Android開發之串口編程原理和實現方式

2020-04-11 12:34:45

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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。這里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：對于串口編程，我們只需對串口進行一系列的設置，然后打開串口，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。這里并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：
關于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云中漫步的技術文章，寫得很好，分析也很全面，那么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是編譯好后生成的文件）
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 style="font-size:18px;"> 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，并在工程目錄下新建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 style="font-size:18px;">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 style="font-size:18px;">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 style="font-size:18px;">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，主要用于讀取串口的信息

復制代碼代碼如下:

 
<span style="font-size:18px;">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; 

/** 
* 這里的read要尤其注意，它會一直等待數據，等到天荒地老，?？菔癄€。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 style="font-size:18px;"> <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 style="font-size:18px;"><?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 style="font-size:18px;"><?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:scrollbarStyle="insideOverlay" > 
</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 style="font-size:18px;">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> 

寫到這里，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，晚了準備見周公去！

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