使用 FileSystem JAVA API 对 HDFS 进行读、写、删除等操作

Hadoop文件系统 
基本的文件系统命令操作, 通过hadoop fs -help可以获取所有的命令的详细帮助文件。 

Java抽象类org.apache.hadoop.fs.FileSystem定义了hadoop的一个文件系统接口。该类是一个抽象类，通过以下两种静态工厂方法可以过去FileSystem实例： 
public static FileSystem.get(Configuration conf) throws IOException 
public static FileSystem.get(URI uri, Configuration conf) throws IOException 

具体方法实现： 
1、public boolean mkdirs(Path f) throws IOException 
一次性新建所有目录（包括父目录）， f是完整的目录路径。 

2、public FSOutputStream create(Path f) throws IOException 
创建指定path对象的一个文件，返回一个用于写入数据的输出流 
create()有多个重载版本，允许我们指定是否强制覆盖已有的文件、文件备份数量、写入文件缓冲区大小、文件块大小以及文件权限。 

3、public boolean copyFromLocal(Path src, Path dst) throws IOException 
将本地文件拷贝到文件系统 

4、public boolean exists(Path f) throws IOException 
检查文件或目录是否存在 

5、public boolean delete(Path f, Boolean recursive) 
永久性删除指定的文件或目录，如果f是一个空目录或者文件，那么recursive的值就会被忽略。只有recursive＝true时，一个非空目录及其内容才会被删除。 

6、FileStatus类封装了文件系统中文件和目录的元数据，包括文件长度、块大小、备份、修改时间、所有者以及权限信息。 

通过"FileStatus.getPath（）"可查看指定HDFS中某个目录下所有文件。 

package hdfsTest;

import java.io.IOException;

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

public class OperatingFiles {
	//initialization
	static Configuration conf = new Configuration();
	static FileSystem hdfs;
	static {
		String path = "/usr/java/hadoop-1.0.3/conf/";
		conf.addResource(new Path(path + "core-site.xml"));
		conf.addResource(new Path(path + "hdfs-site.xml"));
		conf.addResource(new Path(path + "mapred-site.xml"));
		path = "/usr/java/hbase-0.90.3/conf/";
		conf.addResource(new Path(path + "hbase-site.xml"));
		try {
			hdfs = FileSystem.get(conf);
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	
	//create a direction
	public void createDir(String dir) throws IOException {
		Path path = new Path(dir);
		hdfs.mkdirs(path);
		System.out.println("new dir \t" + conf.get("fs.default.name") + dir);
	}	
	
	//copy from local file to HDFS file
	public void copyFile(String localSrc, String hdfsDst) throws IOException{
		Path src = new Path(localSrc);		
		Path dst = new Path(hdfsDst);
		hdfs.copyFromLocalFile(src, dst);
		
		//list all the files in the current direction
		FileStatus files[] = hdfs.listStatus(dst);
		System.out.println("Upload to \t" + conf.get("fs.default.name") + hdfsDst);
		for (FileStatus file : files) {
			System.out.println(file.getPath());
		}
	}
	
	//create a new file
	public void createFile(String fileName, String fileContent) throws IOException {
		Path dst = new Path(fileName);
		byte[] bytes = fileContent.getBytes();
		FSDataOutputStream output = hdfs.create(dst);
		output.write(bytes);
		System.out.println("new file \t" + conf.get("fs.default.name") + fileName);
	}
	
	//list all files
	public void listFiles(String dirName) throws IOException {
		Path f = new Path(dirName);
		FileStatus[] status = hdfs.listStatus(f);
		System.out.println(dirName + " has all files:");
		for (int i = 0; i< status.length; i++) {
			System.out.println(status[i].getPath().toString());
		}
	}

	//judge a file existed? and delete it!
	public void deleteFile(String fileName) throws IOException {
		Path f = new Path(fileName);
		boolean isExists = hdfs.exists(f);
		if (isExists) {	//if exists, delete
			boolean isDel = hdfs.delete(f,true);
			System.out.println(fileName + "  delete? \t" + isDel);
		} else {
			System.out.println(fileName + "  exist? \t" + isExists);
		}
	}

	public static void main(String[] args) throws IOException {
		OperatingFiles ofs = new OperatingFiles();
		System.out.println("\n=======create dir=======");
		String dir = "/test";
		ofs.createDir(dir);
		System.out.println("\n=======copy file=======");
		String src = "/home/ictclas/Configure.xml";
		ofs.copyFile(src, dir);
		System.out.println("\n=======create a file=======");
		String fileContent = "Hello, world! Just a test.";
		ofs.createFile(dir+"/word.txt", fileContent);
	}
}

Using HDFS in java (0.20.0)

Below is a code sample of how to read from and write to HDFS in java.

