Java Agent example

Launcher.java

package me.lotabout;

import com.sun.tools.attach.AgentInitializationException;
import com.sun.tools.attach.AgentLoadException;
import com.sun.tools.attach.AttachNotSupportedException;
import com.sun.tools.attach.VirtualMachine;
import java.io.IOException;
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.Instrumentation;
import java.lang.instrument.UnmodifiableClassException;
import java.security.ProtectionDomain;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.tree.ClassNode;
import org.objectweb.asm.tree.FieldInsnNode;
import org.objectweb.asm.tree.InsnList;
import org.objectweb.asm.tree.LdcInsnNode;
import org.objectweb.asm.tree.MethodInsnNode;

public class Launcher {

  public static void main(String[] args)
      throws IOException, AttachNotSupportedException, AgentLoadException, AgentInitializationException {
    String pid = args[0];
    String path = Launcher.class.getProtectionDomain().getCodeSource().getLocation().getPath();
    VirtualMachine vm = VirtualMachine.attach(pid);
    try {
      vm.loadAgent(path);
    } finally {
      vm.detach();
    }
  }

  public static void premain(String agentArgs, Instrumentation inst)
      throws UnmodifiableClassException {
    inst.addTransformer(new MyTransformer(), true);
    for (var clazz : inst.getAllLoadedClasses()) {
      if (clazz.getName().startsWith("me.lotabout.Main")) {
        inst.retransformClasses(clazz);
      }
    }
  }

  public static void agentmain(String agentArgs, Instrumentation inst)
      throws UnmodifiableClassException {
    premain(agentArgs, inst);
  }

  private static class MyTransformer implements ClassFileTransformer {

    @Override
    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
        ProtectionDomain protectionDomain, byte[] classfileBuffer) {
      if (!className.startsWith("me/lotabout/Main")) {
        return classfileBuffer;
      }

      System.out.println("transforming class: " + className);
      ClassNode cn = new ClassNode(Opcodes.ASM4);
      ClassReader cr = new ClassReader(classfileBuffer);
      cr.accept(cn, 0);

      for (var method : cn.methods) {
        System.out.println("patching Method: " + method.name);
        var list = new InsnList();
        list.add(new FieldInsnNode(Opcodes.GETSTATIC, "java/lang/System", "out",
            "Ljava/io/PrintStream;"));
        list.add(new LdcInsnNode(">> calling Method: " + method.name));
        list.add(new MethodInsnNode(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println",
            "(Ljava/lang/String;)V", false));
        method.instructions.insert(list);
      }

      ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
      cn.accept(cw);
      return cw.toByteArray();
    }
  }
}

MANIFEST.MF

Main-Class: me.lotabout.Launcher
Premain-Class: me.lotabout.Launcher
Agent-Class: me.lotabout.Launcher
Can-Redefine-Classes: true
Can-Retransform-Classes: true
Can-Set-Native-Method-Prefix: true

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>me.lotabout</groupId>
  <artifactId>my-agent</artifactId>
  <version>1.0-SNAPSHOT</version>

  <properties>
    <maven.compiler.source>11</maven.compiler.source>
    <maven.compiler.target>11</maven.compiler.target>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    <dependency>
      <groupId>org.ow2.asm</groupId>
      <artifactId>asm</artifactId>
      <version>9.4</version>
    </dependency>
    <dependency>
      <groupId>org.ow2.asm</groupId>
      <artifactId>asm-tree</artifactId>
      <version>9.4</version>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-assembly-plugin</artifactId>
        <version>3.6.0</version>
        <configuration>
          <descriptorRefs>
            <descriptorRef>jar-with-dependencies</descriptorRef>
          </descriptorRefs>
          <archive>
            <manifestFile>src/main/resources/META-INF/MANIFEST.MF</manifestFile>
          </archive>
        </configuration>
        <executions>
          <execution>
            <phase>package</phase>
            <goals>
              <goal>single</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
</project>