Every Class

This week I wanted to find out which classes are loaded in running environment.

One way of achieving this is replacing the boot classloader and finding a way to keep a reference to every loaded class by every child classloader. This sounded like too much work and I find it very scary to replace system classes with my own which probably don't have the same quality.

Another approach is to write a javaagent. This gives access to the java.lang.instrument.Instrumentation class which in turn has the getAllLoadedClasses() method. Exactly the information I was after and without many nasty side effects or jumping through burning hoops.

Now that I found this information I needed a way to publish it. A colleague of mine pointed me in the direction of JMX. Using JConsole to view which classes are loaded sound great to me so after a little hacking I came up with a javaagent which registers itself as an MBean.

Here's the code:

MANIFEST.MF

Manifest-Version: 1.0
Premain-Class: somecoder.classes.Classes

ClassesMBean.java

package somecoder.classes;

public interface ClassesMBean {
 String[] getClasses();
 long getClassCount();
}

Classes.java

package somecoder.classes;

import java.lang.instrument.Instrumentation;
import java.lang.management.ManagementFactory;

import javax.management.ObjectName;

public final class Classes implements ClassesMBean {
 private Instrumentation inst;

 private Classes(final Instrumentation inst) {
  this.inst = inst;
 }

 public static void premain(final String agentArgs,
   final Instrumentation inst) {
  try {
   final Classes classLister = new Classes(inst);
    ManagementFactory.getPlatformMBeanServer().registerMBean(classLister, new ObjectName("Classes:type=Agent"));
  } catch (Exception e) {
   e.printStackTrace();
  }
 }

 public long getClassCount() {
  return inst.getAllLoadedClasses().length;
 }

 public String[] getClasses() {
  final Class[] classes = inst.getAllLoadedClasses();
  final String[] strings = new String[classes.length];
  for (int i = 0; i < classes.length; i++) {
   strings[i] = classes[i].getName();
  }
  return strings;
 }
}