1. Creating a configuration object:  To be able to read from or write to HDFS, you need to create a Configuration object and pass configuration parameter to it using hadoop configuration files. 
 
    // Conf object will read the HDFS configuration parameters from these  XML
    // files. You may specify the parameters for your own if you want.

    Configuration conf = new Configuration();
    conf.addResource(new Path("/opt/hadoop-0.20.0/conf/core-site.xml"));
    conf.addResource(new Path("/opt/hadoop-0.20.0/conf/hdfs-site.xml"));

    If you do not assign the configurations to conf object (using hadoop xml file) your HDFS operation will be performed on the local file system and not on the HDFS.

2. Adding file to HDFS: Create a FileSystem object and use a file stream to add a file.

    FileSystem fileSystem = FileSystem.get(conf);
    
    // Check if the file already exists
    Path path = new Path("/path/to/file.ext");
    if (fileSystem.exists(path)) {
        System.out.println("File " + dest + " already exists");
        return;
    }

    // Create a new file and write data to it.
    FSDataOutputStream out = fileSystem.create(path);
    InputStream in = new BufferedInputStream(new FileInputStream(
        new File(source)));

    byte[] b = new byte[1024];
    int numBytes = 0;
    while ((numBytes = in.read(b)) > 0) {
        out.write(b, 0, numBytes);
    }

    // Close all the file descripters
    in.close();
    out.close();
    fileSystem.close();

3. Reading file from HDFS: Create a file stream object to a file in HDFS and read it.

    FileSystem fileSystem = FileSystem.get(conf);

    Path path = new Path("/path/to/file.ext");
    if (!fileSystem.exists(path)) {
        System.out.println("File does not exists");
        return;
    }

    FSDataInputStream in = fileSystem.open(path);

    String filename = file.substring(file.lastIndexOf('/') + 1,
        file.length());

    OutputStream out = new BufferedOutputStream(new FileOutputStream(
        new File(filename)));

    byte[] b = new byte[1024];
    int numBytes = 0;
    while ((numBytes = in.read(b)) > 0) {
        out.write(b, 0, numBytes);
    }

    in.close();
    out.close();
    fileSystem.close();

3. Deleting file from HDFS: Create a file stream object to a file in HDFS and delete it.

    FileSystem fileSystem = FileSystem.get(conf);

    Path path = new Path("/path/to/file.ext");
    if (!fileSystem.exists(path)) {
        System.out.println("File does not exists");
        return;
    }

    // Delete file
    fileSystem.delete(new Path(file), true);

    fileSystem.close();

3. Create dir in HDFS: Create a file stream object to a file in HDFS and read it.

    FileSystem fileSystem = FileSystem.get(conf);

    Path path = new Path(dir);
    if (fileSystem.exists(path)) {
        System.out.println("Dir " + dir + " already not exists");
        return;
    }

    // Create directories
    fileSystem.mkdirs(path);

    fileSystem.close();

Code:

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

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

public class HDFSClient {
    public HDFSClient() {

    }

    public void addFile(String source, String dest) throws IOException {
        Configuration conf = new Configuration();

        // Conf object will read the HDFS configuration parameters from these
        // XML files.
        conf.addResource(new Path("/opt/hadoop-0.20.0/conf/core-site.xml"));
        conf.addResource(new Path("/opt/hadoop-0.20.0/conf/hdfs-site.xml"));

        FileSystem fileSystem = FileSystem.get(conf);

        // Get the filename out of the file path
        String filename = source.substring(source.lastIndexOf('/') + 1,
            source.length());

