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Java example source code file (eventHandler.c)

This example Java source code file (eventHandler.c) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

begin_callback, end, end_callback, eventinfo, exit_error, handlernode, jni_false, jnicall, jnienv, jvmti_error_none, log_cb, log_misc, next, null

The eventHandler.c Java example source code

/*
 * Copyright (c) 1998, 2007, 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.
 */
/*
 * eventHandler
 *
 * This module handles events as they come in directly from JVMTI
 * and also maps them to JDI events.  JDI events are those requested
 * at the JDI or JDWP level and seen on those levels.  Mapping is
 * one-to-many, a JVMTI event may map to several JDI events, or
 * to none.  Part of that mapping process is filteration, which
 * eventFilter sub-module handles.  A JDI EventRequest corresponds
 * to a HandlerNode and a JDI filter to the hidden HandlerNode data
 * used by eventFilter.  For example, if at the JDI level the user
 * executed:
 *
 *   EventRequestManager erm = vm.eventRequestManager();
 *   BreakpointRequest bp = erm.createBreakpointRequest();
 *   bp.enable();
 *   ClassPrepareRequest req = erm.createClassPrepareRequest();
 *   req.enable();
 *   req = erm.createClassPrepareRequest();
 *   req.addClassFilter("Foo*");
 *   req.enable();
 *
 * Three handlers would be created, the first with a LocationOnly
 * filter and the last with a ClassMatch  filter.
 * When a JVMTI class prepare event for "Foobar"
 * comes in, the second handler will create one JDI event, the
 * third handler will compare the class signature, and since
 * it matchs create a second event.  There may also be internal
 * events as there are in this case, one created by the front-end
 * and one by the back-end.
 *
 * Each event kind has a handler chain, which is a doublely linked
 * list of handlers for that kind of event.
 */
#include "util.h"
#include "eventHandler.h"
#include "eventHandlerRestricted.h"
#include "eventFilter.h"
#include "eventFilterRestricted.h"
#include "standardHandlers.h"
#include "threadControl.h"
#include "eventHelper.h"
#include "classTrack.h"
#include "commonRef.h"
#include "debugLoop.h"

static HandlerID requestIdCounter;
static jbyte currentSessionID;

/* Counter of active callbacks and flag for vm_death */
static int      active_callbacks   = 0;
static jboolean vm_death_callback_active = JNI_FALSE;
static jrawMonitorID callbackLock;
static jrawMonitorID callbackBlock;

/* Macros to surround callback code (non-VM_DEATH callbacks).
 *   Note that this just keeps a count of the non-VM_DEATH callbacks that
 *   are currently active, it does not prevent these callbacks from
 *   operating in parallel. It's the VM_DEATH callback that will wait
 *   for all these callbacks to finish up, so that it can report the
 *   VM_DEATH in a clean state.
 *   If the VM_DEATH callback is active in the BEGIN macro then this
 *   callback just blocks until released by the VM_DEATH callback.
 *   If the VM_DEATH callback is active in the END macro, then this
 *   callback will notify the VM_DEATH callback if it's the last one,
 *   and then block until released by the VM_DEATH callback.
 *   Why block? These threads are often the threads of the Java program,
 *   not blocking might mean that a return would continue execution of
 *   some java thread in the middle of VM_DEATH, this seems troubled.
 *
 *   WARNING: No not 'return' or 'goto' out of the BEGIN_CALLBACK/END_CALLBACK
 *            block, this will mess up the count.
 */

#define BEGIN_CALLBACK()                                                \
{ /* BEGIN OF CALLBACK */                                               \
    jboolean bypass = JNI_TRUE;                                         \
    debugMonitorEnter(callbackLock); {                                  \
        if (vm_death_callback_active) {                                 \
            /* allow VM_DEATH callback to finish */                     \
            debugMonitorExit(callbackLock);                             \
            /* Now block because VM is about to die */                  \
            debugMonitorEnter(callbackBlock);                           \
            debugMonitorExit(callbackBlock);                            \
        } else {                                                        \
            active_callbacks++;                                         \
            bypass = JNI_FALSE;                                         \
            debugMonitorExit(callbackLock);                             \
        }                                                               \
    }                                                                   \
    if ( !bypass ) {                                                    \
        /* BODY OF CALLBACK CODE */

#define END_CALLBACK() /* Part of bypass if body */                     \
        debugMonitorEnter(callbackLock); {                              \
            active_callbacks--;                                         \
            if (active_callbacks < 0) {                                 \
                EXIT_ERROR(0, "Problems tracking active callbacks");    \
            }                                                           \
            if (vm_death_callback_active) {                             \
                if (active_callbacks == 0) {                            \
                    debugMonitorNotifyAll(callbackLock);                \
                }                                                       \
                /* allow VM_DEATH callback to finish */                 \
                debugMonitorExit(callbackLock);                         \
                /* Now block because VM is about to die */              \
                debugMonitorEnter(callbackBlock);                       \
                debugMonitorExit(callbackBlock);                        \
            } else {                                                    \
                debugMonitorExit(callbackLock);                         \
            }                                                           \
        }                                                               \
    }                                                                   \
} /* END OF CALLBACK */

/*
 * We are starting with a very simple locking scheme
 * for event handling.  All readers and writers of data in
 * the handlers[] chain must own this lock for the duration
 * of its use. If contention becomes a problem, we can:
 *
 * 1) create a lock per event type.
 * 2) move to a readers/writers approach where multiple threads
 * can access the chains simultaneously while reading (the
 * normal activity of an event callback).
 */
static jrawMonitorID handlerLock;

typedef struct HandlerChain_ {
    HandlerNode *first;
    /* add lock here */
} HandlerChain;

/*
 * This array maps event kinds to handler chains.
 * Protected by handlerLock.
 */

static HandlerChain __handlers[EI_max-EI_min+1];

/* Given a HandlerNode, these access our private data.
 */
#define PRIVATE_DATA(node) \
       (&(((EventHandlerRestricted_HandlerNode*)(void*)(node))->private_ehpd))

