Question

In my android application I’ve got a callback from a native thread into Java code which needs to be synchronized with the main UI thread. The intention is that the UI thread display a list of options based on information returned from the native thread. Until the user selects an option the native thread needs to block. After the user selects an option the native thread reads the value and continues running.

I’ve tried to implement this solution using a ConditionVariable however I get a VM error with the comment indicating “Fatal spin-on-suspend, dumping threads”.

It looks as if it’s not possible to use a Java based synchronization object to synchronize these threads. The code works perfectly in the case where I’ve got two Java threads.

In general is there any way to use a Java based synchronization object to synchronize a Java and native thread, or does this need to be implemented using the NDK with a call from the Java thread into an NDK function that implements the synchronization?

Answer 1

The way to do this is not to use a Java based sync object but rather an NDK based sync object as follows:

static pthread_cond_t  uiConditionVariable  = PTHREAD_COND_INITIALIZER;
static pthread_mutex_t uiConditionMutex     = PTHREAD_MUTEX_INITIALIZER;



/**
 * This function opens the condition variable which releases waiting threads.
 */
JNIEXPORT void JNICALL
Java_com_Xxxx_openConditionVariable(JNIEnv *env,jobject o)
{
    pthread_mutex_lock(&uiConditionMutex);
    pthread_cond_signal(&uiConditionVariable);
    pthread_mutex_unlock(&uiConditionMutex);
}

/**
 * This function blocks on the condition variable associated with the 
 */
JNIEXPORT void JNICALL
Java_com_Xxxx_blockConditionVariable(JNIEnv *env,jobject o)
{
    pthread_mutex_lock(&uiConditionMutex);
    pthread_cond_wait(&uiConditionVariable,&uiConditionMutex);
    pthread_mutex_unlock(&uiConditionMutex);
}