关于java:jvm添加自定义dns

 有一个常见的场景，咱们在开发过程中，要配置很多的本地 host，以实现测试环境一些资源的拜访，那么其它人参加进来开发的话，也得在本人电脑上配置，这样既麻烦，又容易出错。那么能不能把这些配置，写到 project 中去实现呢，这样每个人本地的 /etc/hosts 文件中会很洁净，能够随时 clone 启动调试而不需任何配置。答案是必定的，那么接下来咱们就应用 java 反射的形式来实现。咱们次要操作的对象就是 java.net.InetAddress, 能够从源码中看到，该类中有两个外围的变量 cache 和 expirySet。

    // mapping from host name to Addresses - either NameServiceAddresses (while
    // still being looked-up by NameService(s)) or CachedAddresses when cached
    private static final ConcurrentMap<String, Addresses> cache =
        new ConcurrentHashMap<>();

    // CachedAddresses that have to expire are kept ordered in this NavigableSet
    // which is scanned on each access
    private static final NavigableSet<CachedAddresses> expirySet =
        new ConcurrentSkipListSet<>();

这里会有个疑难，为啥还有个 expirySet，这个问题也能够通过源码失去解决，即为了删除生效的条目。具体含意能够通过浏览正文进行了解。

        // remove expired addresses from cache - expirySet keeps them ordered
        // by expiry time so we only need to iterate the prefix of the NavigableSet...
        long now = System.nanoTime();
        for (CachedAddresses caddrs : expirySet) {
            // compare difference of time instants rather than
            // time instants directly, to avoid possible overflow.
            // (see System.nanoTime() recommendations...)
            if ((caddrs.expiryTime - now) < 0L) {
                // ConcurrentSkipListSet uses weakly consistent iterator,
                // so removing while iterating is OK...
                if (expirySet.remove(caddrs)) {
                    // ... remove from cache
                    cache.remove(caddrs.host, caddrs);
                }
            } else {
                // we encountered 1st element that expires in future
                break;
            }
        }

接下来就是如何来实现反射增加 dns 条目了, 本例中基于 java17 实现，其它版本会有相应的变动。

        Class<?> cachedAddresses_Class = Class.forName("java.net.InetAddress$CachedAddresses");
        Constructor<?> constructor = cachedAddresses_Class.getDeclaredConstructors()[0];
        constructor.setAccessible(true);
        Object o = constructor.newInstance(host, toInetAddressArray(host, ip), Long.MAX_VALUE);

        Field cacheField = InetAddress.class.getDeclaredField("cache");
        cacheField.setAccessible(true);
        ConcurrentMap<String, Object> cm = (ConcurrentMap<String, Object>) cacheField.get(null);
        cm.put(host, o);

        Field expirySetField = InetAddress.class.getDeclaredField("expirySet");
        expirySetField.setAccessible(true);
        ConcurrentSkipListSet<Object> cs = (ConcurrentSkipListSet<Object>) expirySetField.get(null);
        cs.add(o);

这样的话，就能够本人封装一下，比方 dns 条目都写在一个文件中，编译打包的时候，按 profile 配置决定是否加载。