JNI基础知识（二）C/C++访问Java的成员

下面介绍下C语言对Java属性以及方法的访问情况以及一些需要注意的地方。

1、访问java静态方法

//java方法
public native static String getStaticStringFromC();

//c代码
//函数实现
JNIEXPORT jstring JNICALL Java_com_zhufk_JniTest_getStaticStringFromC
(JNIEnv * env, jclass jcls){
	return (*env)->NewStringUTF(env, "C String");
}

2、访问java非静态方法

//java
public native String getStringFromC(int i);

//c
//函数实现
JNIEXPORT jstring JNICALL Java_com_zhufk_JniTest_getStringFromC
(JNIEnv * env, jobject jobj,jint num){
	return (*env)->NewStringUTF(env, "C String2");
}

 注：静态方法实现参数是jclass代表的是对象本身，而非静态方法实现参数是jobject代表的是对象实例。 

 3、非静态属性

//java
public String key="zhufk";
public native void accessField();

//c
//访问非静态属性
JNIEXPORT void JNICALL Java_com_zhufk_JniTest_accessField
(JNIEnv * env, jobject jobj){
	//jobj是t的对象，JniTest.class
	jclass cls = (*env)->GetObjectClass(env, jobj);
	//jfield id
	//属性名称、属性签名
	jfieldID fid = (*env)->GetFieldID(env, cls, "key", "Ljava/lang/String;");
	//获取key属性的值
	//get<Type>Field
	jstring jstr = (*env)->GetObjectField(env, jobj, fid);
	//jstring转char C语言
	char* cstr = (*env)->GetStringUTFChars(env, jstr, JNI_FALSE);
	//拼接新字符串
	char text[20] = "good";
	strcat(text, cstr);

	//c char转jni jstring
	jstring new_jstr = (*env)->NewStringUTF(env, text);
	//修改key
	//set<Type>Field
	(*env)->SetObjectField(env, jobj, fid, new_jstr);
}

4、静态属性

//java
public static int age=20;
public native void accessStaticField();

//c
//访问静态属性
JNIEXPORT void JNICALL Java_com_zhufk_JniTest_accessStaticField
(JNIEnv * env, jobject jobj){
	jclass jcls = (*env)->GetObjectClass(env, jobj);
	jfieldID fid = (*env)->GetStaticFieldID(env, jcls, "age", "I");
	jint age = (*env)->GetStaticIntField(env, jobj, fid);
	age++;
	(*env)->SetStaticIntField(env, jobj, fid, age);
}

 5、非静态方法(实现方法)

//java
public native void accessMethod();
public int getRandomInt(int max) {
	System.out.println("genRandomInt 执行了...");
	return new Random().nextInt(max);
}
public static void main(String[] args) {
    JniTest t=new JniTest();
    t.accessMethod();
}

//c
//访问非静态方法
JNIEXPORT void JNICALL Java_com_zhufk_JniTest_accessMethod
(JNIEnv * env, jobject jobj){
	jclass cls = (*env)->GetObjectClass(env, jobj);
	jmethodID mid = (*env)->GetMethodID(env, cls, "getRandomInt", "(I)I");
	jint random = (*env)->CallIntMethod(env, jobj, mid, 200);
	printf("random num:%ld", random);
}

6、静态方法(实现方法)

//java
public native void accessStaticMethod();
public static String getUUID() {
	System.out.println("getUUID 执行了...");
	return UUID.randomUUID().toString();
}
public static void main(String[] args) {
    JniTest t=new JniTest();
    t.accessStaticMethod();
}

//c
//访问静态方法
JNIEXPORT void JNICALL Java_com_zhufk_JniTest_accessStaticMethod
(JNIEnv * env, jobject jobj){
	jclass jcls = (*env)->GetObjectClass(env, jobj);
	jmethodID jmethod_id = (*env)->GetStaticMethodID(env, jcls, "getUUID", "()Ljava/lang/String;");
	//调用
	//callStatic<Type>Method
	jstring uuid = (*env)->CallStaticObjectMethod(env, jcls, jmethod_id);

	char* uuid_str = (*env)->GetStringUTFChars(env, uuid, JNI_FALSE);
	char filename[100];
	sprintf(filename, "D://%s.txt", uuid_str);
	FILE *fp = fopen(filename, "w");
	fputs("i love hongting", fp);
	fclose(fp);
}

