JNI（C访问Java方法）

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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, "<init>", "()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, "<init>", "([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, "<init>", "(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));    }}