前言

总所周知，android应用的编写是基于Java语言的，Java语言的特点决定了Java执行效率较底层语言会低很多，不像机器语言或者C语言这些能通过解释器直接转换成CPU能识别的机器码。

java执行的流程图.png

在一个数据量比较大的业务场景，例如图片、音视频、大文本处理等场景，使用JNI来调用C语言来执行比Java执行会有先天的优势，另一个原因就是在android平台上，C语言的native层的内存使用并不受JVM的内存分配限制。简单的说，能用更多的内存，更底层的语言来处理这些数据量大的业务。

什么是Android_JNI

可以参考Android NDK开发：JNI基础篇
简单的讲Jni就是提供了Java调用C语言的接口，并且屏蔽了Java层和C层之间的数据类型之间的差异，提供了C层和Java层互相调用的接口。

Android_JNI的使用

AndroidStudio在3.0以后集成了NDK开发对应的套件，不需要在单独的使用javac、javah来生成所需的Jni的声明文件(.h文件)和执行文件(.cpp文件)
方法的声明

extern "C"{
jintArray JNICALL
Java_com_wiki_testjni_MainActivity_getImgToGray(JNIEnv *env, jobject instance,
                                                       jintArray data_, jint w, jint h);
}

方法的申明只需要按照上面的格式，即Java_包名_native方法名即可，变量*env和instance是JNI方法默认带的参数，这个后面会提到。
方法的执行格式如下：

JNIEXPORT jintArray JNICALL
Java_com_wiki_testjni_MainActivity_getImgToGray(JNIEnv *env, jobject instance,
                                                       jintArray data_, jint w, jint h) {
    jint *data;
    data = env->GetIntArrayElements(data_, NULL);
    if (data == NULL) {
        return 0; /* exception occurred */
    }
    int alpha = 0xFF << 24;
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            // 获得像素的颜色
            int color = data[w * i + j];
            int red = ((color & 0x00FF0000) >> 16);
            int green = ((color & 0x0000FF00) >> 8);
            int blue = color & 0x000000FF;
            color = (red + green + blue) / 3;
            color = alpha | (color << 16) | (color << 8) | color;
            data[w * i + j] = color;
        }
    }
    int size=w * h;
    jintArray result = env->NewIntArray(size);
    env->SetIntArrayRegion(result, 0, size, data);
    env->ReleaseIntArrayElements(data_, data, 0);
    return result;
}

方法的执行格式如下JNIEXPORT +返回的java数据类型+JNICALL

Android_JNI的使用中重要的方法和变量

androidJNI使用中最重要的变量莫过于JNIEnv *env和jobject instance。
instance实际上是调用jni方法的对象的实例，这个不多讨论，主要讨论一下JNIEnv *env
首先看一下JNIEnv的代码

struct _JNIEnv {
    /* do not rename this; it does not seem to be entirely opaque */
    const struct JNINativeInterface* functions;

#if defined(__cplusplus)

    jint GetVersion()
    { return functions->GetVersion(this); }

    jclass DefineClass(const char *name, jobject loader, const jbyte* buf,
        jsize bufLen)
    { return functions->DefineClass(this, name, loader, buf, bufLen); }

    jclass FindClass(const char* name)
    { return functions->FindClass(this, name); }

    jmethodID FromReflectedMethod(jobject method)
    { return functions->FromReflectedMethod(this, method); }

    jfieldID FromReflectedField(jobject field)
    { return functions->FromReflectedField(this, field); }

    jobject ToReflectedMethod(jclass cls, jmethodID methodID, jboolean isStatic)
    { return functions->ToReflectedMethod(this, cls, methodID, isStatic); }

    jclass GetSuperclass(jclass clazz)
    { return functions->GetSuperclass(this, clazz); }

    jboolean IsAssignableFrom(jclass clazz1, jclass clazz2)
    { return functions->IsAssignableFrom(this, clazz1, clazz2); }

