Instrumentation and Monitoring Sample

package name.abishkar.javaagent;

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

import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class MyClassFileTransformer implements ClassFileTransformer, Opcodes {

    static final Logger logger = LoggerFactory.getLogger(MyClassFileTransformer.class);

    @Override
    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
            ProtectionDomain protectionDomain, byte[] classfileBuffer)
            throws IllegalClassFormatException {
        logger.info("class file transformer invoked for className: {}", className);

        if (className.equals("name/abishkar/user/MyUser")) {

            ClassWriter cw = new ClassWriter(0);
            MethodVisitor mv;

            cw.visit(V1_6, ACC_PUBLIC + ACC_SUPER, "name/abishkar/user/MyUser", null,
                    "java/lang/Object", null);

            cw.visitSource(null, null);

            {
                mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
                mv.visitCode();
                Label l0 = new Label();
                mv.visitLabel(l0);
                mv.visitLineNumber(3, l0);
                mv.visitVarInsn(ALOAD, 0);
                mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
                mv.visitInsn(RETURN);
                Label l1 = new Label();
                mv.visitLabel(l1);
                mv.visitLocalVariable("this", "Lname/abishkar/user/MyUser;", null, l0, l1, 0);
                mv.visitMaxs(1, 1);
                mv.visitEnd();
            }
            {
                mv = cw.visitMethod(ACC_PUBLIC, "getName", "()Ljava/lang/String;", null, null);
                mv.visitCode();
                Label l0 = new Label();
                mv.visitLabel(l0);
                mv.visitLineNumber(6, l0);
                mv.visitLdcInsn("bar");
                mv.visitInsn(ARETURN);
                Label l1 = new Label();
                mv.visitLabel(l1);
                mv.visitLocalVariable("this", "Lname/abishkar/user/MyUser;", null, l0, l1, 0);
                mv.visitMaxs(1, 1);
                mv.visitEnd();
            }
            cw.visitEnd();

            return cw.toByteArray();
        }

        return classfileBuffer;
    }

}


package name.abishkar.javaagent;

import java.lang.instrument.Instrumentation;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class MyJavaAgent {

    static final Logger logger = LoggerFactory.getLogger(MyJavaAgent.class);

    private static Instrumentation instrumentation;

    /**
     * JVM hook to statically load the javaagent at startup.
     *
     * After the Java Virtual Machine (JVM) has initialized, the premain method
     * will be called. Then the real application main method will be called.
     *
     * @param args
     * @param inst
     * @throws Exception
     */
    public static void premain(String args, Instrumentation inst) throws Exception {
        logger.info("premain method invoked with args: {} and inst: {}", args, inst);
        instrumentation = inst;
        instrumentation.addTransformer(new MyClassFileTransformer());
    }

    /**
     * JVM hook to dynamically load javaagent at runtime.
     *
     * The agent class may have an agentmain method for use when the agent is
     * started after VM startup.
     *
     * @param args
     * @param inst
     * @throws Exception
     */
    public static void agentmain(String args, Instrumentation inst) throws Exception {
        logger.info("agentmain method invoked with args: {} and inst: {}", args, inst);
        instrumentation = inst;
        instrumentation.addTransformer(new MyClassFileTransformer());
    }

    /**
     * Programmatic hook to dynamically load javaagent at runtime.
     */
    public static void initialize() {
        if (instrumentation == null) {
            MyJavaAgentLoader.loadAgent();
        }
    }

}


package name.abishkar.javaagent;

import java.lang.management.ManagementFactory;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.sun.tools.attach.VirtualMachine;

public class MyJavaAgentLoader {

    static final Logger logger = LoggerFactory.getLogger(MyJavaAgentLoader.class);

    private static final String jarFilePath = "/home/abishkar/.m2/repository/"
            + "javaagent-examples/javaagent-examples/1.0/"
            + "javaagent-examples-1.0-jar-with-dependencies.jar";

    public static void loadAgent() {
        logger.info("dynamically loading javaagent");
        String nameOfRunningVM = ManagementFactory.getRuntimeMXBean().getName();
        int p = nameOfRunningVM.indexOf('@');
        String pid = nameOfRunningVM.substring(0, p);

        try {
            VirtualMachine vm = VirtualMachine.attach(pid);
            vm.loadAgent(jarFilePath, "");
            vm.detach();
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

}


package name.abishkar.javaagent;

import java.util.Arrays;

import name.abishkar.user.MyUser;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class MyMainClass {

    static final Logger logger = LoggerFactory.getLogger(MyMainClass.class);

    static {
        MyJavaAgent.initialize();
    }

    /**
     * Main method.
     *
     * @param args
     */
    public static void main(String[] args) {
        logger.info("main method invoked with args: {}", Arrays.asList(args));
        logger.info("userName: {}", new MyUser().getName());
    }

}