hadoop SequenceFile 读取例程注释
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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);
- }
- }
- }
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