JNI（C访问Java方法）

JNI（C访问Java方法）

签名char *sig

项目链接地址:https://github.com/jiangwei1517/JniTest1

在项目bin目录下输入：

javap -s -p com.jiangwei.JniTest1

访问属性

JNIEXPORT jstring JNICALL Java_com_jiangwei_JniTest1_changeFieldFromC
(JNIEnv* env, jobject obj){
    jclass clz = (*env)->GetObjectClass(env, obj);
     // 获取java中name属性
    jfieldID fid = (*env)->GetFieldID(env, clz, "name", "Ljava/lang/String;");
    jstring str = (*env)->GetObjectField(env, obj, fid);
    // isCopy是否复制,true:新建一个内存地址，false：不复制，用java原有的地址
    char* old_char = (*env)->GetStringUTFChars (env, str, JNI_FALSE);
    char* new_char = strcat(old_char, "zhouwenkai");
    printf("new char = %s\n",new_char);
    jstring new_str = (*env)->NewStringUTF(env, new_char);
    (*env)->SetObjectField(env, obj, fid, new_str);
    return new_str;
}

静态属性访问

JNIEXPORT void JNICALL Java_com_jiangwei_JniTest1_changeAge
(JNIEnv* env, jobject jobj){
    jclass clz = (*env)->GetObjectClass(env, jobj);
    jfieldID fid = (*env)->GetStaticFieldID(env, clz, "age", "I");
    jint i = (*env)->GetStaticIntField(env, clz, fid);
    i++;
    (*env)->SetStaticIntField(env, clz, fid, i);
}

方法访问

JNIEXPORT void JNICALL Java_com_jiangwei_JniTest1_callMethod
(JNIEnv* env, jobject jobj){
    jclass clz = (*env)->GetObjectClass(env, jobj);
    jmethodID mid = (*env)->GetMethodID(env, clz, "getInt", "(I)I");
    (*env)->CallIntMethod(env, jobj, mid, 200);
}

静态方法访问

JNIEXPORT void JNICALL Java_com_jiangwei_JniTest1_callStaticMethod
(JNIEnv* env, jobject jobj){
    jclass clz = (*env)->GetObjectClass(env, jobj);
    jmethodID mid = (*env)->GetStaticMethodID(env, clz, "getMax", "(Ljava/lang/String;)Ljava/lang/String;");
//    (*env)->NewStringUTF(env, "1990");
    jstring str = (*env)->CallStaticObjectMethod(env, clz, mid, (*env)->NewStringUTF(env, "1990"));
    printf("getMax = %s", str);
}

访问构造方法

• 访问构造方法的method_id是””

• NewObject生成类的对象

  JNIEXPORT jlong JNICALL Java_com_jiangwei_JniTest1_callConstructors
  (JNIEnv* env, jobject jobj){
      jclass clz = (*env)->FindClass(env, "java/util/Date");
      jmethodID mid_constructor = (*env)->GetMethodID(env, clz, "", "()V");
      jobject date = (*env)->NewObject(env, clz, mid_constructor);
      jmethodID mid = (*env)->GetMethodID(env, clz, "getTime", "()J");
      jlong time = (*env)->CallLongMethod(env, date, mid);
      return time;
  }
  

访问父类的方法

C++ 覆盖父类方法需要用virtual关键字.
CallNonvirtualObjectMethod：调用父类的方法。

JNIEXPORT void JNICALL Java_com_jiangwei_JniTest1_callSuperMethod
(JNIEnv* env, jobject jobj){
    jclass clz = (*env)->GetObjectClass(env, jobj);
    jfieldID man_id = (*env)->GetFieldID(env, clz, "man", "Lcom/jiangwei/Man;");
    jobject obj_person = (*env)->GetObjectField(env, jobj, man_id);
    jclass person_clz = (*env)->FindClass(env, "com/jiangwei/Person");
    jmethodID mid = (*env)->GetMethodID(env, person_clz, "getName", "()Ljava/lang/String;");
    (*env)->CallNonvirtualObjectMethod(env, obj_person, person_clz, mid);
}

字符串乱码问题（C转难，从Java中转换）

• Windows下需要将字符串转换成”GB2312”

• Mac下无需转换，默认是UTF-8格式

  JNIEXPORT jstring JNICALL Java_com_jiangwei_JniTest1_getString
  (JNIEnv* env, jobject jobj, jstring str){
      printf("从java传过来的字符串是：%s\n", (*env)->GetStringUTFChars(env, str, JNI_FALSE));
      jstring new_str = (*env)->NewStringUTF(env, "我是小凯");
  
      // 字符串乱码问题，解决方案，从java中将UTF-8转成GB2312类型
      jclass clz = (*env)->FindClass(env, "java/lang/String");
      jmethodID cons_mid = (*env)->GetMethodID(env, clz, "", "([BLjava/lang/String;)V");
      jbyteArray byte_array = (*env)->NewByteArray(env, strlen(new_str));
      (*env)->SetByteArrayRegion(env, byte_array, 0, strlen(new_str), new_str);
      jstring format_str = (*env)->NewObject(env, clz, cons_mid, byte_array, (*env)->NewStringUTF(env, "GB2312"));
      return format_str;
  }
  

C传入数组排序

GetIntArrayElements将jintArray转换成C的jint*类型。ReleaseIntArrayElements修改完后必须释放才能对java代码起作用。