#define NEXT(node) (PRIVATE_DATA(node)->private_next)
#define PREV(node) (PRIVATE_DATA(node)->private_prev)
#define CHAIN(node) (PRIVATE_DATA(node)->private_chain)
#define HANDLER_FUNCTION(node) (PRIVATE_DATA(node)->private_handlerFunction)

static jclass getObjectClass(jobject object);
static jvmtiError freeHandler(HandlerNode *node);

static jvmtiError freeHandlerChain(HandlerChain *chain);

static HandlerChain *
getHandlerChain(EventIndex i)
{
    if ( i < EI_min || i > EI_max ) {
        EXIT_ERROR(AGENT_ERROR_INVALID_EVENT_TYPE,"bad index for handler");
    }
    return &(__handlers[i-EI_min]);
}

static void
insert(HandlerChain *chain, HandlerNode *node)
{
    HandlerNode *oldHead = chain->first;
    NEXT(node) = oldHead;
    PREV(node) = NULL;
    CHAIN(node) = chain;
    if (oldHead != NULL) {
        PREV(oldHead) = node;
    }
    chain->first = node;
}

static HandlerNode *
findInChain(HandlerChain *chain, HandlerID handlerID)
{
    HandlerNode *node = chain->first;
    while (node != NULL) {
        if (node->handlerID == handlerID) {
            return node;
        }
        node = NEXT(node);
    }
    return NULL;
}

static HandlerNode *
find(EventIndex ei, HandlerID handlerID)
{
    return findInChain(getHandlerChain(ei), handlerID);
}

/**
 * Deinsert.  Safe for non-inserted nodes.
 */
static void
deinsert(HandlerNode *node)
{
    HandlerChain *chain = CHAIN(node);

    if (chain == NULL) {
        return;
    }
    if (chain->first == node) {
        chain->first = NEXT(node);
    }
    if (NEXT(node) != NULL) {
        PREV(NEXT(node)) = PREV(node);
    }
    if (PREV(node) != NULL) {
        NEXT(PREV(node)) = NEXT(node);
    }
    CHAIN(node) = NULL;
}

jboolean
eventHandlerRestricted_iterator(EventIndex ei,
                              IteratorFunction func, void *arg)
{
    HandlerChain *chain;
    HandlerNode *node;
    JNIEnv *env;

    chain = getHandlerChain(ei);
    node = chain->first;
    env = getEnv();

    if ( func == NULL ) {
        EXIT_ERROR(AGENT_ERROR_INTERNAL,"iterator function NULL");
    }

    while (node != NULL) {
        if (((func)(env, node, arg))) {
            return JNI_TRUE;
        }
        node = NEXT(node);
    }
    return JNI_FALSE;
}

/* BREAKPOINT, METHOD_ENTRY and SINGLE_STEP events are covered by
 * the co-location of events policy. Of these three co-located
 * events, METHOD_ENTRY is  always reported first and BREAKPOINT
 * is always reported last. Here are the possible combinations and
 * their order:
 *
 * (p1) METHOD_ENTRY, BREAKPOINT (existing)
 * (p2) METHOD_ENTRY, BREAKPOINT (new)
 * (p1) METHOD_ENTRY, SINGLE_STEP
 * (p1) METHOD_ENTRY, SINGLE_STEP, BREAKPOINT (existing)
 * (p1/p2) METHOD_ENTRY, SINGLE_STEP, BREAKPOINT (new)
 * (p1) SINGLE_STEP, BREAKPOINT (existing)
 * (p2) SINGLE_STEP, BREAKPOINT (new)
 *
 * BREAKPOINT (existing) indicates a BREAKPOINT that is set before
 * the other co-located event is posted. BREAKPOINT (new) indicates
 * a BREAKPOINT that is set after the other co-located event is
 * posted and before the thread has resumed execution.
 *
 * Co-location of events policy used to be implemented via
 * temporary BREAKPOINTs along with deferring the reporting of
 * non-BREAKPOINT co-located events, but the temporary BREAKPOINTs
 * caused performance problems on VMs where setting or clearing
 * BREAKPOINTs is expensive, e.g., HotSpot.
 *
 * The policy is now implemented in two phases. Phase 1: when a
 * METHOD_ENTRY or SINGLE_STEP event is received, if there is an
 * existing co-located BREAKPOINT, then the current event is
 * deferred. When the BREAKPOINT event is processed, the event
 * bag will contain the deferred METHOD_ENTRY and/or SINGLE_STEP
 * events along with the BREAKPOINT event. For a METHOD_ENTRY
 * event where there is not an existing co-located BREAKPOINT,
 * if SINGLE_STEP events are also enabled for the thread, then
 * the METHOD_ENTRY event is deferred. When the SINGLE_STEP event
 * is processed, the event bag will also contain the deferred
 * METHOD_ENTRY event. This covers each of the combinations
 * marked with 'p1' above.
 *
 * Phase 2: if there is no existing co-located BREAKPOINT, then the
 * location information for the METHOD_ENTRY or SINGLE_STEP event
 * is recorded in the ThreadNode. If the next event for the thread
 * is a co-located BREAKPOINT, then the first BREAKPOINT event will
 * be skipped since it cannot be delivered in the same event set.
 * This covers each of the combinations marked with 'p2' above.
 *
 * For the combination marked p1/p2, part of the case is handled
 * during phase 1 and the rest is handled during phase 2.
 *
 * The recording of information in the ThreadNode is handled in
 * this routine. The special handling of the next event for the
 * thread is handled in skipEventReport().
 */

static jboolean
deferEventReport(JNIEnv *env, jthread thread,
            EventIndex ei, jclass clazz, jmethodID method, jlocation location)
{
    jboolean deferring = JNI_FALSE;

    switch (ei) {
        case EI_METHOD_ENTRY:
            if (!isMethodNative(method)) {
                jvmtiError error;
                jlocation start;
                jlocation end;
                error = methodLocation(method, &start, &end);
                if (error == JVMTI_ERROR_NONE) {
                    deferring = isBreakpointSet(clazz, method, start) ||
                                threadControl_getInstructionStepMode(thread)
                                    == JVMTI_ENABLE;
                    if (!deferring) {
                        threadControl_saveCLEInfo(env, thread, ei,
                                                  clazz, method, start);
                    }
                }
            }
            break;
        case EI_SINGLE_STEP:
            deferring = isBreakpointSet(clazz, method, location);
            if (!deferring) {
                threadControl_saveCLEInfo(env, thread, ei,
                                          clazz, method, location);
            }
            break;
        default:
            break;
    }
    /* TO DO: Once JVMTI supports a way to know if we're
     * at the end of a method, we should check here for
     * break and step events which precede a method exit
     * event.
     */
    return deferring;
}

