Hadoop源码篇--Client源码

一。前述

今天起剖析源码，先从Client看起，因为Client在MapReduce的过程中承担了很多重要的角色。

二。MapReduce框架主类

代码如下：

public static void main(String[] args) throws Exception {

        Configuration conf = new Configuration(true);
        //job  作业
        Job  job = Job.getInstance(conf);

         // Create a new Job
//         Job job = Job.getInstance();
         job.setJarByClass(MyWC.class);

         // Specify various job-specific parameters
         job.setJobName("myjob");

//         job.setInputPath(new Path("in"));
//         job.setOutputPath(new Path("out"));

         Path input = new Path("/user/root");
        FileInputFormat.addInputPath(job, input );

         Path output = new Path("/output/wordcount");
         if(output.getFileSystem(conf).exists(output)){
             output.getFileSystem(conf).delete(output,true);
         }
        FileOutputFormat.setOutputPath(job, output );

         job.setMapperClass(MyMapper.class);
         job.setMapOutputKeyClass(Text.class);
         job.setMapOutputValueClass(IntWritable.class);
         job.setReducerClass(MyReducer.class);

         // Submit the job, then poll for progress until the job is complete
         job.waitForCompletion(true);

第一步，先分析Job，可以看见源码中Job实现了public class Job extends JobContextImpl implements JobContext

然后JobContext实现了 MRJobConfig,可以看见其中有很多配置

因为job中传的参数为conf,所以这里的配置即对应我们的配置文件中的属性值。

  Job  job = Job.getInstance(conf);

挑几个重要的看下：

public static final int DEFAULT_MAP_MEMORY_MB = 1024;//默认的Mapper任务内存大小。

第二步，分析提交过程 job.waitForCompletion(true);   追踪源码发现主要实现这个类

JobStatus submitJobInternal(Job job, Cluster cluster) 
  throws ClassNotFoundException, InterruptedException, IOException

1. Checking the input and output specifications of the job.//检查输入输出路径
2. Computing the InputSplits for the job.//检查切片
3. Setup the requisite accounting information for the DistributedCache of the job, if necessary.
4. Copying the job's jar and configuration to the map-reduce system directory on the distributed file-system.
5. Submitting the job to the JobTracker and optionally monitoring it's status.

在此方法中，中重点看下此方法 int maps = writeSplits(job, submitJobDir);

追踪后具体实现可知

private <T extends InputSplit>
  int writeNewSplits(JobContext job, Path jobSubmitDir) throws IOException,
      InterruptedException, ClassNotFoundException {
    Configuration conf = job.getConfiguration();
    InputFormat<?, ?> input =
      ReflectionUtils.newInstance(job.getInputFormatClass(), conf);

    List<InputSplit> splits = input.getSplits(job);
    T[] array = (T[]) splits.toArray(new InputSplit[splits.size()]);

    // sort the splits into order based on size, so that the biggest
    // go first
    Arrays.sort(array, new SplitComparator());
    JobSplitWriter.createSplitFiles(jobSubmitDir, conf,
        jobSubmitDir.getFileSystem(conf), array);
    return array.length;
  }

追踪job.getInputFormatClass()可以发现如下代码： 

public Class<? extends InputFormat<?,?>> getInputFormatClass()
     throws ClassNotFoundException {
    return (Class<? extends InputFormat<?,?>>)
      conf.getClass(INPUT_FORMAT_CLASS_ATTR, TextInputFormat.class);
//根据用户配置文件首先取用，如果没有被取用则使用默认输入格式TextInputFormat
  }

所以可得知用户的默认输入类是TextInputformat类并且继承关系如下：

TextInputforMat-->FileinputFormat-->InputFormat

追踪 List<InputSplit> splits = input.getSplits(job);可以得到如下源码：

最为重要的一个源码！！！！！！！！！！！

public List<InputSplit> getSplits(JobContext job) throws IOException {
    Stopwatch sw = new Stopwatch().start();
    long minSize = Math.max(getFormatMinSplitSize(), getMinSplitSize(job));如果用户设置则取用户，没有是1
    long maxSize = getMaxSplitSize(job);//如果用户设置则取用户，没有取最大值