• JNI_COMMIT = 1 java代码修改，但是不释放C的内存空间
• JNI_ABORT = 2 java代码不修改，但是释放C的内存空间。

• 0 java代码修改，释放C的内存空间

  int sort(int* a, int* b){
      return (*a)-(*b);
  }
  
  JNIEXPORT jintArray JNICALL Java_com_jiangwei_JniTest1_sortArray
  (JNIEnv* env, jobject jobj, jintArray arr){
      jint* i = (*env)->GetIntArrayElements(env, arr, JNI_FALSE);
      qsort(i, (*env)->GetArrayLength(env, arr), sizeof(int), sort);
      (*env)->ReleaseIntArrayElements (env, arr, i, JNI_COMMIT);
      return arr;
  }
  

删除C本地引用

在C中，只能删除本地引用，不能创建，也不能获取，区别与全局引用。

JNIEXPORT void JNICALL Java_com_jiangwei_JniTest1_deleteLocalRef
(JNIEnv* env, jobject jobj){
    jclass clz = (*env)->FindClass(env, "java/lang/String");
    jmethodID cons_mid = (*env)->GetMethodID(env, clz, "", "(Ljava/lang/String;)V");
    jstring new_str = (*env)->NewObject(env, clz, cons_mid, (*env)->NewStringUTF(env, "HelloWorld"));
    printf("删除之前new_str:%s\n", (*env)->GetStringUTFChars(env, new_str, JNI_FALSE));
    (*env)->DeleteLocalRef(env, new_str);
    if ((*env)->GetStringUTFChars(env, new_str, JNI_FALSE) == NULL) {
        printf("%s\n", "DeleteLocalRef删除new_str成功！");
    }else{
        printf("%s\n", "DeleteLocalRef删除new_str失败！");
        printf("%s\n", (*env)->GetStringUTFChars(env, new_str, JNI_FALSE));
    }
}

创建、删除、获得全局引用

jstring global_ref;

JNIEXPORT jstring JNICALL Java_com_jiangwei_JniTest1_createGlobalRef
(JNIEnv* env, jobject jobj, jstring str){
    global_ref = (*env)->NewGlobalRef(env, str);
    return global_ref;
}

JNIEXPORT void JNICALL Java_com_jiangwei_JniTest1_deleteGlobalRef
(JNIEnv* env, jobject jobj){
    (*env)->DeleteGlobalRef(env,  global_ref);
}

JNIEXPORT jstring JNICALL Java_com_jiangwei_JniTest1_getGlobalRef
(JNIEnv* env, jobject jobj){
    return global_ref;
}

创建、删除、获得全局弱引用

jstring weak_str;
JNIEXPORT jstring JNICALL Java_com_jiangwei_JniTest1_createWeakGlobalRef
(JNIEnv* env, jobject jobj, jstring str){
    weak_str = (*env)->NewWeakGlobalRef(env, str);
    return weak_str;
}

JNIEXPORT jstring JNICALL Java_com_jiangwei_JniTest1_getWeakGlobalRef
(JNIEnv* env, jobject jobj){
    return weak_str;
}

JNIEXPORT void JNICALL Java_com_jiangwei_JniTest1_deleteWeakGlobalRef
(JNIEnv* env, jobject jobj){
    jclass clz = (*env)->GetObjectClass(env, jobj);

    // 假设出现了异常，C的异常正常是不会在java中补获，但是会阻塞java后续的执行，ExceptionClear清除C本地的异常，不影响java后续的执行
    jfieldID mid = (*env)->GetFieldID(env, clz, "woman", "Lcom/jiangwei/Person");
    jthrowable exception = (*env)->ExceptionOccurred(env);
    if (exception != NULL) {
        (*env)->ExceptionClear(env);
    }
    (*env)->DeleteGlobalRef(env, weak_str);
}

抛出异常，正常C的异常，Java不能补获

JNIEXPORT void JNICALL Java_com_jiangwei_JniTest1_throwException
(JNIEnv* env, jobject jobj){
    // C中抛出异常，交给java（try/catch）处理
    jclass clz = (*env)->FindClass(env, "java/lang/NullPointerException");
    (*env)->ThrowNew(env, clz, "Native error Java_com_jiangwei_JniTest1_throwException");
}

C的异常（清除后，不影响java后续代码执行）

jclass clz = (*env)->GetObjectClass(env, jobj);

// 假设出现了异常，C的异常正常是不会在java中补获，但是会阻塞java后续的执行，ExceptionClear清除C本地的异常，不影响java后续的执行
jfieldID mid = (*env)->GetFieldID(env, clz, "woman", "Lcom/jiangwei/Person");
jthrowable exception = (*env)->ExceptionOccurred(env);
if (exception != NULL) {
    (*env)->ExceptionClear(env);
}

缓存

// 缓存策略
static jstring name;
static jobject man;
JNIEXPORT void JNICALL Java_com_jiangwei_JniTest1_init
(JNIEnv* env, jclass jcls){
    jfieldID id_name = (*env)->GetStaticFieldID(env, jcls, "initA", "Ljava/lang/String;");
    name = (*env)->GetStaticObjectField(env, jcls, id_name);

    jfieldID id_man = (*env)->GetStaticFieldID(env, jcls, "initMan", "Lcom/jiangwei/Man;");
    man = (*env)->GetStaticObjectField(env, jcls, id_man);

    printf("初始化的init字段为%s\n", (*env)->GetStringUTFChars(env, name, JNI_FALSE));
}

JNIEXPORT void JNICALL Java_com_jiangwei_JniTest1_cache
(JNIEnv* env, jobject jobj){
    static jstring name = NULL;
    if (name == NULL) {
        name = (*env)->NewStringUTF(env, "init只执行了一次");
        printf("%s\n", (*env)->GetStringUTFChars(env, name, JNI_FALSE));
    }
}