/* Handle phase 2 of the co-located events policy. See detailed
 * comments in deferEventReport() above.
 */
static jboolean
skipEventReport(JNIEnv *env, jthread thread, EventIndex ei,
                        jclass clazz, jmethodID method, jlocation location)
{
    jboolean skipping = JNI_FALSE;

    if (ei == EI_BREAKPOINT) {
        if (threadControl_cmpCLEInfo(env, thread, clazz, method, location)) {
            LOG_MISC(("Co-located breakpoint event found: "
                "%s,thread=%p,clazz=%p,method=%p,location=%d",
                eventText(ei), thread, clazz, method, location));
            skipping = JNI_TRUE;
        }
    }

    threadControl_clearCLEInfo(env, thread);

    return skipping;
}

static void
reportEvents(JNIEnv *env, jbyte sessionID, jthread thread, EventIndex ei,
             jclass clazz, jmethodID method, jlocation location,
             struct bag *eventBag)
{
    jbyte suspendPolicy;
    jboolean invoking;

    if (bagSize(eventBag) < 1) {
        return;
    }

    /*
     * Never report events before initialization completes
     */
    if (!debugInit_isInitComplete()) {
        return;
    }

    /*
     * Check to see if we should skip reporting this event due to
     * co-location of events policy.
     */
    if (thread != NULL &&
           skipEventReport(env, thread, ei, clazz, method, location)) {
        LOG_MISC(("event report being skipped: "
            "ei=%s,thread=%p,clazz=%p,method=%p,location=%d",
            eventText(ei), thread, clazz, method, location));
        bagDeleteAll(eventBag);
        return;
    }

    /* We delay the reporting of some events so that they can be
     * properly grouped into event sets with upcoming events. If
     * the reporting is to be deferred, the event commands remain
     * in the event bag until a subsequent event occurs.  Event is
     * NULL for synthetic events (e.g. unload).
     */
    if (thread == NULL
         || !deferEventReport(env, thread, ei,
                        clazz, method, location)) {
        struct bag *completedBag = bagDup(eventBag);
        bagDeleteAll(eventBag);
        if (completedBag == NULL) {
            /*
             * TO DO: Report, but don't terminate?
             */
            return;
        } else {
            suspendPolicy = eventHelper_reportEvents(sessionID, completedBag);
            if (thread != NULL && suspendPolicy != JDWP_SUSPEND_POLICY(NONE)) {
                do {
                    /* The events have been reported and this
                     * thread is about to continue, but it may
                     * have been started up up just to perform a
                     * requested method invocation. If so, we do
                     * the invoke now and then stop again waiting
                     * for another continue. By then another
                     * invoke request can be in place, so there is
                     * a loop around this code.
                     */
                    invoking = invoker_doInvoke(thread);
                    if (invoking) {
                        eventHelper_reportInvokeDone(sessionID, thread);
                    }
                } while (invoking);
            }
            bagDestroyBag(completedBag);
        }
    }
}

/* A bagEnumerateFunction.  Create a synthetic class unload event
 * for every class no longer present.  Analogous to event_callback
 * combined with a handler in a unload specific (no event
 * structure) kind of way.
 */
static jboolean
synthesizeUnloadEvent(void *signatureVoid, void *envVoid)
{
    JNIEnv *env = (JNIEnv *)envVoid;
    char *signature = *(char **)signatureVoid;
    char *classname;
    HandlerNode *node;
    jbyte eventSessionID = currentSessionID;
    struct bag *eventBag = eventHelper_createEventBag();

    if (eventBag == NULL) {
        /* TO DO: Report, but don't die
         */
        JDI_ASSERT(eventBag != NULL);
    }

    /* Signature needs to last, so convert extra copy to
     * classname
     */
    classname = jvmtiAllocate((int)strlen(signature)+1);
    (void)strcpy(classname, signature);
    convertSignatureToClassname(classname);

    debugMonitorEnter(handlerLock);

    node = getHandlerChain(EI_GC_FINISH)->first;
    while (node != NULL) {
        /* save next so handlers can remove themselves */
        HandlerNode *next = NEXT(node);
        jboolean shouldDelete;

        if (eventFilterRestricted_passesUnloadFilter(env, classname,
                                                     node,
                                                     &shouldDelete)) {
            /* There may be multiple handlers, the signature will
             * be freed when the event helper thread has written
             * it.  So each event needs a separate allocation.
             */
            char *durableSignature = jvmtiAllocate((int)strlen(signature)+1);
            (void)strcpy(durableSignature, signature);

            eventHelper_recordClassUnload(node->handlerID,
                                          durableSignature,
                                          eventBag);
        }
        if (shouldDelete) {
            /* We can safely free the node now that we are done
             * using it.
             */
            (void)freeHandler(node);
        }
        node = next;
    }

    debugMonitorExit(handlerLock);

    if (eventBag != NULL) {
        reportEvents(env, eventSessionID, (jthread)NULL, 0,
                            (jclass)NULL, (jmethodID)NULL, 0, eventBag);

        /*
         * bag was created locally, destroy it here.
         */
        bagDestroyBag(eventBag);
    }

    jvmtiDeallocate(signature);
    jvmtiDeallocate(classname);

    return JNI_TRUE;
}

/* Garbage Collection Happened */
static unsigned int garbageCollected = 0;

/* The JVMTI generic event callback. Each event is passed to a sequence of
 * handlers in a chain until the chain ends or one handler
 * consumes the event.
 */
static void
event_callback(JNIEnv *env, EventInfo *evinfo)
{
    struct bag *eventBag;
    jbyte eventSessionID = currentSessionID; /* session could change */
    jthrowable currentException;
    jthread thread;

    LOG_MISC(("event_callback(): ei=%s", eventText(evinfo->ei)));
    log_debugee_location("event_callback()", evinfo->thread, evinfo->method, evinfo->location);

    /* We want to preserve any current exception that might get
     * wiped out during event handling (e.g. JNI calls). We have
     * to rely on space for the local reference on the current
     * frame because doing a PushLocalFrame here might itself
     * generate an exception.
     */
    currentException = JNI_FUNC_PTR(env,ExceptionOccurred)(env);
    JNI_FUNC_PTR(env,ExceptionClear)(env);

    /* See if a garbage collection finish event happened earlier.
     *
     * Note: The "if" is an optimization to avoid entering the lock on every
     *       event; garbageCollected may be zapped before we enter
     *       the lock but then this just becomes one big no-op.
     */
    if ( garbageCollected > 0 ) {
        struct bag *unloadedSignatures = NULL;

        /* We want to compact the hash table of all
         * objects sent to the front end by removing objects that have
         * been collected.
         */
        commonRef_compact();

        /* We also need to simulate the class unload events. */

        debugMonitorEnter(handlerLock);

        /* Clear garbage collection counter */
        garbageCollected = 0;

