JNI w/ Threads = Crashes

Question

Hi,
I am creating a Java app using JNI to call C++ libraries a vendor has provided for us. I used a pointer upon instantiation of the C++ object, store it in the Java side, and when I need to call a method in C, would de-reference that pointer to make function call.
This works well in a standalone Java application. However, when I use a thread to call this object, or a servlet, it will get as far as letting me make one function call in C, but other calls afterwards will cause a core dump.
Anyone experience this before? Please share any insight you might have. Thanks in advance.
-Justin
Here's the code:
=======================================
#include <jni.h>
#include <iostream.h>
#include "com_jni_JNIDSM_0005fServer.h"
#include "DSMC_Server.h"
//Java side: public static int cObjectID
static jobject cls = 0;
static jfieldID fid;
* Class: JNIDSM_0005fServer
* Method: DSM_Server
* Signature: ()I
* Create an instance of this object and return a reference handler (pointer).
* REQUIRED: Need to call this method first before DSM_Server object can be used.
JNIEXPORT jint JNICALL Java_com_jni_JNIDSM_1Server_DSM_1Server
(JNIEnv* p_env, jobject p_obj)
DSMC_Server* server;
server = new DSMC_Server();
cout << "ServerPtr Created = " << (jint)server << endl;
Java_com_jni_JNIDSM_1Server_setCID(p_env, p_obj, (jint) server);
return ((jint) server);
* Class: JNIDSM_0005fServer
* Method: connect
* Signature: (Ljava/lang/String;ILjava/lang/String;I)I
* Create connection from the DSM Client to the Server. Pass in host & port of Server and
* name assigned to the module of the DSM Client.
JNIEXPORT jint JNICALL Java_com_jni_JNIDSM_1Server_connect
(JNIEnv* p_env, jobject p_obj, jstring p_host, jint p_port, jstring p_serverModuleName)
// Obtain pointer reference to corresponding DSM objects
DSMC_Server* serverPtr = (DSMC_Server*)
Java_com_jni_JNIDSM_1Server_getCID (p_env, p_obj);
// Convert the Java String to a Char pointer for use
cout << "ServerPtr in JNI_connect() begin = " << (jint) serverPtr << endl;
jboolean isModCopy, isHostCopy;
const char* host = p_env->GetStringUTFChars(p_host, &isHostCopy);
const char* serverModuleName = p_env->GetStringUTFChars(p_serverModuleName, &isModCopy);
// Call the DSM_Server object's Connect() function
jint status = (*serverPtr).Connect (host, p_port, serverModuleName);
// Clean up object allocation for string copy after method is done
if (isHostCopy == JNI_TRUE)
p_env->ReleaseStringUTFChars(p_host, host);
if (isModCopy == JNI_TRUE)
p_env->ReleaseStringUTFChars(p_serverModuleName, serverModuleName);
cout << "ServerPtr in JNI_connect() after = " << (jint) serverPtr << endl;
return status;
* Class: JNIDSM_0005fServer
* Method: disconnect
* Signature: (I)V
* Disconnect from the Server.
JNIEXPORT void JNICALL Java_com_jni_JNIDSM_1Server_disconnect
(JNIEnv* p_env, jobject p_obj)
// Obtain pointer reference to corresponding DSM objects
DSMC_Server* serverPtr = (DSMC_Server*)
Java_com_jni_JNIDSM_1Server_getCID (p_env, p_obj);
cout << "ServerPtr in disconnect = " << (jint) serverPtr << endl;
// Call the DSM_Server object's Disconnect() function
(*serverPtr).Disconnect();
* Class: JNIDSM_0005fServer
* Method: doHeartbeat
* Signature: (I)I
* Check the status of the Server. Return status code that tells the caller Server' status.
JNIEXPORT jint JNICALL Java_com_jni_JNIDSM_1Server_doHeartbeat
(JNIEnv* p_env, jobject p_obj, jint p_serverPtr)
// Obtain pointer reference to corresponding DSM objects
DSMC_Server* serverPtr = (DSMC_Server*)
Java_com_jni_JNIDSM_1Server_getCID (p_env, p_obj);
// Call the DSM_Server object's DoHeartbeat() function
jint status = (*serverPtr).DoHeartbeat();
return status;
* Class: JNIDSM_0005fServer
* Method: getMaxPacketSize
* Signature: (I)I
* Return the packet size that are being sent through this network connection.
JNIEXPORT jint JNICALL Java_com_jni_JNIDSM_1Server_getMaxPacketSize
(JNIEnv* p_env, jclass p_class)
// Obtain pointer reference to corresponding DSM objects
DSMC_Server* serverPtr = (DSMC_Server*)
Java_com_jni_JNIDSM_1Server_getCID (p_env, p_obj);
// Call the DSM_Server object's GetMaxPacketSize() function
jint packetSize = (*serverPtr).GetMaxPacketSize();
return packetSize;
* Class: com_jni_JNIDSM_0005fServer
* Method: getCID
* Signature: ()J
JNIEXPORT jlong JNICALL Java_com_jni_JNIDSM_1Server_getCID
(JNIEnv* p_env, jobject p_obj)
jclass clazz;
clazz = p_env->GetObjectClass(cls);
fid = p_env->GetStaticFieldID(clazz, "cObjectID", "J");
jint id = p_env->GetStaticLongField(clazz, fid);
cout << "ServerPtr retrieved w/ getCID = " << id << endl;
return id;
* Class: com_jni_JNIDSM_0005fServer
* Method: setCID
* Signature: (J)V
JNIEXPORT void JNICALL Java_com_jni_JNIDSM_1Server_setCID
(JNIEnv* p_env, jobject p_obj, jint p_hndlr)
jclass clazz;
//create global reference
if (cls == 0) {
clazz = p_env->GetObjectClass(p_obj);
if (clazz == 0) {
cout << "Error occurred while getting class ref..."<< endl;
cls = p_env->NewGlobalRef(p_obj);
if (cls == 0) {
cout << "Error occurred trying to create Global ref..."<< endl;
fid = p_env->GetStaticFieldID(clazz, "cObjectID", "J");
} else
clazz = p_env->GetObjectClass(cls);
p_env->SetStaticLongField(clazz, fid, p_hndlr);
cout << "ServerPtr saved in setCID = " << p_hndlr << endl;
}

Answer 1

I have experience from the "other side" of JNI - calling into java from c - that may be relevant.
If a particular C thread uses JNI calls to create java objects, and another C thread accesses these java objects (or causes thier destruction), then a crash will ensue.
We use JNI extensively, and are now religious about making sure that only the thread that created an objects access it.