/** A section of an input file. Returned by {@link

* InputFormat#getSplits(JobContext)} and passed to

* {@link InputFormat#createRecordReader(InputSplit,TaskAttemptContext)}.

*

* 文件的一部分，通过InputFormat#getSplits(JobContext)生成

* 作为参数生产RecordReader：InputFormat#createRecordReader(InputSplit,TaskAttemptContext)

* 实现了InputSplit接口

*/

@InterfaceAudience.Public

@InterfaceStability.Stable

public class FileSplit extends InputSplit implements Writable {

private Path file;

private long start;

private long length;

private String[] hosts;

private SplitLocationInfo[] hostInfos;

public FileSplit() {}

/** Constructs a split with host information

*

* @param file the file name。 文件名称

* @param start the position of the first byte in the file to process。第一个byte的偏移量

* @param length the number of bytes in the file to process。 split的长度

* @param hosts the list of hosts containing the block, possibly null。 split所在的主机列表

*/

public FileSplit(Path file, long start, long length, String[] hosts) {

this.file = file;

this.start = start;

this.length = length;

this.hosts = hosts;

}

/** Constructs a split with host and cached-blocks information

*

* @param file the file name。 文件名称

* @param start the position of the first byte in the file to process。第一个byte的偏移量

* @param length the number of bytes in the file to process split的长度

* @param hosts the list of hosts containing the block split所在的主机列表

* @param inMemoryHosts the list of hosts containing the block in memory 在内存中保存block的机器列表

*/

public FileSplit(Path file, long start, long length, String[] hosts,

String[] inMemoryHosts) {

this(file, start, length, hosts);

hostInfos = new SplitLocationInfo[hosts.length];

for (int i = 0; i < hosts.length; i++) {

// because N will be tiny, scanning is probably faster than a HashSet

boolean inMemory = false;

for (String inMemoryHost : inMemoryHosts) {

if (inMemoryHost.equals(hosts[i])) {

inMemory = true;

break;

}

}

hostInfos[i] = new SplitLocationInfo(hosts[i], inMemory);

}

}

/** The file containing this split's data. */

public Path getPath() { return file; }

/** The position of the first byte in the file to process. */

public long getStart() { return start; }

/** The number of bytes in the file to process. */

@Override

public long getLength() { return length; }

@Override

public String toString() { return file + ":" + start + "+" + length; }

////////////////////////////////////////////

// Writable methods

////////////////////////////////////////////

@Override

public void write(DataOutput out) throws IOException {

Text.writeString(out, file.toString());

out.writeLong(start);

out.writeLong(length);

}

@Override

public void readFields(DataInput in) throws IOException {

file = new Path(Text.readString(in));

start = in.readLong();

length = in.readLong();

hosts = null;

}

@Override

public String[] getLocations() throws IOException {

if (this.hosts == null) {

return new String[]{};

} else {

return this.hosts;

}

}

@Override

@Evolving

public SplitLocationInfo[] getLocationInfo() throws IOException {

return hostInfos;

}

}