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JNIEXPORT void JNICALL Java_pkg_Cls_f(JNIEnv *env, jclass cls, jstring jstr) { const jchar *cstr = (*env)->GetStringChars(env, jstr); if (c_str == NULL) { return; } if () { /* exception occurred */ (*env)->ReleaseStringChars(env, jstr, cstr); return; } /* normal return */ (*env)->ReleaseStringChars(env, jstr, cstr); }
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The rst call to ReleaseStringChars is issued when there is a pending exception The native method implementation releases the string resource and returns immediately afterwards without rst clearing the exception 623 Exceptions in Utility Functions Programmers writing utility functions should pay special attention to ensure that exceptions are propagated to the caller native method In particular, we emphasize the following two issues: Preferably, utility functions should provide a special return value to indicate that an exception has occurred This simpli es the caller s task of checking for pending exceptions In addition, utility functions should follow the rules ( 53) for managing local references in exception handling code To illustrate, let us introduce a utility function that performs a callback based on the name and descriptor of an instance method:
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jvalue JNU_CallMethodByName(JNIEnv *env, jboolean *hasException, jobject obj, const char *name, const char *descriptor, ) { va_list args; jclass clazz; jmethodID mid;
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jvalue result; if ((*env)->EnsureLocalCapacity(env, 2) == JNI_OK) { clazz = (*env)->GetObjectClass(env, obj); mid = (*env)->GetMethodID(env, clazz, name, descriptor); if (mid) { const char *p = descriptor; /* skip over argument types to find out the return type */ while (*p != ')') p++; /* skip ')' */ p++; va_start(args, descriptor); switch (*p) { case 'V': (*env)->CallVoidMethodV(env, obj, mid, args); break; case '[': case 'L': resultl = (*env)->CallObjectMethodV( env, obj, mid, args); break; case 'Z': resultz = (*env)->CallBooleanMethodV( env, obj, mid, args); break; case 'B': resultb = (*env)->CallByteMethodV( env, obj, mid, args); break; case 'C': resultc = (*env)->CallCharMethodV( env, obj, mid, args); break; case 'S': results = (*env)->CallShortMethodV( env, obj, mid, args); break; case 'I': resulti = (*env)->CallIntMethodV( env, obj, mid, args); break; case 'J': resultj = (*env)->CallLongMethodV( env, obj, mid, args); break; case 'F': resultf = (*env)->CallFloatMethodV(
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env, obj, mid, args); break; case 'D': resultd = (*env)->CallDoubleMethodV( env, obj, mid, args); break; default: (*env)->FatalError(env, "illegal descriptor"); } va_end(args); } (*env)->DeleteLocalRef(env, clazz); } if (hasException) { *hasException = (*env)->ExceptionCheck(env); } return result; } JNU_CallMethodByName takes, among other arguments, a pointer to a jboolean The jboolean will be set to JNI_FALSE if everything succeeds and to JNI_TRUE if an exception occurs at any point during the execution of this function This gives the caller of JNU_CallMethodByName an obvious way to check for pos-
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JNU_CallMethodByName rst makes sure that it can create two local references: one for the class reference and the other for the result returned from the method call Next, it obtains the class reference from the object and looks up the method ID Depending on the return type, the switch statement dispatches to the corresponding JNI method call function After the callback returns, if hasException is not NULL, we call ExceptionCheck to check for pending exceptions The ExceptionCheck function is new in Java 2 SDK release 12 It is similar to the ExceptionOccurred function The difference is that ExceptionCheck does not return a reference to the exception object, but returns JNI_TRUE when there is a pending exception and returns JNI_FALSE when there is no pending exception ExceptionCheck simpli es local reference management when the native code only needs to know whether an exception has occurred but needs not obtain a reference to the exception object The previous code would have to be rewritten as follows in JDK release 11: if (hasException) { jthrowable exc = (*env)->ExceptionOccurred(env); *hasException = exc != NULL; (*env)->DeleteLocalRef(env, exc); }
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The additional DeleteLocalRef call is necessary in order to delete the local reference to the exception object Using the JNU_CallMethodByName function we can rewrite the implementation of InstanceMethodCallnativeMethod in Section 42 as follows:
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JNIEXPORT void JNICALL Java_InstanceMethodCall_nativeMethod(JNIEnv *env, jobject obj) { printf("In C\n"); JNU_CallMethodByName(env, NULL, obj, "callback", "()V"); }
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We need not check for exceptions after the JNU_CallMethodByName call because the native method returns immediately afterwards
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