ZXing是一个开放源码的，用Java实现的多种格式的条码扫描与生成库。
Zxing的GITHUB地址：Zxing

ZXing支持的格式：

1D product 1D industrial 2D
UPC-A Code 39 QR Code
UPC-E Code 93 Data Matrix
EAN-8 Code 128 Aztec (beta)
EAN-13 Codabar PDF 417 (beta)
ITF MaxiCode
RSS-14
RSS-Expanded

zxing代码库结构及其介绍如下：

Module Description
core The core image decoding library, and test code
javase JavaSE-specific client code
android Android client Barcode Scanner
android-integration Supports integration with Barcode Scanner via Intent
android-core Android-related code shared among android, other Android apps
zxingorg The source behind zxing.org
zxing.appspot.com The source behind web-based barcode generator at zxing.appspot.com

这篇文章主要分析android上是如何调用zxing去识别二维码的。
先在GITHUB上下载源码，目前最新的版本是3.3.3。解压并打开文件夹，我们主要看android、android-core和core这三个文件夹。

android文件夹：

在“zxing-zxing-3.3.3\android\src\com\google\zxing\client\android”下有android的src代码。

CaptureActivity.java

主界面是CaptureActivity.java。在CaptureActivity.java中，cameraManager用于调用摄像头。
摄像头的初始化：

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private void initCamera(SurfaceHolder surfaceHolder) {
  if (surfaceHolder == null) {
    throw new IllegalStateException("No SurfaceHolder provided");
  }
  if (cameraManager.isOpen()) {
    Log.w(TAG, "initCamera() while already open -- late SurfaceView callback?");
    return;
  }
  try {
    cameraManager.openDriver(surfaceHolder);
    // Creating the handler starts the preview, which can also throw a RuntimeException.
    if (handler == null) {
      handler = new CaptureActivityHandler(this, decodeFormats, decodeHints, characterSet, cameraManager);
    }
    decodeOrStoreSavedBitmap(null, null);
  } catch (IOException ioe) {
    Log.w(TAG, ioe);
    displayFrameworkBugMessageAndExit();
  } catch (RuntimeException e) {
    // Barcode Scanner has seen crashes in the wild of this variety:
    // java.?lang.?RuntimeException: Fail to connect to camera service
    Log.w(TAG, "Unexpected error initializing camera", e);
    displayFrameworkBugMessageAndExit();
  }
}

可以看到在打开摄像头后，即调用CaptureActivityHandler进行识别操作：

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cameraManager.openDriver(surfaceHolder);
// Creating the handler starts the preview, which can also throw a RuntimeException.
if (handler == null) {
  handler = new CaptureActivityHandler(this, decodeFormats, decodeHints, characterSet, cameraManager);
}

摄像头预览的内容是放在名为preview_view的SurfaceView容器里的。CaptureActivity extends Activity implements SurfaceHolder.Callback并重写了这三个Surface的Callback函数：

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@Override
public void surfaceCreated(SurfaceHolder holder) {
  if (holder == null) {
    Log.e(TAG, "*** WARNING *** surfaceCreated() gave us a null surface!");
  }
  if (!hasSurface) {
    hasSurface = true;
    initCamera(holder);
  }
}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {
  hasSurface = false;
}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
  // do nothing
}

使用 public void handleDecode(Result rawResult, Bitmap barcode, float scaleFactor)来处理返回到这层的识别的数据：

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/**
 * A valid barcode has been found, so give an indication of success and show the results.
 *
 * @param rawResult The contents of the barcode.
 * @param scaleFactor amount by which thumbnail was scaled
 * @param barcode   A greyscale bitmap of the camera data which was decoded.
 */
public void handleDecode(Result rawResult, Bitmap barcode, float scaleFactor) {

