／var／logmarcus chiu

Java - Java Agent (Creating Agent)

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Since a java agent makes use of the Java Instrumentation API, let’s see some of the most used methods:

  • addTransformer – adds a transformer to the instrumentation engine
  • getAllLoadedClasses – returns an array of all classes currently loaded by the JVM
  • retransformClasses – facilitates the instrumentation of already loaded classes by adding byte-code
  • removeTransformer – unregisters the supplied transformer
  • redefineClasses – redefine the supplied set of classes using the supplied class files, meaning that the class will be fully replaced, not modified as with retransformClasses

Creating an Agent

every Java agent needs at least one of:

  • premain - used to statically load an agent into JVM
  • agentmain - used to dynamically load an agent into JVM

Example

public class MyInstrumentationAgent {
    private static Logger LOGGER = LoggerFactory.getLogger(MyInstrumentationAgent.class);

    public static void premain(String agentArgs, Instrumentation inst) {
        LOGGER.info("[Agent] In premain method");
        String className = "com.baeldung.instrumentation.application.MyAtm";
        transformClass(className,inst);
    }

    public static void agentmain(String agentArgs, Instrumentation inst) {
        LOGGER.info("[Agent] In agentmain method");
        String className = "com.baeldung.instrumentation.application.MyAtm";
        transformClass(className,inst);
    }

    private static void transformClass(String className, Instrumentation instrumentation) {
        Class<?> targetCls = null;
        ClassLoader targetClassLoader = null;

        // see if we can get the class using forName
        try {
            targetCls = Class.forName(className);
            targetClassLoader = targetCls.getClassLoader();
            transform(targetCls, targetClassLoader, instrumentation);
            return;
        } catch (Exception ex) {
            LOGGER.error("Class [{}] not found with Class.forName");
        }

        // otherwise iterate all loaded classes and find what we want
        for(Class<?> clazz: instrumentation.getAllLoadedClasses()) {
            if(clazz.getName().equals(className)) {
                targetCls = clazz;
                targetClassLoader = targetCls.getClassLoader();
                transform(targetCls, targetClassLoader, instrumentation);
                return;
            }
        }

        throw new RuntimeException("Failed to find class [" + className + "]");
    }

    private static void transform(Class<?> clazz, ClassLoader classLoader, Instrumentation instrumentation) {
        AtmTransformer dt = new AtmTransformer(clazz.getName(), classLoader);
        instrumentation.addTransformer(dt, true);

        try {
            instrumentation.retransformClasses(clazz);
        } catch (Exception ex) {
            throw new RuntimeException("Transform failed for class: [" + clazz.getName() + "]", ex);
        }
    }
}

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