1/*
2 * Copyright (C) 2010 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package java.net;
18
19import libcore.util.BasicLruCache;
20
21/**
22 * Implements caching for {@code InetAddress}. We use a unified cache for both positive and negative
23 * cache entries.
24 *
25 * TODO: benchmark and optimize InetAddress until we get to the point where we can just rely on
26 * the C library level caching. The main thing caching at this level buys us is avoiding repeated
27 * conversions from 'struct sockaddr's to InetAddress[].
28 */
29class AddressCache {
30    /**
31     * When the cache contains more entries than this, we start dropping the oldest ones.
32     * This should be a power of two to avoid wasted space in our custom map.
33     */
34    private static final int MAX_ENTRIES = 16;
35
36    // The TTL for the Java-level cache is short, just 2s.
37    private static final long TTL_NANOS = 2 * 1000000000L;
38
39    // The actual cache.
40    private final BasicLruCache<String, AddressCacheEntry> cache
41            = new BasicLruCache<String, AddressCacheEntry>(MAX_ENTRIES);
42
43    static class AddressCacheEntry {
44        // Either an InetAddress[] for a positive entry,
45        // or a String detail message for a negative entry.
46        final Object value;
47
48        /**
49         * The absolute expiry time in nanoseconds. Nanoseconds from System.nanoTime is ideal
50         * because -- unlike System.currentTimeMillis -- it can never go backwards.
51         *
52         * We don't need to worry about overflow with a TTL_NANOS of 2s.
53         */
54        final long expiryNanos;
55
56        AddressCacheEntry(Object value) {
57            this.value = value;
58            this.expiryNanos = System.nanoTime() + TTL_NANOS;
59        }
60    }
61
62    /**
63     * Removes all entries from the cache.
64     */
65    public void clear() {
66        cache.evictAll();
67    }
68
69    /**
70     * Returns the cached InetAddress[] associated with 'hostname'. Returns null if nothing is known
71     * about 'hostname'. Returns a String suitable for use as an UnknownHostException detail
72     * message if 'hostname' is known not to exist.
73     */
74    public Object get(String hostname) {
75        AddressCacheEntry entry = cache.get(hostname);
76        // Do we have a valid cache entry?
77        if (entry != null && entry.expiryNanos >= System.nanoTime()) {
78            return entry.value;
79        }
80        // Either we didn't find anything, or it had expired.
81        // No need to remove expired entries: the caller will provide a replacement shortly.
82        return null;
83    }
84
85    /**
86     * Associates the given 'addresses' with 'hostname'. The association will expire after a
87     * certain length of time.
88     */
89    public void put(String hostname, InetAddress[] addresses) {
90        cache.put(hostname, new AddressCacheEntry(addresses));
91    }
92
93    /**
94     * Records that 'hostname' is known not to have any associated addresses. (I.e. insert a
95     * negative cache entry.)
96     */
97    public void putUnknownHost(String hostname, String detailMessage) {
98        cache.put(hostname, new AddressCacheEntry(detailMessage));
99    }
100}
101