7、构造方法

//java
public native Date accessConstructor();

//c
JNIEXPORT jobject JNICALL Java_com_zhufk_JniTest_accessConstructor
(JNIEnv * env, jobject jobj){
	jclass cls = (*env)->FindClass(env, "java/util/Date");
	jmethodID constructor_mid = (*env)->GetMethodID(env, cls, "<init>", "()V");
	jobject data_obj = (*env)->NewObject(env, cls, constructor_mid);
	jmethodID get_time_mid = (*env)->GetMethodID(env, cls, "getTime", "()J");
	jlong time = (*env)->CallLongMethod(env, data_obj, get_time_mid);
	printf("\ntime:%lld", time);

	return data_obj;
}

8、父类方法

//java
public Human human=new Man();
public native void accessNonvirtualMethod();

//c
//调用父类的方法
JNIEXPORT void JNICALL Java_com_zhufk_JniTest_accessNonvirtualMethod
(JNIEnv *env, jobject jobj){
	jclass cls = (*env)->GetObjectClass(env, jobj);
	//获取man属性（对象）
	jfieldID fid = (*env)->GetFieldID(env, cls, "human", "Lcom/zhufk/jni/Human;");
	//获取
	jobject human_obj = (*env)->GetObjectField(env, jobj, fid);

	//执行sayHi方法
	jclass human_cls = (*env)->FindClass(env, "com/zhufk/jni/Human"); //注意：传父类的名称
	jmethodID mid = (*env)->GetMethodID(env, human_cls, "sayHi", "()V");

	//执行
	//(*env)->CallObjectMethod(env, human_obj, mid);
	//调用的父类的方法
	(*env)->CallNonvirtualObjectMethod(env, human_obj, human_cls, mid);
}

 9、中文乱码问题

//java
public native String chineseChars(String in);

//c
JNIEXPORT jobject JNICALL Java_com_zhufk_JniTest_chineseChars
(JNIEnv * env, jobject jobj,jstring in){
	/*char* cstr = (*env)->GetStringUTFChars(env, in, JNI_FALSE);
	printf("%s\n", cstr);*/

	char* c_str = "李晨和范冰冰";
	//jstring jstr = (*env)->NewStringUTF(env, c);

	jclass str_cls = (*env)->FindClass(env, "java/lang/String");
	jmethodID str_mid = (*env)->GetMethodID(env, str_cls, "<init>", "([BLjava/lang/String;)V");

	jbyteArray bytes = (*env)->NewByteArray(env, strlen(c_str));
	(*env)->SetByteArrayRegion(env, bytes, 0, strlen(c_str), c_str);
	jstring charsetName = (*env)->NewStringUTF(env, "GB2312");

	return (*env)->NewObject(env, str_cls, str_mid, bytes, charsetName);
}

 10、数组

//java
public native void giveArray(int[] array);
public native int[] getArray(int len);

//c
int compare(int *a, int *b){
	return (*a) - (*b);
}

JNIEXPORT void JNICALL Java_com_zhufk_JniTest_giveArray
(JNIEnv * env, jobject jobj, jintArray array){
	jint* elems = (*env)->GetIntArrayElements(env, array, NULL);

	jint size = (*env)->GetArrayLength(env, array);

	qsort(elems, size, sizeof(jint), compare);

	(*env)->ReleaseIntArrayElements(env, array, elems, 0);
}

JNIEXPORT jintArray JNICALL Java_com_zhufk_JniTest_getArray
(JNIEnv * env, jobject jobj, jint len){
	jintArray jint_arr = (*env)->NewIntArray(env, len);

	jint *elems = (*env)->GetIntArrayElements(env, jint_arr, NULL);

	int i = 0;
	for (; i < len;i++){
		elems[i] = i;
	}
	(*env)->ReleaseIntArrayElements(env, jint_arr, elems, 0);
	return jint_arr;
}

