can open the file by simply opening this .ipr file in IDEA.

Note : As per Mavens official site mvn idea:idea is obsolete. You can directly do File -> Import project and select pom.xml file.

I am getting "Package name does not correspond to the file path" error after importing project in my IDE.

This is because dash(-) is not allowed in package name and your -DgroupId contained  dash(-) which was used as package name. You can refactor and replace dash(-) with underscores(_) which is legal in a package name.

 Useful Links

• Can't execute jar- file: “no main manifest attribute”
• Building Java Projects with Maven
• Maven + Spring hello world example
• Package name does not correspond to the file path