Redirecting Input in Maven Java Programs | Generated by AI

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Redirecting input for exec:java programs can be a bit tricky because exec:java runs the Java process within the same JVM as Maven. This means you can’t use standard shell redirection operators (<, >) in the same way you would for a separate process.

However, you still have a few options to provide input to your exec:java program:

1. Programmatic Input (Standard System.in)

The most straightforward way is to design your Java program to read from System.in as usual. When you run mvn exec:java, whatever you type in your console will be directed to System.in of your Java program.

Example Java Program (MyInputProgram.java):

package com.example.app;

import java.util.Scanner;

public class MyInputProgram {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter your name: ");
        String name = scanner.nextLine();
        System.out.println("Hello, " + name + "!");

        System.out.print("Enter your age: ");
        int age = scanner.nextInt();
        System.out.println("You are " + age + " years old.");

        scanner.close();
    }
}

Running from the command line:

mvn compile exec:java -Dexec.mainClass="com.example.app.MyInputProgram"

When you run this, Maven will start, and then your program will prompt you for input. You type directly into the console.

2. Using the exec.args Property for Input (for simple cases)

If your program expects very simple, non-interactive input, you might be able to pass it as an argument, and your program then reads that argument instead of System.in. This isn’t true standard input redirection, but it serves a similar purpose for simple data.

Example Java Program (MyArgProgram.java):

package com.example.app;

public class MyArgProgram {
    public static void main(String[] args) {
        if (args.length > 0) {
            String inputData = args[0];
            System.out.println("Received input from argument: " + inputData);
        } else {
            System.out.println("No input argument provided.");
        }
    }
}

Running from the command line:

mvn compile exec:java -Dexec.mainClass="com.example.app.MyArgProgram" -Dexec.args="MyCustomInputString"

Or with spaces:

mvn compile exec:java -Dexec.mainClass="com.example.app.MyArgProgram" -Dexec.args="\"Input with spaces\""

This approach is only suitable if your program can be modified to accept its “input” as command-line arguments.

3. Redirecting Input using Shell Redirection (with exec:exec)

If you absolutely need to redirect input from a file or pipe, you must use the exec:exec goal instead of exec:java. The exec:exec goal spawns a separate process, which allows the shell to handle standard input/output redirection.

pom.xml configuration for exec:exec:

<project>
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.yourcompany</groupId>
    <artifactId>my-maven-app</artifactId>
    <version>1.0-SNAPSHOT</version>

    <build>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>3.2.0</version> <executions>
                    <execution>
                        <id>run-my-java-app</id>
                        <phase>package</phase> <goals>
                            <goal>exec</goal>
                        </goals>
                        <configuration>
                            <executable>java</executable>
                            <arguments>
                                <argument>-classpath</argument>
                                <classpath/> <argument>com.example.app.MyInputProgram</argument>
                                </arguments>
                            <workingDirectory>${project.build.directory}/classes</workingDirectory>
                            </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

    <properties>
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
    </properties>

</project>

Example Java Program (same MyInputProgram.java as above):

package com.example.app;

import java.util.Scanner;

public class MyInputProgram {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter your name: ");
        String name = scanner.nextLine();
        System.out.println("Hello, " + name + "!");

        // Assuming you want to read another line for age after name
        System.out.print("Enter your age: ");
        int age = scanner.nextInt();
        System.out.println("You are " + age + " years old.");

        scanner.close();
    }
}

Input File (input.txt):

Alice
30

Running with exec:exec and input redirection:

First, compile your project:

mvn compile

Then, run the exec:exec goal and redirect input using your shell:

mvn exec:exec@run-my-java-app < input.txt

Explanation:

Important Notes for exec:exec:

In summary:

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