每日总结11.14

（二）编程实现一个类“MyFSDataInputStream”，该类继承“org.apache.hadoop.fs.FSDataInputStream”，

要求如下：实现按行读取HDFS中指定文件的方法“readLine()”，如果读到文件末尾，则返回空，否则返回文件一行的文本。

首先启动hdfs集群

然后创建maven项目

MyFSDataInputStream类：

package DataTutorial.Second;

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

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

public class ShowTheContent extends FSDataInputStream{
    public ShowTheContent(InputStream in) {
        super(in);
    }

    /**
     * 实现按行读取 每次读入一个字符，遇到"\n"结束，返回一行内容
     */
    public static String readline(BufferedReader br) throws IOException {
        char[] data = new char[1024];
        int read = -1;
        int off = 0;

        while ((read = br.read(data, off, 1)) != -1) {
            if (String.valueOf(data[off]).equals("\n")) {
                off += 1;
                break;
            }
            off += 1;
        }
        if (off > 0) {
            return String.valueOf(data, 0, off);
        } else {
            return null;
        }
    }

    /**
     * 读取文件内容
     */
    public static void cat(Configuration conf, String remoteFilePath) throws IOException {
        FileSystem fs = FileSystem.get(conf);
        Path remotePath = new Path(remoteFilePath);

        try (FSDataInputStream in = fs.open(remotePath);
             BufferedReader br = new BufferedReader(new InputStreamReader(in))) {
            String line;
            while ((line = ShowTheContent.readline(br)) != null) {
                System.out.println(line);
            }
        }
    }

    /**
     * 主函数
     */
    public static void main(String[] args) {
        Configuration conf = new Configuration();
        conf.setLong("ipc.maximum.data.length", 67108864); // 设置为合适的值
        conf.set("fs.default.name", "hdfs://192.168.88.101:8020");
        String remoteFilePath = "/test/test2.txt"; // HDFS 路径

        try {
            ShowTheContent.cat(conf, remoteFilePath);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

（三）查看Java帮助手册或其它资料，用“java.net.URL”和“org.apache.hadoop.fs.FsURLStreamHandlerFactory”编程完成输出HDFS中指定文件的文本到终端中。

package DataTutorial.Third;

import org.apache.hadoop.fs.*;
import org.apache.hadoop.io.IOUtils;
import java.io.*;
import java.net.URL;

public class HDFSApi {
    static {
        URL.setURLStreamHandlerFactory(new FsUrlStreamHandlerFactory());
    }

    /**
     * 主函数
     */
    public static void main(String[] args) throws Exception {

        String remoteFilePath = "hdfs://192.168.88.101:8020/test/test2.txt"; // HDFS 文件
        InputStream in = null;
        try {
            /* 通过 URL 对象打开数据流，从中读取数据 */
            in = new URL(remoteFilePath).openStream();
            IOUtils.copyBytes(in, System.out, 4096, false);
        } finally {
            IOUtils.closeStream(in);
        }
    }
}