在 Reducer 在我的mapreduce程序中，我想读一本 JobCounter 和一个 FileSystemCounter . 运行命令时 mapred job -status <job id> ，我需要的计数器按其显示名称列出：

...
File System Counters
    FILE: Number of bytes read=148874
    FILE: Number of bytes written=22010065
    FILE: Number of read operations=0
    FILE: Number of large read operations=0
    FILE: Number of write operations=0
    HDFS: Number of bytes read=135823
    HDFS: Number of bytes written=44423504133
    HDFS: Number of read operations=2185
    HDFS: Number of large read operations=0
    HDFS: Number of write operations=1316
Job Counters 
    Launched map tasks=1
    Launched reduce tasks=200
    Rack-local map tasks=1
    Total time spent by all maps in occupied slots (ms)=5293
    Total time spent by all reduces in occupied slots (ms)=972893
    Total time spent by all map tasks (ms)=5293
    Total time spent by all reduce tasks (ms)=972893
    Total vcore-seconds taken by all map tasks=5293
    Total vcore-seconds taken by all reduce tasks=972893
    Total megabyte-seconds taken by all map tasks=5420032
    Total megabyte-seconds taken by all reduce tasks=996242432
...

如何在运行时从系统内部访问这些计数器 Reducer 的代码？
使用google，我找不到任何关于如何访问这些计数器的有用信息。使用context.getcounter（string groupname，string countername）的直接尝试无法检索 Counter 示例，因此抛出 NullPointerException 一经召唤 getValue() :

long bytes = context.getCounter(
    FileSystemCounter.class.getName(),
    FileSystemCounter.BYTES_WRITTEN.name()
).getValue();
long milliseconds = context.getCounter(
    JobCounter.class.getName(),
    JobCounter.MILLIS_REDUCES.name()
).getValue();