<h4>Issues with JNI that use FindClass</h4>

<p>

There may be issues when using a JNI implementation that uses FindClass 

in a function where the JNIEnv pointer is not available, such as in a C 

callback (bug <a href="https://bugs.eclipse.org/bugs/show_bug.cgi?id=125250">125250</a>).  The reason is that FindClass, in this case, uses the application

class loader to find the class.

If the desired class is in the classpath used for the application classloader

(e.g. defined by the VM argument -cp <classpath>), as it would typically be in 

a stand-alone application, there is no problem.  However, under

Eclipse, the application classloader does not have access to classes 

contained in plug-ins.  Eclipse uses its own class loader to find classes 

contained in plug-ins.

</p>

<p>

The proper plug-in class loader is used by FindClass in JNI functions which are 

passed the JNIEnv pointer, but not when you have to use AttachCurrentThread to get the 

JNIEnv pointer.  In this case the application classloader is used.

</p>

<p>

For example, the following will fail because AttachCurrentThread is used to 

get the JNIEnv pointer:

<pre>

static JavaVM* jvm;  // Global variable

void myCallback(void) {

    JNIEnv* env;

    jvm->AttachCurrentThread((void**)&env, NULL);

    // Fails if some/class is not in the application classloader:

    jclass cls = env->FindClass("some/class");

    jmethodID methodID = env->GetMethodID(cls, "methodName",

      "(Ljava/lang/String;)V or whatever signature");

    env->CallVoidMethod(callback, methodID, ...);

    jvm->DetachCurrentThread();

  }

}

</pre>

<p>

A solution is to cache the method ID, for example:

</p>

<pre>

static jmethodID mid;  // Global variable

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {

...

  // Store the JavaVM pointer

    jvm = vm;

  // Find the class and store the method ID

  // Will use the class loader that loaded the JNI library

    jclass cls = env->FindClass(className"some/class");

    if(!cls) goto ERR;

    mid = env->GetMethodID(cls, "methodName",

      "(Ljava/lang/String;)V or whatever signature");

    if(!mid) goto ERR;

...

}

void myCallback(void) {

    JNIEnv* env;

    jvm->AttachCurrentThread((void**)&env, NULL);

    env->CallVoidMethod(callback, mid, ...);

     // Handle error ...

    jvm->DetachCurrentThread();

  }

}

</pre>