hadoop SequenceFile 读取例程注释

最新推荐文章于 2022-05-22 19:21:37 发布

转载最新推荐文章于 2022-05-22 19:21:37 发布 · 439 阅读

本文提供了一个使用Java实现的Hadoop SequenceFile读取示例程序，展示了如何通过SequenceFile.Reader类来读取Hadoop文件系统中的SequenceFile文件，并逐条打印出文件中的键值对。

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import java.io.IOException;
import java.net.URI;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.util.ReflectionUtils;
// vv SequenceFileReadDemo
public class SequenceFileReadDemo {
public static void main(String[] args) throws IOException {
String uri = args[0];
Configuration conf = new Configuration();
FileSystem fs = FileSystem.get(URI.create(uri), conf);
Path path = new Path(uri);
SequenceFile.Reader reader = null;
try {
reader = new SequenceFile.Reader(fs, path, conf);//返回 SequenceFile.Reader 对象
Writable key = (Writable)
ReflectionUtils.newInstance(reader.getKeyClass(), conf);//getKeyClass()获得Sequence中使用的类型
Writable value = (Writable)//ReflectionUtils.newInstace 得到常见的键值的实例
ReflectionUtils.newInstance(reader.getValueClass(), conf);//同上
long position = reader.getPosition();
while (reader.next(key, value)) { //next（）方法迭代读取记录直到读完返回false
String syncSeen = reader.syncSeen() ? "*" : "";//替换特殊字符同步
System.out.printf("[%s%s]\t%s\t%s\n", position, syncSeen, key, value);
position = reader.getPosition(); // beginning of next record
}
} finally {
IOUtils.closeStream(reader);
}
}
}