Android笔记之解决OkHttp解析dns超时时间无法设置的问题

问题

使用OkHttp，设备切换路由后，访问网络出现长时间无响应，很久以后才抛出UnknownHostException，这明显不是我们想要的，我们设置的connectTimeout属性似乎对dns的解析不起作用。

如何解决

我们先看看OkHttpClient有没有关于Dns的相关设置，发现OkHttpClient的Builder类存在dns()方法可以设置一个Dns类型参数。
Dns类源码如下：

package okhttp3;

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.Arrays;
import java.util.List;

/**
 * A domain name service that resolves IP addresses for host names. Most applications will use the
 * {@linkplain #SYSTEM system DNS service}, which is the default. Some applications may provide
 * their own implementation to use a different DNS server, to prefer IPv6 addresses, to prefer IPv4
 * addresses, or to force a specific known IP address.
 *
 * <p>Implementations of this interface must be safe for concurrent use.
 */
public interface Dns {
  /**
   * A DNS that uses {@link InetAddress#getAllByName} to ask the underlying operating system to
   * lookup IP addresses. Most custom {@link Dns} implementations should delegate to this instance.
   */
  Dns SYSTEM = new Dns() {
    @Override public List<InetAddress> lookup(String hostname) throws UnknownHostException {
      if (hostname == null) throw new UnknownHostException("hostname == null");
      try {
        return Arrays.asList(InetAddress.getAllByName(hostname));
      } catch (NullPointerException e) {
        UnknownHostException unknownHostException =
            new UnknownHostException("Broken system behaviour for dns lookup of " + hostname);
        unknownHostException.initCause(e);
        throw unknownHostException;
      }
    }
  };

  /**
   * Returns the IP addresses of {@code hostname}, in the order they will be attempted by OkHttp. If
   * a connection to an address fails, OkHttp will retry the connection with the next address until
   * either a connection is made, the set of IP addresses is exhausted, or a limit is exceeded.
   */
  List<InetAddress> lookup(String hostname) throws UnknownHostException;
}

这是一个抽象类，并且其中已经写好了默认实现并赋值给了SYSTEM对象。
透过代码我们可以看出，解析dns主要是靠这行代码：

InetAddress.getAllByName(hostname)

Ok，我们已经知道如何解析dns并设置给OkHttpClient了，现在我们只需要重新实现这个过程，插入超时控制，并让OKHttp调用新的Dns解析就好了。

实现

实现抽象类Dns

package com.x.http;

import okhttp3.Dns;

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;
import java.util.concurrent.TimeUnit;

public class XDns implements Dns {
    private long timeout;

    public XDns(long timeout) {
        this.timeout = timeout;
    }

    @Override
    public List<InetAddress> lookup(final String hostname) throws UnknownHostException {
        if (hostname == null) {
            throw new UnknownHostException("hostname == null");
        } else {
            try {
                FutureTask<List<InetAddress>> task = new FutureTask<>(
                        new Callable<List<InetAddress>>() {
                            @Override
                            public List<InetAddress> call() throws Exception {
                                return Arrays.asList(InetAddress.getAllByName(hostname));
                            }
                        });
                new Thread(task).start();
                return task.get(timeout, TimeUnit.MILLISECONDS);
            } catch (Exception var4) {
                UnknownHostException unknownHostException =
                        new UnknownHostException("Broken system behaviour for dns lookup of " + hostname);
                unknownHostException.initCause(var4);
                throw unknownHostException;
            }
        }
    }
}

 上面的实现主要依赖了FutureTask可以设置任务执行时间的特性，不明白的同学可以自行学习一下。
然后把新的dns解析类设置给OkHttpClient

    public static OkHttpClient createClient(long timeout, long writeTimeout,
     long readTimeout, boolean bRetry) {
        OkHttpClient.Builder builder = new OkHttpClient.Builder()
                .connectTimeout(timeout, TimeUnit.MILLISECONDS)
                .dns(new XDns(timeout))
                .retryOnConnectionFailure(bRetry);
        if (writeTimeout > 0)
            builder.writeTimeout(writeTimeout, TimeUnit.MILLISECONDS);
        if (readTimeout > 0)
            builder.readTimeout(readTimeout, TimeUnit.MILLISECONDS);
        OkHttpClient client = builder.build();
        client.dispatcher().setMaxRequestsPerHost(10);
        client.dispatcher().setMaxRequests(30);
        return client;
    }

转载于：https://blog.youkuaiyun.com/quwei3930921/article/details/85336552#commentBox 感谢作者的辛苦付出