    jobject ToReflectedField(jclass cls, jfieldID fieldID, jboolean isStatic)
    { return functions->ToReflectedField(this, cls, fieldID, isStatic); }

    jint Throw(jthrowable obj)
    { return functions->Throw(this, obj); }

    jint ThrowNew(jclass clazz, const char* message)
    { return functions->ThrowNew(this, clazz, message); }

    jthrowable ExceptionOccurred()
    { return functions->ExceptionOccurred(this); }

    void ExceptionDescribe()
    { functions->ExceptionDescribe(this); }

    void ExceptionClear()
    { functions->ExceptionClear(this); }

    void FatalError(const char* msg)
    { functions->FatalError(this, msg); }

    jint PushLocalFrame(jint capacity)
    { return functions->PushLocalFrame(this, capacity); }

    jobject PopLocalFrame(jobject result)
    { return functions->PopLocalFrame(this, result); }

    jobject NewGlobalRef(jobject obj)
    { return functions->NewGlobalRef(this, obj); }

    void DeleteGlobalRef(jobject globalRef)
    { functions->DeleteGlobalRef(this, globalRef); }

    void DeleteLocalRef(jobject localRef)
    { functions->DeleteLocalRef(this, localRef); }

    jboolean IsSameObject(jobject ref1, jobject ref2)
    { return functions->IsSameObject(this, ref1, ref2); }

    jobject NewLocalRef(jobject ref)
    { return functions->NewLocalRef(this, ref); }

    jint EnsureLocalCapacity(jint capacity)
    { return functions->EnsureLocalCapacity(this, capacity); }

    jobject AllocObject(jclass clazz)
    { return functions->AllocObject(this, clazz); }

    jobject NewObject(jclass clazz, jmethodID methodID, ...)
    {
        va_list args;
        va_start(args, methodID);
        jobject result = functions->NewObjectV(this, clazz, methodID, args);
        va_end(args);
        return result;
    }

    jobject NewObjectV(jclass clazz, jmethodID methodID, va_list args)
    { return functions->NewObjectV(this, clazz, methodID, args); }

    jobject NewObjectA(jclass clazz, jmethodID methodID, const jvalue* args)
    { return functions->NewObjectA(this, clazz, methodID, args); }

    jclass GetObjectClass(jobject obj)
    { return functions->GetObjectClass(this, obj); }

    jboolean IsInstanceOf(jobject obj, jclass clazz)
    { return functions->IsInstanceOf(this, obj, clazz); }

    jmethodID GetMethodID(jclass clazz, const char* name, const char* sig)
    { return functions->GetMethodID(this, clazz, name, sig); }

#define CALL_TYPE_METHOD(_jtype, _jname)                                    \
    _jtype Call##_jname##Method(jobject obj, jmethodID methodID, ...)       \
    {                                                                       \
        _jtype result;                                                      \
        va_list args;                                                       \
        va_start(args, methodID);                                           \
        result = functions->Call##_jname##MethodV(this, obj, methodID,      \
                    args);                                                  \
        va_end(args);                                                       \
        return result;                                                      \
    }
#define CALL_TYPE_METHODV(_jtype, _jname)                                   \
    _jtype Call##_jname##MethodV(jobject obj, jmethodID methodID,           \
        va_list args)                                                       \
    { return functions->Call##_jname##MethodV(this, obj, methodID, args); }
#define CALL_TYPE_METHODA(_jtype, _jname)                                   \
    _jtype Call##_jname##MethodA(jobject obj, jmethodID methodID,           \
                                 const jvalue* args)                        \
    { return functions->Call##_jname##MethodA(this, obj, methodID, args); }

#define CALL_TYPE(_jtype, _jname)                                           \
    CALL_TYPE_METHOD(_jtype, _jname)                                        \
    CALL_TYPE_METHODV(_jtype, _jname)                                       \
    CALL_TYPE_METHODA(_jtype, _jname)

