tools/jdi/EventSetImpl.java

/*
 * Copyright (c) 1998, 2020, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package com.sun.tools.jdi;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Spliterator;
import java.util.Spliterators;

import com.sun.jdi.Field;
import com.sun.jdi.InternalException;
import com.sun.jdi.Locatable;
import com.sun.jdi.Location;
import com.sun.jdi.Method;
import com.sun.jdi.ObjectReference;
import com.sun.jdi.ReferenceType;
import com.sun.jdi.ThreadReference;
import com.sun.jdi.VMDisconnectedException;
import com.sun.jdi.Value;
import com.sun.jdi.VirtualMachine;
import com.sun.jdi.event.AccessWatchpointEvent;
import com.sun.jdi.event.BreakpointEvent;
import com.sun.jdi.event.ClassPrepareEvent;
import com.sun.jdi.event.ClassUnloadEvent;
import com.sun.jdi.event.Event;
import com.sun.jdi.event.EventIterator;
import com.sun.jdi.event.EventSet;
import com.sun.jdi.event.ExceptionEvent;
import com.sun.jdi.event.MethodEntryEvent;
import com.sun.jdi.event.MethodExitEvent;
import com.sun.jdi.event.ModificationWatchpointEvent;
import com.sun.jdi.event.MonitorContendedEnterEvent;
import com.sun.jdi.event.MonitorContendedEnteredEvent;
import com.sun.jdi.event.MonitorWaitEvent;
import com.sun.jdi.event.MonitorWaitedEvent;
import com.sun.jdi.event.StepEvent;
import com.sun.jdi.event.ThreadDeathEvent;
import com.sun.jdi.event.ThreadStartEvent;
import com.sun.jdi.event.VMDeathEvent;
import com.sun.jdi.event.VMDisconnectEvent;
import com.sun.jdi.event.VMStartEvent;
import com.sun.jdi.event.WatchpointEvent;
import com.sun.jdi.request.EventRequest;

enum EventDestination {UNKNOWN_EVENT, INTERNAL_EVENT, CLIENT_EVENT};

/*
 * An EventSet is normally created by the transport reader thread when
 * it reads a JDWP Composite command.  The constructor doesn't unpack
 * the events contained in the Composite command and create EventImpls
 * for them because that process might involve calling back into the back-end
 * which should not be done by the transport reader thread.  Instead,
 * the raw bytes of the packet are read and stored in the EventSet.
 * The EventSet is then added to each EventQueue. When an EventSet is
 * removed from an EventQueue, the EventSetImpl.build() method is called.
 * This method reads the packet bytes and creates the actual EventImpl objects.
 * build() also filters out events for our internal handler and puts them in
 * their own EventSet.  This means that the EventImpls that are in the EventSet
 * that is on the queues are all for client requests.
 */
public class EventSetImpl extends ArrayList<Event> implements EventSet {
    private static final long serialVersionUID = -4857338819787924570L;
    private VirtualMachineImpl vm; // we implement Mirror
    private Packet pkt;
    private byte suspendPolicy;
    private EventSetImpl internalEventSet;

    public String toString() {
        String string = "event set, policy:" + suspendPolicy +
                        ", count:" + this.size() + " = {";
        boolean first = true;
        for (Event event : this) {
            if (!first) {
                string += ", ";
            }
            string += event.toString();
            first = false;
        }
        string += "}";
        return string;
    }

    abstract class EventImpl extends MirrorImpl implements Event {

        private final byte eventCmd;
        private final int requestID;
        // This is set only for client requests, not internal requests.
        private final EventRequest request;

        /**
         * Constructor for events.
         */
        protected EventImpl(JDWP.Event.Composite.Events.EventsCommon evt,
                            int requestID) {
            super(EventSetImpl.this.vm);
            this.eventCmd = evt.eventKind();
            this.requestID = requestID;
            EventRequestManagerImpl ermi = EventSetImpl.this.
                vm.eventRequestManagerImpl();
            this.request =  ermi.request(eventCmd, requestID);
        }

        /*
         * Override superclass back to default equality
         */
        public boolean equals(Object obj) {
            return this == obj;
        }

        public int hashCode() {
            return System.identityHashCode(this);
        }

