JavaでのHDFSおよびローカルファイルの読み取り
HDFSかローカルかに関係なく、ファイルパスを読み取りたい。現在、接頭辞file：//のローカルパスと接頭辞hdfs：//のHDFSパスを渡し、次のようなコードを記述します
Configuration configuration = new Configuration();
FileSystem fileSystem = null;
if (filePath.startsWith("hdfs://")) {
fileSystem = FileSystem.get(configuration);
} else if (filePath.startsWith("file://")) {
fileSystem = FileSystem.getLocal(configuration).getRawFileSystem();
}
ここから、ファイルシステムのAPIを使用してファイルを読み取ります。
これよりも良い方法があるかどうか教えてください。
これは理にかなっていますか、
public static void main(String[] args) throws IOException {
Configuration conf = new Configuration();
conf.addResource(new Path("/hadoop/projects/hadoop-1.0.4/conf/core-site.xml"));
conf.addResource(new Path("/hadoop/projects/hadoop-1.0.4/conf/hdfs-site.xml"));
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.println("Enter the file path...");
String filePath = br.readLine();
Path path = new Path(filePath);
FileSystem fs = path.getFileSystem(conf);
FSDataInputStream inputStream = fs.open(path);
System.out.println(inputStream.available());
fs.close();
}
この方法で行っても、そのチェックを入れる必要はありません。 FileSystemをPathから直接取得し、好きなように実行します。
次の方法でFileSystemを取得できます。
Configuration conf = new Configuration();
Path path = new Path(stringPath);
FileSystem fs = FileSystem.get(path.toUri(), conf);
パスがhdfs://またはfile://で始まるかどうかを判断する必要はありません。このAPIが作業を行います。
HDFSパスのファイルをリストする以下のコードスニペットを確認してください。つまり、hdfs://で始まるパス文字列です。 Hadoop構成とローカルパスを提供できる場合、ローカルファイルシステムのファイルも一覧表示されます。つまり、file://で始まるパス文字列です。
//helper method to get the list of files from the HDFS path
public static List<String> listFilesFromHDFSPath(Configuration hadoopConfiguration, String hdfsPath,
boolean recursive)
{
//resulting list of files
List<String> filePaths = new ArrayList<String>();
FileSystem fs = null;
//try-catch-finally all possible exceptions
try
{
//get path from string and then the filesystem
Path path = new Path(hdfsPath); //throws IllegalArgumentException, all others will only throw IOException
fs = path.getFileSystem(hadoopConfiguration);
//resolve hdfsPath first to check whether the path exists => either a real directory or o real file
//resolvePath() returns fully-qualified variant of the path
path = fs.resolvePath(path);
//if recursive approach is requested
if (recursive)
{
//(heap issues with recursive approach) => using a queue
Queue<Path> fileQueue = new LinkedList<Path>();
//add the obtained path to the queue
fileQueue.add(path);
//while the fileQueue is not empty
while (!fileQueue.isEmpty())
{
//get the file path from queue
Path filePath = fileQueue.remove();
//filePath refers to a file
if (fs.isFile(filePath))
{
filePaths.add(filePath.toString());
}
else //else filePath refers to a directory
{
//list paths in the directory and add to the queue
FileStatus[] fileStatuses = fs.listStatus(filePath);
for (FileStatus fileStatus : fileStatuses)
{
fileQueue.add(fileStatus.getPath());
} // for
} // else
} // while
} // if
else //non-recursive approach => no heap overhead
{
//if the given hdfsPath is actually directory
if (fs.isDirectory(path))
{
FileStatus[] fileStatuses = fs.listStatus(path);
//loop all file statuses
for (FileStatus fileStatus : fileStatuses)
{
//if the given status is a file, then update the resulting list
if (fileStatus.isFile())
filePaths.add(fileStatus.getPath().toString());
} // for
} // if
else //it is a file then
{
//return the one and only file path to the resulting list
filePaths.add(path.toString());
} // else
} // else
} // try
catch(Exception ex) //will catch all exception including IOException and IllegalArgumentException
{
ex.printStackTrace();
//if some problem occurs return an empty array list
return new ArrayList<String>();
} //
finally
{
//close filesystem; not more operations
try
{
if(fs != null)
fs.close();
} catch (IOException e)
{
e.printStackTrace();
} // catch
} // finally
//return the resulting list; list can be empty if given path is an empty directory without files and sub-directories
return filePaths;
} // listFilesFromHDFSPath
本当にJava.io.File APIを使用したい場合、次のメソッドはローカルファイルシステムからのみファイルをリストするのに役立ちます。つまり、file://で始まるパス文字列です。
//helper method to list files from the local path in the local file system
public static List<String> listFilesFromLocalPath(String localPathString, boolean recursive)
{
//resulting list of files
List<String> localFilePaths = new ArrayList<String>();
//get the Java file instance from local path string
File localPath = new File(localPathString);
//this case is possible if the given localPathString does not exit => which means neither file nor a directory
if(!localPath.exists())
{
System.err.println("\n" + localPathString + " is neither a file nor a directory; please provide correct local path");
//return with empty list
return new ArrayList<String>();
} // if
//at this point localPath does exist in the file system => either as a directory or a file
//if recursive approach is requested
if (recursive)
{
//recursive approach => using a queue
Queue<File> fileQueue = new LinkedList<File>();
//add the file in obtained path to the queue
fileQueue.add(localPath);
//while the fileQueue is not empty
while (!fileQueue.isEmpty())
{
//get the file from queue
File file = fileQueue.remove();
//file instance refers to a file
if (file.isFile())
{
//update the list with file absolute path
localFilePaths.add(file.getAbsolutePath());
} // if
else //else file instance refers to a directory
{
//list files in the directory and add to the queue
File[] listedFiles = file.listFiles();
for (File listedFile : listedFiles)
{
fileQueue.add(listedFile);
} // for
} // else
} // while
} // if
else //non-recursive approach
{
//if the given localPathString is actually a directory
if (localPath.isDirectory())
{
File[] listedFiles = localPath.listFiles();
//loop all listed files
for (File listedFile : listedFiles)
{
//if the given listedFile is actually a file, then update the resulting list
if (listedFile.isFile())
localFilePaths.add(listedFile.getAbsolutePath());
} // for
} // if
else //it is a file then
{
//return the one and only file absolute path to the resulting list
localFilePaths.add(localPath.getAbsolutePath());
} // else
} // else
//return the resulting list; list can be empty if given path is an empty directory without files and sub-directories
return localFilePaths;
} // listFilesFromLocalPath