ImageCache
Date: 2016-04-28

org.apache.xmlgraphics.image.loader.cache

## Class ImageCache

- java.lang.Object
- - org.apache.xmlgraphics.image.loader.cache.ImageCache

-     public class ImageCache
      extends java.lang.Object

  This class provides a cache for images. The main key into the images is the original URI the image was accessed with.

  Don't use one ImageCache instance in the context of multiple base URIs because relative URIs would not work correctly anymore.

  By default, the URIs of inaccessible images are remembered but these entries are discarded after 60 seconds (which causes a retry next time the same URI is requested). This allows to counteract performance loss when accessing invalid or temporarily unavailable images over slow connections.

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected static org.apache.commons.logging.Log` | `log` logger

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `ImageCache()` Default constructor with default settings.
    `ImageCache(org.apache.xmlgraphics.image.loader.cache.TimeStampProvider timeStampProvider, ExpirationPolicy invalidURIExpirationPolicy)` Constructor for customized behaviour and testing.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `clearCache()` Clears the image cache (all ImageInfo and Image objects).
    `void` | `doHouseKeeping()` Triggers some house-keeping, i.e.
    `Image` | `getImage(ImageInfo info, ImageFlavor flavor)` Returns an image from the cache or null if it wasn't found.
    `Image` | `getImage(java.lang.String uri, ImageFlavor flavor)` Returns an image from the cache or null if it wasn't found.
    `protected ImageInfo` | `getImageInfo(java.lang.String uri)` Returns an ImageInfo instance from the cache or null if none is found.
    `boolean` | `isInvalidURI(java.lang.String uri)` Indicates whether a URI has previously been identified as an invalid URI.
    `ImageInfo` | `needImageInfo(java.lang.String uri, ImageSessionContext session, ImageManager manager)` Returns an ImageInfo instance for a given URI.
    `void` | `putImage(Image img)` Registers an image with the cache.
    `protected void` | `putImageInfo(ImageInfo info)` Registers an ImageInfo instance with the cache.
    `void` | `setCacheListener(ImageCacheListener listener)` Sets an ImageCacheListener instance so the events in the image cache can be observed.

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

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

- - ### Field Detail

    - #### log

          protected static final org.apache.commons.logging.Log log

      logger

  - ### Constructor Detail

    - #### ImageCache

          public ImageCache()

      Default constructor with default settings.

    - #### ImageCache

          public ImageCache(org.apache.xmlgraphics.image.loader.cache.TimeStampProvider timeStampProvider,
                    ExpirationPolicy invalidURIExpirationPolicy)

      Constructor for customized behaviour and testing.
      Parameters:
        `timeStampProvider` - the time stamp provider to use
        `invalidURIExpirationPolicy` - the expiration policy for invalid URIs

  - ### Method Detail

    - #### setCacheListener

          public void setCacheListener(ImageCacheListener listener)

      Sets an ImageCacheListener instance so the events in the image cache can be observed.
      Parameters:
        `listener` - the listener instance

    - #### needImageInfo

          public ImageInfo needImageInfo(java.lang.String uri,
                                ImageSessionContext session,
                                ImageManager manager)
                                  throws ImageException,
                                         java.io.IOException

      Returns an ImageInfo instance for a given URI.
      Parameters:
        `uri` - the image's URI
        `session` - the session context
        `manager` - the ImageManager handling the images
      Returns:
        the ImageInfo instance
      Throws:
        `ImageException` - if an error occurs while parsing image data
        `java.io.IOException` - if an I/O error occurs while loading image data

    - #### isInvalidURI

          public boolean isInvalidURI(java.lang.String uri)

      Indicates whether a URI has previously been identified as an invalid URI.
      Parameters:
        `uri` - the image's URI
      Returns:
        true if the URI is invalid

    - #### getImageInfo

          protected ImageInfo getImageInfo(java.lang.String uri)

      Returns an ImageInfo instance from the cache or null if none is found.
      Parameters:
        `uri` - the image's URI
      Returns:
        the ImageInfo instance or null if the requested information is not in the cache

    - #### putImageInfo

          protected void putImageInfo(ImageInfo info)

      Registers an ImageInfo instance with the cache.
      Parameters:
        `info` - the ImageInfo instance

    - #### getImage

          public Image getImage(ImageInfo info,
                       ImageFlavor flavor)

      Returns an image from the cache or null if it wasn't found.
      Parameters:
        `info` - the ImageInfo instance representing the image
        `flavor` - the requested ImageFlavor for the image
      Returns:
        the requested image or null if the image is not in the cache

    - #### getImage

          public Image getImage(java.lang.String uri,
                       ImageFlavor flavor)

      Returns an image from the cache or null if it wasn't found.
      Parameters:
        `uri` - the image's URI
        `flavor` - the requested ImageFlavor for the image
      Returns:
        the requested image or null if the image is not in the cache

    - #### putImage

          public void putImage(Image img)

      Registers an image with the cache.
      Parameters:
        `img` - the image

    - #### clearCache

          public void clearCache()

      Clears the image cache (all ImageInfo and Image objects).

    - #### doHouseKeeping

          public void doHouseKeeping()

      Triggers some house-keeping, i.e. removes stale entries.