Debugging any reasonably sized program with the following JDI client results, eventually, in a hang.

import com.sun.jdi.*;
import com.sun.jdi.connect.*;
import com.sun.jdi.request.*;
import com.sun.jdi.event.*;
import java.util.*;
import java.io.IOException;


class Dbg {

  private String mainClass = null;

  // possible values for Connector's startMode parameter
  private static final String sInterrupted = "interrupted";
  private static final String sRunning = "running";

  private VirtualMachineManager vmm = Bootstrap.virtualMachineManager();
  private LaunchingConnector conn = vmm.defaultConnector();
  private Map args = conn.defaultArguments();
  private VirtualMachine vm = null;

  private void printArgs(Map args) {
    Iterator is = args.values().iterator();
    while (is.hasNext()) {
      Connector.Argument arg = (Connector.Argument) is.next();
      System.out.println(" Name: " + arg.name() );
      System.out.println(" Must Specify: " + arg.mustSpecify() );
      System.out.println(" Description: " + arg.description() );
      System.out.println(" Value: " + arg.value() );
      System.out.println("======================================");
    }
  }

  private void setArgs() {
    Connector.Argument main = (Connector.Argument)
args.get("main");
    Connector.Argument startMode = (Connector.Argument)
args.get("startMode");

    if ((main == null) || (startMode == null)) {
      Dbg.FATAL("Unable to understand default args...");
    }

    main.setValue(mainClass);
    startMode.setValue(sInterrupted);
    //startMode.setValue(sRunning);
  }

  public static void FATAL(String s) {
    System.out.println(s);
    System.exit(1);
  }

  void go() {
    setArgs();
    printArgs(args);

    try {
      vm = conn.launch(args);
    }
    catch (IllegalConnectorArgumentsException e) {
      Dbg.FATAL("Error: " + e);
    }
    catch (VMStartException e) {
      Dbg.FATAL("Error: " + e);
    }
    catch (IOException e) {
      Dbg.FATAL("Error: " + e);
    }

    prepareStep();
    while (true) {
      doStep();
    }
  }

  void prepareStep() {
    List threads = vm.allThreads();
    Iterator ts = threads.iterator();
    while (ts.hasNext()) {
      ThreadReference t = (ThreadReference) ts.next();
      requestStep(t);
    }
  }

  void requestStep(ThreadReference t) {
    EventRequestManager erm = vm.eventRequestManager();
    StepRequest request =
erm.createStepRequest(t,StepRequest.STEP_LINE,StepRequest.STEP_INTO);
    //request.addCountFilter(1);
    request.enable();
  }

  void doStep() {
    vm.resume();
    EventQueue eq = vm.eventQueue();
    EventSet es = null;
    try {
      es = eq.remove();
    } catch (InterruptedException e) {
      Dbg.FATAL("Interrupted: " + e);
    }

    EventIterator ei = es.eventIterator();

    while (ei.hasNext()) {
      Event event = ei.nextEvent();

      if (event instanceof StepEvent) {
        StepEvent se = (StepEvent) event;
        Location l = se.location();
        StepRequest request = (StepRequest) se.request();
        String src = "<unknown>";
        try { src = l.sourceName(); } catch (AbsentInformationException e) {}

        int n = l.lineNumber();
        String name = l.method().name();

        System.out.println("File: " + src + " Line: " + n + " Method: " +
name);
        
        // requeue the request
        //requestStep(request.thread());
      } else {
          System.out.println(event);
      }
    }
  }

  Dbg(String mainClass) {
    this.mainClass = mainClass;
  }

  ////////////////////////////////////////////////////////

  public static void usage() {
      System.out.println("Usage:");
      System.out.println(" ");
      System.out.println(" java Dbg <classname> <args>");
      System.out.println(" ");
      System.out.println(" ");
  }

  public static void main(String[] args) {
    if (args.length < 1) {
      usage();
    }

    String mainClass = args[0];
    System.out.println("class = " + mainClass);
    Dbg dbg = new Dbg(mainClass);
    dbg.go();
  }

}