【NDK】【009】JNI类型和Java类型对应关系，C++类对象和Java类对象互转

JNI类型和Java类型对应关系

Java类型JNI类型真实C++类型booleanjbooleanuint8_t，unsigned charbytejbyteint8_t，signed charcharjcharuint16_t，unsigned shortshortjshortint16_t，shortintjintint32_t，intlongjlongint64_t，long longfloatjfloatfloatdoublejdoubledoubleObjectjobjectc++ classClassjclassc++ class，继承jobjectStringjstringc++ class，继承jobjectboolean[]jbooleanArrayc++ class，继承jarray和jobjectbyte[]jbyteArrayc++ class，继承jarray和jobjectchar[]jcharArrayc++ class，继承jarray和jobjectshort[]jshortArrayc++ class，继承jarray和jobjectint[]jintArrayc++ class，继承jarray和jobjectlong[]jlongArrayc++ class，继承jarray和jobjectfloat[]jfloatArrayc++ class，继承jarray和jobjectdouble[]jdoubleArrayc++ class，继承jarray和jobjectObject[]jobjectArrayc++ class，继承jarray和jobjectClass[]jobjectArrayc++ class，继承jarray和jobjectString[]jobjectArrayc++ class，继承jarray和jobject

JNI类型和方法签名

在C++代码中，可以通过反射获取Java中的方法和类型

Java中的每个类型和方法，都对应一个唯一字符串，这个就是签名

Java类型签名booleanZbyteBcharCshortSintIlongLfloatFdoubleDObjectL+包名+分号[][+类型签名[][][[+类型签名方法(参数签名)+返回值签名

签名示例如下

Java类型签名StringLjava/lang/String;ObjectLjava/lang/Object;String[][Ljava/lang/String;int func(int i, Object object)(ILjava/lang/Object;)I

C++接收和创建Java类对象

    //通过JNI复制一个Java对象
    native Point native_copy_object(Point object);

	//调用JNI方法复制对象
    Point srcObject = new Point(5, 10);
    Point dstObject = native_copy_object(srcObject);

	//C++实现JNI函数
	extern "C" JNIEXPORT jobject JNICALL
	Java_easing_android_media_MediaPlayer_native_1copy_1object(JNIEnv *env, jobject obj, jobject srcObject) {
	   //找到Point对应的Class
	   jclass dstClazz = env->FindClass("android/graphics/Point");
	   //获取srcObject的字段值
	   jfieldID xFieldId = env->GetFieldID(dstClazz, "x", "I");
	   jfieldID yFieldId = env->GetFieldID(dstClazz, "y", "I");
	   jint x = env->GetIntField(srcObject, xFieldId);
	   jint y = env->GetIntField(srcObject, yFieldId);
	   //创建一个新的Point对象
	   jobject dstObject = env->AllocObject(dstClazz);
	   //拷贝srcObject的字段给dstObject
	   env->SetIntField(dstObject, xFieldId, x);
	   env->SetIntField(dstObject, yFieldId, y);
	   //返回新的Point给Java
	   return dstObject;
	}

Java接收和创建C++类对象

C++对象是没法传递给Java层的

因为在C++中，有jobject对应Java中的类型，但在Java中，却没有数据类型能够对应C++中的类类型

但我们却可以通过方法封装，快速将C++对象转为Java对象，再传递给Java，或是将收到的Java对象，快速转为C++对象

    struct Point {

        public:

        int x;
        int y;

        //从JavaObject创建一个C++Object
        static Point *fromJavaObject(JNIEnv *&env, jobject &interface, jobject &javaObject) {
            Point *point = new Point();
            jclass clazz = env->FindClass("android/graphics/Point");
            jfieldID xFieldId = env->GetFieldID(clazz, "x", "I");
            jfieldID yFieldId = env->GetFieldID(clazz, "y", "I");
            point->x = env->GetIntField(javaObject, xFieldId);
            point->y = env->GetIntField(javaObject, yFieldId);
            return point;
        }

        //从C++Object创建一个JavaObject
        static jobject toJavaObject(JNIEnv *&env, jobject &interface, Point *&cppObject) {
            Point *point = new Point();
            jclass clazz = env->FindClass("android/graphics/Point");
            jfieldID xFieldId = env->GetFieldID(clazz, "x", "I");
            jfieldID yFieldId = env->GetFieldID(clazz, "y", "I");
            jobject javaObject = env->AllocObject(clazz);
            env->SetIntField(javaObject, xFieldId, cppObject->x);
            env->SetIntField(javaObject, yFieldId, cppObject->y);
            return javaObject;
        }
    };