这其中，处理又有两种方式：内部方式和外部方式。

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switch (source) {
  case NATIVE_APP_INTENT:
  case PRODUCT_SEARCH_LINK:
    handleDecodeExternally(rawResult, resultHandler, barcode);
    break;
  case ZXING_LINK:
    if (scanFromWebPageManager == null || !scanFromWebPageManager.isScanFromWebPage()) {
      handleDecodeInternally(rawResult, resultHandler, barcode);
    } else {
      handleDecodeExternally(rawResult, resultHandler, barcode);
    }
    break;
  case NONE:
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
    if (fromLiveScan && prefs.getBoolean(PreferencesActivity.KEY_BULK_MODE, false)) {
      Toast.makeText(getApplicationContext(),
                     getResources().getString(R.string.msg_bulk_mode_scanned) + " (" + rawResult.getText() + ')',
                     Toast.LENGTH_SHORT).show();
      maybeSetClipboard(resultHandler);
      // Wait a moment or else it will scan the same barcode continuously about 3 times
      restartPreviewAfterDelay(BULK_MODE_SCAN_DELAY_MS);
    } else {
      handleDecodeInternally(rawResult, resultHandler, barcode);
    }
    break;+
}

handleDecodeInternally主要是在UI上做显示：

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// Put up our own UI for how to handle the decoded contents.
private void handleDecodeInternally(Result rawResult, ResultHandler resultHandler, Bitmap barcode) {

而handleDecodeExternally则进一步往上一层传，如果外部有调用的话：

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// Briefly show the contents of the barcode, then handle the result outside Barcode Scanner.
private void handleDecodeExternally(Result rawResult, ResultHandler resultHandler, Bitmap barcode) {

CaptureActivityHandler.java

在CaptureActivity调用了CaptureActivityHandler后，参数仅一步传递到DecodeThread:

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CaptureActivityHandler(CaptureActivity activity,
                       Collection<BarcodeFormat> decodeFormats,
                       Map<DecodeHintType,?> baseHints,
                       String characterSet,
                       CameraManager cameraManager) {
  this.activity = activity;
  decodeThread = new DecodeThread(activity, decodeFormats, baseHints, characterSet,
      new ViewfinderResultPointCallback(activity.getViewfinderView()));
  decodeThread.start();
  state = State.SUCCESS;

  // Start ourselves capturing previews and decoding.
  this.cameraManager = cameraManager;
  cameraManager.startPreview();
  restartPreviewAndDecode();
}

此外，CaptureActivityHandler继承了handle，并重写了handleMessage方法，从而使用CaptureActivityHandler来为CaptureActivity处理底层的消息。调用过程：
在CaptureActivity的onActivityResult方法中获取返回的信息：

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@Override
public void onActivityResult(int requestCode, int resultCode, Intent intent) {
  if (resultCode == RESULT_OK && requestCode == HISTORY_REQUEST_CODE && historyManager != null) {
    int itemNumber = intent.getIntExtra(Intents.History.ITEM_NUMBER, -1);
    if (itemNumber >= 0) {
      HistoryItem historyItem = historyManager.buildHistoryItem(itemNumber);
      decodeOrStoreSavedBitmap(null, historyItem.getResult());
    }
  }
}

这其中，返回的信息被保存至历史记录，并在decodeOrStoreSavedBitmap函数里将消息传递到CaptureActivityHandler的handleMessage：

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private void decodeOrStoreSavedBitmap(Bitmap bitmap, Result result) {
  // Bitmap isn't used yet -- will be used soon
  if (handler == null) {
    savedResultToShow = result;
  } else {
    if (result != null) {
      savedResultToShow = result;
    }
    if (savedResultToShow != null) {
      Message message = Message.obtain(handler, R.id.decode_succeeded, savedResultToShow);
      handler.sendMessage(message);
    }
    savedResultToShow = null;
  }
}

在handleMessage里调用了前面所说的CaptureActivity里的handleDecode的方法：

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case R.id.decode_succeeded:
  state = State.SUCCESS;
  Bundle bundle = message.getData();
  Bitmap barcode = null;
  float scaleFactor = 1.0f;
  if (bundle != null) {
    byte[] compressedBitmap = bundle.getByteArray(DecodeThread.BARCODE_BITMAP);
    if (compressedBitmap != null) {
      barcode = BitmapFactory.decodeByteArray(compressedBitmap, 0, compressedBitmap.length, null);
      // Mutable copy:
      barcode = barcode.copy(Bitmap.Config.ARGB_8888, true);
    }
    scaleFactor = bundle.getFloat(DecodeThread.BARCODE_SCALED_FACTOR);          
  }
  activity.handleDecode((Result) message.obj, barcode, scaleFactor);
  break;

