java Context类

首先,Context是一个内部类,从哪看出来的呢?如下

public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {/*** The <code>Context</code> passed on to the {@link Mapper} implementations.*/public abstract class Contextimplements MapContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> {}}

内部类有啥好处呢?也就是Mapper类的参数那么Context可以很方便的访问到.

Context类实现了MapContext接口.
看一下.有一个获取分片的方法.至于context怎么来的,还需要继续溯源

public interface MapContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> extends TaskInputOutputContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> {/*** Get the input split for this map.*/public InputSplit getInputSplit();}

获取是否有下一个键值对,以及当前键值对.还有写入方法在这里定义了,就是把k2,v2写出去.写到哪里呢?

public interface TaskInputOutputContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> extends TaskAttemptContext {/*** Advance to the next key, value pair, returning null if at end.* @return the key object that was read into, or null if no more*/public boolean nextKeyValue() throws IOException, InterruptedException;/*** Get the current key.* @return the current key object or null if there isn't one* @throws IOException* @throws InterruptedException*/public KEYIN getCurrentKey() throws IOException, InterruptedException;/*** Get the current value.* @return the value object that was read into* @throws IOException* @throws InterruptedException*/public VALUEIN getCurrentValue() throws IOException, InterruptedException;/*** Generate an output key/value pair.*/public void write(KEYOUT key, VALUEOUT value) throws IOException, InterruptedException;/*** Get the {@link OutputCommitter} for the task-attempt.* @return the <code>OutputCommitter</code> for the task-attempt*/public OutputCommitter getOutputCommitter();
}

继续溯源.TaskInputOutputContext又继承了TaskAttemptContext接口,这个接口中包含一些job信息.

public interface TaskAttemptContext extends JobContext, Progressable {/*** Get the unique name for this task attempt.*/public TaskAttemptID getTaskAttemptID();/*** Set the current status of the task to the given string.*/public void setStatus(String msg);/*** Get the last set status message.* @return the current status message*/public String getStatus();/*** The current progress of the task attempt.* @return a number between 0.0 and 1.0 (inclusive) indicating the attempt's* progress.*/public abstract float getProgress();/*** Get the {@link Counter} for the given <code>counterName</code>.* @param counterName counter name* @return the <code>Counter</code> for the given <code>counterName</code>*/public Counter getCounter(Enum<?> counterName);/*** Get the {@link Counter} for the given <code>groupName</code> and * <code>counterName</code>.* @param counterName counter name* @return the <code>Counter</code> for the given <code>groupName</code> and *         <code>counterName</code>*/public Counter getCounter(String groupName, String counterName);}

