build a simple debugger with jdi to set breakpoints and retrieve the value of a variable

Question

I am looking to build a debugger using java debug interface.
My objective is to set a breakpoint and get the value of a variable.
I found this answer close to what

Answer 1

found this article useful. here is also an good example that would help you.

alternatively, you can check the following project

and here is an example code for you to play on.

/*
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package jdidebugger;

import com.sun.jdi.AbsentInformationException;
import com.sun.jdi.Bootstrap;
import com.sun.jdi.ClassType;
import com.sun.jdi.IncompatibleThreadStateException;
import com.sun.jdi.LocalVariable;
import com.sun.jdi.Location;
import com.sun.jdi.Method;
import com.sun.jdi.StackFrame;
import com.sun.jdi.ThreadReference;
import com.sun.jdi.Value;
import com.sun.jdi.VirtualMachine;
import com.sun.jdi.VirtualMachineManager;
import com.sun.jdi.connect.Connector;
import com.sun.jdi.connect.IllegalConnectorArgumentsException;
import com.sun.jdi.connect.LaunchingConnector;
import com.sun.jdi.connect.VMStartException;
import com.sun.jdi.event.BreakpointEvent;
import com.sun.jdi.event.ClassPrepareEvent;
import com.sun.jdi.event.Event;
import com.sun.jdi.event.EventIterator;
import com.sun.jdi.event.EventQueue;
import com.sun.jdi.event.EventSet;
import com.sun.jdi.request.BreakpointRequest;
import com.sun.jdi.request.ClassPrepareRequest;
import com.sun.jdi.request.EventRequestManager;
import java.io.IOException;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author bonnie
 */
public class JdiDebugger {

    /**
     * @param options
     * @param main
     * @param classPattern
     * @param methodName
     * @param lineNumber
     * @throws java.io.IOException
     * @throws com.sun.jdi.connect.IllegalConnectorArgumentsException
     * @throws com.sun.jdi.connect.VMStartException
     * @throws java.lang.InterruptedException
     * @throws com.sun.jdi.AbsentInformationException
     * @throws com.sun.jdi.IncompatibleThreadStateException
     */
    public static void onMethodExit(String options, String main, String classPattern, String methodName) throws IOException, IllegalConnectorArgumentsException, VMStartException, InterruptedException, AbsentInformationException, IncompatibleThreadStateException {

        // create and launch a virtual machine
        VirtualMachineManager vmm = Bootstrap.virtualMachineManager();
        LaunchingConnector lc = vmm.defaultConnector();
        Map env = lc.defaultArguments();
        env.get("options").setValue(options);
        env.get("main").setValue(main);
        VirtualMachine vm = lc.launch(env);

        // create a class prepare request
        EventRequestManager erm = vm.eventRequestManager();
        ClassPrepareRequest r = erm.createClassPrepareRequest();
        r.addClassFilter(classPattern);
        r.enable();

        EventQueue queue = vm.eventQueue();
        while (true) {
            EventSet eventSet = queue.remove();
            EventIterator it = eventSet.eventIterator();
            while (it.hasNext()) {
                Event event = it.nextEvent();
                if (event instanceof ClassPrepareEvent) {
                    ClassPrepareEvent evt = (ClassPrepareEvent) event;
                    ClassType classType = (ClassType) evt.referenceType();

                    classType.methodsByName(methodName).forEach(new Consumer() {
                        @Override
                        public void accept(Method m) {
                            List locations = null;
                            try {
                                locations = m.allLineLocations();
                            } catch (AbsentInformationException ex) {
                                Logger.getLogger(JdiDebuggerOld.class.getName()).log(Level.SEVERE, null, ex);
                            }
                            // get the last line location of the function and enable the 
                            // break point
                            Location location = locations.get(locations.size() - 1);
                            BreakpointRequest bpReq = erm.createBreakpointRequest(location);
                            bpReq.enable();
                        }
                    });

                }
                if (event instanceof BreakpointEvent) {
                    // disable the breakpoint event
                    event.request().disable();

                    ThreadReference thread = ((BreakpointEvent) event).thread();
                    StackFrame stackFrame = thread.frame(0);

                    // print all the visible variables with the respective values
                    Map visibleVariables = (Map) stackFrame.getValues(stackFrame.visibleVariables());
                    for (Map.Entry entry : visibleVariables.entrySet()) {
                        System.out.println(entry.getKey() + ":" + entry.getValue());
                    }
                }
                vm.resume();
            }
        }
    }
}

and this is how you call the method

new jdiDebugger().onMehtodeExit("-cp ", "", "", "");