        /**
         * Constructor for VM disconnected events.
         */
        protected EventImpl(byte eventCmd) {
            super(EventSetImpl.this.vm);
            this.eventCmd = eventCmd;
            this.requestID = 0;
            this.request = null;
        }

        public EventRequest request() {
            return request;
        }

        int requestID() {
            return requestID;
        }

        EventDestination destination() {
            /*
             * We need to decide if this event is for
             * 1. an internal request
             * 2. a client request that is no longer available, ie
             *    it has been deleted, or disabled and re-enabled
             *    which gives it a new ID.
             * 3. a current client request that is disabled
             * 4. a current enabled client request.
             *
             * We will filter this set into a set
             * that contains only 1s for our internal queue
             * and a set that contains only 4s for our client queue.
             * If we get an EventSet that contains only 2 and 3
             * then we have to resume it if it is not SUSPEND_NONE
             * because no one else will.
             */
            if (requestID == 0) {
                /* An unsolicited event.  These have traditionally
                 * been treated as client events.
                 */
                return EventDestination.CLIENT_EVENT;
            }

            // Is this an event for a current client request?
            if (request == null) {
                // Nope.  Is it an event for an internal request?
                EventRequestManagerImpl ermi = this.vm.getInternalEventRequestManager();
                if (ermi.request(eventCmd, requestID) != null) {
                    // Yep
                    return EventDestination.INTERNAL_EVENT;
                }
                return EventDestination.UNKNOWN_EVENT;
            }

            // We found a client request
            if (request.isEnabled()) {
                return EventDestination.CLIENT_EVENT;
            }
            return EventDestination.UNKNOWN_EVENT;
        }

        abstract String eventName();

        public String toString() {
            return eventName();
        }

    }

    abstract class ThreadedEventImpl extends EventImpl {
        private ThreadReference thread;

        ThreadedEventImpl(JDWP.Event.Composite.Events.EventsCommon evt,
                          int requestID, ThreadReference thread) {
            super(evt, requestID);
            this.thread = thread;
        }

        public ThreadReference thread() {
            return thread;
        }

        public String toString() {
            return eventName() + " in thread " + thread.name();
        }
    }

    abstract class LocatableEventImpl extends ThreadedEventImpl
                                      implements Locatable {
        private Location location;

        LocatableEventImpl(JDWP.Event.Composite.Events.EventsCommon evt,
                           int requestID,
                           ThreadReference thread, Location location) {
            super(evt, requestID, thread);
            this.location = location;
        }

        public Location location() {
            return location;
        }

        /**
         * For MethodEntry and MethodExit
         */
        public Method method() {
            return location.method();
        }

        public String toString() {
            return eventName() + "@" +
                   ((location() == null) ? " null" : location().toString()) +
                   " in thread " + thread().name();
        }
    }

    class BreakpointEventImpl extends LocatableEventImpl
                              implements BreakpointEvent {
        BreakpointEventImpl(JDWP.Event.Composite.Events.Breakpoint evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
        }

        String eventName() {
            return "BreakpointEvent";
        }
    }

    class StepEventImpl extends LocatableEventImpl implements StepEvent {
        StepEventImpl(JDWP.Event.Composite.Events.SingleStep evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
        }

        String eventName() {
            return "StepEvent";
        }
    }

    class MethodEntryEventImpl extends LocatableEventImpl
                               implements MethodEntryEvent {
        MethodEntryEventImpl(JDWP.Event.Composite.Events.MethodEntry evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
        }

        String eventName() {
            return "MethodEntryEvent";
        }
    }

    class MethodExitEventImpl extends LocatableEventImpl
                            implements MethodExitEvent {
        private Value returnVal = null;

        MethodExitEventImpl(JDWP.Event.Composite.Events.MethodExit evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
        }

        MethodExitEventImpl(JDWP.Event.Composite.Events.MethodExitWithReturnValue evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            returnVal = evt.value;
        }

        String eventName() {
            return "MethodExitEvent";
        }

        public Value returnValue() {
            if (!this.vm.canGetMethodReturnValues()) {
                throw new UnsupportedOperationException(
                "target does not support return values in MethodExit events");
            }
            return returnVal;
        }

    }

    class MonitorContendedEnterEventImpl extends LocatableEventImpl
                            implements MonitorContendedEnterEvent {
        private ObjectReference monitor = null;

        MonitorContendedEnterEventImpl(JDWP.Event.Composite.Events.MonitorContendedEnter evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            this.monitor = evt.object;
        }

