Interface Cache

  • All Known Implementing Classes:
    SimpleCache

    public interface Cache
    A cache that supports partial caching of resources.

    Terminology

    • A resource is a complete piece of logical data, for example a complete media file.
    • A cache key uniquely identifies a resource. URIs are often suitable for use as cache keys, however this is not always the case. URIs are not suitable when caching resources obtained from a service that generates multiple URIs for the same underlying resource, for example because the service uses expiring URIs as a form of access control.
    • A cache span is a byte range within a resource, which may or may not be cached. A cache span that's not cached is called a hole span. A cache span that is cached corresponds to a single underlying file in the cache.
    • Field Detail

      • UID_UNSET

        static final long UID_UNSET
        Returned by getUid() if initialization failed before the unique identifier was read or generated.
        See Also:
        Constant Field Values
    • Method Detail

      • getUid

        long getUid()
        Returns a non-negative unique identifier for the cache, or UID_UNSET if initialization failed before the unique identifier was determined.

        Implementations are expected to generate and store the unique identifier alongside the cached content. If the location of the cache is deleted or swapped, it is expected that a new unique identifier will be generated when the cache is recreated.

      • release

        @WorkerThread
        void release()
        Releases the cache. This method must be called when the cache is no longer required. The cache must not be used after calling this method.

        This method may be slow and shouldn't normally be called on the main thread.

      • addListener

        NavigableSet<CacheSpan> addListener​(String key,
                                            Cache.Listener listener)
        Registers a listener to listen for changes to a given resource.

        No guarantees are made about the thread or threads on which the listener is called, but it is guaranteed that listener methods will be called in a serial fashion (i.e. one at a time) and in the same order as events occurred.

        Parameters:
        key - The cache key of the resource.
        listener - The listener to add.
        Returns:
        The current spans for the resource.
      • removeListener

        void removeListener​(String key,
                            Cache.Listener listener)
        Unregisters a listener.
        Parameters:
        key - The cache key of the resource.
        listener - The listener to remove.
      • getCachedSpans

        NavigableSet<CacheSpan> getCachedSpans​(String key)
        Returns the cached spans for a given resource.
        Parameters:
        key - The cache key of the resource.
        Returns:
        The spans for the key.
      • getKeys

        Set<String> getKeys()
        Returns the cache keys of all of the resources that are at least partially cached.
      • getCacheSpace

        long getCacheSpace()
        Returns the total disk space in bytes used by the cache.
      • startReadWrite

        @WorkerThread
        CacheSpan startReadWrite​(String key,
                                 long position,
                                 long length)
                          throws InterruptedException,
                                 Cache.CacheException
        A caller should invoke this method when they require data starting from a given position in a given resource.

        If there is a cache entry that overlaps the position, then the returned CacheSpan defines the file in which the data is stored. CacheSpan.isCached is true. The caller may read from the cache file, but does not acquire any locks.

        If there is no cache entry overlapping position, then the returned CacheSpan defines a hole in the cache starting at position into which the caller may write as it obtains the data from some other source. The returned CacheSpan serves as a lock. Whilst the caller holds the lock it may write data into the hole. It may split data into multiple files. When the caller has finished writing a file it should commit it to the cache by calling commitFile(File, long). When the caller has finished writing, it must release the lock by calling releaseHoleSpan(com.google.android.exoplayer2.upstream.cache.CacheSpan).

        This method may be slow and shouldn't normally be called on the main thread.

        Parameters:
        key - The cache key of the resource.
        position - The starting position in the resource from which data is required.
        length - The length of the data being requested, or C.LENGTH_UNSET if unbounded. The length is ignored if there is a cache entry that overlaps the position. Else, it defines the maximum length of the hole CacheSpan that's returned. Cache implementations may support parallel writes into non-overlapping holes, and so passing the actual required length should be preferred to passing C.LENGTH_UNSET when possible.
        Returns:
        The CacheSpan.
        Throws:
        InterruptedException - If the thread was interrupted.
        Cache.CacheException - If an error is encountered.
      • startReadWriteNonBlocking

        @WorkerThread
        @Nullable
        CacheSpan startReadWriteNonBlocking​(String key,
                                            long position,
                                            long length)
                                     throws Cache.CacheException
        Same as startReadWrite(String, long, long). However, if the cache entry is locked, then instead of blocking, this method will return null as the CacheSpan.

