使用 Java API 读取 HDFS 上的文件并输出

答案：可以使用 org.apache.hadoop.fs.FileSystem 类的 open() 方法来打开 HDFS 上的文件流，然后读取并输出文件内容。下面是 Java 代码示例：

import java.io.IOException;
import java.io.InputStream;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;

public class ReadHdfsFile {
    public static void main(String[] args) {
        Configuration conf = new Configuration();
        conf.set("fs.defaultFS", "hdfs://localhost:9000");
        FileSystem fs = null;
        InputStream in = null;
        try {
            fs = FileSystem.get(conf);
            Path file = new Path("/test/input/test.txt");
            in = fs.open(file);
            int bytesRead = 0;
            byte[] buffer = new byte[4096];
            while ((bytesRead = in.read(buffer)) > 0) {
                System.out.write(buffer, 0, bytesRead);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                in.close();
                fs.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}