    CALL_TYPE(jobject, Object)
    CALL_TYPE(jboolean, Boolean)
    CALL_TYPE(jbyte, Byte)
    CALL_TYPE(jchar, Char)
    CALL_TYPE(jshort, Short)
    CALL_TYPE(jint, Int)
    CALL_TYPE(jlong, Long)
    CALL_TYPE(jfloat, Float)
    CALL_TYPE(jdouble, Double)

    void CallVoidMethod(jobject obj, jmethodID methodID, ...)
    {
        va_list args;
        va_start(args, methodID);
        functions->CallVoidMethodV(this, obj, methodID, args);
        va_end(args);
    }
    void CallVoidMethodV(jobject obj, jmethodID methodID, va_list args)
    { functions->CallVoidMethodV(this, obj, methodID, args); }
    void CallVoidMethodA(jobject obj, jmethodID methodID, const jvalue* args)
    { functions->CallVoidMethodA(this, obj, methodID, args); }

#define CALL_NONVIRT_TYPE_METHOD(_jtype, _jname)                            \
    _jtype CallNonvirtual##_jname##Method(jobject obj, jclass clazz,        \
        jmethodID methodID, ...)                                            \
    {                                                                       \
        _jtype result;                                                      \
        va_list args;                                                       \
        va_start(args, methodID);                                           \
        result = functions->CallNonvirtual##_jname##MethodV(this, obj,      \
                    clazz, methodID, args);                                 \
        va_end(args);                                                       \
        return result;                                                      \
    }
#define CALL_NONVIRT_TYPE_METHODV(_jtype, _jname)                           \
    _jtype CallNonvirtual##_jname##MethodV(jobject obj, jclass clazz,       \
        jmethodID methodID, va_list args)                                   \
    { return functions->CallNonvirtual##_jname##MethodV(this, obj, clazz,   \
        methodID, args); }
#define CALL_NONVIRT_TYPE_METHODA(_jtype, _jname)                           \
    _jtype CallNonvirtual##_jname##MethodA(jobject obj, jclass clazz,       \
        jmethodID methodID, const jvalue* args)                             \
    { return functions->CallNonvirtual##_jname##MethodA(this, obj, clazz,   \
        methodID, args); }

#define CALL_NONVIRT_TYPE(_jtype, _jname)                                   \
    CALL_NONVIRT_TYPE_METHOD(_jtype, _jname)                                \
    CALL_NONVIRT_TYPE_METHODV(_jtype, _jname)                               \
    CALL_NONVIRT_TYPE_METHODA(_jtype, _jname)

    CALL_NONVIRT_TYPE(jobject, Object)
    CALL_NONVIRT_TYPE(jboolean, Boolean)
    CALL_NONVIRT_TYPE(jbyte, Byte)
    CALL_NONVIRT_TYPE(jchar, Char)
    CALL_NONVIRT_TYPE(jshort, Short)
    CALL_NONVIRT_TYPE(jint, Int)
    CALL_NONVIRT_TYPE(jlong, Long)
    CALL_NONVIRT_TYPE(jfloat, Float)
    CALL_NONVIRT_TYPE(jdouble, Double)

    void CallNonvirtualVoidMethod(jobject obj, jclass clazz,
        jmethodID methodID, ...)
    {
        va_list args;
        va_start(args, methodID);
        functions->CallNonvirtualVoidMethodV(this, obj, clazz, methodID, args);
        va_end(args);
    }
    void CallNonvirtualVoidMethodV(jobject obj, jclass clazz,
        jmethodID methodID, va_list args)
    { functions->CallNonvirtualVoidMethodV(this, obj, clazz, methodID, args); }
    void CallNonvirtualVoidMethodA(jobject obj, jclass clazz,
        jmethodID methodID, const jvalue* args)
    { functions->CallNonvirtualVoidMethodA(this, obj, clazz, methodID, args); }

    jfieldID GetFieldID(jclass clazz, const char* name, const char* sig)
    { return functions->GetFieldID(this, clazz, name, sig); }