        /* Analyze which class unloads occurred */
        unloadedSignatures = classTrack_processUnloads(env);

        debugMonitorExit(handlerLock);

        /* Generate the synthetic class unload events and/or just cleanup.  */
        if ( unloadedSignatures != NULL ) {
            (void)bagEnumerateOver(unloadedSignatures, synthesizeUnloadEvent,
                             (void *)env);
            bagDestroyBag(unloadedSignatures);
        }
    }

    thread = evinfo->thread;
    if (thread != NULL) {
        /*
         * Record the fact that we're entering an event
         * handler so that thread operations (status, interrupt,
         * stop) can be done correctly and so that thread
         * resources can be allocated.  This must be done before
         * grabbing any locks.
         */
        eventBag = threadControl_onEventHandlerEntry(eventSessionID,
                                 evinfo->ei, thread, currentException);
        if ( eventBag == NULL ) {
            jboolean invoking;
            do {
                /* The event has been 'handled' and this
                 * thread is about to continue, but it may
                 * have been started up just to perform a
                 * requested method invocation. If so, we do
                 * the invoke now and then stop again waiting
                 * for another continue. By then another
                 * invoke request can be in place, so there is
                 * a loop around this code.
                 */
                invoking = invoker_doInvoke(thread);
                if (invoking) {
                    eventHelper_reportInvokeDone(eventSessionID, thread);
                }
            } while (invoking);
            return; /* Do nothing, event was consumed */
        }
    } else {
        eventBag = eventHelper_createEventBag();
        if (eventBag == NULL) {
            /*
             * TO DO: Report, but don't die
             */
            eventBag = NULL;  /* to shut up lint */
        }
    }

    debugMonitorEnter(handlerLock);
    {
        HandlerNode *node;
        char        *classname;

        /* We must keep track of all classes prepared to know what's unloaded */
        if (evinfo->ei == EI_CLASS_PREPARE) {
            classTrack_addPreparedClass(env, evinfo->clazz);
        }

        node = getHandlerChain(evinfo->ei)->first;
        classname = getClassname(evinfo->clazz);

        while (node != NULL) {
            /* save next so handlers can remove themselves */
            HandlerNode *next = NEXT(node);
            jboolean shouldDelete;

            if (eventFilterRestricted_passesFilter(env, classname,
                                                   evinfo, node,
                                                   &shouldDelete)) {
                HandlerFunction func;

                func = HANDLER_FUNCTION(node);
                if ( func == NULL ) {
                    EXIT_ERROR(AGENT_ERROR_INTERNAL,"handler function NULL");
                }
                (*func)(env, evinfo, node, eventBag);
            }
            if (shouldDelete) {
                /* We can safely free the node now that we are done
                 * using it.
                 */
                (void)freeHandler(node);
            }
            node = next;
        }
        jvmtiDeallocate(classname);
    }
    debugMonitorExit(handlerLock);

    if (eventBag != NULL) {
        reportEvents(env, eventSessionID, thread, evinfo->ei,
                evinfo->clazz, evinfo->method, evinfo->location, eventBag);
    }

    /* we are continuing after VMDeathEvent - now we are dead */
    if (evinfo->ei == EI_VM_DEATH) {
        gdata->vmDead = JNI_TRUE;
    }

    /*
     * If the bag was created locally, destroy it here.
     */
    if (thread == NULL) {
        bagDestroyBag(eventBag);
    }

    /* Always restore any exception that was set beforehand.  If
     * there is a pending async exception, StopThread will be
     * called from threadControl_onEventHandlerExit immediately
     * below.  Depending on VM implementation and state, the async
     * exception might immediately overwrite the currentException,
     * or it might be delayed until later.  */
    if (currentException != NULL) {
        JNI_FUNC_PTR(env,Throw)(env, currentException);
    } else {
        JNI_FUNC_PTR(env,ExceptionClear)(env);
    }

    /*
     * Release thread resources and perform any delayed operations.
     */
    if (thread != NULL) {
        threadControl_onEventHandlerExit(evinfo->ei, thread, eventBag);
    }
}

/* Returns a local ref to the declaring class for an object. */
static jclass
getObjectClass(jobject object)
{
    jclass clazz;
    JNIEnv *env = getEnv();

    clazz = JNI_FUNC_PTR(env,GetObjectClass)(env, object);

    return clazz;
}

/* Returns a local ref to the declaring class for a method, or NULL. */
jclass
getMethodClass(jvmtiEnv *jvmti_env, jmethodID method)
{
    jclass clazz = NULL;
    jvmtiError error;

    if ( method == NULL ) {
        return NULL;
    }
    error = methodClass(method, &clazz);
    if ( error != JVMTI_ERROR_NONE ) {
        EXIT_ERROR(error,"Can't get jclass for a methodID, invalid?");
        return NULL;
    }
    return clazz;
}

/* Event callback for JVMTI_EVENT_SINGLE_STEP */
static void JNICALL
cbSingleStep(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jmethodID method, jlocation location)
{
    EventInfo info;

    LOG_CB(("cbSingleStep: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_SINGLE_STEP;
        info.thread     = thread;
        info.clazz      = getMethodClass(jvmti_env, method);
        info.method     = method;
        info.location   = location;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbSingleStep"));
}

/* Event callback for JVMTI_EVENT_BREAKPOINT */
static void JNICALL
cbBreakpoint(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jmethodID method, jlocation location)
{
    EventInfo info;

    LOG_CB(("cbBreakpoint: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_BREAKPOINT;
        info.thread     = thread;
        info.clazz      = getMethodClass(jvmti_env, method);
        info.method     = method;
        info.location   = location;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbBreakpoint"));
}

/* Event callback for JVMTI_EVENT_FRAME_POP */
static void JNICALL
cbFramePop(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jmethodID method,
                        jboolean wasPoppedByException)
{
    EventInfo info;

    /* JDWP does not return these events when popped due to an exception. */
    if ( wasPoppedByException ) {
        return;
    }

    LOG_CB(("cbFramePop: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_FRAME_POP;
        info.thread     = thread;
        info.clazz      = getMethodClass(jvmti_env, method);
        info.method     = method;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbFramePop"));
}

/* Event callback for JVMTI_EVENT_EXCEPTION */
static void JNICALL
cbException(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jmethodID method,
                        jlocation location, jobject exception,
                        jmethodID catch_method, jlocation catch_location)
{
    EventInfo info;