DecodeThread.java：

DecodeThread是一个线程，继承了Thread。
其初始化里添加了decodeFormats的内容：

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DecodeThread(CaptureActivity activity,
             Collection<BarcodeFormat> decodeFormats,
             Map<DecodeHintType,?> baseHints,
             String characterSet,
             ResultPointCallback resultPointCallback) {

  this.activity = activity;
  handlerInitLatch = new CountDownLatch(1);

  hints = new EnumMap<>(DecodeHintType.class);
  if (baseHints != null) {
    hints.putAll(baseHints);
  }

  // The prefs can't change while the thread is running, so pick them up once here.
  if (decodeFormats == null || decodeFormats.isEmpty()) {
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(activity);
    decodeFormats = EnumSet.noneOf(BarcodeFormat.class);
    if (prefs.getBoolean(PreferencesActivity.KEY_DECODE_1D_PRODUCT, true)) {
      decodeFormats.addAll(DecodeFormatManager.PRODUCT_FORMATS);
    }
    if (prefs.getBoolean(PreferencesActivity.KEY_DECODE_1D_INDUSTRIAL, true)) {
      decodeFormats.addAll(DecodeFormatManager.INDUSTRIAL_FORMATS);
    }
    if (prefs.getBoolean(PreferencesActivity.KEY_DECODE_QR, true)) {
      decodeFormats.addAll(DecodeFormatManager.QR_CODE_FORMATS);
    }
    if (prefs.getBoolean(PreferencesActivity.KEY_DECODE_DATA_MATRIX, true)) {
      decodeFormats.addAll(DecodeFormatManager.DATA_MATRIX_FORMATS);
    }
    if (prefs.getBoolean(PreferencesActivity.KEY_DECODE_AZTEC, false)) {
      decodeFormats.addAll(DecodeFormatManager.AZTEC_FORMATS);
    }
    if (prefs.getBoolean(PreferencesActivity.KEY_DECODE_PDF417, false)) {
      decodeFormats.addAll(DecodeFormatManager.PDF417_FORMATS);
    }
  }
  hints.put(DecodeHintType.POSSIBLE_FORMATS, decodeFormats);

  if (characterSet != null) {
    hints.put(DecodeHintType.CHARACTER_SET, characterSet);
  }
  hints.put(DecodeHintType.NEED_RESULT_POINT_CALLBACK, resultPointCallback);
  Log.i("DecodeThread", "Hints: " + hints);
}

在其run方法里通向下一层调用的方法DecodeHandler：

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@Override
public void run() {
  Looper.prepare();
  handler = new DecodeHandler(activity, hints);
  handlerInitLatch.countDown();
  Looper.loop();
}

DecodeHandler.java：

DecodeHandler继承Handler，在handleMessage里调用该class里的decode函数：

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@Override
public void handleMessage(Message message) {
  if (message == null || !running) {
    return;
  }
  switch (message.what) {
    case R.id.decode:
      decode((byte[]) message.obj, message.arg1, message.arg2);
      break;
    case R.id.quit:
      running = false;
      Looper.myLooper().quit();
      break;
  }
}

decode函数完成了解码和解码结果的数据回传，android的调用core的解码的接口就在decode里：

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private void decode(byte[] data, int width, int height) {
  long start = System.currentTimeMillis();
  Result rawResult = null;
  PlanarYUVLuminanceSource source = activity.getCameraManager().buildLuminanceSource(data, width, height);
  if (source != null) {
    BinaryBitmap bitmap = new BinaryBitmap(new HybridBinarizer(source));
    try {
      rawResult = multiFormatReader.decodeWithState(bitmap);
    } catch (ReaderException re) {
      // continue
    } finally {
      multiFormatReader.reset();
    }
  }

  Handler handler = activity.getHandler();
  if (rawResult != null) {
    // Don't log the barcode contents for security.
    long end = System.currentTimeMillis();
    Log.d(TAG, "Found barcode in " + (end - start) + " ms");
    if (handler != null) {
      Message message = Message.obtain(handler, R.id.decode_succeeded, rawResult);
      Bundle bundle = new Bundle();
      bundleThumbnail(source, bundle);        
      message.setData(bundle);
      message.sendToTarget();
    }
  } else {
    if (handler != null) {
      Message message = Message.obtain(handler, R.id.decode_failed);
      message.sendToTarget();
    }
  }
}

decode调用的底层函数为：rawResult = multiFormatReader.decodeWithState(bitmap);
安卓层扫码大致流程如下图所示：
安卓层扫码大致流程