        // Create the destination path including the filename.
        if (dest.charAt(dest.length() - 1) != '/') {
            dest = dest + "/" + filename;
        } else {
            dest = dest + filename;
        }

        // System.out.println("Adding file to " + destination);

        // Check if the file already exists
        Path path = new Path(dest);
        if (fileSystem.exists(path)) {
            System.out.println("File " + dest + " already exists");
            return;
        }

        // Create a new file and write data to it.
        FSDataOutputStream out = fileSystem.create(path);
        InputStream in = new BufferedInputStream(new FileInputStream(
            new File(source)));

        byte[] b = new byte[1024];
        int numBytes = 0;
        while ((numBytes = in.read(b)) > 0) {
            out.write(b, 0, numBytes);
        }

        // Close all the file descripters
        in.close();
        out.close();
        fileSystem.close();
    }

    public void readFile(String file) throws IOException {
        Configuration conf = new Configuration();
        conf.addResource(new Path("/opt/hadoop-0.20.0/conf/core-site.xml"));

        FileSystem fileSystem = FileSystem.get(conf);

        Path path = new Path(file);
        if (!fileSystem.exists(path)) {
            System.out.println("File " + file + " does not exists");
            return;
        }

        FSDataInputStream in = fileSystem.open(path);

        String filename = file.substring(file.lastIndexOf('/') + 1,
            file.length());

        OutputStream out = new BufferedOutputStream(new FileOutputStream(
            new File(filename)));

        byte[] b = new byte[1024];
        int numBytes = 0;
        while ((numBytes = in.read(b)) > 0) {
            out.write(b, 0, numBytes);
        }

        in.close();
        out.close();
        fileSystem.close();
    }

    public void deleteFile(String file) throws IOException {
        Configuration conf = new Configuration();
        conf.addResource(new Path("/opt/hadoop-0.20.0/conf/core-site.xml"));

        FileSystem fileSystem = FileSystem.get(conf);

        Path path = new Path(file);
        if (!fileSystem.exists(path)) {
            System.out.println("File " + file + " does not exists");
            return;
        }

        fileSystem.delete(new Path(file), true);

        fileSystem.close();
    }

    public void mkdir(String dir) throws IOException {
        Configuration conf = new Configuration();
        conf.addResource(new Path("/opt/hadoop-0.20.0/conf/core-site.xml"));

        FileSystem fileSystem = FileSystem.get(conf);

        Path path = new Path(dir);
        if (fileSystem.exists(path)) {
            System.out.println("Dir " + dir + " already not exists");
            return;
        }

        fileSystem.mkdirs(path);

        fileSystem.close();
    }

    public static void main(String[] args) throws IOException {

        if (args.length < 1) {
            System.out.println("Usage: hdfsclient add/read/delete/mkdir" +
                " [<local_path> <hdfs_path>]");
            System.exit(1);
        }

        HDFSClient client = new HDFSClient();
        if (args[0].equals("add")) {
            if (args.length < 3) {
                System.out.println("Usage: hdfsclient add <local_path> " + 
                "<hdfs_path>");
                System.exit(1);
            }

            client.addFile(args[1], args[2]);
        } else if (args[0].equals("read")) {
            if (args.length < 2) {
                System.out.println("Usage: hdfsclient read <hdfs_path>");
                System.exit(1);
            }

            client.readFile(args[1]);
        } else if (args[0].equals("delete")) {
            if (args.length < 2) {
                System.out.println("Usage: hdfsclient delete <hdfs_path>");
                System.exit(1);
            }

            client.deleteFile(args[1]);
        } else if (args[0].equals("mkdir")) {
            if (args.length < 2) {
                System.out.println("Usage: hdfsclient mkdir <hdfs_path>");
                System.exit(1);
            }

            client.mkdir(args[1]);
        } else {   
            System.out.println("Usage: hdfsclient add/read/delete/mkdir" +
                " [<local_path> <hdfs_path>]");
            System.exit(1);
        }

        System.out.println("Done!");
    }
}

from：http://smallwildpig.iteye.com/blog/1705039   Java对HDFS的操作

http://blog.rajeevsharma.in/2009/06/using-hdfs-in-java-0200.html   Using HDFS in java (0.20.0)