Class DeferredFileOutputStream

All Implemented Interfaces:
Closeable, Flushable, AutoCloseable

public class DeferredFileOutputStream extends ThresholdingOutputStream
An output stream which will retain data in memory until a specified threshold is reached, and only then commit it to disk. If the stream is closed before the threshold is reached, the data will not be written to disk at all.

This class originated in FileUpload processing. In this use case, you do not know in advance the size of the file being uploaded. If the file is small you want to store it in memory (for speed), but if the file is large you want to store it to file (to avoid memory issues).

  • Constructor Details

    • DeferredFileOutputStream

      public DeferredFileOutputStream(int threshold, File outputFile)
      Constructs an instance of this class which will trigger an event at the specified threshold, and save data to a file beyond that point. The initial buffer size will default to 1024 bytes which is ByteArrayOutputStream's default buffer size.
      threshold - The number of bytes at which to trigger an event.
      outputFile - The file to which data is saved beyond the threshold.
  • Method Details

    • getStream

      protected OutputStream getStream() throws IOException
      Returns the current output stream. This may be memory based or disk based, depending on the current state with respect to the threshold.
      Specified by:
      getStream in class ThresholdingOutputStream
      The underlying output stream.
      IOException - if an error occurs.
    • thresholdReached

      protected void thresholdReached() throws IOException
      Switches the underlying output stream from a memory based stream to one that is backed by disk. This is the point at which we realise that too much data is being written to keep in memory, so we elect to switch to disk-based storage.
      Specified by:
      thresholdReached in class ThresholdingOutputStream
      IOException - if an error occurs.
    • isInMemory

      public boolean isInMemory()
      Determines whether or not the data for this output stream has been retained in memory.
      true if the data is available in memory; false otherwise.
    • getData

      public byte[] getData()
      Returns the data for this output stream as an array of bytes, assuming that the data has been retained in memory. If the data was written to disk, this method returns null.
      The data for this output stream, or null if no such data is available.
    • getFile

      public File getFile()
      Returns either the output file specified in the constructor or the temporary file created or null.

      If the constructor specifying the file is used then it returns that same output file, even when threshold has not been reached.

      If constructor specifying a temporary file prefix/suffix is used then the temporary file created once the threshold is reached is returned If the threshold was not reached then null is returned.

      The file for this output stream, or null if no such file exists.
    • close

      public void close() throws IOException
      Closes underlying output stream, and mark this as closed
      Specified by:
      close in interface AutoCloseable
      Specified by:
      close in interface Closeable
      close in class ThresholdingOutputStream
      IOException - if an error occurs.