# Getting Started with Java Agent

23 Nov 2020  Sergio Martin Rubio  2 mins read.

Java Agent is an instrumentation API which is usually overlooked and allows you to modify bytecode at runtime. This means that is a powerful feature but “with great power there must also come great responsibility”.

The Java Agent feature was added as part of JDK 1.5 and has some use cases like metrics generation, for testing, in Aspect Oriented Programming and in any other scenario that requires modifying the bytecode at runtime.

A Java Agent is simply a class with a premain method that runs when the JVM is launched. We will show how to create a simple Java Agent in the next section.

## Implementation

1. Create a class with a premain method like this:
public class MyJavaAgent {

public static void premain(String agentArgs, Instrumentation inst) {
System.out.println("This is a Java Agent");
}
}

1. Create a resources/META-INF/MANIFEST.MF file where you specify the location of the agent class.
Manifest-Version: 1.0
Main-Class: com.sergiomartinrubio.javaagentexample.Main
Premain-Class: com.sergiomartinrubio.javaagentexample.MyJavaAgent


The MANIFEST.MF is a file that contains information about the files included in the .jar file. i.e. You can set up the entry point of the application with Main-Class or a Java agent class with Premain-Class.

1. Compile the application. You can use Maven with maven-jar-plugin in your pom.xml file.
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-jar-plugin</artifactId>
<version>3.2.0</version>
<configuration>
<archive>
<manifestFile>src/main/resources/META-INF/MANIFEST.MF</manifestFile>
</archive>
</configuration>
</plugin>
</plugins>
</build>

1. Run the generated Jar file with -javaagent that points to the .jar file that contains the Java Agent (in this case is the same Jar file).
java -javaagent:/Users/sergiomartinrubio/workspace/java-agent-example/target/java-agent-example-1.0-SNAPSHOT.jar -jar target/java-agent-example-1.0-SNAPSHOT.jar


It is important that the -javaagent parameter goes before the -jar parameter.