11、引用

//java
//局部引用
public native void localRef();
	
//全局引用
public native void createGlobalRef();
	
public native String getGlobalRef();
	
public native void deleteGlobalRef();
	
//弱全局引用
public native void creatWeakGlobalRef();
public native String getWeakGlobalRef();
public native void deleteWeakGlobalRef();

//c
//JNI 引用变量
//引用类型：局部引用和全局引用
//作用：在JNI中告知虚拟机何时收回一个JNI变量
JNIEXPORT void JNICALL Java_com_zhufk_JniTest_localRef
(JNIEnv * env, jobject jobj){
	int i = 0;
	for (; i < 5; i++){
		jclass cls = (*env)->FindClass(env, "java/lang/Date");
		jmethodID constructor_mid = (*env)->GetMethodID(env, cls, "<init>", "()V");
		jobject obj = (*env)->NewObject(env, cls, constructor_mid);

		(*env)->DeleteLocalRef(env, obj);
	}
}

//全局引用
jstring global_str;

JNIEXPORT void JNICALL Java_com_zhufk_JniTest_createGlobalRef
(JNIEnv * env, jobject jobj){
	jstring str = (*env)->NewStringUTF(env, "jni development is powerful!");
	global_str = (*env)->NewGlobalRef(env, str);
}

JNIEXPORT jobject JNICALL Java_com_zhufk_JniTest_getGlobalRef
(JNIEnv * env, jobject jobj){
	return global_str;
}

JNIEXPORT void JNICALL Java_com_zhufk_JniTest_deleteGlobalRef
(JNIEnv * env, jobject jobj){
	(*env)->DeleteGlobalRef(env, global_str);
}

 12、异常

//java
public native void exeception();
public static void main(String[] args) {
    JniTest t=new JniTest();
    try {
		t.exeception();						
	} catch (Throwable e) {
		System.out.println("发生异常："+e.getMessage());
	}
}


//c
JNIEXPORT void JNICALL Java_com_zhufk_JniTest_exeception
(JNIEnv * env, jobject jobj){
	jclass cls = (*env)->GetObjectClass(env, jobj);
	jfieldID field_id = (*env)->GetFieldID(env, cls, "key2", "Ljava/lang/String;");
	//检测是否发生Java异常
	jthrowable exception = (*env)->ExceptionOccurred(env);
	if (exception != NULL){
		(*env)->ExceptionClear(env);

		field_id = (*env)->GetFieldID(env, cls, "key", "Ljava/lang/String;");
	}

	jstring j_str = (*env)->GetObjectField(env, jobj, field_id);
	char* c_str = (*env)->GetStringUTFChars(env, j_str, NULL);

	if (_stricmp(c_str, "superzhufk_") != 0)
	{
		jclass newExcCls = (*env)->FindClass(env, "java/lang/IllegalArgumentException");
		(*env)->ThrowNew(env, newExcCls, "key's value is invalid!");
	}
}

13、缓存

//java
public native void cached();
	
//初始化全局变量
public static native void initIds();
public static void main(String[] args) {
    for (int i = 0; i < 5; i++) {
		t.cached();
	}
}

//加载动态库
static{
    System.loadLibrary("JniTest");
    initIds();
}

//c
//缓存策略
JNIEXPORT void JNICALL Java_com_zhufk_JniTest_cached
(JNIEnv * env, jobject jobj){
	jclass cls = (*env)->GetObjectClass(env, jobj);
	//局部静态变量
	static jfieldID fid = NULL;
	if (fid == NULL){
		fid = (*env)->GetFieldID(env, cls, "key", "Ljava/lang/String;");
		printf("-----GetFieldID----\n");
	}
}

//初始化全局变量
jfieldID key_fid;
jmethodID random_mid;
JNIEXPORT void JNICALL Java_com_zhufk_JniTest_initIds
(JNIEnv * env, jclass cls){
	key_fid = (*env)->GetFieldID(env, cls, "key", "Ljava/lang/String;");
	random_mid = (*env)->GetMethodID(env, cls, "getRandomInt", "(I)I");
}

 

---

 

---