    jobject GetObjectField(jobject obj, jfieldID fieldID)
    { return functions->GetObjectField(this, obj, fieldID); }
    jboolean GetBooleanField(jobject obj, jfieldID fieldID)
    { return functions->GetBooleanField(this, obj, fieldID); }
    jbyte GetByteField(jobject obj, jfieldID fieldID)
    { return functions->GetByteField(this, obj, fieldID); }
    jchar GetCharField(jobject obj, jfieldID fieldID)
    { return functions->GetCharField(this, obj, fieldID); }
    jshort GetShortField(jobject obj, jfieldID fieldID)
    { return functions->GetShortField(this, obj, fieldID); }
    jint GetIntField(jobject obj, jfieldID fieldID)
    { return functions->GetIntField(this, obj, fieldID); }
    jlong GetLongField(jobject obj, jfieldID fieldID)
    { return functions->GetLongField(this, obj, fieldID); }
    jfloat GetFloatField(jobject obj, jfieldID fieldID)
    { return functions->GetFloatField(this, obj, fieldID); }
    jdouble GetDoubleField(jobject obj, jfieldID fieldID)
    { return functions->GetDoubleField(this, obj, fieldID); }

    void SetObjectField(jobject obj, jfieldID fieldID, jobject value)
    { functions->SetObjectField(this, obj, fieldID, value); }
    void SetBooleanField(jobject obj, jfieldID fieldID, jboolean value)
    { functions->SetBooleanField(this, obj, fieldID, value); }
    void SetByteField(jobject obj, jfieldID fieldID, jbyte value)
    { functions->SetByteField(this, obj, fieldID, value); }
    void SetCharField(jobject obj, jfieldID fieldID, jchar value)
    { functions->SetCharField(this, obj, fieldID, value); }
    void SetShortField(jobject obj, jfieldID fieldID, jshort value)
    { functions->SetShortField(this, obj, fieldID, value); }
    void SetIntField(jobject obj, jfieldID fieldID, jint value)
    { functions->SetIntField(this, obj, fieldID, value); }
    void SetLongField(jobject obj, jfieldID fieldID, jlong value)
    { functions->SetLongField(this, obj, fieldID, value); }
    void SetFloatField(jobject obj, jfieldID fieldID, jfloat value)
    { functions->SetFloatField(this, obj, fieldID, value); }
    void SetDoubleField(jobject obj, jfieldID fieldID, jdouble value)
    { functions->SetDoubleField(this, obj, fieldID, value); }

    jmethodID GetStaticMethodID(jclass clazz, const char* name, const char* sig)
    { return functions->GetStaticMethodID(this, clazz, name, sig); }

#define CALL_STATIC_TYPE_METHOD(_jtype, _jname)                             \
    _jtype CallStatic##_jname##Method(jclass clazz, jmethodID methodID,     \
        ...)                                                                \
    {                                                                       \
        _jtype result;                                                      \
        va_list args;                                                       \
        va_start(args, methodID);                                           \
        result = functions->CallStatic##_jname##MethodV(this, clazz,        \
                    methodID, args);                                        \
        va_end(args);                                                       \
        return result;                                                      \
    }
#define CALL_STATIC_TYPE_METHODV(_jtype, _jname)                            \
    _jtype CallStatic##_jname##MethodV(jclass clazz, jmethodID methodID,    \
        va_list args)                                                       \
    { return functions->CallStatic##_jname##MethodV(this, clazz, methodID,  \
        args); }
#define CALL_STATIC_TYPE_METHODA(_jtype, _jname)                            \
    _jtype CallStatic##_jname##MethodA(jclass clazz, jmethodID methodID,    \
                                       const jvalue* args)                  \
    { return functions->CallStatic##_jname##MethodA(this, clazz, methodID,  \
        args); }

