Android開發之串口編程原理和實現方式

提到串口編程，就不得不提到JNI，不得不提到JavaAPI中的文件描述符類：FileDescriptor。下面我分別對JNI、FileDescriptor以及串口的一些知識點和實現的源碼進行分析說明。這裡主要是參考了開源項目android-serialport-api。

串口編程需要了解的基本知識點：對於串口編程，我們只需對串口進行一系列的設置，然後打開串口，這些操作我們可以參考串口調試助手的源碼進行學習。在Java中如果要實現串口的讀寫功能只需操作文件設備類:FileDescriptor即可，其他的事都由驅動來完成不用多管！當然，你想了解，那就得看驅動代碼了。這裡並不打算對驅動進行說明，只初略闡述應用層的實現方式。

（一）JNI：
關於JNI的文章網上有很多，不再多做解釋，想詳細了解的朋友可以查看雲中漫步的技術文章，寫得很好，分析也很全面，那麼在這篇拙文中我強調3點：
1、如何將編譯好的SO文件打包到APK中？（方法很簡單，直接在工程目錄下新建文件夾 libs/armeabi，將SO文件Copy到此目錄即可）
2、命名要注意的地方？（在編譯好的SO文件中，將文件重命名為：libfilename.so即可。其中filename.so是編譯好後生成的文件）
3、MakeFile文件的編寫（不用多說，可以直接參考package/apps目錄下用到JNI的相關項目寫法）
這是關鍵的代碼：
復制代碼 代碼如下:
<span > 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;
}
}
</span>

（二）FileDescritor：
文件描述符類的實例用作與基礎機器有關的某種結構的不透明句柄，該結構表示開放文件、開放套接字或者字節的另一個源或接收者。文件描述符的主要實際用途是創建一個包含該結構的FileInputStream 或FileOutputStream。這是API的描述，不太好理解，其實可簡單的理解為：FileDescritor就是對一個文件進行讀寫。
（三）實現串口通信細節
1) 建工程：SerialDemo包名：org.winplus.serial，並在工程目錄下新建jni和libs兩個文件夾和一個org.winplus.serial.utils，如下圖：
2) 新建一個類：SerialPortFinder，添加如下代碼：
復制代碼 代碼如下:
<span >package org.winplus.serial.utils;

import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.LineNumberReader;
import java.util.Iterator;
import java.util.Vector;

import android.util.Log;

public class SerialPortFinder {

private static final String TAG = "SerialPort";

private Vector<Driver> mDrivers = null;

public class Driver {
public Driver(String name, String root) {
mDriverName = name;
mDeviceRoot = root;
}

private String mDriverName;
private String mDeviceRoot;
Vector<File> mDevices = null;

public Vector<File> getDevices() {
if (mDevices == null) {
mDevices = new Vector<File>();
File dev = new File("/dev");
File[] files = dev.listFiles();
int i;
for (i = 0; i < files.length; i++) {
if (files[i].getAbsolutePath().startsWith(mDeviceRoot)) {
Log.d(TAG, "Found new device: " + files[i]);
mDevices.add(files[i]);
}
}
}
return mDevices;
}

public String getName() {
return mDriverName;
}
}

Vector<Driver> getDrivers() throws IOException {
if (mDrivers == null) {
mDrivers = new Vector<Driver>();
LineNumberReader r = new LineNumberReader(new FileReader(
"/proc/tty/drivers"));
String l;
while ((l = r.readLine()) != null) {
// Issue 3:
// Since driver name may contain spaces, we do not extract
// driver name with split()
String drivername = l.substring(0, 0x15).trim();
String[] w = l.split(" +");
if ((w.length >= 5) && (w[w.length - 1].equals("serial"))) {
Log.d(TAG, "Found new driver " + drivername + " on "
+ w[w.length - 4]);
mDrivers.add(new Driver(drivername, w[w.length - 4]));
}
}
r.close();
}
return mDrivers;
}

public String[] getAllDevices() {
Vector<String> devices = new Vector<String>();
// Parse each driver
Iterator<Driver> itdriv;
try {
itdriv = getDrivers().iterator();
while (itdriv.hasNext()) {
Driver driver = itdriv.next();
Iterator<File> itdev = driver.getDevices().iterator();
while (itdev.hasNext()) {
String device = itdev.next().getName();
String value = String.format("%s (%s)", device,
driver.getName());
devices.add(value);
}
}
} catch (IOException e) {
e.printStackTrace();
}
return devices.toArray(new String[devices.size()]);
}

public String[] getAllDevicesPath() {
Vector<String> devices = new Vector<String>();
// Parse each driver
Iterator<Driver> itdriv;
try {
itdriv = getDrivers().iterator();
while (itdriv.hasNext()) {
Driver driver = itdriv.next();
Iterator<File> itdev = driver.getDevices().iterator();
while (itdev.hasNext()) {
String device = itdev.next().getAbsolutePath();
devices.add(device);
}
}
} catch (IOException e) {
e.printStackTrace();
}
return devices.toArray(new String[devices.size()]);
}
}
</span>

