如何从 jni 函数返回多个变量，避免数组 mangaling 的开销？

最佳答案

我可以想出三种不同的方法。

回调

从您的 JNI 代码中调用一个带有多个参数的 Java 方法，在您的 Java 代码中的某处设置一个变量，您可以在从该方法返回时检索该变量。

JNIEXPORT void JNICALL Java_my_package_name_JNIReturnExample_returnWithJavaCallback(JNIEnv *env, jobject javaThis, jfloat param1, jfloat param2)
{
    // Get the class of the current calling object
    jclass clazz = (*env)->GetObjectClass(env, javaThis);

    // Get the method id of the instance method: void javaCallback(float, float) in my.package.name.JNIReturnExample
    jmethodID callback = (*env)->GetMethodID(env, clazz, "javaCallback", "(FF)V");

    // Calls my.package.name.JNIReturnExample#javaCallback(float, float);
    (*env)->CallVoidMethod(env, javaThis, callback, param1, param2);
}

返回一个新的 Java 对象

在JNI中实例化一个Java对象(my.package.name.JNIReturnExample)，并返回给Java。

JNIEXPORT jobject JNICALL Java_my_package_name_JNIReturnExample_returnObjectValue(JNIEnv *env, jobject javaThis, jfloat param1, jfloat param2)
{
    // Get the class we wish to return an instance of
    jclass clazz = (*env)->FindClass(env, "my/package/name/JNIReturnObject");

    // Get the method id of an empty constructor in clazz
    jmethodID constructor = (*env)->GetMethodID(env, clazz, "<init>", "()V");

    // Create an instance of clazz
    jobject obj = (*env)->NewObject(env, clazz, constructor);

    // Get Field references
    jfieldID param1Field = (*env)->GetFieldID(env, clazz, "param1", "F");
    jfieldID param2Field = (*env)->GetFieldID(env, clazz, "param2", "F");

    // Set fields for object
    (*env)->SetFloatField(env, obj, param1Field, param1);
    (*env)->SetFloatField(env, obj, param2Field, param2);

    // return object
    return obj;
}

传递一个 Java 对象作为参数，并设置它的字段

在您的 Java 代码中创建一个 Java 对象的新实例，并将该对象作为参数传递给您的 JNI 函数。

JNIEXPORT void JNICALL Java_my_package_name_JNIReturnExample_setObjectFields(JNIEnv *env, jobject javaThis, jobject obj, jfloat param1, jfloat param2)
{
    // Get the class of the input object
    jclass clazz = (*env)->GetObjectClass(env, obj);

    // Get Field references
    jfieldID param1Field = (*env)->GetFieldID(env, clazz, "param1", "F");
    jfieldID param2Field = (*env)->GetFieldID(env, clazz, "param2", "F");

    // Set fields for object
    (*env)->SetFloatField(env, obj, param1Field, param1);
    (*env)->SetFloatField(env, obj, param2Field, param2);
}

请注意，无论您决定使用哪种方法，都应该缓存各种 JNI 类型 jclass、jmethodID、jfieldID，因为 JNI 查找操作很慢，而且实际上只需要执行一次。

缓存

在回调方法中缓存 JNI 引用，并使用方法调用它们:

static jclass myCallbackClass;
static jmethodID myCallbackMethod;

/**
 * Call this method in JNI_OnLoad
 */ 
void CacheCallback()
{
    // Get a reference to the Callback class
    jclass clazz = (*env)->FindClass(env, "my/package/name/JNIReturnExample");

    // Store a global reference, since the local one will be freed when returning from the function.
    myCallbackClass = (*env)->NewGlobalRef(env, clazz);

    // Get a reference to the static callback method
    jmethodID callback = (*env)->GetStaticMethodID(env, myCallbackClass, "jniCallback", "(II)V");

    // jmethodID doesn't need a NewGlobalRef call
    myCallbackMethod = callback;
}

/**
 * Call this method in JNI_OnUnload
 */
void ReleaseCallback()
{
    (*env)->DeleteGlobalRef(env, myCallbackClass);
    myCallbackClass = NULL;

    // jmethodIDs are safe to keep without an explicit global reference, for this reason, we don't need to delete the reference either.
    myCallbackMethod = NULL;
}

关于java - android jni返回多个变量，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/29043872/