Hadoop源码分析：HDFS读取文件

Hadoop源码分析：HDFS读取文件

上一篇博客http://blog.csdn.net/chengyuqiang/article/details/78636721分析了HDFS的DistributedFileSystem对象的创建过程。
然后就可以按照HDFS的API对HDFS中的文件和目录进行操作了，如列出某个目录中的文件和子目录、读取文件、写入文件等。

1.1 FileSystem.open()

与使用Java IO读取本地文件类似，读取HDFS文件其实就是创建一个文件输入流，在Hadoop中使用FileSystem.open()方法来创建输入流，open()方法是我们此次源码分析的入口。

    public static void readFile(String filePath) throws IOException{        FileSystem fs = getFileSystem(filePath);        InputStream in=null;        try{            in=fs.open(new Path(filePath));            IOUtils.copyBytes(in, System.out,4096,false);        }catch(Exception e){            System.out.println(e.getMessage());        }finally{            IOUtils.closeStream(in);        }    }

1.2 进入open(Path f)方法

该方法返回的是一个FSDataInputStream对象。

  /**   * Opens an FSDataInputStream at the indicated Path.   * @param f the file to open   */  public FSDataInputStream open(Path f) throws IOException {    return open(f, getConf().getInt("io.file.buffer.size", 4096));  }

1.3 进入 open(Path f, int bufferSize)方法

发现这是个抽象方法

  /**   * Opens an FSDataInputStream at the indicated Path.   * @param f the file name to open   * @param bufferSize the size of the buffer to be used.   */  public abstract FSDataInputStream open(Path f, int bufferSize)    throws IOException;

下面进入DistributedFileSystem类的open(Path f, int bufferSize)方法

  @Override  public FSDataInputStream open(Path f, final int bufferSize)      throws IOException {    //统计信息    statistics.incrementReadOps(1);    //相对路径转换为绝对路径    Path absF = fixRelativePart(f);    return new FileSystemLinkResolver<FSDataInputStream>() {      @Override//核心方法      public FSDataInputStream doCall(final Path p)          throws IOException, UnresolvedLinkException {        final DFSInputStream dfsis =          dfs.open(getPathName(p), bufferSize, verifyChecksum);        return dfs.createWrappedInputStream(dfsis);      }      @Override      public FSDataInputStream next(final FileSystem fs, final Path p)          throws IOException {        return fs.open(p, bufferSize);      }    }.resolve(this, absF);  }

在该方法中，statistics是一个org.apache.hadoop.fs.FileSystem.Statistics类型，它实现了文件系统读写过程中的一些统计，例如自从该HDFS对象建立以来，读了多少字节、写了多少字节等。在返回结果的时候，创建了一个FileSystemLinkResolver对象，并实现了此类的两个抽象方法， 最后调用了resolve()方法，其中doCall()方法和next()方法都在resolve()方法里用到了，只是next()方法只是在resolve()方法异常捕获时才调用。所以跟踪doCall()方法，doCall()方法里的open()方法有3个参数其中src表示要打开的文件路径，buffersize表示缓冲大小，verifyChecksum表示是否校验和。

1.4 进入dfs.open(String src, int buffersize, boolean verifyChecksum)方法

  /**   * Create an input stream that obtains a nodelist from the   * namenode, and then reads from all the right places.  Creates   * inner subclass of InputStream that does the right out-of-band   * work.   */  public DFSInputStream open(String src, int buffersize, boolean verifyChecksum)      throws IOException, UnresolvedLinkException {    checkOpen();    //    Get block info from namenode    TraceScope scope = getPathTraceScope("newDFSInputStream", src);    try {      return new DFSInputStream(this, src, verifyChecksum);    } finally {      scope.close();    }  }

checkOpen()方法表示检查文件系统是否已经打开，如果没有打开，则抛出异常–FileSystem closed。最后，在这个方法中调用了DFSClient.DFSInputStream()的构造方法，创建DFSInputStream输入流对象并返回(DFSInputStream是对客户端读取的输入流的抽象)。

1.5 进入该DFSInputStream构造方法

该方法先是做了一些准备工作，然后调用openInfo()方法，openInfo()方法是一个线程安全的方法，作用是从namenode获取要打开的文件的数据块信息。也就是说主要是为locatedBlocks对象赋值。

@InterfaceAudience.Privatepublic class DFSClient implements java.io.Closeable, RemotePeerFactory,    DataEncryptionKeyFactory {  ...  DFSInputStream(DFSClient dfsClient, String src, boolean verifyChecksum                 ) throws IOException, UnresolvedLinkException {    this.dfsClient = dfsClient;    this.verifyChecksum = verifyChecksum;    this.src = src;    synchronized (infoLock) {      this.cachingStrategy = dfsClient.getDefaultReadCachingStrategy();    }    openInfo();  }  ...}