core文件夹：

在“zxing-zxing-3.3.3\core\src\main\java\com\google\zxing”下有core的src代码。

MultiFormatReader.java

MultiFormatReader支持读取多种格式的码的内容，我们只看二维码部分：

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if (formats.contains(BarcodeFormat.QR_CODE)) {
  readers.add(new QRCodeReader());
}

new一个QRCodeReader()实例，解码在decodeInternal里调用reader.decode(image, hints)：

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private Result decodeInternal(BinaryBitmap image) throws NotFoundException {
  if (readers != null) {
    for (Reader reader : readers) {
      try {
        return reader.decode(image, hints);
      } catch (ReaderException re) {
        // continue
      }
    }
  }
  throw NotFoundException.getNotFoundInstance();
}

QRCodeReader.java

QRCodeReader.java在“zxing-zxing-3.3.3\core\src\main\java\com\google\zxing\qrcode”文件夹下：
实现解码的函数为 public final Result decode(BinaryBitmap image, Map<DecodeHintType,?> hints)，在
public Result decode(BinaryBitmap image) throws NotFoundException, ChecksumException, FormatException中被调用：

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@Override
public final Result decode(BinaryBitmap image, Map<DecodeHintType,?> hints)
    throws NotFoundException, ChecksumException, FormatException {
  DecoderResult decoderResult;
  ResultPoint[] points;
  if (hints != null && hints.containsKey(DecodeHintType.PURE_BARCODE)) {
    BitMatrix bits = extractPureBits(image.getBlackMatrix());
    decoderResult = decoder.decode(bits, hints);
    points = NO_POINTS;
  } else {
    DetectorResult detectorResult = new Detector(image.getBlackMatrix()).detect(hints);
    decoderResult = decoder.decode(detectorResult.getBits(), hints);
    points = detectorResult.getPoints();
  }

  // If the code was mirrored: swap the bottom-left and the top-right points.
  if (decoderResult.getOther() instanceof QRCodeDecoderMetaData) {
    ((QRCodeDecoderMetaData) decoderResult.getOther()).applyMirroredCorrection(points);
  }

  Result result = new Result(decoderResult.getText(), decoderResult.getRawBytes(), points, BarcodeFormat.QR_CODE);
  List<byte[]> byteSegments = decoderResult.getByteSegments();
  if (byteSegments != null) {
    result.putMetadata(ResultMetadataType.BYTE_SEGMENTS, byteSegments);
  }
  String ecLevel = decoderResult.getECLevel();
  if (ecLevel != null) {
    result.putMetadata(ResultMetadataType.ERROR_CORRECTION_LEVEL, ecLevel);
  }
  if (decoderResult.hasStructuredAppend()) {
    result.putMetadata(ResultMetadataType.STRUCTURED_APPEND_SEQUENCE,
                       decoderResult.getStructuredAppendSequenceNumber());
    result.putMetadata(ResultMetadataType.STRUCTURED_APPEND_PARITY,
                       decoderResult.getStructuredAppendParity());
  }
  return result;
}

可以看到该函数先检测图片中的二维码，检测到之后再进行解码：
检测二维码：

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DetectorResult detectorResult = new Detector(image.getBlackMatrix()).detect(hints);

解码二维码：

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decoderResult = decoder.decode(detectorResult.getBits(), hints);

在这里我们可以看到上层得到的rawResult里包含的是什么内容：

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Result result = new Result(decoderResult.getText(), decoderResult.getRawBytes(), points, BarcodeFormat.QR_CODE);

points为Detector检测到的二维码的位置信息：

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points = detectorResult.getPoints();

在QRCodeReader.java里还有一个特别的函数，对一张干净的二维码图片进行快速解码，得到bits信息：

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/**
 * This method detects a code in a "pure" image -- that is, pure monochrome image
 * which contains only an unrotated, unskewed, image of a code, with some white border
 * around it. This is a specialized method that works exceptionally fast in this special
 * case.
 *
 * @see com.google.zxing.datamatrix.DataMatrixReader#extractPureBits(BitMatrix)
 */
private static BitMatrix extractPureBits(BitMatrix image) throws NotFoundException

Decoder.java:

Decoder.java在“zxing-zxing-3.3.3\core\src\main\java\com\google\zxing\qrcode\decoder”文件夹下：
主要的解码函数是private DecoderResult decode(BitMatrix bits, Map<DecodeHintType,?> hints)和
private DecoderResult decode(BitMatrixParser parser, Map<DecodeHintType,?> hints)：