#define CALL_STATIC_TYPE(_jtype, _jname)                                    \
    CALL_STATIC_TYPE_METHOD(_jtype, _jname)                                 \
    CALL_STATIC_TYPE_METHODV(_jtype, _jname)                                \
    CALL_STATIC_TYPE_METHODA(_jtype, _jname)

    CALL_STATIC_TYPE(jobject, Object)
    CALL_STATIC_TYPE(jboolean, Boolean)
    CALL_STATIC_TYPE(jbyte, Byte)
    CALL_STATIC_TYPE(jchar, Char)
    CALL_STATIC_TYPE(jshort, Short)
    CALL_STATIC_TYPE(jint, Int)
    CALL_STATIC_TYPE(jlong, Long)
    CALL_STATIC_TYPE(jfloat, Float)
    CALL_STATIC_TYPE(jdouble, Double)

    void CallStaticVoidMethod(jclass clazz, jmethodID methodID, ...)
    {
        va_list args;
        va_start(args, methodID);
        functions->CallStaticVoidMethodV(this, clazz, methodID, args);
        va_end(args);
    }
    void CallStaticVoidMethodV(jclass clazz, jmethodID methodID, va_list args)
    { functions->CallStaticVoidMethodV(this, clazz, methodID, args); }
    void CallStaticVoidMethodA(jclass clazz, jmethodID methodID, const jvalue* args)
    { functions->CallStaticVoidMethodA(this, clazz, methodID, args); }

    jfieldID GetStaticFieldID(jclass clazz, const char* name, const char* sig)
    { return functions->GetStaticFieldID(this, clazz, name, sig); }

    jobject GetStaticObjectField(jclass clazz, jfieldID fieldID)
    { return functions->GetStaticObjectField(this, clazz, fieldID); }
    jboolean GetStaticBooleanField(jclass clazz, jfieldID fieldID)
    { return functions->GetStaticBooleanField(this, clazz, fieldID); }
    jbyte GetStaticByteField(jclass clazz, jfieldID fieldID)
    { return functions->GetStaticByteField(this, clazz, fieldID); }
    jchar GetStaticCharField(jclass clazz, jfieldID fieldID)
    { return functions->GetStaticCharField(this, clazz, fieldID); }
    jshort GetStaticShortField(jclass clazz, jfieldID fieldID)
    { return functions->GetStaticShortField(this, clazz, fieldID); }
    jint GetStaticIntField(jclass clazz, jfieldID fieldID)
    { return functions->GetStaticIntField(this, clazz, fieldID); }
    jlong GetStaticLongField(jclass clazz, jfieldID fieldID)
    { return functions->GetStaticLongField(this, clazz, fieldID); }
    jfloat GetStaticFloatField(jclass clazz, jfieldID fieldID)
    { return functions->GetStaticFloatField(this, clazz, fieldID); }
    jdouble GetStaticDoubleField(jclass clazz, jfieldID fieldID)
    { return functions->GetStaticDoubleField(this, clazz, fieldID); }

    void SetStaticObjectField(jclass clazz, jfieldID fieldID, jobject value)
    { functions->SetStaticObjectField(this, clazz, fieldID, value); }
    void SetStaticBooleanField(jclass clazz, jfieldID fieldID, jboolean value)
    { functions->SetStaticBooleanField(this, clazz, fieldID, value); }
    void SetStaticByteField(jclass clazz, jfieldID fieldID, jbyte value)
    { functions->SetStaticByteField(this, clazz, fieldID, value); }
    void SetStaticCharField(jclass clazz, jfieldID fieldID, jchar value)
    { functions->SetStaticCharField(this, clazz, fieldID, value); }
    void SetStaticShortField(jclass clazz, jfieldID fieldID, jshort value)
    { functions->SetStaticShortField(this, clazz, fieldID, value); }
    void SetStaticIntField(jclass clazz, jfieldID fieldID, jint value)
    { functions->SetStaticIntField(this, clazz, fieldID, value); }
    void SetStaticLongField(jclass clazz, jfieldID fieldID, jlong value)
    { functions->SetStaticLongField(this, clazz, fieldID, value); }
    void SetStaticFloatField(jclass clazz, jfieldID fieldID, jfloat value)
    { functions->SetStaticFloatField(this, clazz, fieldID, value); }
    void SetStaticDoubleField(jclass clazz, jfieldID fieldID, jdouble value)
    { functions->SetStaticDoubleField(this, clazz, fieldID, value); }