    LOG_CB(("cbException: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei                         = EI_EXCEPTION;
        info.thread                     = thread;
        info.clazz                      = getMethodClass(jvmti_env, method);
        info.method                     = method;
        info.location                   = location;
        info.object                     = exception;
        info.u.exception.catch_clazz    = getMethodClass(jvmti_env, catch_method);
        info.u.exception.catch_method   = catch_method;
        info.u.exception.catch_location = catch_location;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbException"));
}

/* Event callback for JVMTI_EVENT_THREAD_START */
static void JNICALL
cbThreadStart(jvmtiEnv *jvmti_env, JNIEnv *env, jthread thread)
{
    EventInfo info;

    LOG_CB(("cbThreadStart: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_THREAD_START;
        info.thread     = thread;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbThreadStart"));
}

/* Event callback for JVMTI_EVENT_THREAD_END */
static void JNICALL
cbThreadEnd(jvmtiEnv *jvmti_env, JNIEnv *env, jthread thread)
{
    EventInfo info;

    LOG_CB(("cbThreadEnd: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_THREAD_END;
        info.thread     = thread;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbThreadEnd"));
}

/* Event callback for JVMTI_EVENT_CLASS_PREPARE */
static void JNICALL
cbClassPrepare(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jclass klass)
{
    EventInfo info;

    LOG_CB(("cbClassPrepare: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_CLASS_PREPARE;
        info.thread     = thread;
        info.clazz      = klass;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbClassPrepare"));
}

/* Event callback for JVMTI_EVENT_GARBAGE_COLLECTION_FINISH */
static void JNICALL
cbGarbageCollectionFinish(jvmtiEnv *jvmti_env)
{
    LOG_CB(("cbGarbageCollectionFinish"));
    ++garbageCollected;
    LOG_MISC(("END cbGarbageCollectionFinish"));
}

/* Event callback for JVMTI_EVENT_CLASS_LOAD */
static void JNICALL
cbClassLoad(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jclass klass)
{
    EventInfo info;

    LOG_CB(("cbClassLoad: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_CLASS_LOAD;
        info.thread     = thread;
        info.clazz      = klass;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbClassLoad"));
}

/* Event callback for JVMTI_EVENT_FIELD_ACCESS */
static void JNICALL
cbFieldAccess(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jmethodID method,
                        jlocation location, jclass field_klass,
                        jobject object, jfieldID field)
{
    EventInfo info;

    LOG_CB(("cbFieldAccess: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei                         = EI_FIELD_ACCESS;
        info.thread                     = thread;
        info.clazz                      = getMethodClass(jvmti_env, method);
        info.method                     = method;
        info.location                   = location;
        info.u.field_access.field_clazz = field_klass;
        info.object                     = object;
        info.u.field_access.field       = field;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbFieldAccess"));
}

/* Event callback for JVMTI_EVENT_FIELD_MODIFICATION */
static void JNICALL
cbFieldModification(jvmtiEnv *jvmti_env, JNIEnv *env,
        jthread thread, jmethodID method,
        jlocation location, jclass field_klass, jobject object, jfieldID field,
        char signature_type, jvalue new_value)
{
    EventInfo info;

    LOG_CB(("cbFieldModification: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei                                 = EI_FIELD_MODIFICATION;
        info.thread                             = thread;
        info.clazz                              = getMethodClass(jvmti_env, method);
        info.method                             = method;
        info.location                           = location;
        info.u.field_modification.field         = field;
        info.u.field_modification.field_clazz   = field_klass;
        info.object                             = object;
        info.u.field_modification.signature_type= signature_type;
        info.u.field_modification.new_value     = new_value;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbFieldModification"));
}

/* Event callback for JVMTI_EVENT_EXCEPTION_CATCH */
static void JNICALL
cbExceptionCatch(jvmtiEnv *jvmti_env, JNIEnv *env, jthread thread,
        jmethodID method, jlocation location, jobject exception)
{
    EventInfo info;

    LOG_CB(("cbExceptionCatch: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_EXCEPTION_CATCH;
        info.thread     = thread;
        info.clazz      = getMethodClass(jvmti_env, method);
        info.method     = method;
        info.location   = location;
        info.object     = exception;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbExceptionCatch"));
}

/* Event callback for JVMTI_EVENT_METHOD_ENTRY */
static void JNICALL
cbMethodEntry(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jmethodID method)
{
    EventInfo info;

    LOG_CB(("cbMethodEntry: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_METHOD_ENTRY;
        info.thread     = thread;
        info.clazz      = getMethodClass(jvmti_env, method);
        info.method     = method;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbMethodEntry"));
}

/* Event callback for JVMTI_EVENT_METHOD_EXIT */
static void JNICALL
cbMethodExit(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jmethodID method,
                        jboolean wasPoppedByException, jvalue return_value)
{
    EventInfo info;

    /* JDWP does not return these events when popped due to an exception. */
    if ( wasPoppedByException ) {
        return;
    }

    LOG_CB(("cbMethodExit: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_METHOD_EXIT;
        info.thread     = thread;
        info.clazz      = getMethodClass(jvmti_env, method);
        info.method     = method;
        info.u.method_exit.return_value = return_value;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbMethodExit"));
}

/* Event callback for JVMTI_EVENT_MONITOR_CONTENDED_ENTER */
static void JNICALL
cbMonitorContendedEnter(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jobject object)
{
    EventInfo info;
    jvmtiError error;
    jmethodID  method;
    jlocation  location;

    LOG_CB(("cbMonitorContendedEnter: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_MONITOR_CONTENDED_ENTER;
        info.thread     = thread;
        info.object     = object;
        /* get current location of contended monitor enter */
        error = JVMTI_FUNC_PTR(gdata->jvmti,GetFrameLocation)
                (gdata->jvmti, thread, 0, &method, &location);
        if (error == JVMTI_ERROR_NONE) {
            info.location = location;
            info.method   = method;
            info.clazz    = getMethodClass(jvmti_env, method);
        } else {
            info.location = -1;
        }
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbMonitorContendedEnter"));
}

/* Event callback for JVMTI_EVENT_MONITOR_CONTENDED_ENTERED */
static void JNICALL
cbMonitorContendedEntered(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jobject object)
{
    EventInfo info;
    jvmtiError error;
    jmethodID  method;
    jlocation  location;

    LOG_CB(("cbMonitorContendedEntered: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_MONITOR_CONTENDED_ENTERED;
        info.thread     = thread;
        info.object     = object;
        /* get current location of contended monitor enter */
        error = JVMTI_FUNC_PTR(gdata->jvmti,GetFrameLocation)
                (gdata->jvmti, thread, 0, &method, &location);
        if (error == JVMTI_ERROR_NONE) {
            info.location = location;
            info.method   = method;
            info.clazz    = getMethodClass(jvmti_env, method);
        } else {
            info.location = -1;
        }
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbMonitorContendedEntered"));
}