        This method may be slow and shouldn't normally be called on the main thread.

        Parameters:
        key - The cache key of the resource.
        position - The starting position in the resource from which data is required.
        length - The length of the data being requested, or C.LENGTH_UNSET if unbounded. The length is ignored if there is a cache entry that overlaps the position. Else, it defines the range of data locked by the returned CacheSpan.
        Returns:
        The CacheSpan. Or null if the cache entry is locked.
        Throws:
        Cache.CacheException - If an error is encountered.
      • startFile

        @WorkerThread
        File startFile​(String key,
                       long position,
                       long length)
                throws Cache.CacheException
        Obtains a cache file into which data can be written. Must only be called when holding a corresponding hole CacheSpan obtained from startReadWrite(String, long, long).

        This method may be slow and shouldn't normally be called on the main thread.

        Parameters:
        key - The cache key of the resource being written.
        position - The starting position in the resource from which data will be written.
        length - The length of the data being written, or C.LENGTH_UNSET if unknown. Used only to ensure that there is enough space in the cache.
        Returns:
        The file into which data should be written.
        Throws:
        Cache.CacheException - If an error is encountered.
      • commitFile

        @WorkerThread
        void commitFile​(File file,
                        long length)
                 throws Cache.CacheException
        Commits a file into the cache. Must only be called when holding a corresponding hole CacheSpan obtained from startReadWrite(String, long, long).

        This method may be slow and shouldn't normally be called on the main thread.

        Parameters:
        file - A newly written cache file.
        length - The length of the newly written cache file in bytes.
        Throws:
        Cache.CacheException - If an error is encountered.
      • removeResource

        @WorkerThread
        void removeResource​(String key)
        Removes all CacheSpans for a resource, deleting the underlying files.
        Parameters:
        key - The cache key of the resource being removed.
      • removeSpan

        @WorkerThread
        void removeSpan​(CacheSpan span)
        Removes a cached CacheSpan from the cache, deleting the underlying file.

        This method may be slow and shouldn't normally be called on the main thread.

        Parameters:
        span - The CacheSpan to remove.
      • isCached

        boolean isCached​(String key,
                         long position,
                         long length)
        Returns whether the specified range of data in a resource is fully cached.
        Parameters:
        key - The cache key of the resource.
        position - The starting position of the data in the resource.
        length - The length of the data.
        Returns:
        true if the data is available in the Cache otherwise false;
      • getCachedLength

        long getCachedLength​(String key,
                             long position,
                             long length)
        Returns the length of continuously cached data starting from position, up to a maximum of maxLength, of a resource. If position isn't cached then -holeLength is returned, where holeLength is the length of continuously uncached data starting from position, up to a maximum of maxLength.
        Parameters:
        key - The cache key of the resource.
        position - The starting position of the data in the resource.
        length - The maximum length of the data or hole to be returned. C.LENGTH_UNSET is permitted, and is equivalent to passing Long.MAX_VALUE.
        Returns:
        The length of the continuously cached data, or -holeLength if position isn't cached.
      • getCachedBytes

        long getCachedBytes​(String key,
                            long position,
                            long length)
        Returns the total number of cached bytes between position (inclusive) and (position + length) (exclusive) of a resource.
        Parameters:
        key - The cache key of the resource.
        position - The starting position of the data in the resource.
        length - The length of the data to check. C.LENGTH_UNSET is permitted, and is equivalent to passing Long.MAX_VALUE.
        Returns:
        The total number of cached bytes.
      • applyContentMetadataMutations

        @WorkerThread
        void applyContentMetadataMutations​(String key,
                                           ContentMetadataMutations mutations)
                                    throws Cache.CacheException
        Applies mutations to the ContentMetadata for the given resource. A new CachedContent is added if there isn't one already for the resource.

        This method may be slow and shouldn't normally be called on the main thread.

        Parameters:
        key - The cache key of the resource.
        mutations - Contains mutations to be applied to the metadata.
        Throws:
        Cache.CacheException - If an error is encountered.