1 Disk LRU Cache 2 ============== 3 4 A cache that uses a bounded amount of space on a filesystem. Each cache entry 5 has a string key and a fixed number of values. Each key must match the regex 6 `[a-z0-9_-]{1,64}`. Values are byte sequences, accessible as streams or files. 7 Each value must be between `0` and `Integer.MAX_VALUE` bytes in length. 8 9 The cache stores its data in a directory on the filesystem. This directory must 10 be exclusive to the cache; the cache may delete or overwrite files from its 11 directory. It is an error for multiple processes to use the same cache 12 directory at the same time. 13 14 This cache limits the number of bytes that it will store on the filesystem. 15 When the number of stored bytes exceeds the limit, the cache will remove 16 entries in the background until the limit is satisfied. The limit is not 17 strict: the cache may temporarily exceed it while waiting for files to be 18 deleted. The limit does not include filesystem overhead or the cache journal so 19 space-sensitive applications should set a conservative limit. 20 21 Clients call `edit` to create or update the values of an entry. An entry may 22 have only one editor at one time; if a value is not available to be edited then 23 `edit` will return null. 24 25 * When an entry is being **created** it is necessary to supply a full set of 26 values; the empty value should be used as a placeholder if necessary. 27 * When an entry is being **edited**, it is not necessary to supply data for 28 every value; values default to their previous value. 29 30 Every `edit` call must be matched by a call to `Editor.commit` or 31 `Editor.abort`. Committing is atomic: a read observes the full set of values as 32 they were before or after the commit, but never a mix of values. 33 34 Clients call `get` to read a snapshot of an entry. The read will observe the 35 value at the time that `get` was called. Updates and removals after the call do 36 not impact ongoing reads. 37 38 This class is tolerant of some I/O errors. If files are missing from the 39 filesystem, the corresponding entries will be dropped from the cache. If an 40 error occurs while writing a cache value, the edit will fail silently. Callers 41 should handle other problems by catching `IOException` and responding 42 appropriately. 43 44 *Note: This implementation specifically targets Android compatibility.* 45 46 License 47 ======= 48 49 Copyright 2012 Jake Wharton 50 Copyright 2011 The Android Open Source Project 51 52 Licensed under the Apache License, Version 2.0 (the "License"); 53 you may not use this file except in compliance with the License. 54 You may obtain a copy of the License at 55 56 http://www.apache.org/licenses/LICENSE-2.0 57 58 Unless required by applicable law or agreed to in writing, software 59 distributed under the License is distributed on an "AS IS" BASIS, 60 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 61 See the License for the specific language governing permissions and 62 limitations under the License. 63 64