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/**
 * <p>Decodes a QR Code represented as a {@link BitMatrix}. A 1 or "true" is taken to mean a black module.</p>
 *
 * @param bits booleans representing white/black QR Code modules
 * @param hints decoding hints that should be used to influence decoding
 * @return text and bytes encoded within the QR Code
 * @throws FormatException if the QR Code cannot be decoded
 * @throws ChecksumException if error correction fails
 */
public DecoderResult decode(BitMatrix bits, Map<DecodeHintType,?> hints)
    throws FormatException, ChecksumException {

  // Construct a parser and read version, error-correction level
  BitMatrixParser parser = new BitMatrixParser(bits);
  FormatException fe = null;
  ChecksumException ce = null;
  try {
    return decode(parser, hints);
  } catch (FormatException e) {
    fe = e;
  } catch (ChecksumException e) {
    ce = e;
  }

  try {

    // Revert the bit matrix
    parser.remask();

    // Will be attempting a mirrored reading of the version and format info.
    parser.setMirror(true);

    // Preemptively read the version.
    parser.readVersion();

    // Preemptively read the format information.
    parser.readFormatInformation();

    /*
     * Since we're here, this means we have successfully detected some kind
     * of version and format information when mirrored. This is a good sign,
     * that the QR code may be mirrored, and we should try once more with a
     * mirrored content.
     */
    // Prepare for a mirrored reading.
    parser.mirror();

    DecoderResult result = decode(parser, hints);

    // Success! Notify the caller that the code was mirrored.
    result.setOther(new QRCodeDecoderMetaData(true));

    return result;

  } catch (FormatException | ChecksumException e) {
    // Throw the exception from the original reading
    if (fe != null) {
      throw fe;
    }
    throw ce; // If fe is null, this can't be
  }
}

private DecoderResult decode(BitMatrixParser parser, Map<DecodeHintType,?> hints)
    throws FormatException, ChecksumException {
  Version version = parser.readVersion();
  ErrorCorrectionLevel ecLevel = parser.readFormatInformation().getErrorCorrectionLevel();

  // Read codewords
  byte[] codewords = parser.readCodewords();
  // Separate into data blocks
  DataBlock[] dataBlocks = DataBlock.getDataBlocks(codewords, version, ecLevel);

  // Count total number of data bytes
  int totalBytes = 0;
  for (DataBlock dataBlock : dataBlocks) {
    totalBytes += dataBlock.getNumDataCodewords();
  }
  byte[] resultBytes = new byte[totalBytes];
  int resultOffset = 0;

  // Error-correct and copy data blocks together into a stream of bytes
  for (DataBlock dataBlock : dataBlocks) {
    byte[] codewordBytes = dataBlock.getCodewords();
    int numDataCodewords = dataBlock.getNumDataCodewords();
    correctErrors(codewordBytes, numDataCodewords);
    for (int i = 0; i < numDataCodewords; i++) {
      resultBytes[resultOffset++] = codewordBytes[i];
    }
  }

  // Decode the contents of that stream of bytes
  return DecodedBitStreamParser.decode(resultBytes, version, ecLevel, hints);
}

Detector.java:

Detector.java在“zxing-zxing-3.3.3\core\src\main\java\com\google\zxing\qrcode\detector”文件夹下：
在这里可以看到points里面存放的位置信息的结构：

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if (alignmentPattern == null) {
  points = new ResultPoint[]{bottomLeft, topLeft, topRight};
} else {
  points = new ResultPoint[]{bottomLeft, topLeft, topRight, alignmentPattern};
}

bottomLeft, topLeft, topRight为二维码的三个定位点：

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FinderPattern topLeft = info.getTopLeft();
FinderPattern topRight = info.getTopRight();
FinderPattern bottomLeft = info.getBottomLeft();

对FinderPattern的定义在FinderPattern.java里：

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FinderPattern(float posX, float posY, float estimatedModuleSize) {
  this(posX, posY, estimatedModuleSize, 1);
}

private FinderPattern(float posX, float posY, float estimatedModuleSize, int count) {
  super(posX, posY);
  this.estimatedModuleSize = estimatedModuleSize;
  this.count = count;
}

可以看到，topLeft的x,y两个值为float类型的。