/* Event callback for JVMTI_EVENT_MONITOR_WAIT */
static void JNICALL
cbMonitorWait(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jobject object,
                        jlong timeout)
{
    EventInfo info;
    jvmtiError error;
    jmethodID  method;
    jlocation  location;

    LOG_CB(("cbMonitorWait: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_MONITOR_WAIT;
        info.thread     = thread;
        info.object     = object;
        /* The info.clazz is used for both class filtering and for location info.
         * For monitor wait event the class filtering is done for class of monitor
         * object. So here info.clazz is set to class of monitor object here and it
         * is reset to class of method before writing location info.
         * See writeMonitorEvent in eventHelper.c
         */
        info.clazz      = getObjectClass(object);
        info.u.monitor.timeout = timeout;

        /* get location of monitor wait() method. */
        error = JVMTI_FUNC_PTR(gdata->jvmti,GetFrameLocation)
                (gdata->jvmti, thread, 0, &method, &location);
        if (error == JVMTI_ERROR_NONE) {
            info.location = location;
            info.method   = method;
        } else {
            info.location = -1;
        }
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbMonitorWait"));
}

/* Event callback for JVMTI_EVENT_MONITOR_WAIT */
static void JNICALL
cbMonitorWaited(jvmtiEnv *jvmti_env, JNIEnv *env,
                        jthread thread, jobject object,
                        jboolean timed_out)
{
    EventInfo info;
    jvmtiError error;
    jmethodID  method;
    jlocation  location;

    LOG_CB(("cbMonitorWaited: thread=%p", thread));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_MONITOR_WAITED;
        info.thread     = thread;
        info.object     = object;
        /* The info.clazz is used for both class filtering and for location info.
         * For monitor waited event the class filtering is done for class of monitor
         * object. So here info.clazz is set to class of monitor object here and it
         * is reset to class of method before writing location info.
         * See writeMonitorEvent in eventHelper.c
         */
        info.clazz      = getObjectClass(object);
        info.u.monitor.timed_out = timed_out;

        /* get location of monitor wait() method */
        error = JVMTI_FUNC_PTR(gdata->jvmti,GetFrameLocation)
                (gdata->jvmti, thread, 0, &method, &location);
        if (error == JVMTI_ERROR_NONE) {
            info.location = location;
            info.method   = method;
        } else {
            info.location = -1;
        }
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbMonitorWaited"));
}

/* Event callback for JVMTI_EVENT_VM_INIT */
static void JNICALL
cbVMInit(jvmtiEnv *jvmti_env, JNIEnv *env, jthread thread)
{
    EventInfo info;

    LOG_CB(("cbVMInit"));

    BEGIN_CALLBACK() {
        (void)memset(&info,0,sizeof(info));
        info.ei         = EI_VM_INIT;
        info.thread     = thread;
        event_callback(env, &info);
    } END_CALLBACK();

    LOG_MISC(("END cbVMInit"));
}

/* Event callback for JVMTI_EVENT_VM_DEATH */
static void JNICALL
cbVMDeath(jvmtiEnv *jvmti_env, JNIEnv *env)
{
    jvmtiError error;
    EventInfo info;
    LOG_CB(("cbVMDeath"));

    /* Clear out ALL callbacks at this time, we don't want any more. */
    /*    This should prevent any new BEGIN_CALLBACK() calls. */
    (void)memset(&(gdata->callbacks),0,sizeof(gdata->callbacks));
    error = JVMTI_FUNC_PTR(gdata->jvmti,SetEventCallbacks)
                (gdata->jvmti, &(gdata->callbacks), sizeof(gdata->callbacks));
    if (error != JVMTI_ERROR_NONE) {
        EXIT_ERROR(error,"Can't clear event callbacks on vm death");
    }

    /* Now that no new callbacks will be made, we need to wait for the ones
     *   that are still active to complete.
     *   The BEGIN_CALLBACK/END_CALLBACK macros implement the VM_DEATH
     *   callback protocol. Once the callback table is cleared (above),
     *   we can have callback threads in different stages:
     *   1) after callback function entry and before BEGIN_CALLBACK
     *      macro; we catch these threads with callbackBlock in the
     *      BEGIN_CALLBACK macro
     *   2) after BEGIN_CALLBACK macro and before END_CALLBACK macro; we
     *      catch these threads with callbackBlock in the END_CALLBACK
     *      macro
     *   3) after END_CALLBACK macro; these threads have made it past
     *      callbackBlock and callbackLock and don't count as active
     *
     *   Since some of the callback threads could be blocked or suspended
     *   we will resume all threads suspended by the debugger for a short
     *   time to flush out all callbacks. Note that the callback threads
     *   will block from returning to the VM in both macros. Some threads
     *   not associated with callbacks, but suspended by the debugger may
     *   continue on, but not for long.
     *   Once the last callback finishes, it will notify this thread and
     *   we fall out of the loop below and actually process the VM_DEATH
     *   event.
     */
    debugMonitorEnter(callbackBlock); {
        debugMonitorEnter(callbackLock); {
            vm_death_callback_active = JNI_TRUE;
            (void)threadControl_resumeAll();
            while (active_callbacks > 0) {
                /* wait for active CALLBACKs to check in (and block) */
                debugMonitorWait(callbackLock);
            }
        } debugMonitorExit(callbackLock);

        /* Only now should we actually process the VM death event */
        (void)memset(&info,0,sizeof(info));
        info.ei                 = EI_VM_DEATH;
        event_callback(env, &info);

        /* Here we unblock all the callbacks and let them return to the
         *   VM.  It's not clear this is necessary, but leaving threads
         *   blocked doesn't seem like a good idea. They don't have much
         *   life left anyway.
         */
    } debugMonitorExit(callbackBlock);

    /*
     * The VM will die soon after the completion of this callback - we
     * may need to do a final synchronization with the command loop to
     * avoid the VM terminating with replying to the final (resume)
     * command.
     */
    debugLoop_sync();

    LOG_MISC(("END cbVMDeath"));
}

/**
 * Delete this handler (do not delete permanent handlers):
 * Deinsert handler from active list,
 * make it inactive, and free it's memory
 * Assumes handlerLock held.
 */
static jvmtiError
freeHandler(HandlerNode *node) {
    jvmtiError error = JVMTI_ERROR_NONE;