    jstring NewString(const jchar* unicodeChars, jsize len)
    { return functions->NewString(this, unicodeChars, len); }

    jsize GetStringLength(jstring string)
    { return functions->GetStringLength(this, string); }

    const jchar* GetStringChars(jstring string, jboolean* isCopy)
    { return functions->GetStringChars(this, string, isCopy); }

    void ReleaseStringChars(jstring string, const jchar* chars)
    { functions->ReleaseStringChars(this, string, chars); }

    jstring NewStringUTF(const char* bytes)
    { return functions->NewStringUTF(this, bytes); }

    jsize GetStringUTFLength(jstring string)
    { return functions->GetStringUTFLength(this, string); }

    const char* GetStringUTFChars(jstring string, jboolean* isCopy)
    { return functions->GetStringUTFChars(this, string, isCopy); }

    void ReleaseStringUTFChars(jstring string, const char* utf)
    { functions->ReleaseStringUTFChars(this, string, utf); }

    jsize GetArrayLength(jarray array)
    { return functions->GetArrayLength(this, array); }

    jobjectArray NewObjectArray(jsize length, jclass elementClass,
        jobject initialElement)
    { return functions->NewObjectArray(this, length, elementClass,
        initialElement); }

    jobject GetObjectArrayElement(jobjectArray array, jsize index)
    { return functions->GetObjectArrayElement(this, array, index); }

    void SetObjectArrayElement(jobjectArray array, jsize index, jobject value)
    { functions->SetObjectArrayElement(this, array, index, value); }

    jbooleanArray NewBooleanArray(jsize length)
    { return functions->NewBooleanArray(this, length); }
    jbyteArray NewByteArray(jsize length)
    { return functions->NewByteArray(this, length); }
    jcharArray NewCharArray(jsize length)
    { return functions->NewCharArray(this, length); }
    jshortArray NewShortArray(jsize length)
    { return functions->NewShortArray(this, length); }
    jintArray NewIntArray(jsize length)
    { return functions->NewIntArray(this, length); }
    jlongArray NewLongArray(jsize length)
    { return functions->NewLongArray(this, length); }
    jfloatArray NewFloatArray(jsize length)
    { return functions->NewFloatArray(this, length); }
    jdoubleArray NewDoubleArray(jsize length)
    { return functions->NewDoubleArray(this, length); }

    jboolean* GetBooleanArrayElements(jbooleanArray array, jboolean* isCopy)
    { return functions->GetBooleanArrayElements(this, array, isCopy); }
    jbyte* GetByteArrayElements(jbyteArray array, jboolean* isCopy)
    { return functions->GetByteArrayElements(this, array, isCopy); }
    jchar* GetCharArrayElements(jcharArray array, jboolean* isCopy)
    { return functions->GetCharArrayElements(this, array, isCopy); }
    jshort* GetShortArrayElements(jshortArray array, jboolean* isCopy)
    { return functions->GetShortArrayElements(this, array, isCopy); }
    jint* GetIntArrayElements(jintArray array, jboolean* isCopy)
    { return functions->GetIntArrayElements(this, array, isCopy); }
    jlong* GetLongArrayElements(jlongArray array, jboolean* isCopy)
    { return functions->GetLongArrayElements(this, array, isCopy); }
    jfloat* GetFloatArrayElements(jfloatArray array, jboolean* isCopy)
    { return functions->GetFloatArrayElements(this, array, isCopy); }
    jdouble* GetDoubleArrayElements(jdoubleArray array, jboolean* isCopy)
    { return functions->GetDoubleArrayElements(this, array, isCopy); }