        String eventName() {
            return "MonitorContendedEnter";
        }

        public ObjectReference  monitor() {
            return monitor;
        };

    }

    class MonitorContendedEnteredEventImpl extends LocatableEventImpl
                            implements MonitorContendedEnteredEvent {
        private ObjectReference monitor = null;

        MonitorContendedEnteredEventImpl(JDWP.Event.Composite.Events.MonitorContendedEntered evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            this.monitor = evt.object;
        }

        String eventName() {
            return "MonitorContendedEntered";
        }

        public ObjectReference  monitor() {
            return monitor;
        };

    }

    class MonitorWaitEventImpl extends LocatableEventImpl
                            implements MonitorWaitEvent {
        private ObjectReference monitor = null;
        private long timeout;

        MonitorWaitEventImpl(JDWP.Event.Composite.Events.MonitorWait evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            this.monitor = evt.object;
            this.timeout = evt.timeout;
        }

        String eventName() {
            return "MonitorWait";
        }

        public ObjectReference  monitor() {
            return monitor;
        };

        public long timeout() {
            return timeout;
        }
    }

    class MonitorWaitedEventImpl extends LocatableEventImpl
                            implements MonitorWaitedEvent {
        private ObjectReference monitor = null;
        private boolean timed_out;

        MonitorWaitedEventImpl(JDWP.Event.Composite.Events.MonitorWaited evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            this.monitor = evt.object;
            this.timed_out = evt.timed_out;
        }

        String eventName() {
            return "MonitorWaited";
        }

        public ObjectReference  monitor() {
            return monitor;
        };

        public boolean timedout() {
            return timed_out;
        }
    }

    class ClassPrepareEventImpl extends ThreadedEventImpl
                            implements ClassPrepareEvent {
        private ReferenceType referenceType;

        ClassPrepareEventImpl(JDWP.Event.Composite.Events.ClassPrepare evt) {
            super(evt, evt.requestID, evt.thread);
            referenceType = this.vm.referenceType(evt.typeID, evt.refTypeTag,
                                                  evt.signature);
            ((ReferenceTypeImpl)referenceType).setStatus(evt.status);
        }

        public ReferenceType referenceType() {
            return referenceType;
        }

        String eventName() {
            return "ClassPrepareEvent";
        }
    }

    class ClassUnloadEventImpl extends EventImpl implements ClassUnloadEvent {
        private String classSignature;

        ClassUnloadEventImpl(JDWP.Event.Composite.Events.ClassUnload evt) {
            super(evt, evt.requestID);
            this.classSignature = evt.signature;
        }

        public String className() {
            return JNITypeParser.convertSignatureToClassname(classSignature);
        }

        public String classSignature() {
            return classSignature;
        }

        String eventName() {
            return "ClassUnloadEvent";
        }
    }

    class ExceptionEventImpl extends LocatableEventImpl
                                             implements ExceptionEvent {
        private ObjectReference exception;
        private Location catchLocation;

        ExceptionEventImpl(JDWP.Event.Composite.Events.Exception evt) {
            super(evt, evt.requestID, evt.thread, evt.location);
            this.exception = evt.exception;
            this.catchLocation = evt.catchLocation;
        }

        public ObjectReference exception() {
            return exception;
        }

        public Location catchLocation() {
            return catchLocation;
        }

        String eventName() {
            return "ExceptionEvent";
        }
    }

    class ThreadDeathEventImpl extends ThreadedEventImpl
                                        implements ThreadDeathEvent {
        ThreadDeathEventImpl(JDWP.Event.Composite.Events.ThreadDeath evt) {
            super(evt, evt.requestID, evt.thread);
        }

        String eventName() {
            return "ThreadDeathEvent";
        }
    }

    class ThreadStartEventImpl extends ThreadedEventImpl
                                        implements ThreadStartEvent {
        ThreadStartEventImpl(JDWP.Event.Composite.Events.ThreadStart evt) {
            super(evt, evt.requestID, evt.thread);
        }

        String eventName() {
            return "ThreadStartEvent";
        }
    }

    class VMStartEventImpl extends ThreadedEventImpl
                                        implements VMStartEvent {
        VMStartEventImpl(JDWP.Event.Composite.Events.VMStart evt) {
            super(evt, evt.requestID, evt.thread);
        }