    /* deinsert the handler node before disableEvents() to make
     * sure the event will be disabled when no other event
     * handlers are installed.
     */
    if (node != NULL && (!node->permanent)) {
        deinsert(node);
        error = eventFilterRestricted_deinstall(node);
        jvmtiDeallocate(node);
    }

    return error;
}

/**
 * Delete all the handlers on this chain (do not delete permanent handlers).
 * Assumes handlerLock held.
 */
static jvmtiError
freeHandlerChain(HandlerChain *chain)
{
    HandlerNode *node;
    jvmtiError   error;

    error = JVMTI_ERROR_NONE;
    node  = chain->first;
    while ( node != NULL ) {
        HandlerNode *next;
        jvmtiError   singleError;

        next = NEXT(node);
        singleError = freeHandler(node);
        if ( singleError != JVMTI_ERROR_NONE ) {
            error = singleError;
        }
        node = next;
    }
    return error;
}

/**
 * Deinsert and free all memory.  Safe for non-inserted nodes.
 */
jvmtiError
eventHandler_free(HandlerNode *node)
{
    jvmtiError error;

    debugMonitorEnter(handlerLock);

    error = freeHandler(node);

    debugMonitorExit(handlerLock);

    return error;
}

/**
 * Free all handlers of this kind created by the JDWP client,
 * that is, doesn't free handlers internally created by back-end.
 */
jvmtiError
eventHandler_freeAll(EventIndex ei)
{
    jvmtiError error = JVMTI_ERROR_NONE;
    HandlerNode *node;

    debugMonitorEnter(handlerLock);
    node = getHandlerChain(ei)->first;
    while (node != NULL) {
        HandlerNode *next = NEXT(node);    /* allows node removal */
        if (node->handlerID != 0) {        /* don't free internal handlers */
            error = freeHandler(node);
            if (error != JVMTI_ERROR_NONE) {
                break;
            }
        }
        node = next;
    }
    debugMonitorExit(handlerLock);
    return error;
}

/***
 * Delete all breakpoints on "clazz".
 */
void
eventHandler_freeClassBreakpoints(jclass clazz)
{
    HandlerNode *node;
    JNIEnv *env = getEnv();

    debugMonitorEnter(handlerLock);
    node = getHandlerChain(EI_BREAKPOINT)->first;
    while (node != NULL) {
        HandlerNode *next = NEXT(node); /* allows node removal */
        if (eventFilterRestricted_isBreakpointInClass(env, clazz,
                                                      node)) {
            (void)freeHandler(node);
        }
        node = next;
    }
    debugMonitorExit(handlerLock);
}

jvmtiError
eventHandler_freeByID(EventIndex ei, HandlerID handlerID)
{
    jvmtiError error;
    HandlerNode *node;

    debugMonitorEnter(handlerLock);
    node = find(ei, handlerID);
    if (node != NULL) {
        error = freeHandler(node);
    } else {
        /* already freed */
        error = JVMTI_ERROR_NONE;
    }
    debugMonitorExit(handlerLock);
    return error;
}

void
eventHandler_initialize(jbyte sessionID)
{
    jvmtiError error;
    jint i;

    requestIdCounter = 1;
    currentSessionID = sessionID;

    /* This is for BEGIN_CALLBACK/END_CALLBACK handling, make sure this
     *   is done while none of these callbacks are active.
     */
    active_callbacks = 0;
    vm_death_callback_active = JNI_FALSE;
    callbackLock = debugMonitorCreate("JDWP Callback Lock");
    callbackBlock = debugMonitorCreate("JDWP Callback Block");

    handlerLock = debugMonitorCreate("JDWP Event Handler Lock");

    for (i = EI_min; i <= EI_max; ++i) {
        getHandlerChain(i)->first = NULL;
    }

    /*
     * Permanently enabled some events.
     */
    error = threadControl_setEventMode(JVMTI_ENABLE,
                                      EI_VM_INIT, NULL);
    if (error != JVMTI_ERROR_NONE) {
        EXIT_ERROR(error,"Can't enable vm init events");
    }
    error = threadControl_setEventMode(JVMTI_ENABLE,
                                      EI_VM_DEATH, NULL);
    if (error != JVMTI_ERROR_NONE) {
        EXIT_ERROR(error,"Can't enable vm death events");
    }
    error = threadControl_setEventMode(JVMTI_ENABLE,
                                      EI_THREAD_START, NULL);
    if (error != JVMTI_ERROR_NONE) {
        EXIT_ERROR(error,"Can't enable thread start events");
    }
    error = threadControl_setEventMode(JVMTI_ENABLE,
                                       EI_THREAD_END, NULL);
    if (error != JVMTI_ERROR_NONE) {
        EXIT_ERROR(error,"Can't enable thread end events");
    }
    error = threadControl_setEventMode(JVMTI_ENABLE,
                                       EI_CLASS_PREPARE, NULL);
    if (error != JVMTI_ERROR_NONE) {
        EXIT_ERROR(error,"Can't enable class prepare events");
    }
    error = threadControl_setEventMode(JVMTI_ENABLE,
                                       EI_GC_FINISH, NULL);
    if (error != JVMTI_ERROR_NONE) {
        EXIT_ERROR(error,"Can't enable garbage collection finish events");
    }

