Cookbook - Hadoop in Action - page 163

...
int myproperty = job.getInt("myjob.myproperty", 0);
if (myproperty < 0) {
System.err.println("Invalid myjob.myproperty: " + myproperty);
System.exit(0);
}
JobClient.runJob(job);
return 0;
}

When you allow the user to specify custom properties, it's good practice for the driver
to validate any user input. The example above ensures that the user will not be allowed
to specify a negative value for myjob.myproperty .

7.2 Probing for task-specific information

In addition to retrieving custom properties and global configuration, we can also use
the getter methods

on the configuration object to obtain certain state information
about the current task and job. For example, in the Mapper you can grab the map.
input.file property to get the file path to the current map task. This is exactly what
the configure() method in the datajoin package's DataJoinMapperBase does to
infer a tag for the data source.
this.inputFile = job.get("map.input.file");
this.inputTag = generateInputTag(this.inputFile);

Table 7.1 lists some of the other task-specific state information.

Table 7.1 Task-specific state information one can get in the configuration object

Property Type Description

String The job ID

mapred.job.id

mapred.jar

String The jar location in job directory

job.local.dir

String The job's local scratch space

mapred.tip.id

String The task ID

mapred.task.id

String The task attempt ID

mapred.task.is.map

boolean Flag denoting whether this is a map task

int The ID of the task within the job

mapred.task.partition

map.input.file

String The file path that the mapper is reading from

long The offset into the file of the start of the current

mapper's input split

map.input.start

map.input.length

long The number of bytes in the current

mapper's input split

mapred.work.output.dir String

The task's working (i.e., temporary) output directory