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Disk LRU Cache

A cache that uses a bounded amount of space on a filesystem. Each cache entry has a string key and a fixed number of values. Each key must match the regex [a-z0-9_-]{1,64}. Values are byte sequences, accessible as streams or files. Each value must be between 0 and Integer.MAX_VALUE bytes in length.

The cache stores its data in a directory on the filesystem. This directory must be exclusive to the cache; the cache may delete or overwrite files from its directory. It is an error for multiple processes to use the same cache directory at the same time.

This cache limits the number of bytes that it will store on the filesystem. When the number of stored bytes exceeds the limit, the cache will remove entries in the background until the limit is satisfied. The limit is not strict: the cache may temporarily exceed it while waiting for files to be deleted. The limit does not include filesystem overhead or the cache journal so space-sensitive applications should set a conservative limit.

Clients call edit to create or update the values of an entry. An entry may have only one editor at one time; if a value is not available to be edited then edit will return null.

  • When an entry is being created it is necessary to supply a full set of values; the empty value should be used as a placeholder if necessary.
  • When an entry is being edited, it is not necessary to supply data for every value; values default to their previous value.

Every edit call must be matched by a call to Editor.commit or Editor.abort. Committing is atomic: a read observes the full set of values as they were before or after the commit, but never a mix of values.

Clients call get to read a snapshot of an entry. The read will observe the value at the time that get was called. Updates and removals after the call do not impact ongoing reads.

This class is tolerant of some I/O errors. If files are missing from the filesystem, the corresponding entries will be dropped from the cache. If an error occurs while writing a cache value, the edit will fail silently. Callers should handle other problems by catching IOException and responding appropriately.

Note: This implementation specifically targets Android compatibility.

License

Copyright 2012 Jake Wharton
Copyright 2011 The Android Open Source Project

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.