    (void)memset(&(gdata->callbacks),0,sizeof(gdata->callbacks));
    /* Event callback for JVMTI_EVENT_SINGLE_STEP */
    gdata->callbacks.SingleStep                 = &cbSingleStep;
    /* Event callback for JVMTI_EVENT_BREAKPOINT */
    gdata->callbacks.Breakpoint                 = &cbBreakpoint;
    /* Event callback for JVMTI_EVENT_FRAME_POP */
    gdata->callbacks.FramePop                   = &cbFramePop;
    /* Event callback for JVMTI_EVENT_EXCEPTION */
    gdata->callbacks.Exception                  = &cbException;
    /* Event callback for JVMTI_EVENT_THREAD_START */
    gdata->callbacks.ThreadStart                = &cbThreadStart;
    /* Event callback for JVMTI_EVENT_THREAD_END */
    gdata->callbacks.ThreadEnd                  = &cbThreadEnd;
    /* Event callback for JVMTI_EVENT_CLASS_PREPARE */
    gdata->callbacks.ClassPrepare               = &cbClassPrepare;
    /* Event callback for JVMTI_EVENT_CLASS_LOAD */
    gdata->callbacks.ClassLoad                  = &cbClassLoad;
    /* Event callback for JVMTI_EVENT_FIELD_ACCESS */
    gdata->callbacks.FieldAccess                = &cbFieldAccess;
    /* Event callback for JVMTI_EVENT_FIELD_MODIFICATION */
    gdata->callbacks.FieldModification          = &cbFieldModification;
    /* Event callback for JVMTI_EVENT_EXCEPTION_CATCH */
    gdata->callbacks.ExceptionCatch             = &cbExceptionCatch;
    /* Event callback for JVMTI_EVENT_METHOD_ENTRY */
    gdata->callbacks.MethodEntry                = &cbMethodEntry;
    /* Event callback for JVMTI_EVENT_METHOD_EXIT */
    gdata->callbacks.MethodExit                 = &cbMethodExit;
    /* Event callback for JVMTI_EVENT_MONITOR_CONTENDED_ENTER */
    gdata->callbacks.MonitorContendedEnter      = &cbMonitorContendedEnter;
    /* Event callback for JVMTI_EVENT_MONITOR_CONTENDED_ENTERED */
    gdata->callbacks.MonitorContendedEntered    = &cbMonitorContendedEntered;
    /* Event callback for JVMTI_EVENT_MONITOR_WAIT */
    gdata->callbacks.MonitorWait                = &cbMonitorWait;
    /* Event callback for JVMTI_EVENT_MONITOR_WAITED */
    gdata->callbacks.MonitorWaited              = &cbMonitorWaited;
    /* Event callback for JVMTI_EVENT_VM_INIT */
    gdata->callbacks.VMInit                     = &cbVMInit;
    /* Event callback for JVMTI_EVENT_VM_DEATH */
    gdata->callbacks.VMDeath                    = &cbVMDeath;
    /* Event callback for JVMTI_EVENT_GARBAGE_COLLECTION_FINISH */
    gdata->callbacks.GarbageCollectionFinish    = &cbGarbageCollectionFinish;

    error = JVMTI_FUNC_PTR(gdata->jvmti,SetEventCallbacks)
                (gdata->jvmti, &(gdata->callbacks), sizeof(gdata->callbacks));
    if (error != JVMTI_ERROR_NONE) {
        EXIT_ERROR(error,"Can't set event callbacks");
    }

    /* Notify other modules that the event callbacks are in place */
    threadControl_onHook();

    /* Get the event helper thread initialized */
    eventHelper_initialize(sessionID);
}

void
eventHandler_reset(jbyte sessionID)
{
    int i;

    debugMonitorEnter(handlerLock);

    /* We must do this first so that if any invokes complete,
     * there will be no attempt to send them to the front
     * end. Waiting for threadControl_reset leaves a window where
     * the invoke completions can sneak through.
     */
    threadControl_detachInvokes();

    /* Reset the event helper thread, purging all queued and
     * in-process commands.
     */
    eventHelper_reset(sessionID);

    /* delete all handlers */
    for (i = EI_min; i <= EI_max; i++) {
        (void)freeHandlerChain(getHandlerChain(i));
    }

    requestIdCounter = 1;
    currentSessionID = sessionID;

    debugMonitorExit(handlerLock);
}

void
eventHandler_lock(void)
{
    debugMonitorEnter(handlerLock);
}

void
eventHandler_unlock(void)
{
    debugMonitorExit(handlerLock);
}

/***** handler creation *****/

HandlerNode *
eventHandler_alloc(jint filterCount, EventIndex ei, jbyte suspendPolicy)
{
    HandlerNode *node = eventFilterRestricted_alloc(filterCount);

    if (node != NULL) {
        node->ei = ei;
        node->suspendPolicy = suspendPolicy;
        node->permanent = JNI_FALSE;
    }

    return node;
}


HandlerID
eventHandler_allocHandlerID(void)
{
    jint handlerID;
    debugMonitorEnter(handlerLock);
    handlerID = ++requestIdCounter;
    debugMonitorExit(handlerLock);
    return handlerID;
}


static jvmtiError
installHandler(HandlerNode *node,
              HandlerFunction func,
              jboolean external)
{
    jvmtiError error;

    if ( func == NULL ) {
        return AGENT_ERROR_INVALID_EVENT_TYPE;
    }

    debugMonitorEnter(handlerLock);

    HANDLER_FUNCTION(node) = func;

    node->handlerID = external? ++requestIdCounter : 0;
    error = eventFilterRestricted_install(node);
    if (error == JVMTI_ERROR_NONE) {
        insert(getHandlerChain(node->ei), node);
    }

    debugMonitorExit(handlerLock);

    return error;
}

static HandlerNode *
createInternal(EventIndex ei, HandlerFunction func,
               jthread thread, jclass clazz, jmethodID method,
               jlocation location, jboolean permanent)
{
    jint index = 0;
    jvmtiError error = JVMTI_ERROR_NONE;
    HandlerNode *node;

    /*
     * Start with necessary allocations
     */
    node = eventHandler_alloc(
        ((thread == NULL)? 0 : 1) + ((clazz == NULL)? 0 : 1),
        ei, JDWP_SUSPEND_POLICY(NONE));
    if (node == NULL) {
        return NULL;
    }

    node->permanent = permanent;

    if (thread != NULL) {
        error = eventFilter_setThreadOnlyFilter(node, index++, thread);
    }

    if ((error == JVMTI_ERROR_NONE) && (clazz != NULL)) {
        error = eventFilter_setLocationOnlyFilter(node, index++, clazz,
                                                  method, location);
    }
    /*
     * Create the new handler node
     */
    error = installHandler(node, func, JNI_FALSE);

    if (error != JVMTI_ERROR_NONE) {
        (void)eventHandler_free(node);
        node = NULL;
    }
    return node;
}

HandlerNode *
eventHandler_createPermanentInternal(EventIndex ei, HandlerFunction func)
{
    return createInternal(ei, func, NULL,
                          NULL, NULL, (jlocation)NULL, JNI_TRUE);
}

HandlerNode *
eventHandler_createInternalThreadOnly(EventIndex ei,
                                      HandlerFunction func,
                                      jthread thread)
{
    return createInternal(ei, func, thread,
                          NULL, NULL, (jlocation)NULL, JNI_FALSE);
}

HandlerNode *
eventHandler_createInternalBreakpoint(HandlerFunction func,
                                      jthread thread,
                                      jclass clazz,
                                      jmethodID method,
                                      jlocation location)
{
    return createInternal(EI_BREAKPOINT, func, thread,
                          clazz, method, location, JNI_FALSE);
}

jvmtiError
eventHandler_installExternal(HandlerNode *node)
{
    return installHandler(node,
                          standardHandlers_defaultHandler(node->ei),
                          JNI_TRUE);
}

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