android_serialport_api hacking

/************************************************************************************
 *
 *                    android_serialport_api hacking
 *
 *  声明:
 *      1. 这是android_serialport_api的jni源代码解读;
 *      2. 源代码url: https://code.google.com/p/android-serialport-api/
 *      3. 可以从中知道JNI是如何查找类，创建对象，访问对象的属性等等内容;
 *
 *
 * Copyright 2009-2011 Cedric Priscal
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 *
 *
 * 相关参考：
 *    1. 访问JAVA中的字段(jfieldID): http://www.cnblogs.com/lijunamneg/archive/2012/12/22/2829023.html
 *    2. JNIEnv解析: http://blog.csdn.net/freechao/article/details/7692239
 *    3. The Java™ Native Interface Programmer’s Guide and Specification
 *
 *****************************************************************************/

#include <termios.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <jni.h>

#include "SerialPort.h"

#include "android/log.h"

/**
 * 定义一些宏，方便写调试代码
 */
static const char *TAG="serial_port";
#define LOGI(fmt, args...) __android_log_print(ANDROID_LOG_INFO,  TAG, fmt, ##args)
#define LOGD(fmt, args...) __android_log_print(ANDROID_LOG_DEBUG, TAG, fmt, ##args)
#define LOGE(fmt, args...) __android_log_print(ANDROID_LOG_ERROR, TAG, fmt, ##args)

static speed_t getBaudrate(jint baudrate)
{
    switch(baudrate) {
    case 0: return B0;
    case 50: return B50;
    case 75: return B75;
    case 110: return B110;
    case 134: return B134;
    case 150: return B150;
    case 200: return B200;
    case 300: return B300;
    case 600: return B600;
    case 1200: return B1200;
    case 1800: return B1800;
    case 2400: return B2400;
    case 4800: return B4800;
    case 9600: return B9600;
    case 19200: return B19200;
    case 38400: return B38400;
    case 57600: return B57600;
    case 115200: return B115200;
    case 230400: return B230400;
    case 460800: return B460800;
    case 500000: return B500000;
    case 576000: return B576000;
    case 921600: return B921600;
    case 1000000: return B1000000;
    case 1152000: return B1152000;
    case 1500000: return B1500000;
    case 2000000: return B2000000;
    case 2500000: return B2500000;
    case 3000000: return B3000000;
    case 3500000: return B3500000;
    case 4000000: return B4000000;
    default: return -1;
    }
}

/*
 * Class:     com_android_aplex_SerialPort
 * Method:    open
 * Signature: (Ljava/lang/String;II)Ljava/io/FileDescriptor;
 */
JNIEXPORT jobject JNICALL Java_com_android_aplex_SerialPort_open
  (JNIEnv *env, jclass thiz, jstring path, jint baudrate, jint flags)
{
    
    int fd;
    speed_t speed;
    jobject mFileDescriptor;    //保存文件描述符的对象引用

    // Check arguments 
    {
        speed = getBaudrate(baudrate);
        if (speed == -1) {
            // TODO: throw an exception 
            LOGE("Invalid baudrate");
            return NULL;
        }
    }

    // Opening device 
    {
        jboolean iscopy;
        /**
         * 将Java的字符串转换成C中的字符串
         */
        const char *path_utf = (*env)->GetStringUTFChars(env, path, &iscopy);
        LOGD("Opening serial port %s with flags 0x%x", path_utf, O_RDWR | flags);
        fd = open(path_utf, O_RDWR | flags);
        LOGD("open() fd = %d", fd);
        /**
         * 和前面的GetStringUTFChars一对用法，相当于malloc和free
         */
        (*env)->ReleaseStringUTFChars(env, path, path_utf);
        if (fd == -1)
        {
            // Throw an exception 
            LOGE("Cannot open port");
            // TODO: throw an exception 
            return NULL;
        }
    }

    // Configure device 
    {
        struct termios cfg;
        LOGD("Configuring serial port");
        if (tcgetattr(fd, &cfg))
        {
            LOGE("tcgetattr() failed");
            close(fd);
            // TODO: throw an exception 
            return NULL;
        }

        cfmakeraw(&cfg);
        cfsetispeed(&cfg, speed);
        cfsetospeed(&cfg, speed);

        if (tcsetattr(fd, TCSANOW, &cfg))
        {
            LOGE("tcsetattr() failed");
            close(fd);
            // TODO: throw an exception 
            return NULL;
        }
    }

    // Create a corresponding file descriptor 
    {
        /**
         * Returns a reference to the named class or interface.
         * 这个相当于在当前虚拟机加载的所有的类中找这个类：java/io/FileDescriptor
         */
        jclass cFileDescriptor = (*env)->FindClass(env, "java/io/FileDescriptor");
        /**
         * To obtain the method ID of a constructor, supply "<init>" as the method name and “V” as the return type.
         * 获取类中的无参构造函数
         */
        jmethodID iFileDescriptor = (*env)->GetMethodID(env, cFileDescriptor, "<init>", "()V");
        jfieldID descriptorID = (*env)->GetFieldID(env, cFileDescriptor, "descriptor", "I");
        /**
         * Constructs a new object. The method ID indicates which constructor method to invoke. This ID may be obtained by calling
         * GetMethodID with "<init>" as the method name and “V” as the return type. The constructor must be defined in the class
         * referred to by clazz, not one of its superclasses.
         * 生成一个类对象
         */
        mFileDescriptor = (*env)->NewObject(env, cFileDescriptor, iFileDescriptor);
        /**
         * Sets the value of an instance field of an object. The obj reference must not be NULL.
         * 设置对象的值
         */
        (*env)->SetIntField(env, mFileDescriptor, descriptorID, (jint)fd);
    }

    return mFileDescriptor;
    
}

/*
 * Class:     com_android_aplex_SerialPort
 * Method:    close
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_com_android_aplex_SerialPort_close
  (JNIEnv *env, jobject thiz)
{
    /**
     * Returns the class of an object. The obj reference must not be NULL.
     * thiz为java层传入的对象，GetObjectClass相当于获得这个对象的类，名字取得不错
     */
    jclass SerialPortClass = (*env)->GetObjectClass(env, thiz);
    /**
     * FindClass initializes the class or interface it returns.
     * 这个相当于在当前虚拟机加载的所有的类中找这个类：java/io/FileDescriptor
     */
    jclass FileDescriptorClass = (*env)->FindClass(env, "java/io/FileDescriptor");
    /**
     * Returns the field ID for an instance field of a class
     * 通过域名、域类型获取类对应的域ID号
     */
    jfieldID mFdID = (*env)->GetFieldID(env, SerialPortClass, "mFd", "Ljava/io/FileDescriptor;");
    jfieldID descriptorID = (*env)->GetFieldID(env, FileDescriptorClass, "descriptor", "I");
    /**
     * Returns the value of a field of an instance. The field to access is specified by a field ID.
     * 通过对象对应域的ID号获取域对象，或者值
     */
    jobject mFd = (*env)->GetObjectField(env, thiz, mFdID);
    jint descriptor = (*env)->GetIntField(env, mFd, descriptorID);

    LOGD("close(fd = %d)", descriptor);
    close(descriptor);
    
}