My Java Notebook

My BIRT report viewer which is deployed on a Tomcat server used to work without any problem. Lately I have been getting the following error:
Exception
version mismatch
and "show exception stack trace" link.

When I clicked on the link "show exception stack trace" I got an empty stack trace. This was strange because nothing was changed on the server which runs the database or the BIRT report viewer. The only thing that did changed was my operating system. I upgraded my Ubuntu Linux to Ubuntu Hardy Heron 8.04 LTS which comes with Firefox 3 beta. It appears that the BIRT report viewer version mismatch problem is somehow related to Firefox 3. This version mismatch problem does not exists when using a different browser.

BIRT is an open source Eclipse-based Java reporting system.

This problem has been fixed in BIRT 2.3

Hadoop Distributed File System (HDFS) Java tutorial.

This Java tutorial contains examples and Java code on how to create, rename, delete and do much more on Hadoop Distributed File System using the Haddop Java API.

Copy a file from the local file system to HDFS

The srcFile variable needs to contain the full name (path + file name) of the file in the local file system. The dstFile variable needs to contain the desired full name of the file in the Hadoop file system.

  Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path srcPath = new Path(srcFile);
  Path dstPath = new Path(dstFile);
  hdfs.copyFromLocalFile(srcPath, dstPath);

Create HDFS file

The fileName variable contains the file name and path in the Hadoop file system. The content of the file is the buff variable which is an array of bytes.

  //byte[] buff - The content of the file
  Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  FSDataOutputStream outputStream = hdfs.create(path);
  outputStream.write(buff, 0, buff.length);

Rename HDFS file

In order to rename a file in Hadoop file system, we need the full name (path + name) of the file we want to rename. The rename method returns true if the file was renamed, otherwise false.

  Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path fromPath = new Path(fromFileName);
  Path toPath = new Path(toFileName);
  boolean isRenamed = hdfs.rename(fromPath, toPath);

Delete HDFS file

In order to delete a file in Hadoop file system, we need the full name (path + name) of the file we want to delete. The delete method returns true if the file was deleted, otherwise false.

  Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  boolean isDeleted = hdfs.delete(path, false);

Recursive delete:

  Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  boolean isDeleted = hdfs.delete(path, true);

Get HDFS file last modification time

In order to get the last modification time of a file in Hadoop file system, we need the full name (path + name) of the file.

  Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  FileStatus fileStatus = hdfs.getFileStatus(path);
  long modificationTime = fileStatus.getModificationTime

Check if a file exists in HDFS

In order to check the existance of a file in Hadoop file system, we need the full name (path + name) of the file we want to check. The exists methods returns true if the file exists, otherwise false.

  Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  boolean isExists = hdfs.exists(path);

Get the locations of a file in the HDFS cluster

A file can exist on more than one node in the Hadoop file system cluster for two reasons:

1. Based on the HDFS cluster configuration, Hadoop saves parts of files on different nodes in the cluster.
2. Based on the HDFS cluster configuration, Hadoop saves more than one copy of each file on different nodes for redundancy (The default is three).

  Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  FileStatus fileStatus = hdfs.getFileStatus(path);

  BlockLocation[] blkLocations = hdfs.getFileBlockLocations(path, 0, fileStatus.getLen());
    
  int blkCount = blkLocations.length;
  for (int i=0; i < blkCount; i++) {
    String[] hosts = blkLocations[i].getHosts();
    // Do something with the block hosts
  }

Get a list of all the nodes host names in the HDFS cluster

This method casts the FileSystem Object to a DistributedFileSystem Object. This method will work only when Hadoop is configured as a cluster. Running Hadoop on the local machine only, in a non cluster configuration will cause this method to throw an Exception.

  Configuration config = new Configuration();
  FileSystem fs = FileSystem.get(config);
  DistributedFileSystem hdfs = (DistributedFileSystem) fs;
  DatanodeInfo[] dataNodeStats = hdfs.getDataNodeStats();
  String[] names = new String[dataNodeStats.length];
  for (int i = 0; i < dataNodeStats.length; i++) {
      names[i] = dataNodeStats[i].getHostName();
  }

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