一、OutputFormat

OutputFormat描述的是MapReduce的輸出格式，它主要的任務是：

1.驗證job輸出格式的有效性，如：檢查輸出的目錄是否存在。

2.通過實現RecordWriter，將輸出的結果寫到檔案系統的檔案中。

OutputFormat的主要是由三個抽象方法組成，下面根據原始碼介紹每個方法的功能，原始碼詳解如下：

 public abstract class OutputFormat<K, V> {

   /**

    * Get the {@link RecordWriter} for the given task.

    *  得到給定任務的K-V對，即RecordWriter。

    * @param context the information about the current task.

    * @return a {@link RecordWriter} to write the output for the job.

    * @throws IOException

    */

   public abstract RecordWriter<K, V> getRecordWriter(TaskAttemptContext context)

           throws IOException, InterruptedException;

   /**

    * Check for validity of the output-specification for the job.

    * 為job檢查輸出格式的有效性。

    * <p>This is to validate the output specification for the job when it is

    * a job is submitted.  Typically checks that it does not already exist,

    * throwing an exception when it already exists, so that output is not

    * overwritten.</p>

    * 這裡，當job被提交時驗證輸出格式。實際上檢查輸出目錄是否已經存在，當存在時丟擲exception。

    * 以至於原來的輸出不會被覆蓋。

    * @param context information about the job

    * @throws IOException when output should not be attempted

    */

   public abstract void checkOutputSpecs(JobContext context) throws IOException, InterruptedException;

   /**

    * Get the output committer for this output format. This is responsible

    * for ensuring the output is committed correctly.

    * 獲得一個OutPutCommitter物件。這是用來確保輸出被正確的提交。

    * @param context the task context

    * @return an output committer

    * @throws IOException

    * @throws InterruptedException

    */

   public abstract OutputCommitter getOutputCommitter(TaskAttemptContext context)

           throws IOException, InterruptedException;

 }