TTL time to live

        在通过DNS查找域名的过程中，可能会经过多台中间DNS服务器才能找到指定的域名，因此，在DNS服务器上查找域名是非常昂贵的操作。在Java中为了缓解这个问题，提供了DNS缓存。当InetAddress类第一次使用某个域名（如www.google.com）创建InetAddress对象后，JVM就会将这个域名和它从DNS上获得的信息（如IP地址）都保存在DNS缓存中。当下一次InetAddress类再使用这个域名时，就直接从DNS缓存里获得所需的信息，而无需再访问DNS服务器。DNS缓存在默认时将永远保留曾经访问过的域名信息，但我们可以修改这个默认值。

LINUX/android/libcore/luni/src/main/java/java/net/AddressCache.java

//The TTL for the Java-level cache is short, just 2s.

privatestaticfinallongTTL_NANOS= 2 * 1000000000L;

InetAddress.getByName(“www.baidu.com”);

在这个时间内，断网了还能返回结果，超过后会抛异常。没网络也会异常。

当前类型网络连接断开后会把map里面缓存给清除，所有在解MMIOT的bug的时候就自己写个map，把mms的缓存到自己写的map中，永远不删除，因为还有个时间的判断，所以不影响

package java.net;


import libcore.util.BasicLruCache;


/**
 * Implements caching for {@code InetAddress}. We use a unified cache for both positive and negative
 * cache entries.
 *
 * TODO: benchmark and optimize InetAddress until we get to the point where we can just rely on
 * the C library level caching. The main thing caching at this level buys us is avoiding repeated
 * conversions from 'struct sockaddr's to InetAddress[].
 */
class AddressCache {
    /**
     * When the cache contains more entries than this, we start dropping the oldest ones.
     * This should be a power of two to avoid wasted space in our custom map.
     */
    private static final int MAX_ENTRIES = 16;


// Aaron@20150514 change to fix MMS bug. Default value is 2S. 2.13.4 MMSC
// URL DNS HISTORY
// The TTL for the Java-level cache is short, just 2s.
// Aaron@20150515 change for MMIOT TTL bug
private static final long TTL_VZW_MMS = 15 * 60 * 1000000000L;
private static final long TTL_NANOS = 2 * 1000000000L;
// end


    // The actual cache.
    private final BasicLruCache<AddressCacheKey, AddressCacheEntry> cache
            = new BasicLruCache<AddressCacheKey, AddressCacheEntry>(MAX_ENTRIES);
// Aaron@20150514 add for MMIOT TTL bug
private final BasicLruCache<AddressCacheKey, AddressCacheEntry> mcache = new BasicLruCache<AddressCacheKey, AddressCacheEntry>(
MAX_ENTRIES);


// end


    static class AddressCacheKey {
        private final String mHostname;
        private final int mNetId;


        AddressCacheKey(String hostname, int netId) {
            mHostname = hostname;
            mNetId = netId;
        }


        @Override public boolean equals(Object o) {
            if (this == o) {
                return true;
            }
            if (!(o instanceof AddressCacheKey)) {
                return false;
            }
            AddressCacheKey lhs = (AddressCacheKey) o;
            return mHostname.equals(lhs.mHostname) && mNetId == lhs.mNetId;
        }


        @Override public int hashCode() {
            int result = 17;
            result = 31 * result + mNetId;
            result = 31 * result + mHostname.hashCode();
            return result;
        }
    }


    static class AddressCacheEntry {
        // Either an InetAddress[] for a positive entry,
        // or a String detail message for a negative entry.
        final Object value;


        /**
         * The absolute expiry time in nanoseconds. Nanoseconds from System.nanoTime is ideal
         * because -- unlike System.currentTimeMillis -- it can never go backwards.
         *
         * We don't need to worry about overflow with a TTL_NANOS of 2s.
         */
        final long expiryNanos;


        AddressCacheEntry(Object value) {
            this.value = value;
// Aaron@20150514 change for MMIOT TTL bug
if (InetAddress.isMMSconnect) {
this.expiryNanos = System.nanoTime() + TTL_VZW_MMS;
} else
// end
            this.expiryNanos = System.nanoTime() + TTL_NANOS;
        }
    }


    /**
     * Removes all entries from the cache.
     */
    public void clear() {
        cache.evictAll();
    }


    /**
     * Returns the cached InetAddress[] for 'hostname' on network 'netId'. Returns null
     * if nothing is known about 'hostname'. Returns a String suitable for use as an
     * UnknownHostException detail message if 'hostname' is known not to exist.
     */
    public Object get(String hostname, int netId) {
        AddressCacheEntry entry = cache.get(new AddressCacheKey(hostname, netId));
// Aaron@20150514 add for MMIOT TTL bug
if (InetAddress.isMMSconnect) {
entry = mcache.get(new AddressCacheKey(hostname, netId));
}
// end
        // Do we have a valid cache entry?
        if (entry != null && entry.expiryNanos >= System.nanoTime()) {
            return entry.value;
        }
        // Either we didn't find anything, or it had expired.
        // No need to remove expired entries: the caller will provide a replacement shortly.
        return null;
    }


    /**
     * Associates the given 'addresses' with 'hostname'. The association will expire after a
     * certain length of time.
     */
    public void put(String hostname, int netId, InetAddress[] addresses) {
        cache.put(new AddressCacheKey(hostname, netId), new AddressCacheEntry(addresses));
// Aaron@20150514 add for MMIOT TTL bug
if (InetAddress.isMMSconnect) {
mcache.put(new AddressCacheKey(hostname, netId),
new AddressCacheEntry(addresses));
}
// end
    }


    /**
     * Records that 'hostname' is known not to have any associated addresses. (I.e. insert a
     * negative cache entry.)
     */
    public void putUnknownHost(String hostname, int netId, String detailMessage) {
        cache.put(new AddressCacheKey(hostname, netId), new AddressCacheEntry(detailMessage));
    }
}