    void ReleaseBooleanArrayElements(jbooleanArray array, jboolean* elems,
        jint mode)
    { functions->ReleaseBooleanArrayElements(this, array, elems, mode); }
    void ReleaseByteArrayElements(jbyteArray array, jbyte* elems,
        jint mode)
    { functions->ReleaseByteArrayElements(this, array, elems, mode); }
    void ReleaseCharArrayElements(jcharArray array, jchar* elems,
        jint mode)
    { functions->ReleaseCharArrayElements(this, array, elems, mode); }
    void ReleaseShortArrayElements(jshortArray array, jshort* elems,
        jint mode)
    { functions->ReleaseShortArrayElements(this, array, elems, mode); }
    void ReleaseIntArrayElements(jintArray array, jint* elems,
        jint mode)
    { functions->ReleaseIntArrayElements(this, array, elems, mode); }
    void ReleaseLongArrayElements(jlongArray array, jlong* elems,
        jint mode)
    { functions->ReleaseLongArrayElements(this, array, elems, mode); }
    void ReleaseFloatArrayElements(jfloatArray array, jfloat* elems,
        jint mode)
    { functions->ReleaseFloatArrayElements(this, array, elems, mode); }
    void ReleaseDoubleArrayElements(jdoubleArray array, jdouble* elems,
        jint mode)
    { functions->ReleaseDoubleArrayElements(this, array, elems, mode); }

    void GetBooleanArrayRegion(jbooleanArray array, jsize start, jsize len,
        jboolean* buf)
    { functions->GetBooleanArrayRegion(this, array, start, len, buf); }
    void GetByteArrayRegion(jbyteArray array, jsize start, jsize len,
        jbyte* buf)
    { functions->GetByteArrayRegion(this, array, start, len, buf); }
    void GetCharArrayRegion(jcharArray array, jsize start, jsize len,
        jchar* buf)
    { functions->GetCharArrayRegion(this, array, start, len, buf); }
    void GetShortArrayRegion(jshortArray array, jsize start, jsize len,
        jshort* buf)
    { functions->GetShortArrayRegion(this, array, start, len, buf); }
    void GetIntArrayRegion(jintArray array, jsize start, jsize len,
        jint* buf)
    { functions->GetIntArrayRegion(this, array, start, len, buf); }
    void GetLongArrayRegion(jlongArray array, jsize start, jsize len,
        jlong* buf)
    { functions->GetLongArrayRegion(this, array, start, len, buf); }
    void GetFloatArrayRegion(jfloatArray array, jsize start, jsize len,
        jfloat* buf)
    { functions->GetFloatArrayRegion(this, array, start, len, buf); }
    void GetDoubleArrayRegion(jdoubleArray array, jsize start, jsize len,
        jdouble* buf)
    { functions->GetDoubleArrayRegion(this, array, start, len, buf); }

    void SetBooleanArrayRegion(jbooleanArray array, jsize start, jsize len,
        const jboolean* buf)
    { functions->SetBooleanArrayRegion(this, array, start, len, buf); }
    void SetByteArrayRegion(jbyteArray array, jsize start, jsize len,
        const jbyte* buf)
    { functions->SetByteArrayRegion(this, array, start, len, buf); }
    void SetCharArrayRegion(jcharArray array, jsize start, jsize len,
        const jchar* buf)
    { functions->SetCharArrayRegion(this, array, start, len, buf); }
    void SetShortArrayRegion(jshortArray array, jsize start, jsize len,
        const jshort* buf)
    { functions->SetShortArrayRegion(this, array, start, len, buf); }
    void SetIntArrayRegion(jintArray array, jsize start, jsize len,
        const jint* buf)
    { functions->SetIntArrayRegion(this, array, start, len, buf); }
    void SetLongArrayRegion(jlongArray array, jsize start, jsize len,
        const jlong* buf)
    { functions->SetLongArrayRegion(this, array, start, len, buf); }
    void SetFloatArrayRegion(jfloatArray array, jsize start, jsize len,
        const jfloat* buf)
    { functions->SetFloatArrayRegion(this, array, start, len, buf); }
    void SetDoubleArrayRegion(jdoubleArray array, jsize start, jsize len,
        const jdouble* buf)
    { functions->SetDoubleArrayRegion(this, array, start, len, buf); }