        String eventName() {
            return "VMStartEvent";
        }
    }

    class VMDeathEventImpl extends EventImpl implements VMDeathEvent {

        VMDeathEventImpl(JDWP.Event.Composite.Events.VMDeath evt) {
            super(evt, evt.requestID);
        }

        String eventName() {
            return "VMDeathEvent";
        }
    }

    class VMDisconnectEventImpl extends EventImpl
                                         implements VMDisconnectEvent {

        VMDisconnectEventImpl() {
            super((byte)JDWP.EventKind.VM_DISCONNECTED);
        }

        String eventName() {
            return "VMDisconnectEvent";
        }
    }

    abstract class WatchpointEventImpl extends LocatableEventImpl
                                            implements WatchpointEvent {
        private final ReferenceTypeImpl refType;
        private final long fieldID;
        private final ObjectReference object;
        private Field field = null;

        WatchpointEventImpl(JDWP.Event.Composite.Events.EventsCommon evt,
                            int requestID,
                            ThreadReference thread, Location location,
                            byte refTypeTag, long typeID, long fieldID,
                            ObjectReference object) {
            super(evt, requestID, thread, location);
            this.refType = this.vm.referenceType(typeID, refTypeTag);
            this.fieldID = fieldID;
            this.object = object;
        }

        public Field field() {
            if (field == null) {
                field = refType.getFieldMirror(fieldID);
            }
            return field;
        }

        public ObjectReference object() {
            return object;
        }

        public Value valueCurrent() {
            if (object == null) {
                return refType.getValue(field());
            } else {
                return object.getValue(field());
            }
        }
    }

    class AccessWatchpointEventImpl extends WatchpointEventImpl
                                            implements AccessWatchpointEvent {

        AccessWatchpointEventImpl(JDWP.Event.Composite.Events.FieldAccess evt) {
            super(evt, evt.requestID, evt.thread, evt.location,
                  evt.refTypeTag, evt.typeID, evt.fieldID, evt.object);
        }

        String eventName() {
            return "AccessWatchpoint";
        }
    }

    class ModificationWatchpointEventImpl extends WatchpointEventImpl
                           implements ModificationWatchpointEvent {
        Value newValue;

        ModificationWatchpointEventImpl(
                        JDWP.Event.Composite.Events.FieldModification evt) {
            super(evt, evt.requestID, evt.thread, evt.location,
                  evt.refTypeTag, evt.typeID, evt.fieldID, evt.object);
            this.newValue = evt.valueToBe;
        }

        public Value valueToBe() {
            return newValue;
        }

        String eventName() {
            return "ModificationWatchpoint";
        }
    }

    /**
     * Events are constructed on the thread which reads all data from the
     * transport. This means that the packet cannot be converted to real
     * JDI objects as that may involve further communications with the
     * back end which would deadlock.
     *
     * Hence the {@link #build()} method below called by EventQueue.
     */
    EventSetImpl(VirtualMachine aVm, Packet pkt) {
        super();

        // From "MirrorImpl":
        // Yes, its a bit of a hack. But by doing it this
        // way, this is the only place we have to change
        // typing to substitute a new impl.
        vm = (VirtualMachineImpl)aVm;

        this.pkt = pkt;
    }

    /**
     * Constructor for special events like VM disconnected
     */
    EventSetImpl(VirtualMachine aVm, byte eventCmd) {
        this(aVm, null);
        suspendPolicy = JDWP.SuspendPolicy.NONE;
        switch (eventCmd) {
            case JDWP.EventKind.VM_DISCONNECTED:
                addEvent(new VMDisconnectEventImpl());
                break;

            default:
                throw new InternalException("Bad singleton event code");
        }
    }

    private void addEvent(EventImpl evt) {
        // Note that this class has a public add method that throws
        // an exception so that clients can't modify the EventSet
        super.add(evt);
    }