继续溯源

public interface JobContext extends MRJobConfig {/*** Return the configuration for the job.* @return the shared configuration object*/public Configuration getConfiguration();/*** Get credentials for the job.* @return credentials for the job*/public Credentials getCredentials();/*** Get the unique ID for the job.* @return the object with the job id*/public JobID getJobID();/*** Get configured the number of reduce tasks for this job. Defaults to * <code>1</code>.* @return the number of reduce tasks for this job.*/public int getNumReduceTasks();/*** Get the current working directory for the default file system.* * @return the directory name.*/public Path getWorkingDirectory() throws IOException;/*** Get the key class for the job output data.* @return the key class for the job output data.*/public Class<?> getOutputKeyClass();/*** Get the value class for job outputs.* @return the value class for job outputs.*/public Class<?> getOutputValueClass();/*** Get the key class for the map output data. If it is not set, use the* (final) output key class. This allows the map output key class to be* different than the final output key class.* @return the map output key class.*/public Class<?> getMapOutputKeyClass();/*** Get the value class for the map output data. If it is not set, use the* (final) output value class This allows the map output value class to be* different than the final output value class.*  * @return the map output value class.*/public Class<?> getMapOutputValueClass();/*** Get the user-specified job name. This is only used to identify the * job to the user.* * @return the job's name, defaulting to "".*/public String getJobName();/*** Get the {@link InputFormat} class for the job.* * @return the {@link InputFormat} class for the job.*/public Class<? extends InputFormat<?,?>> getInputFormatClass() throws ClassNotFoundException;/*** Get the {@link Mapper} class for the job.* * @return the {@link Mapper} class for the job.*/public Class<? extends Mapper<?,?,?,?>> getMapperClass() throws ClassNotFoundException;/*** Get the combiner class for the job.* * @return the combiner class for the job.*/public Class<? extends Reducer<?,?,?,?>> getCombinerClass() throws ClassNotFoundException;/*** Get the {@link Reducer} class for the job.* * @return the {@link Reducer} class for the job.*/public Class<? extends Reducer<?,?,?,?>> getReducerClass() throws ClassNotFoundException;/*** Get the {@link OutputFormat} class for the job.* * @return the {@link OutputFormat} class for the job.*/public Class<? extends OutputFormat<?,?>> getOutputFormatClass() throws ClassNotFoundException;/*** Get the {@link Partitioner} class for the job.* * @return the {@link Partitioner} class for the job.*/public Class<? extends Partitioner<?,?>> getPartitionerClass() throws ClassNotFoundException;/*** Get the {@link RawComparator} comparator used to compare keys.* * @return the {@link RawComparator} comparator used to compare keys.*/public RawComparator<?> getSortComparator();/*** Get the pathname of the job's jar.* @return the pathname*/public String getJar();/*** Get the user defined {@link RawComparator} comparator for* grouping keys of inputs to the combiner.** @return comparator set by the user for grouping values.* @see Job#setCombinerKeyGroupingComparatorClass(Class)*/public RawComparator<?> getCombinerKeyGroupingComparator();/*** Get the user defined {@link RawComparator} comparator for* grouping keys of inputs to the reduce.** @return comparator set by the user for grouping values.* @see Job#setGroupingComparatorClass(Class)* @see #getCombinerKeyGroupingComparator()*/public RawComparator<?> getGroupingComparator();/*** Get whether job-setup and job-cleanup is needed for the job * * @return boolean */public boolean getJobSetupCleanupNeeded();/*** Get whether task-cleanup is needed for the job * * @return boolean */public boolean getTaskCleanupNeeded();/*** Get whether the task profiling is enabled.* @return true if some tasks will be profiled*/public boolean getProfileEnabled();/*** Get the profiler configuration arguments.** The default value for this property is* "-agentlib:hprof=cpu=samples,heap=sites,force=n,thread=y,verbose=n,file=%s"* * @return the parameters to pass to the task child to configure profiling*/public String getProfileParams();/*** Get the range of maps or reduces to profile.* @param isMap is the task a map?* @return the task ranges*/public IntegerRanges getProfileTaskRange(boolean isMap);/*** Get the reported username for this job.* * @return the username*/public String getUser();/*** Originally intended to check if symlinks should be used, but currently* symlinks cannot be disabled.* @return true*/@Deprecatedpublic boolean getSymlink();/*** Get the archive entries in classpath as an array of Path*/public Path[] getArchiveClassPaths();/*** Get cache archives set in the Configuration* @return A URI array of the caches set in the Configuration* @throws IOException*/public URI[] getCacheArchives() throws IOException;/*** Get cache files set in the Configuration* @return A URI array of the files set in the Configuration* @throws IOException*/public URI[] getCacheFiles() throws IOException;/*** Return the path array of the localized caches* @return A path array of localized caches* @throws IOException* @deprecated the array returned only includes the items the were * downloaded. There is no way to map this to what is returned by* {@link #getCacheArchives()}.*/@Deprecatedpublic Path[] getLocalCacheArchives() throws IOException;/*** Return the path array of the localized files* @return A path array of localized files* @throws IOException* @deprecated the array returned only includes the items the were * downloaded. There is no way to map this to what is returned by* {@link #getCacheFiles()}.*/@Deprecatedpublic Path[] getLocalCacheFiles() throws IOException;/*** Get the file entries in classpath as an array of Path*/public Path[] getFileClassPaths();/*** Get the timestamps of the archives.  Used by internal* DistributedCache and MapReduce code.* @return a string array of timestamps */public String[] getArchiveTimestamps();/*** Get the timestamps of the files.  Used by internal* DistributedCache and MapReduce code.* @return a string array of timestamps */public String[] getFileTimestamps();/** * Get the configured number of maximum attempts that will be made to run a* map task, as specified by the <code>mapred.map.max.attempts</code>* property. If this property is not already set, the default is 4 attempts.*  * @return the max number of attempts per map task.*/public int getMaxMapAttempts();/** * Get the configured number of maximum attempts  that will be made to run a* reduce task, as specified by the <code>mapred.reduce.max.attempts</code>* property. If this property is not already set, the default is 4 attempts.* * @return the max number of attempts per reduce task.*/public int getMaxReduceAttempts();}

总结一下,就是

context是一个Mapper的内部类
context可以获取一些Job相关的信息.

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