最近在Android项目中要使用到串口编程,开始的时候为了省事,直接把以前在linux下用纯C写得串口程序封装成so库,再在JNI中调用so库,一点也没有问题。
虽说没有什么问题,总觉得在JAVA中使用纯C实现串口所有的操作很像是在“挂羊头卖狗肉”,而且也有点繁琐,想说JAVA应该把这些东西直接封装成API,于是在网上查资料,想找到类似于windows下的CreateFile的API接口,未果。
还好JAVA之中有个FileDescriptor类,可以把串口当作一个FileDescriptor,打开之后操作FileDescriptor就相当于操作串口。
不管是windows、linux、或是android操作系统,串口编程无非是以下几步:
- 打开串口
- 串口配置
- 串口操作(读写)
- 关闭串口
这是典型的流驱动设备,操作很简单,在C语言里无非就是open、write、read、close几个操作就能轻松搞定的东西。这跟文件的输入输出流其实是一个概念,所以在windows中可以用CreateFile操作串口,同样的道理,在java中可以使用FileDescriptor操作串口。
下面是android 串口编程的具体步骤,先创建一个串口类SerialPort:
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- private static final String TAG = "SerialPort";
- /*
- * Do not remove or rename the field mFd: it is used by native method close();
- */
- private FileDescriptor mFd;
- private FileInputStream mFileInputStream;
- private FileOutputStream mFileOutputStream;
- public SerialPort(File device, int baudrate, int flags) throws SecurityException, IOException {
- /* Check access permission */
- if (!device.canRead() || !device.canWrite()) {
- try {
- /* Missing read/write permission, trying to chmod the file */
- Process su;
- su = Runtime.getRuntime().exec("/system/bin/su");
- String cmd = "chmod 666 " + device.getAbsolutePath() + "\n"
- + "exit\n";
- su.getOutputStream().write(cmd.getBytes());
- if ((su.waitFor() != 0) || !device.canRead()
- || !device.canWrite()) {
- throw new SecurityException();
- }
- } catch (Exception e) {
- e.printStackTrace();
- throw new SecurityException();
- }
- }
- mFd = open(device.getAbsolutePath(), baudrate, flags);
- if (mFd == null) {
- Log.e(TAG, "native open returns null");
- throw new IOException();
- }
- mFileInputStream = new FileInputStream(mFd);
- mFileOutputStream = new FileOutputStream(mFd);
- }
- // Getters and setters
- public InputStream getInputStream() {
- return mFileInputStream;
- }
- public OutputStream getOutputStream() {
- return mFileOutputStream;
- }
- // JNI
- private native static FileDescriptor open(String path, int baudrate, int flags);
- public native void close();
- static {
- System.loadLibrary("serial_port");
- }
这个类中主要干了两件事情:
创建了打开串口和关闭串口的本地方法
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- private native static FileDescriptor open(String path, int baudrate, int flags);
- public native void close();
关联串口的文件描述符(FileDescriptor),并把文件的输入输出流与之关联,在代码之中
[java] view plain copy
- private FileDescriptor mFd;
- private FileInputStream mFileInputStream;
- private FileOutputStream mFileOutputStream;
- mFd = open(device.getAbsolutePath(), baudrate, flags);
- mFileInputStream = new FileInputStream(mFd);
- mFileOutputStream = new FileOutputStream(mFd);
这段代码的意思就是把串口打开,并取得串口的输入输出操作(读写操作)。
到了这里,有个问题,如何打开串口?
代码上可知,打开串口的操作用的是一个本地方法open,那只能在JNI中实现这个接口,下面是jni中的实现:
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- 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: android_serialport_SerialPort
- * Method: open
- * Signature: (Ljava/lang/String;II)Ljava/io/FileDescriptor;
- */
- JNIEXPORT jobject JNICALL Java_android_1serialport_1api_SerialPort_open
- (JNIEnv *env, jclass thiz, jstring path, jint baudrate, jint flags)
- {
- int fd;
- speed_t speed;
- jobject mFileDescriptor;
- /* Check arguments */
- {
- speed = getBaudrate(baudrate);
- if (speed == -1) {
- /* TODO: throw an exception */
- LOGE("Invalid baudrate");
- return NULL;
- }
- }
- /* Opening device */
- {
- jboolean iscopy;
- 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);
- (*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 */
- {
- jclass cFileDescriptor = (*env)->FindClass(env, "java/io/FileDescriptor");
- jmethodID iFileDescriptor = (*env)->GetMethodID(env, cFileDescriptor, "<init>", "()V");
- jfieldID descriptorID = (*env)->GetFieldID(env, cFileDescriptor, "descriptor", "I");
- mFileDescriptor = (*env)->NewObject(env, cFileDescriptor, iFileDescriptor);
- (*env)->SetIntField(env, mFileDescriptor, descriptorID, (jint)fd);
- }
- return mFileDescriptor;
- }
- /*
- * Class: cedric_serial_SerialPort
- * Method: close
- * Signature: ()V
- */
- JNIEXPORT void JNICALL Java_android_1serialport_1api_SerialPort_close
- (JNIEnv *env, jobject thiz)
- {
- jclass SerialPortClass = (*env)->GetObjectClass(env, thiz);
- jclass FileDescriptorClass = (*env)->FindClass(env, "java/io/FileDescriptor");
- jfieldID mFdID = (*env)->GetFieldID(env, SerialPortClass, "mFd", "Ljava/io/FileDescriptor;");
- jfieldID descriptorID = (*env)->GetFieldID(env, FileDescriptorClass, "descriptor", "I");
- jobject mFd = (*env)->GetObjectField(env, thiz, mFdID);
- jint descriptor = (*env)->GetIntField(env, mFd, descriptorID);
- LOGD("close(fd = %d)", descriptor);
- close(descriptor);
- }
在Java_android_1serialport_1api_SerialPort_open接口中,能发现一些熟悉的东西:
[cpp] view plain copy
- fd = open(path_utf, O_RDWR | flags);
- tcgetattr(fd, &cfg)
- cfmakeraw(&cfg);
- cfsetispeed(&cfg, speed);
- cfsetospeed(&cfg, speed);
[cpp] view plain copy
- ...
这些都是在linux下的串口编程,分别是打开串口设备节点,配置数据流控、波特率等,这些东西其实不是重点,重点是如何把打开的串口设备与FileDescriptor关联,即把fd封装到FileDescriptor中去:
[cpp] view plain copy
- /* Create a corresponding file descriptor */
- {
- jclass cFileDescriptor = (*env)->FindClass(env, "java/io/FileDescriptor");
- jmethodID iFileDescriptor = (*env)->GetMethodID(env, cFileDescriptor, "<init>", "()V");
- jfieldID descriptorID = (*env)->GetFieldID(env, cFileDescriptor, "descriptor", "I");
- mFileDescriptor = (*env)->NewObject(env, cFileDescriptor, iFileDescriptor);
- (*env)->SetIntField(env, mFileDescriptor, descriptorID, (jint)fd);
- }
- return mFileDescriptor;
上面的代码简单解释来说就是mFileDescriptor与fd关联并返回给JAVA层。
至此,其实串口的JAVA封装已经完成,这个类其实已经可以完成串口的所有操作(打开,配置,读写,关闭),接下来的事情只是逻辑层面的事情了。