Java example source code file (gctest.c)

This example Java source code file (gctest.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

agent_onunload, cannot, jni_ok, jnicall, jnienv, jniexport, jvmti_enable, jvmti_error_none, jvmti_event_garbage_collection_finish, jvmti_event_garbage_collection_start, null, thread, unable

The gctest.c Java example source code

/*
 * Copyright (c) 2004, 2011, Oracle and/or its affiliates. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 *   - Neither the name of Oracle nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * This source code is provided to illustrate the usage of a given feature
 * or technique and has been deliberately simplified. Additional steps
 * required for a production-quality application, such as security checks,
 * input validation and proper error handling, might not be present in
 * this sample code.
 */


/* Example of using JVMTI_EVENT_GARBAGE_COLLECTION_START and
 *                  JVMTI_EVENT_GARBAGE_COLLECTION_FINISH events.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "jni.h"
#include "jvmti.h"

/* For stdout_message(), fatal_error(), and check_jvmti_error() */
#include "agent_util.h"

/* Global static data */
static jvmtiEnv     *jvmti;
static int           gc_count;
static jrawMonitorID lock;

/* Worker thread that waits for garbage collections */
static void JNICALL
worker(jvmtiEnv* jvmti, JNIEnv* jni, void *p)
{
    jvmtiError err;

    stdout_message("GC worker started...\n");

    for (;;) {
        err = (*jvmti)->RawMonitorEnter(jvmti, lock);
        check_jvmti_error(jvmti, err, "raw monitor enter");
        while (gc_count == 0) {
            err = (*jvmti)->RawMonitorWait(jvmti, lock, 0);
            if (err != JVMTI_ERROR_NONE) {
                err = (*jvmti)->RawMonitorExit(jvmti, lock);
                check_jvmti_error(jvmti, err, "raw monitor wait");
                return;
            }
        }
        gc_count = 0;

        err = (*jvmti)->RawMonitorExit(jvmti, lock);
        check_jvmti_error(jvmti, err, "raw monitor exit");

        /* Perform arbitrary JVMTI/JNI work here to do post-GC cleanup */
        stdout_message("post-GarbageCollectionFinish actions...\n");
    }
}

/* Creates a new jthread */
static jthread
alloc_thread(JNIEnv *env)
{
    jclass    thrClass;
    jmethodID cid;
    jthread   res;

    thrClass = (*env)->FindClass(env, "java/lang/Thread");
    if ( thrClass == NULL ) {
        fatal_error("Cannot find Thread class\n");
    }
    cid      = (*env)->GetMethodID(env, thrClass, "<init>", "()V");
    if ( cid == NULL ) {
        fatal_error("Cannot find Thread constructor method\n");
    }
    res      = (*env)->NewObject(env, thrClass, cid);
    if ( res == NULL ) {
        fatal_error("Cannot create new Thread object\n");
    }
    return res;
}

/* Callback for JVMTI_EVENT_VM_INIT */
static void JNICALL
vm_init(jvmtiEnv *jvmti, JNIEnv *env, jthread thread)
{
    jvmtiError err;

    stdout_message("VMInit...\n");

    err = (*jvmti)->RunAgentThread(jvmti, alloc_thread(env), &worker, NULL,
        JVMTI_THREAD_MAX_PRIORITY);
    check_jvmti_error(jvmti, err, "running agent thread");
}

/* Callback for JVMTI_EVENT_GARBAGE_COLLECTION_START */
static void JNICALL
gc_start(jvmtiEnv* jvmti_env)
{
    stdout_message("GarbageCollectionStart...\n");
}

/* Callback for JVMTI_EVENT_GARBAGE_COLLECTION_FINISH */
static void JNICALL
gc_finish(jvmtiEnv* jvmti_env)
{
    jvmtiError err;

    stdout_message("GarbageCollectionFinish...\n");

    err = (*jvmti)->RawMonitorEnter(jvmti, lock);
    check_jvmti_error(jvmti, err, "raw monitor enter");
    gc_count++;
    err = (*jvmti)->RawMonitorNotify(jvmti, lock);
    check_jvmti_error(jvmti, err, "raw monitor notify");
    err = (*jvmti)->RawMonitorExit(jvmti, lock);
    check_jvmti_error(jvmti, err, "raw monitor exit");
}

/* Agent_OnLoad() is called first, we prepare for a VM_INIT event here. */
JNIEXPORT jint JNICALL
Agent_OnLoad(JavaVM *vm, char *options, void *reserved)
{
    jint                rc;
    jvmtiError          err;
    jvmtiCapabilities   capabilities;
    jvmtiEventCallbacks callbacks;

    /* Get JVMTI environment */
    rc = (*vm)->GetEnv(vm, (void **)&jvmti, JVMTI_VERSION);
    if (rc != JNI_OK) {
        fatal_error("ERROR: Unable to create jvmtiEnv, rc=%d\n", rc);
        return -1;
    }

    /* Get/Add JVMTI capabilities */
    (void)memset(&capabilities, 0, sizeof(capabilities));
    capabilities.can_generate_garbage_collection_events = 1;
    err = (*jvmti)->AddCapabilities(jvmti, &capabilities);
    check_jvmti_error(jvmti, err, "add capabilities");

    /* Set callbacks and enable event notifications */
    memset(&callbacks, 0, sizeof(callbacks));
    callbacks.VMInit                  = &vm_init;
    callbacks.GarbageCollectionStart  = &gc_start;
    callbacks.GarbageCollectionFinish = &gc_finish;
    err = (*jvmti)->SetEventCallbacks(jvmti, &callbacks, sizeof(callbacks));
    check_jvmti_error(jvmti, err, "set event callbacks");
    err = (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE,
                        JVMTI_EVENT_VM_INIT, NULL);
    check_jvmti_error(jvmti, err, "set event notification");
    err = (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE,
                        JVMTI_EVENT_GARBAGE_COLLECTION_START, NULL);
    check_jvmti_error(jvmti, err, "set event notification");
    err = (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE,
                        JVMTI_EVENT_GARBAGE_COLLECTION_FINISH, NULL);
    check_jvmti_error(jvmti, err, "set event notification");

    /* Create the necessary raw monitor */
    err = (*jvmti)->CreateRawMonitor(jvmti, "lock", &lock);
    check_jvmti_error(jvmti, err, "create raw monitor");
    return 0;
}

/* Agent_OnUnload() is called last */
JNIEXPORT void JNICALL
Agent_OnUnload(JavaVM *vm)
{
}