    // generate splits
    List<InputSplit> splits = new ArrayList<InputSplit>();
    List<FileStatus> files = listStatus(job);
    for (FileStatus file: files) {
      Path path = file.getPath();//取输入文件的大小和路径
      long length = file.getLen();
      if (length != 0) {
        BlockLocation[] blkLocations;
        if (file instanceof LocatedFileStatus) {
          blkLocations = ((LocatedFileStatus) file).getBlockLocations();
        } else {
          FileSystem fs = path.getFileSystem(job.getConfiguration());
          blkLocations = fs.getFileBlockLocations(file, 0, length);//获得所有块的位置。
        }
        if (isSplitable(job, path)) {
          long blockSize = file.getBlockSize();
          long splitSize = computeSplitSize(blockSize, minSize, maxSize);//获得切片大小

          long bytesRemaining = length;
          while (((double) bytesRemaining)/splitSize > SPLIT_SLOP) {
            int blkIndex = getBlockIndex(blkLocations, length-bytesRemaining);//这一块传参传的是切块的偏移量，返回这个块的索引
            splits.add(makeSplit(path, length-bytesRemaining, splitSize,
                        blkLocations[blkIndex].getHosts(),//根据当前块的索引号取出来块的位置包括副本的位置 然后传递给切片，然后切片知道往哪运算。即往块的位置信息计算
                        blkLocations[blkIndex].getCachedHosts()));
            bytesRemaining -= splitSize;
          }

          if (bytesRemaining != 0) {
            int blkIndex = getBlockIndex(blkLocations, length-bytesRemaining);
            splits.add(makeSplit(path, length-bytesRemaining, bytesRemaining,
                       blkLocations[blkIndex].getHosts(),
                       blkLocations[blkIndex].getCachedHosts()));
          }
        } else { // not splitable
          splits.add(makeSplit(path, 0, length, blkLocations[0].getHosts(),
                      blkLocations[0].getCachedHosts()));
        }
      } else {
        //Create empty hosts array for zero length files
        splits.add(makeSplit(path, 0, length, new String[0]));
      }
    }
    // Save the number of input files for metrics/loadgen
    job.getConfiguration().setLong(NUM_INPUT_FILES, files.size());
    sw.stop();
    if (LOG.isDebugEnabled()) {
      LOG.debug("Total # of splits generated by getSplits: " + splits.size()
          + ", TimeTaken: " + sw.elapsedMillis());
    }
    return splits;
  }

 1.long splitSize = computeSplitSize(blockSize, minSize, maxSize);追踪源码发现

protected long computeSplitSize(long blockSize, long minSize, long maxSize) {
    return Math.max(minSize, Math.min(maxSize, blockSize));
  }

 切片大小默认是块的大小！！！！

假如让切片大小 < 块的大小则更改配置的最大值MaxSize，让其小于blocksize

假如让切片大小 > 块的大小则更改配置的最小值MinSize，让其大于blocksize

通过FileInputFormat.setMinInputSplitSize即可。

2. int blkIndex = getBlockIndex(blkLocations, length-bytesRemaining) 追踪源码发现

 protected int getBlockIndex(BlockLocation[] blkLocations,
                              long offset) {
    for (int i = 0 ; i < blkLocations.length; i++) {
      // is the offset inside this block?
      if ((blkLocations[i].getOffset() <= offset) &&
          (offset < blkLocations[i].getOffset() + blkLocations[i].getLength())){//切片要大于>=块的起始量，小于一个块的末尾量。
        return i;//返回这个块
      }
    }
    BlockLocation last = blkLocations[blkLocations.length -1];
    long fileLength = last.getOffset() + last.getLength() -1;
    throw new IllegalArgumentException("Offset " + offset +
                                       " is outside of file (0.." +
                                       fileLength + ")");
  }

3. splits.add(makeSplit(path, length-bytesRemaining, splitSize, blkLocations[blkIndex].getHosts()

创建切片的时候，一个切片对应一个mapperr任务，所以创建切片的四个位置（path,0,10,host）

根据host可知mapper任务的计算位置，则对应计算向数据移动！！！！块是逻辑的，并没有真正切割数据。！！

4.上述getSplits方法最终得到一个切片的清单,清单的数目就是mapper的数量！！即开始方法的入口 int maps = writeSplits(job, submitJobDir);返回值。

5.计算向数据移动时会拉取只属于自己的文件。

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