Executing Multiple Main Classes Using Maven

In many Java projects, you may need to run different main classes for various purposes, such as running a tool or sending emails. Maven provides a flexible way to configure and execute multiple main classes using the exec-maven-plugin. Here's how you can achieve this.

Adding Multiple Main Classes in `pom.xml`

To configure multiple main classes, you can add separate executions in the exec-maven-plugin section of your pom.xml. Below is an example configuration:

<build>
    <plugins>
        <!-- Maven Compiler Plugin -->
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.8.0</version>
            <configuration>
                <source>1.8</source>
                <target>1.8</target>
            </configuration>
        </plugin>

        <!-- Exec Maven Plugin -->
        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>exec-maven-plugin</artifactId>
            <version>3.0.0</version>
            <executions>
                <!-- Execution for the First Main Class -->
                <execution>
                    <id>execute-main-class-1</id>
                    <goals>
                        <goal>java</goal>
                    </goals>
                    <configuration>
                        <mainClass>com.example.MainRunner</mainClass>
                        <cleanupDaemonThreads>false</cleanupDaemonThreads>
                    </configuration>
                </execution>

                <!-- Execution for the Second Main Class -->
                <execution>
                    <id>execute-main-class-2</id>
                    <goals>
                        <goal>java</goal>
                    </goals>
                    <configuration>
                        <mainClass>com.example.MailSender</mainClass>
                        <cleanupDaemonThreads>false</cleanupDaemonThreads>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

Running the Main Classes with Maven

You can execute each main class by specifying its configuration or dynamically passing the mainClass at runtime.

1. Execute Preconfigured Main Classes

Use the id defined in the <execution> block:

To execute the first main class (com.example.MainRunner):
```
mvn exec:java@execute-main-class-1
```
To execute the second main class (com.example.MailSender):
```
mvn exec:java@execute-main-class-2
```

2. Override the Main Class Dynamically

Alternatively, you can pass the mainClass as a parameter without modifying the pom.xml:

To run any main class, for example, com.example.MainRunner:

mvn exec:java -Dexec.mainClass="com.example.MainRunner"

For another main class, like com.example.MailSender:

mvn exec:java -Dexec.mainClass="com.example.MailSender"

Key Points to Consider

Separate Executions: Using <execution> blocks allows you to configure each main class independently, which is helpful if each requires specific arguments or settings.
Dynamic Flexibility: Passing mainClass dynamically using -Dexec.mainClass makes the setup more flexible, especially if you frequently add or modify classes to execute.
Resource Management: Setting cleanupDaemonThreads to false ensures that any background threads started by your application are not forcibly terminated by Maven after execution.

Use Case Example

Imagine you have two main classes:

MainRunner: A utility tool to process data.
MailSender: A program to send email notifications.

By using the above Maven setup, you can easily execute each class separately using a Maven command, either preconfigured or dynamic, as required.

Conclusion

This approach simplifies managing and executing multiple main classes in your project. It provides the flexibility to predefine executions or run any class dynamically. Whether you're running tools, performing tests, or sending notifications, this setup can streamline your development workflow.

Let me know if you need further clarification or enhancements!

Executing Multiple Main Classes Using Maven