上面這個類在“android-serialport-api串口工具測試隨筆”中有詳細的說明，我就不多說了。
3）新建SerialPort類，這個類主要用來加載SO文件，通過JNI的方式打開關閉串口
復制代碼 代碼如下:
<span >package org.winplus.serial.utils;

import java.io.File;
import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import android.util.Log;

public class SerialPort {
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");
}
}
</span>

4） 新建一個MyApplication 繼承android.app.Application，用來對串口進行初始化和關閉串口
復制代碼 代碼如下:
<span >package org.winplus.serial;

import java.io.File;
import java.io.IOException;
import java.security.InvalidParameterException;

import org.winplus.serial.utils.SerialPort;
import org.winplus.serial.utils.SerialPortFinder;

import android.content.SharedPreferences;

public class MyApplication extends android.app.Application {
public SerialPortFinder mSerialPortFinder = new SerialPortFinder();
private SerialPort mSerialPort = null;

public SerialPort getSerialPort() throws SecurityException, IOException, InvalidParameterException {
if (mSerialPort == null) {
/* Read serial port parameters */
SharedPreferences sp = getSharedPreferences("android_serialport_api.sample_preferences", MODE_PRIVATE);
String path = sp.getString("DEVICE", "");
int baudrate = Integer.decode(sp.getString("BAUDRATE", "-1"));

/* Check parameters */
if ( (path.length() == 0) || (baudrate == -1)) {
throw new InvalidParameterException();
}

/* Open the serial port */
mSerialPort = new SerialPort(new File(path), baudrate, 0);
}
return mSerialPort;
}

public void closeSerialPort() {
if (mSerialPort != null) {
mSerialPort.close();
mSerialPort = null;
}
}
}
</span>

5） 新建一個繼承抽象的Activity類，主要用於讀取串口的信息
復制代碼 代碼如下:
<span >package org.winplus.serial;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.security.InvalidParameterException;

import org.winplus.serial.utils.SerialPort;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.DialogInterface.OnClickListener;
import android.os.Bundle;

public abstract class SerialPortActivity extends Activity {
protected MyApplication mApplication;
protected SerialPort mSerialPort;
protected OutputStream mOutputStream;
private InputStream mInputStream;
private ReadThread mReadThread;

private class ReadThread extends Thread {

@Override
public void run() {
super.run();
while (!isInterrupted()) {
int size;
try {
byte[] buffer = new byte[64];
if (mInputStream == null)
return;

/**
* 這裡的read要尤其注意，它會一直等待數據，等到天荒地老，海枯石爛。如果要判斷是否接受完成，只有設置結束標識，或作其他特殊的處理。
*/
size = mInputStream.read(buffer);
if (size > 0) {
onDataReceived(buffer, size);
}
} catch (IOException e) {
e.printStackTrace();
return;
}
}
}
}

private void DisplayError(int resourceId) {
AlertDialog.Builder b = new AlertDialog.Builder(this);
b.setTitle("Error");
b.setMessage(resourceId);
b.setPositiveButton("OK", new OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
SerialPortActivity.this.finish();
}
});
b.show();
}

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mApplication = (MyApplication) getApplication();
try {
mSerialPort = mApplication.getSerialPort();
mOutputStream = mSerialPort.getOutputStream();
mInputStream = mSerialPort.getInputStream();

/* Create a receiving thread */
mReadThread = new ReadThread();
mReadThread.start();
} catch (SecurityException e) {
DisplayError(R.string.error_security);
} catch (IOException e) {
DisplayError(R.string.error_unknown);
} catch (InvalidParameterException e) {
DisplayError(R.string.error_configuration);
}
}

protected abstract void onDataReceived(final byte[] buffer, final int size);

@Override
protected void onDestroy() {
if (mReadThread != null)
mReadThread.interrupt();
mApplication.closeSerialPort();
mSerialPort = null;
super.onDestroy();
}
}
</span>