    jint RegisterNatives(jclass clazz, const JNINativeMethod* methods,
        jint nMethods)
    { return functions->RegisterNatives(this, clazz, methods, nMethods); }

    jint UnregisterNatives(jclass clazz)
    { return functions->UnregisterNatives(this, clazz); }

    jint MonitorEnter(jobject obj)
    { return functions->MonitorEnter(this, obj); }

    jint MonitorExit(jobject obj)
    { return functions->MonitorExit(this, obj); }

    jint GetJavaVM(JavaVM** vm)
    { return functions->GetJavaVM(this, vm); }

    void GetStringRegion(jstring str, jsize start, jsize len, jchar* buf)
    { functions->GetStringRegion(this, str, start, len, buf); }

    void GetStringUTFRegion(jstring str, jsize start, jsize len, char* buf)
    { return functions->GetStringUTFRegion(this, str, start, len, buf); }

    void* GetPrimitiveArrayCritical(jarray array, jboolean* isCopy)
    { return functions->GetPrimitiveArrayCritical(this, array, isCopy); }

    void ReleasePrimitiveArrayCritical(jarray array, void* carray, jint mode)
    { functions->ReleasePrimitiveArrayCritical(this, array, carray, mode); }

    const jchar* GetStringCritical(jstring string, jboolean* isCopy)
    { return functions->GetStringCritical(this, string, isCopy); }

    void ReleaseStringCritical(jstring string, const jchar* carray)
    { functions->ReleaseStringCritical(this, string, carray); }

    jweak NewWeakGlobalRef(jobject obj)
    { return functions->NewWeakGlobalRef(this, obj); }

    void DeleteWeakGlobalRef(jweak obj)
    { functions->DeleteWeakGlobalRef(this, obj); }

    jboolean ExceptionCheck()
    { return functions->ExceptionCheck(this); }

    jobject NewDirectByteBuffer(void* address, jlong capacity)
    { return functions->NewDirectByteBuffer(this, address, capacity); }

    void* GetDirectBufferAddress(jobject buf)
    { return functions->GetDirectBufferAddress(this, buf); }

    jlong GetDirectBufferCapacity(jobject buf)
    { return functions->GetDirectBufferCapacity(this, buf); }

    /* added in JNI 1.6 */
    jobjectRefType GetObjectRefType(jobject obj)
    { return functions->GetObjectRefType(this, obj); }
#endif /*__cplusplus*/
};

代码比较长，可以看到JNIEnv实际上就是一个函数列表的指针，函数列表里面封装了开发者用到的常用函数，这些函数大体分成两类，一类是类似GetStaticFieldID这些于java层交互的函数，这些函数的用途大体上就是生成java对象，调用java方法，设置java类实例的变量等等，另一类则是GetIntArrayElements，ReleaseIntArrayElements这些对于内存的分配回收等使用方法的封装，方便了开发者对于内存的管理。
值得注意的是，本人在android源码中并没有搜索到这些函数列表方法的具体实现，这些函数的实现应该是在系统初始化的时候从bootloader启动后从固态存储器读取系统后也把相应的这些函数实现读取到内存的不可修改的数据段中。

实现原理

足记里的图片处理加字幕，实际上就是针对图片添加相应的滤镜，对图片的每个像素进行修正，达到特点的效果，例如灰度效果，冷暖色调，底片效果等等。关于图片的处理大多是基于open gl的，本实例的代码也有一部分是根据open gl的代码转换成c代码。至于字幕的添加就比较简单了，通过布局将文本控件覆盖在图片上。

jni效果图.gif