使用JNI将字符串从java传递到c++

Passing strings from java to c++ using JNI

本文关键字：c++ java JNI 字符串 使用      更新时间：2023-10-16

我有一个android应用程序，可以接收未读Facebook通知和收件箱。应用程序必须在QT中完成，但我几乎不知道QT c++，所以我在java中开发应用程序，只是使用JNI从QT调用java类。这是工作正常，但事情是我需要发送一个插槽(在QT方面)，每次有一个新的Facebook通知/消息。

所以我的问题是:每分钟，我如何通知QT从Java，我有一个新的消息和发送字符串?

这是我的java代码:

主类:

public class MainActivity extends FragmentActivity {
...
static public void startFacebookActivity() {
    String msgTag = "FACEBOOK_APP";
    try {
        Activity mother = QtNative.activity();
        Intent intent = new Intent(mother, MainActivity.class);
        mother.startActivity(intent);
    } catch (Exception e) {
        Log.e(msgTag, e.toString());
        e.printStackTrace();
    }
}
}

FRAGMENT CLASS(每分钟验证是否有新的facebook消息，如果有，应该通知QT并发送消息，以便QT能够发送一个插槽)

private static native void publishNotification(String notification);
....
if (newNotification==true)
    publishNotification(responseNotification);
...

QT侧

facebookAndroid.cpp 

#include "facebookAndroid.h"
#include <QtAndroidExtras>
FacebookAndroid* FacebookAndroid::s_instance = 0;
FacebookAndroid::FacebookAndroid(QObject *parent) : QObject(parent) { s_instance = this;}
void FacebookAndroid::startAndroidFacebook() {
    QAndroidJniObject::callStaticMethod<void>("org.qtproject.example.MainActivity",
                                              "startFacebookActivity",
                                              "()V");
}
FacebookAndroid* FacebookAndroid::instance() {
    return s_instance;
}
static void publishNotification(JNIEnv *env, jclass /*clazz*/, jstring notification)      {
    const char* nativeString = env->GetStringUTFChars(notification, 0);
    FacebookAndroid::instance()->handleNewNotification(QString(nativeString));
}
static JNINativeMethod methods[] = {
    {"publishNotification", "(Ljava/lang/String;)V", (void *)publishNotification}
};
jint JNICALL JNI_OnLoad(JavaVM *vm, void *) {
    JNIEnv *env;
    if (vm->GetEnv(reinterpret_cast<void **>(&env), JNI_VERSION_1_4) != JNI_OK)
        return JNI_FALSE;
    jclass clazz = env->FindClass("org/qtproject/example/MainActivity");
    if (env->RegisterNatives(clazz, methods, sizeof(methods) / sizeof(methods[0])) < 0)
        return JNI_FALSE;
    return JNI_VERSION_1_4;
}

main.cpp 

 #include <QtGui/QGuiApplication>
 #include "qtquick2applicationviewer.h"
 #include <QtQuick>
 #include "facebookAndroid.h"
 int main(int argc, char *argv[])
 {
     QGuiApplication app(argc, argv);
     QtQuick2ApplicationViewer viewer;
     FacebookAndroid sa(&viewer);
     viewer.rootContext()->setContextProperty(QString("iniciaFacebook"), &sa);
     viewer.setMainQmlFile(QStringLiteral("qml/FacebookTry/main.qml"));
     viewer.showExpanded();
     return app.exec();
 }

facebookAndroid.h 

    #ifndef FACEBOOKANDROID_H
    #define FACEBOOKANDROID_H
    #include <QObject>
    #include <jni.h>
    class FacebookAndroid : public QObject {
        Q_OBJECT
    public:
        FacebookAndroid(QObject *parent = 0);
        FacebookAndroid* instance();
            void handleNewNotification(QString notification);
            protected:
            static FacebookAndroid *s_instance;
    public slots:
        void startAndroidFacebook();
    };
    #endif // FACEBOOKANDROID_H


In function 'void publisNotification(JNIEnv*, jclass,jstring)'
cannot call member function 'FacebookAnddroid::instance()' without object
   FacebookAndroid::instance()->handleNewNotification(QString(nativeString));
   in facebookAndroid.cpp

每个帮助都会很有帮助

如果我必须这样做，我可能会选择以下路径:

    定义一个本地Java方法。它将被用作来自Java端的"信号"为你的原生方法实现一个c++实现。操作要发布的实例(槽所有者)
  • 注册本地方法

在你的Java类中:

class Main Activity {
// ...
private static native void publishNotification(String notification);
// Call it from your Java code as it has a proper implementation
//...
    if (newNotification) {
        publishNotification(notification);
    }
//...

在c++/Qt端:

单例实现:

//in facebookandroid.h
class FacebookAndroid {
    public:
    FacebookAndroid* instance();
    void handleNewNotification(QString notification);
    protected:
    static FacebookAndroid *s_instance;
};
//in facebookandroid.cpp
FacebookAndroid* FacebookAndroid::s_instance = 0;
FacebookAndroid::FacebookAndroid(QObject *parent) : QObject(parent) {
    s_instance = this; // remind your first instanciation here
}
FacebookAndroid* FacebookAndroid::instance() {
    return s_instance;
}

本地方法实现:

//In facebookandroid.cpp
static void publishNotifcation(JNIEnv *env, jclass /*clazz*/, jstring notification)      {
    const char* nativeString = env->GetStringUTFChars(notification, 0);
    FacebookAndroid::instance()->handleNewNotification(QString(nativeString));
}

由于我们只能通过静态方法来做到这一点，我必须在我的类中定义一个单例，以特定地只访问一个实例。

注册方法:

//In facebookandroid.cpp
static JNINativeMethod methods[] = {
    {"publishNotification", "(Ljava/lang/String;)V", (void *)publishNotification}
};
jint JNICALL JNI_OnLoad(JavaVM *vm, void *) {
    JNIEnv *env;
    if (vm->GetEnv(reinterpret_cast<void **>(&env), JNI_VERSION_1_4) != JNI_OK)
        return JNI_FALSE;
    jclass clazz = env->FindClass("org/qtproject/example/MainActivity");
    if (env->RegisterNatives(clazz, methods, sizeof(methods) / sizeof(methods[0])) < 0)
        return JNI_FALSE;
    return JNI_VERSION_1_4;
}

经过一番研究，我发现了一个特别有用和完整的例子。它的目的是实现InApp购买，但其机制与我在这个答案中描述的完全相同。

既然你在Java中定期检查通知，你是否有可能从Qt/c++中定期调用Java类，然后接收数据?这可以很容易地完成使用QTimer，我看到你已经实现了java类从Qt的调用。

我认为RegisterNatives是这里的关键:

http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/functions.html wp17734 

static void sendToQT(JNIEnv *env, jclass clazz, jstring notification) {
  const char *GotInQTTheNotification = (*env)->GetStringUTFChars(env, notification, NULL);
      printf("Hello %sn", GotInQTTheNotification );
      (*env)->ReleaseStringUTFChars(env, notification, GotInQTTheNotification );
  }
}
static JNINativeMethod method_table[] = {
  { "sendToQT", "(Ljava/lang/String;I)V", (void *) sendToQT }
};

int main(int argc, char *argv[])
{
    JavaVM *vm;
    JNIEnv *env;
    /*
     * more of code
     */
    jclass clazz = (*env)->FindClass(env, "org/qtproject/example/MainActivity");
    jint ret = (*env)->RegisterNatives(env, clazz, method_table, method_table_size);    
    vm->DestroyJavaVM();
    return 0;
}

在mainactivity .java中包含一个本机声明。

public static void native sendToQT(String notification);
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