6）編寫string.xml 以及baudrates.xml文件
在string.xml文件中添加：
復制代碼 代碼如下:
<span > <string name="error_configuration">Please configure your serial port first.</string>
<string name="error_security">You do not have read/write permission to the serial port.</string>
<string name="error_unknown">The serial port can not be opened for an unknown reason.</string>
</span>

在baudrates.xml文件中添加
復制代碼 代碼如下:
<span ><?xml version="1.0" encoding="utf-8"?>
<resources>

<string-array name="baudrates_name">
<item>50</item>
<item>75</item>
<item>110</item>
<item>134</item>
<item>150</item>
<item>200</item>
<item>300</item>
<item>600</item>
<item>1200</item>
<item>1800</item>
<item>2400</item>
<item>4800</item>
<item>9600</item>
<item>19200</item>
<item>38400</item>
<item>57600</item>
<item>115200</item>
<item>230400</item>
<item>460800</item>
<item>500000</item>
<item>576000</item>
<item>921600</item>
<item>1000000</item>
<item>1152000</item>
<item>1500000</item>
<item>2000000</item>
<item>2500000</item>
<item>3000000</item>
<item>3500000</item>
<item>4000000</item>
</string-array>
<string-array name="baudrates_value">
<item>50</item>
<item>75</item>
<item>110</item>
<item>134</item>
<item>150</item>
<item>200</item>
<item>300</item>
<item>600</item>
<item>1200</item>
<item>1800</item>
<item>2400</item>
<item>4800</item>
<item>9600</item>
<item>19200</item>
<item>38400</item>
<item>57600</item>
<item>115200</item>
<item>230400</item>
<item>460800</item>
<item>500000</item>
<item>576000</item>
<item>921600</item>
<item>1000000</item>
<item>1152000</item>
<item>1500000</item>
<item>2000000</item>
<item>2500000</item>
<item>3000000</item>
<item>3500000</item>
<item>4000000</item>
</string-array>

</resources>
</span>

7）開始編寫界面了：在main.xml布局文件中添加兩個編輯框，一個用來發送命令，一個用來接收命令：
復制代碼 代碼如下:
<span ><?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >

<EditText
android:id="@+id/EditTextReception"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:layout_weight="1"
android:gravity="top"
android:hint="Reception"
android:isScrollContainer="true"
android:scrollbar >
</EditText>

<EditText
android:id="@+id/EditTextEmission"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:hint="Emission"
android:lines="1" >
</EditText>

</LinearLayout>
</span>

8） SerialDemoActivity類的實現：
復制代碼 代碼如下:
<span >package org.winplus.serial;

import java.io.IOException;

import android.os.Bundle;
import android.view.KeyEvent;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.TextView.OnEditorActionListener;

public class SerialDemoActivity extends SerialPortActivity{
EditText mReception;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

// setTitle("Loopback test");
mReception = (EditText) findViewById(R.id.EditTextReception);

EditText Emission = (EditText) findViewById(R.id.EditTextEmission);
Emission.setOnEditorActionListener(new OnEditorActionListener() {
public boolean onEditorAction(TextView v, int actionId, KeyEvent event) {
int i;
CharSequence t = v.getText();
char[] text = new char[t.length()];
for (i=0; i<t.length(); i++) {
text[i] = t.charAt(i);
}
try {
mOutputStream.write(new String(text).getBytes());
mOutputStream.write('\n');
} catch (IOException e) {
e.printStackTrace();
}
return false;
}
});
}

@Override
protected void onDataReceived(final byte[] buffer, final int size) {
runOnUiThread(new Runnable() {
public void run() {
if (mReception != null) {
mReception.append(new String(buffer, 0, size));
}
}
});
}
}
</span>

寫到這裡，代碼基本上寫完了。下面就是要實現JNI層的功能了，要實現JNI，必須首先生成頭文件，頭文件的生成方式也很簡單， 我們編譯工程，在終端輸入 javah org.winplus.serial.utils.SerialPort 則會生成頭文件：org_winplus_serial_utils_SerialPort.h,這個頭文件的名字可以隨意命名。我們將它命名為：SerialPort.h拷貝到新建的目錄jni中，新建SerialPort.c 文件，這兩個文件的代碼就不貼出來了。直接到上傳的代碼中看吧。
（四）串口的應用，可實現掃描頭，指紋識別等外圍USB轉串口的特色應用
還蠻繁瑣的，以上只是對開源項目android-serialport-api 進行精簡想了解此項目請點擊此處！就這樣吧，晚了准備見周公去！