1.6 进入openInfo()方法

该方法中如果读取数据块信息失败，则会再次读取3次，主要调用了方法fetchLocatedBlocksAndGetLastBlockLength()方法来读取数据块的信息。该方法名字虽然长，但是说的很明白，即读取数据块信息并且获得最后一个数据块的长度。为什么偏偏要获取最后一个数据块的长度呢？因为之前的数据块大小固定嘛，如果是默认的，那就是128M，而最后一块大小就不一定了，有必要获取下。

 /**   * Grab the open-file info from namenode   */  void openInfo() throws IOException, UnresolvedLinkException {    synchronized(infoLock) {      lastBlockBeingWrittenLength = fetchLocatedBlocksAndGetLastBlockLength();      int retriesForLastBlockLength = dfsClient.getConf().retryTimesForGetLastBlockLength;      while (retriesForLastBlockLength > 0) {        // Getting last block length as -1 is a special case. When cluster        // restarts, DNs may not report immediately. At this time partial block        // locations will not be available with NN for getting the length. Lets        // retry for 3 times to get the length.        if (lastBlockBeingWrittenLength == -1) {          DFSClient.LOG.warn("Last block locations not available. "              + "Datanodes might not have reported blocks completely."              + " Will retry for " + retriesForLastBlockLength + " times");          waitFor(dfsClient.getConf().retryIntervalForGetLastBlockLength);          lastBlockBeingWrittenLength = fetchLocatedBlocksAndGetLastBlockLength();        } else {          break;        }        retriesForLastBlockLength--;      }      if (retriesForLastBlockLength == 0) {        throw new IOException("Could not obtain the last block locations.");      }    }  }

1.7 进入fetchLocatedBlocksAndGetLastBlockLength()方法

  private long fetchLocatedBlocksAndGetLastBlockLength() throws IOException {    //此处又出现了dfsClient,该对象负责与NameNode通信，由此可知getLocatedBlocks()方法实现了远程通信    final LocatedBlocks newInfo = dfsClient.getLocatedBlocks(src, 0);    if (DFSClient.LOG.isDebugEnabled()) {      DFSClient.LOG.debug("newInfo = " + newInfo);    }    if (newInfo == null) {      throw new IOException("Cannot open filename " + src);    }    if (locatedBlocks != null) {      Iterator<LocatedBlock> oldIter = locatedBlocks.getLocatedBlocks().iterator();      Iterator<LocatedBlock> newIter = newInfo.getLocatedBlocks().iterator();      while (oldIter.hasNext() && newIter.hasNext()) {        if (! oldIter.next().getBlock().equals(newIter.next().getBlock())) {          throw new IOException("Blocklist for " + src + " has changed!");        }      }    }    locatedBlocks = newInfo;    long lastBlockBeingWrittenLength = 0;    if (!locatedBlocks.isLastBlockComplete()) {      final LocatedBlock last = locatedBlocks.getLastLocatedBlock();      if (last != null) {        if (last.getLocations().length == 0) {          if (last.getBlockSize() == 0) {            // if the length is zero, then no data has been written to            // datanode. So no need to wait for the locations.            return 0;          }          return -1;        }        final long len = readBlockLength(last);        last.getBlock().setNumBytes(len);        lastBlockBeingWrittenLength = len;       }    }    fileEncryptionInfo = locatedBlocks.getFileEncryptionInfo();    return lastBlockBeingWrittenLength;  }

1.8 进入getLocatedBlocks(String src, long start)方法

  public LocatedBlocks getLocatedBlocks(String src, long start)      throws IOException {    return getLocatedBlocks(src, start, dfsClientConf.prefetchSize);  }

进入getLocatedBlocks(String src, long start, long length)方法

  /*   * This is just a wrapper around callGetBlockLocations, but non-static so that   * we can stub it out for tests.   */  @VisibleForTesting  public LocatedBlocks getLocatedBlocks(String src, long start, long length)      throws IOException {    TraceScope scope = getPathTraceScope("getBlockLocations", src);    try {      //此处终于见到了namenode       return callGetBlockLocations(namenode, src, start, length);    } finally {      scope.close();    }  }

1.9 进入callGetBlockLocations()方法

进入callGetBlockLocations(ClientProtocol namenode,String src, long start, long length)方法

  /**   * @see ClientProtocol#getBlockLocations(String, long, long)   */  static LocatedBlocks callGetBlockLocations(ClientProtocol namenode,      String src, long start, long length)       throws IOException {    try {      //调用namenode对象，进行远程调用      return namenode.getBlockLocations(src, start, length);    } catch(RemoteException re) {      throw re.unwrapRemoteException(AccessControlException.class,                                     FileNotFoundException.class,                                     UnresolvedPathException.class);    }  }

callGetBlockLocations(ClientProtocol namenode,String src, long start, long length)方法涉及RPC的远程调用，后面再继续研读。