    /*
     * Complete the construction of an EventSet.  This is called from
     * an event handler thread.  It upacks the JDWP events inside
     * the packet and creates EventImpls for them.  The EventSet is already
     * on EventQueues when this is called, so it has to be synch.
     */
    synchronized void build() {
        if (pkt == null) {
            return;
        }
        PacketStream ps = new PacketStream(vm, pkt);
        JDWP.Event.Composite compEvt = new JDWP.Event.Composite(vm, ps);
        suspendPolicy = compEvt.suspendPolicy;
        if ((vm.traceFlags & VirtualMachine.TRACE_EVENTS) != 0) {
            switch(suspendPolicy) {
                case JDWP.SuspendPolicy.ALL:
                    vm.printTrace("EventSet: SUSPEND_ALL");
                    break;

                case JDWP.SuspendPolicy.EVENT_THREAD:
                    vm.printTrace("EventSet: SUSPEND_EVENT_THREAD");
                    break;

                case JDWP.SuspendPolicy.NONE:
                    vm.printTrace("EventSet: SUSPEND_NONE");
                    break;
            }
        }

        ThreadReference fix6485605 = null;
        for (int i = 0; i < compEvt.events.length; i++) {
            EventImpl evt = createEvent(compEvt.events[i]);
            if ((vm.traceFlags & VirtualMachine.TRACE_EVENTS) != 0) {
                try {
                    vm.printTrace("Event: " + evt);
                } catch (VMDisconnectedException ee) {
                    // ignore - see bug 6502716
                }
            }

            switch (evt.destination()) {
                case UNKNOWN_EVENT:
                    // Ignore disabled, deleted, unknown events, but
                    // save the thread if there is one since we might
                    // have to resume it.  Note that events for different
                    // threads can't be in the same event set.
                    if (evt instanceof ThreadedEventImpl &&
                        suspendPolicy == JDWP.SuspendPolicy.EVENT_THREAD) {
                        fix6485605 = ((ThreadedEventImpl)evt).thread();
                    }
                    continue;
                case CLIENT_EVENT:
                    addEvent(evt);
                    break;
                case INTERNAL_EVENT:
                    if (internalEventSet == null) {
                        internalEventSet = new EventSetImpl(this.vm, null);
                    }
                    internalEventSet.addEvent(evt);
                    break;
                default:
                    throw new InternalException("Invalid event destination");
            }
        }
        pkt = null; // No longer needed - free it up

        // Avoid hangs described in 6296125, 6293795
        if (super.size() == 0) {
            // This set has no client events.  If we don't do
            // needed resumes, no one else is going to.
            if (suspendPolicy == JDWP.SuspendPolicy.ALL) {
                vm.resume();
            } else if (suspendPolicy == JDWP.SuspendPolicy.EVENT_THREAD) {
                // See bug 6485605.
                if (fix6485605 != null) {
                    fix6485605.resume();
                } else {
                    // apparently, there is nothing to resume.
                }
            }
            suspendPolicy = JDWP.SuspendPolicy.NONE;

        }

    }

    /**
     * Filter out internal events
     */
    EventSet userFilter() {
        return this;
    }

    /**
     * Filter out user events.
     */
    EventSet internalFilter() {
        return this.internalEventSet;
    }

    EventImpl createEvent(JDWP.Event.Composite.Events evt) {
        JDWP.Event.Composite.Events.EventsCommon comm = evt.aEventsCommon;
        switch (evt.eventKind) {
            case JDWP.EventKind.THREAD_START:
                return new ThreadStartEventImpl(
                      (JDWP.Event.Composite.Events.ThreadStart)comm);

            case JDWP.EventKind.THREAD_END:
                return new ThreadDeathEventImpl(
                      (JDWP.Event.Composite.Events.ThreadDeath)comm);

            case JDWP.EventKind.EXCEPTION:
                return new ExceptionEventImpl(
                      (JDWP.Event.Composite.Events.Exception)comm);

            case JDWP.EventKind.BREAKPOINT:
                return new BreakpointEventImpl(
                      (JDWP.Event.Composite.Events.Breakpoint)comm);

            case JDWP.EventKind.METHOD_ENTRY:
                return new MethodEntryEventImpl(
                      (JDWP.Event.Composite.Events.MethodEntry)comm);

            case JDWP.EventKind.METHOD_EXIT:
                return new MethodExitEventImpl(
                      (JDWP.Event.Composite.Events.MethodExit)comm);

            case JDWP.EventKind.METHOD_EXIT_WITH_RETURN_VALUE:
                return new MethodExitEventImpl(
                      (JDWP.Event.Composite.Events.MethodExitWithReturnValue)comm);

            case JDWP.EventKind.FIELD_ACCESS:
                return new AccessWatchpointEventImpl(
                      (JDWP.Event.Composite.Events.FieldAccess)comm);

