《Hadoop权威指南》学习笔记（一）_in = new url(args[i]).openstream();表示什么功能?

本博文是我学习《Hadoop权威指南》3.5节的笔记，主要是里面范例程序的实现，部分实现有修改

1 从Hadoop读取数据

首先新建一个文本文件test.txt作为测试

hadoop fs -mkdir /poems  //在Hadoop集群上新建一个目录poems
hadoop fs -copyFromLocal test.txt /poems/test.txt  //将本地test.txt文件上传到Hadoop集群
hadoop fs -ls /poems  //检查是否成功上传

上传无误就可以在poems目录下看见刚才上传的文件，准备工作完成，接下来用两种方法读取Hadoop集群上的test.txt文件

1.1 URL读取

新建类URLCat如下

package com.tuan.hadoopLearn.hdfs;
 
import java.net.URL;
import java.io.InputStream;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.fs.FsUrlStreamHandlerFactory;
 
public class URLCat {
    static {
	URL.setURLStreamHandlerFactory(new FsUrlStreamHandlerFactory());
    }
    public static void main(String[] args) {
        InputStream in = null;
        try {
        	in = new URL(args[0]).openStream();
        	IOUtils.copyBytes(in, System.out, 4096, false);
        } catch (Exception e) {
        	System.out.println(e);
        } finally {
        	IOUtils.closeStream(in);
        }
    }
}

maven install后，进入工程jar包目录下（一般是.m2目录下），cmd执行命令，注意两个路径要写对，一个是URLCat类的路径，一个是Hadoop集群的路径

hadoop jar hadoopLearn-0.0.1-SNAPSHOT.jar com.tuan.hadoopLearn.URLCat hdfs:/poems/test.txt  //执行jar包下的URLCat类

这里可能会出现找不到或无法加载主类的错误，一般有两个原因，一个是CLASSPATH环境变量没有配置，我使用的Java1.8配置CLASSPATH应该为配置为“.;%JAVA_HOME\lib\dt.jar;%JAVA_HOME%\lib\tools.jar”，最前面有一个“.”，表示当前目录；另一个原因是类的路径没有写完整

执行成功后就可以通过URLCat类读取到刚才上传的test.txt文件并在控制台打印

1.2 FileSystem读取

类似地，写一个FileSystemCat类

package com.tuan.hadoopLearn.hdfs;
 
import java.io.InputStream;
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;
 
public class FileSystemCat {
    public static void main(String[] args) {
	String uri = args[0];
	Configuration conf = new Configuration();
	InputStream in = null;
	try {
	    FileSystem fs = FileSystem.get(URI.create(uri), conf);
	    in = fs.open(new Path(uri));
	    IOUtils.copyBytes(in, System.out, 4096, false);
	} catch (Exception e) {
	    System.out.println(e);
	} finally {
	    IOUtils.closeStream(in);
	}
    }		
}

同样maven install后，cmd输入下列命令运行，可以看到输出test.txt内容

hadoop jar hadoopLearn-0.0.1-SNAPSHOT.jar com.tuan.hadoopLearn.FileSystemCat hdfs:/poems/test.txt

1.3 读取位置定位

将java.io的InputStream替换为hadoop的FsDataInputStream，可以实现按指定偏移量读取文件的功能，新建FileSystemDoubleCat类如下

package com.tuan.hadoopLearn.hdfs;
 
import java.net.URI;
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 org.apache.hadoop.io.IOUtils;
 
public class FileSystemDoubleCat {
    public static void main(String[] args) {
	String uri = args[0];
	Configuration conf = new Configuration();
	FSDataInputStream in = null;
	try {
	    FileSystem fs = FileSystem.get(URI.create(uri), conf);
	    in = fs.open(new Path(uri));
	    IOUtils.copyBytes(in, System.out, 4096, false);
	    System.out.println("\n------------------------------------------------------------------");
	    in.seek(4);
	    IOUtils.copyBytes(in, System.out, 4096, false);
	} catch (Exception e) {
	    System.out.println(e);
	} finally {
	    IOUtils.closeStream(in);
	}
    }
}

同样执行类似的cmd命令，结果如图，第二次输出的时候开始的4个字母没有输出，因为执行了seek(4)

hadoop jar hadoopLearn-0.0.1-SNAPSHOT.jar com.tuan.hadoopLearn.FileSystemDoubleCat hdfs:/poems/test.txt

