【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
boolean	jboolean	uint8_t，unsigned char
byte	jbyte	int8_t，signed char
char	jchar	uint16_t，unsigned short
short	jshort	int16_t，short
int	jint	int32_t，int
long	jlong	int64_t，long long
float	jfloat	float
double	jdouble	double
Object	jobject	c++ class
Class	jclass	c++ class，继承jobject
String	jstring	c++ class，继承jobject
boolean[]	jbooleanArray	c++ class，继承jarray和jobject
byte[]	jbyteArray	c++ class，继承jarray和jobject
char[]	jcharArray	c++ class，继承jarray和jobject
short[]	jshortArray	c++ class，继承jarray和jobject
int[]	jintArray	c++ class，继承jarray和jobject
long[]	jlongArray	c++ class，继承jarray和jobject
float[]	jfloatArray	c++ class，继承jarray和jobject
double[]	jdoubleArray	c++ class，继承jarray和jobject
Object[]	jobjectArray	c++ class，继承jarray和jobject
Class[]	jobjectArray	c++ class，继承jarray和jobject
String[]	jobjectArray	c++ 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
boolean	Z
byte	B
char	C
short	S
int	I
long	L
float	F
double	D
Object	L+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
String	Ljava/lang/String;
Object	Ljava/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;
        }
    };