# Class DeferredFileOutputStream

java.lang.Object
java.io.OutputStream
org.apache.tomcat.util.http.fileupload.ThresholdingOutputStream
org.apache.tomcat.util.http.fileupload.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 Summary

  Constructors
  Constructor
  Description
  `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.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `close()`
  Closes underlying output stream, and mark this as closed
  `byte[]`
  `getData()`
  Returns the data for this output stream as an array of bytes, assuming that the data has been retained in memory.
  `File`
  `getFile()`
  Returns either the output file specified in the constructor or the temporary file created or null.
  `protected OutputStream`
  `getStream()`
  Returns the current output stream.
  `boolean`
  `isInMemory()`
  Determines whether or not the data for this output stream has been retained in memory.
  `protected void`
  `thresholdReached()`
  Switches the underlying output stream from a memory based stream to one that is backed by disk.

  ### Methods inherited from class org.apache.tomcat.util.http.fileupload.ThresholdingOutputStream

  `checkThreshold, flush, isThresholdExceeded, write, write, write`

  ### Methods inherited from class java.io.OutputStream

  `nullOutputStream`

  ### Methods inherited from class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## 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.
    Parameters:
      `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`
    Returns:
      The underlying output stream.
    Throws:
      `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`
    Throws:
      `IOException` - if an error occurs.

  - ### isInMemory

    public boolean isInMemory()
    Determines whether or not the data for this output stream has been retained in memory.
    Returns:
      `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`.
    Returns:
      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.

    Returns:
      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`
    Overrides:
      `close` in class `ThresholdingOutputStream`
    Throws:
      `IOException` - if an error occurs.