2 向Hadoop写入数据

在本地新建一个poem1.txt文件，准备等会通过Java传到Hadoop集群上

新建类FileCopyWithProgress

package com.tuan.hadoopLearn.hdfs;
 
import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.OutputStream;
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.util.Progressable;
 
public class FileCopyWithProgress {
    public static void main(String[] args) {
    	String localSrc = args[0];
    	String dst = args[1];
    	InputStream in = null;
    	OutputStream out = null;
    	Configuration conf = new Configuration();
    	try {
    		in = new BufferedInputStream(new FileInputStream(localSrc));
    		FileSystem fs = FileSystem.get(URI.create(dst), conf);
    		out = fs.create(new Path(dst), new Progressable() {
    			@Override
    			public void progress() {  //回调函数，显示上传过程
    				System.out.print(".");
    			}
    		});
    		IOUtils.copyBytes(in, out, 4096, true);
    	} catch (Exception e) {
    		System.out.println(e);
    	}
    }
}

输入cmd命令，将文件poems.txt传到hadoop集群上

hadoop jar hadoopLearn-0.0.1-SNAPSHOT.jar com.tuan.hadoopLearn.FileCopyWithProgress C:\Users\Lenovo\Desktop\poem1.txt hdfs:/poems/poem1.txt
hadoop fs -ls /poems  //检查是否文件成功上传

3 文件信息查询

3.1 读取文件状态

这个和书上的例子不太一样，书上做了一个测试小集群，这里直接在集群上测试，新建类ShowFileStatusTest

package com.tuan.hadoopLearn.hdfs;
 
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.fs.FileStatus;
 
public class ShowFileStatusTest {
    public static void main(String[] args) {
        String uri = args[0];
        try {
            FileSystem fs = FileSystem.get(URI.create(uri), new Configuration());
            FileStatus stat = fs.getFileStatus(new Path(uri));
            if (stat.isDir()) {
                System.out.println(uri + " is a directory");
            } else {
                System.out.println(uri + " is a file");
            }
            System.out.println("The path is " + stat.getPath());
            System.out.println("The length is " + stat.getLen());
            System.out.println("The modification time is " + stat.getModificationTime());
            System.out.println("The permission is " + stat.getPermission().toString());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

 输入cmd命令

hadoop jar hadoopLearn-0.0.1-SNAPSHOT.jar com.tuan.hadoopLearn.ShowFileStatusTest hdfs:/poems  //读取poems目录信息
hadoop jar hadoopLearn-0.0.1-SNAPSHOT.jar com.tuan.hadoopLearn.ShowFileStatusTest hdfs:/poems/poem1.txt  //读取poem1.txt文件信息

3.2 列出目录下文件

这次同时列出两个指定目录下的文件，再新建一个jokes目录，放入一个joke1.txt

写好类ListStatus

package com.tuan.hadoopLearn.hdfs;
 
import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileStatus;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.FileUtil;
import org.apache.hadoop.fs.Path;
 
public class ListStatus {
	public static void main(String[] args) {
        try {
            FileSystem fs = FileSystem.get( new Configuration());
            Path[] paths = new Path[args.length];
            for (int i = 0; i < args.length; i ++) {
            	paths[i] = new Path(args[i]);
            }
            FileStatus[] stats = fs.listStatus(paths);
            Path[] listPaths = FileUtil.stat2Paths(stats);
            for (Path p : listPaths) {
            	System.out.println(p);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

cmd命令

hadoop jar hadoopLearn-0.0.1-SNAPSHOT.jar com.tuan.hadoopLearn.ListStatus hdfs:/poems hdfs:/jokes

3.3 目录过滤

新建类RegexExcludedPathFilter，将指定目录下符合条件的目录过滤并将其他目录列出

package com.tuan.hadoopLearn.hdfs;
 
import java.io.IOException;
 
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileStatus;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.FileUtil;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.fs.PathFilter;
 
public class RegexExcludedPathFilter implements PathFilter{
    private final String reg;
    
    public RegexExcludedPathFilter(String reg) {
        this.reg = reg;
    }
    
    @Override
    public boolean accept(Path path) {
        return !path.toString().matches(reg);
    }
    
    public static void main(String[] args) {
        String uri = args[0];
        String reg = args[1];
        try {
            FileSystem fs = FileSystem.get(new Configuration());
            FileStatus[] stats = fs.globStatus(new Path(uri), new RegexExcludedPathFilter(reg));
            Path[] listPaths = FileUtil.stat2Paths(stats);
            for (Path p : listPaths) {
                System.out.println(p);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

输入cmd命令，这里我卡了很久，本来poems目录下有两个文件poem1.txt和test.txt，我试图用/poems/t.*过滤掉test.txt，一直不成功，后来改成.*/poems/t.*就成功了，似乎globStatus()两个参数目录的输入方式不同，PathFilter的目录是完成的目录，这仅是本人不负责任的推测，留待以后继续研究

hadoop jar hadoopLearn-0.0.1-SNAPSHOT.jar com.tuan.hadoopLearn.RegexExcludedPathFilter /poems/* .*/poems/t.*