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 : return B0;
     case : return B50;
     case : return B75;
     case : return B110;
     case : return B134;
     case : return B150;
     case : return B200;
     case : return B300;
     case : return B600;
     case : return B1200;
     case : return B1800;
     case : return B2400;
     case : return B4800;
     case : return B9600;
     case : return B19200;
     case : return B38400;
     case : return B57600;
     case : return B115200;
     case : return B230400;
     case : return B460800;
     case : return B500000;
     case : return B576000;
     case : return B921600;
     case : return B1000000;
     case : return B1152000;
     case : return B1500000;
     case : return B2000000;
     case : return B2500000;
     case : return B3000000;
     case : return B3500000;
     case : return B4000000;
     default: return -;
     }
 }

 /*
  * 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 == -) {
             // 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 == -)
         {
             // 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);

 }