java - JNI 回调调用失败

Question

我目前正在为 Tox 聊天库编写 JNI 包装器。很多事情都是通过回调来处理的(比如接收消息、好友请求等)。为了处理这些问题，我已经实现了一些有效的东西(到目前为止，我不明白为什么它停止工作):

在主类中，我定义了一个允许设置回调的方法，如下所示:

    /**
 * Native call to tox_callback_friendrequest
 * 
 * @param messengerPointer
 *            pointer to the internal messenger struct
 * @param callback
 *            the callback to set for receiving friend requests
 */
private native void tox_onfriendrequest(long messengerPointer,
        OnFriendRequestCallback callback);

/**
 * Method used to set a callback method for receiving friend requests. Any
 * time a friend request is received on this Tox instance, the
 * {@link OnFriendRequestCallback#execute(String, String)} method will be
 * executed.
 * 
 * @param callback
 *            the callback to set for receiving friend requests
 * @throws ToxException
 *             if the instance has been killed
 */
public void setOnFriendRequestCallback(OnFriendRequestCallback callback)
        throws ToxException {
    lock.lock();
    try {
        checkPointer();

        tox_onfriendrequest(this.messengerPointer, callback);
    } finally {
        lock.unlock();
    }
}

公共(public)java方法只需调用以下 native 代码:

JNIEXPORT void JNICALL Java_im_tox_jtoxcore_JTox_tox_1onfriendrequest(
        JNIEnv * env, jobject obj, jlong messenger, jobject callback) {
    tox_jni_globals_t *_messenger = (tox_jni_globals_t *) messenger;
    if (_messenger->frqc) {
        if (_messenger->frqc->jobj) {
            (*env)->DeleteGlobalRef(env, _messenger->frqc->jobj);
        }
        free(_messenger->frqc);
    }

    friendrequest_callback_t *data = malloc(sizeof(friendrequest_callback_t));
    data->env = env;
    data->jobj = (*env)->NewGlobalRef(env, callback);
    (*env)->DeleteLocalRef(env, callback);
    _messenger->frqc = data;
    tox_callback_friendrequest(_messenger->tox, (void *) callback_friendrequest,
            data);
}

static void callback_friendrequest(uint8_t *pubkey, uint8_t *message,
        uint16_t length, void *ptr) {
    friendrequest_callback_t *data = ptr;

    jclass clazz = (*data->env)->GetObjectClass(data->env, data->jobj);
    jmethodID meth = (*data->env)->GetMethodID(data->env, clazz, "execute",
            "(Ljava/lang/String;[B)V");

    char buf[ADDR_SIZE_HEX] = { 0 };
    addr_to_hex(pubkey, buf);
    jstring _pubkey = (*data->env)->NewStringUTF(data->env, buf);
    jbyteArray _message = (*data->env)->NewByteArray(data->env, length);
    (*data->env)->SetByteArrayRegion(data->env, _message, 0, length, message);

    printf("definitely happening"); //printf debugging. This IS being printed.
    fflush(stdout);
    (*data->env)->CallVoidMethod(data->env, data->jobj, meth, _pubkey, message); //this does not seem to be called
}

这段代码相当复杂，但它的作用如下:首先，它检查是否已经为此操作设置了回调，如果是，则将其删除。然后，它为收到的 Callback 对象创建一个新的全局引用，并保存该引用以及指向信使结构的 JNIEnv 指针。最后，它调用Tox-API提供的回调方法，并注册回调static void callback_friendrequest

在不透明的void指针ptr中，我们存储JNIEnv和Callback对象。回调类如下所示:

package im.tox.jtoxcore.test;

import im.tox.jtoxcore.JTox;
import im.tox.jtoxcore.ToxException;
import im.tox.jtoxcore.callbacks.OnFriendRequestCallback;

public class TestOnFriendRequestCallback extends OnFriendRequestCallback {

    public TestOnFriendRequestCallback(JTox jtox) {
        super(jtox);
    }

    @Override
    public void execute(String publicKey, byte[] message) {
        String msg;
        if (message == null) {
            msg = "No mesage";
        } else {
            try {
                msg = JTox.getByteString(message);
            } catch (ToxException e) {
                msg = "ERROR: Unable to get message";
            }
        }
        System.out.println("We received a friend request from " + publicKey
                + " with the following message: " + msg);

        try {
            jtox.confirmRequest(publicKey);
            System.out.println("confirmed!");
        } catch (ToxException e) {
            System.out.println("Unable to confirm friend request");
        }
    }
}

现在，当我通过另一个客户端发送好友请求时，注册回调后，它会打印“肯定会发生”，这是我引入的调试语句。 CallVoidMetod 调用不会被执行，因为它绝对不会打印我收到了 FriendRequest，也不会打印它已被确认。

对我来说，这意味着回调已成功注册，但由于某种原因，Java 方法从未被调用。

完整代码可在 GitHub 上获取:https://github.com/Tox/jToxcore

Answer 1

我怀疑失败的原因是您将 Java 环境指针存储在名为 data 的对象中。 env 指针无法以这种方式缓存，并且在调用回调时可能无效。有关如何处理此问题的更多信息，请参阅 this question .