JavaAgent.java

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.lang.instrument.Instrumentation;
import java.security.ProtectionDomain;

public class JavaAgent {

    //This method is called by the jvm once the agent is attached
    public static void premain(String args, Instrumentation instrumentation) {

        System.out.println("Instrumentatin Agent Added ...");
        instrumentation.addTransformer(new SimpleClassTransformer());
    }

    //This class is added to register as a transformer
    private static class SimpleClassTransformer implements ClassFileTransformer {

        //This method is called for every class (Except special classes) before loaded by the JVM
        public byte[] transform(ClassLoader loader, String className,
                                Class<?> classBeingRedefined, ProtectionDomain protectionDomain,
                                byte[] classfileBuffer) throws IllegalClassFormatException {

            if(className.equals("Main")) {  // We are only interested in the Main class
                try {
                    // Javassist class pool
                    ClassPool classPool = ClassPool.getDefault();
                    CtClass ctMainClass = classPool.get("Main");

                    // get the methods of the Main class as an array
                    CtMethod[] methods = ctMainClass.getMethods();

                    for(int i = 0 ; i < methods.length; i++) {

                        // Iterate through the array and select the methods that we are interested
                        CtMethod selectedMethod = methods[i];
                        if(selectedMethod.getName().equals("addition")
                                || selectedMethod.getName().equals("subtraction")
                                || selectedMethod.getName().equals("multiplication")
                                || selectedMethod.getName().equals("division")) {

                            //change the byte code of that method body
                            //add a local variable of long type
                            selectedMethod.addLocalVariable("nanoTime" , CtClass.longType);
                            //initialize the declared variable with the system time (nano seconds) at the beginning of the method
                            selectedMethod.insertBefore("nanoTime = System.nanoTime();");
                            //print the difference of the time at the end of the method
                            selectedMethod.insertAfter("System.out.println(\"Method execution time in Millis : \" + (1.0*System.nanoTime() - nanoTime)/1000000);");
                        }
                    }

                    return ctMainClass.toBytecode(); // return the instrumented class

                } catch (Exception e) {
                    e.printStackTrace();
                }
            }

            return new byte[0];
        }
    }

}

MANIFEST.MF

Premain-Class: JavaAgent
// new line (please remove these characters and keep the new line blank)

pom.xml

<?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/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.alltechviewer.agent</groupId>
    <artifactId>JavaAgent</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>org.javassist</groupId>
            <artifactId>javassist</artifactId>
            <version>3.18.0-GA</version>
        </dependency>
    </dependencies>
  
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>3.0.2</version>
                <configuration>
                    <archive>
                        <manifestFile>src/main/resources/META-INF/MANIFEST.MF</manifestFile>
                    </archive>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Structure

src
 |------main
 |        '---java
 |        |     '---JavaAgent.java
 |        '---resources
 |                 '---META-INF
 |                       '---MANIFEST.MF
pom.xml