            case JDWP.EventKind.FIELD_MODIFICATION:
                return new ModificationWatchpointEventImpl(
                      (JDWP.Event.Composite.Events.FieldModification)comm);

            case JDWP.EventKind.SINGLE_STEP:
                return new StepEventImpl(
                      (JDWP.Event.Composite.Events.SingleStep)comm);

            case JDWP.EventKind.CLASS_PREPARE:
                return new ClassPrepareEventImpl(
                      (JDWP.Event.Composite.Events.ClassPrepare)comm);

            case JDWP.EventKind.CLASS_UNLOAD:
                return new ClassUnloadEventImpl(
                      (JDWP.Event.Composite.Events.ClassUnload)comm);

            case JDWP.EventKind.MONITOR_CONTENDED_ENTER:
                return new MonitorContendedEnterEventImpl(
                      (JDWP.Event.Composite.Events.MonitorContendedEnter)comm);

            case JDWP.EventKind.MONITOR_CONTENDED_ENTERED:
                return new MonitorContendedEnteredEventImpl(
                      (JDWP.Event.Composite.Events.MonitorContendedEntered)comm);

            case JDWP.EventKind.MONITOR_WAIT:
                return new MonitorWaitEventImpl(
                      (JDWP.Event.Composite.Events.MonitorWait)comm);

            case JDWP.EventKind.MONITOR_WAITED:
                return new MonitorWaitedEventImpl(
                      (JDWP.Event.Composite.Events.MonitorWaited)comm);

            case JDWP.EventKind.VM_START:
                return new VMStartEventImpl(
                      (JDWP.Event.Composite.Events.VMStart)comm);

            case JDWP.EventKind.VM_DEATH:
                return new VMDeathEventImpl(
                      (JDWP.Event.Composite.Events.VMDeath)comm);

            default:
                // Ignore unknown event types
                System.err.println("Ignoring event cmd " +
                                   evt.eventKind + " from the VM");
                return null;
        }
    }

    public VirtualMachine virtualMachine() {
        return vm;
    }

    public int suspendPolicy() {
        return EventRequestManagerImpl.JDWPtoJDISuspendPolicy(suspendPolicy);
    }

    private ThreadReference eventThread() {
        for (Event event : this) {
            if (event instanceof ThreadedEventImpl) {
                return ((ThreadedEventImpl)event).thread();
            }
        }
        return null;
    }

    public void resume() {
        switch (suspendPolicy()) {
            case EventRequest.SUSPEND_ALL:
                vm.resume();
                break;
            case EventRequest.SUSPEND_EVENT_THREAD:
                ThreadReference thread = eventThread();
                if (thread == null) {
                    throw new InternalException("Inconsistent suspend policy");
                }
                thread.resume();
                break;
            case EventRequest.SUSPEND_NONE:
                // Do nothing
                break;
            default:
                throw new InternalException("Invalid suspend policy");
        }
    }

    public Iterator<Event> iterator() {
        return new Itr();
    }

    public EventIterator eventIterator() {
        return new Itr();
    }

    public class Itr implements EventIterator {
        /**
         * Index of element to be returned by subsequent call to next.
         */
        int cursor = 0;

        public boolean hasNext() {
            return cursor != size();
        }

        public Event next() {
            try {
                Event nxt = get(cursor);
                ++cursor;
                return nxt;
            } catch(IndexOutOfBoundsException e) {
                throw new NoSuchElementException();
            }
        }

        public Event nextEvent() {
            return next();
        }

        public void remove() {
            throw new UnsupportedOperationException();
        }
    }

    @Override
    public Spliterator<Event> spliterator() {
        return Spliterators.spliterator(this, Spliterator.DISTINCT);
    }

    /* below make this unmodifiable */

    public boolean add(Event o){
        throw new UnsupportedOperationException();
    }
    public boolean remove(Object o) {
        throw new UnsupportedOperationException();
    }
    public boolean addAll(Collection<? extends Event> coll) {
        throw new UnsupportedOperationException();
    }
    public boolean removeAll(Collection<?> coll) {
        throw new UnsupportedOperationException();
    }
    public boolean retainAll(Collection<?> coll) {
        throw new UnsupportedOperationException();
    }
    public void clear() {
        throw new UnsupportedOperationException();
    }
}