JNI各种类型转换总结

JNI本身并不复杂，难点在于类型转换上，下就对类型转换进行一个总结。

1、基本类型，直接转换

static jbyte byte_test_jni(JNIEnv *env, jclass clazz, jbyte bt)
{
	LOGD("%s:%u, static_use_byte!------bt :%d", __func__, __LINE__,(int)bt);
	jbyte ret = 't';
	return ret;
}

2、返回String

static jstring static_hello_jni(JNIEnv *env, jclass clazz)
{
	LOGD("%s:%u, static_hello", __func__, __LINE__);
	return (env)->NewStringUTF("static hello jni.");
}

3、类作为参数

（1）通过实例对象获取类

（2）通过类获得方法与字段ID

（3）进行处理

static void class_arg_test_jni(JNIEnv *env, jclass clazz, jobject obj)
{
	LOGD("%s:%u, class_arg_test_jni", __func__, __LINE__);
	//通过实例对象获取类
	jclass objClass = env->GetObjectClass(obj);
	if(objClass)
	{
	    /***通过类获取字段ID**********int*****/
	    jfieldID intID = env->GetFieldID(objClass,"nValue","I");
	    /***通过字段ID获取对象实例字段的值*********/
	    jint nValue = (int)env->GetIntField(obj,intID);
	    LOGD("%s:%u, class_arg_test_jni, nValue :%d", __func__, __LINE__, nValue );
	    /*****设置对象实例的字段的值********/
	    env->SetIntField(obj,intID,222);

	    //str
	    jfieldID strID = env->GetFieldID(objClass,"strValue","Ljava/lang/String;");
	    jstring str = env->NewStringUTF("XXXXXXXXXXXXOOOOOOOOOOOOOOOOO");
	    env->SetObjectField(obj,strID,str);

	    //array-----------如何设置nArr呢
	    jfieldID intArrID = env->GetFieldID(objClass,"nArr","[I");
	    jint nArr[8] = {8,7,6,5,4,3,2,1};
	    jintArray jnArray = env->NewIntArray(8);
	    env->SetIntArrayRegion(jnArray,0,8,nArr);
	    env->SetObjectField(obj,intArrID,jnArray);

	    //处理enum对象
	    /*******1、通过类获得字段ID***************/
	    jfieldID enumID = env->GetFieldID(objClass,"mdate","Lcom/example/data/TestClass$DateIn;");
	    /*******2、通过ID获取实例对象相应字段的值(对象实例)************/
	    jobject jenum = (jclass)env->GetObjectField(obj,enumID);
	    if(jenum == NULL) {
	    	LOGD("%s:%u, get enum failed", __func__, __LINE__);
	    	return;
	    }
	    LOGD("%s:%u, ok", __func__, __LINE__ );
	    /********3、通过类获取方法ID**************/
	    jclass jenumClass = (env)->FindClass("com/example/data/TestClass$DateIn");
	    jmethodID getVal = env->GetMethodID(jenumClass, "name", "()Ljava/lang/String;");

	    jstring value = (jstring)env->CallObjectMethod(jenum, getVal);
	    const char * valueNative = env->GetStringUTFChars(value, 0);

	    if (strcmp(valueNative, "MONDAY") == 0)
	    	LOGI("%s:%u, TODAY IS MONDAY!", __func__, __LINE__);
	    else if(strcmp(valueNative, "TUESDAY") == 0)
	    	LOGI("%s:%u, TODAY IS NOT TUESDAY!", __func__, __LINE__);
	   	else
	    	LOGI("%s:%u, TODAY", __func__, __LINE__);
	}
}

4、返回类

static jobject class_return_test_jni(JNIEnv *env, jclass clazz)
{
	LOGD("%s:%u, class_return_test_jni", __func__, __LINE__);
	//通过实例对象获取类
	jclass objectClass = (env)->FindClass("com/example/data/TestClass");
	//通过类获得构造函数
	jmethodID mid = (env)->GetMethodID(objectClass,"<init>","()V");

	/*********生成java中的类TestClass********/
	jobject  m_obj   = env->NewObject(objectClass,mid);
	LOGI("%s:%u, static_deal----GetMethodID", __func__, __LINE__ );

	/*******************通过类获取字段ID******************/
	jfieldID ival = (env)->GetFieldID(objectClass,"nValue","I");
	//jfieldID strID = env->GetFieldID(objectClass,"strValue","Ljava/lang/String;");

	/*****************在jni层操作java类中的变量*****************/
	(env)->SetIntField(m_obj,ival,100);
	return m_obj;
}

5、返回枚举

void enum_arg_test_jni(JNIEnv *env, jclass clazz, jobject obj2)
{
	LOGD("%s:%u, enum_arg_test_jni", __func__, __LINE__);
	//通过对象实例来获取类
	jclass enumclass= env->GetObjectClass(obj2);
	if(enumclass == NULL) {
		LOGD("%s:%u, get enum failed", __func__, __LINE__);
	       return;
	}
	 //name()函数是enum类型自带的函数，()Ljava/lang/String;是对应的签名
	//1、通过类获取方法ID
	jmethodID getVal = env->GetMethodID(enumclass, "name", "()Ljava/lang/String;");
	//2、获取根据ID方法，调用实例的name()方法
	jstring value = (jstring)env->CallObjectMethod(obj2, getVal);
	const char * valueNative = env->GetStringUTFChars(value, 0);

	if (strcmp(valueNative, "MONDAY") == 0)
		LOGI("%s:%u, TODAY IS MONDAY!", __func__, __LINE__);
	else if(strcmp(valueNative, "TUESDAY") == 0)
		LOGI("%s:%u, TODAY IS NOT TUESDAY!", __func__, __LINE__);
	else
		LOGI("%s:%u, TODAY", __func__, __LINE__);
}

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