hadoop Head 源码
haddop Head 代码
文件路径：/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/shell/Head.java
/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.hadoop.fs.shell;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.PathIsDirectoryException;
import org.apache.hadoop.io.IOUtils;
import java.io.IOException;
import java.util.LinkedList;
import java.util.List;
import static org.apache.hadoop.fs.Options.OpenFileOptions.FS_OPTION_OPENFILE_READ_POLICY_SEQUENTIAL;
/**
* Show the first 1KB of the file.
*/
@InterfaceAudience.Private
@InterfaceStability.Unstable
class Head extends FsCommand {
public static void registerCommands(CommandFactory factory) {
factory.addClass(Head.class, "-head");
}
public static final String NAME = "head";
public static final String USAGE = "<file>";
public static final String DESCRIPTION =
"Show the first 1KB of the file.\n";
private long endingOffset = 1024;
@Override
protected void processOptions(LinkedList<String> args) throws IOException {
CommandFormat cf = new CommandFormat(1, 1);
cf.parse(args);
}
@Override
protected List<PathData> expandArgument(String arg) throws IOException {
List<PathData> items = new LinkedList<PathData>();
items.add(new PathData(arg, getConf()));
return items;
}
@Override
protected void processPath(PathData item) throws IOException {
if (item.stat.isDirectory()) {
throw new PathIsDirectoryException(item.toString());
}
dumpToOffset(item);
}
private void dumpToOffset(PathData item) throws IOException {
try (FSDataInputStream in = item.openFile(
FS_OPTION_OPENFILE_READ_POLICY_SEQUENTIAL)) {
IOUtils.copyBytes(in, System.out, endingOffset, false);
}
}
}
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