问题
使用OkHttp，设备切换路由后，访问网络出现长时间无响应，很久后来才抛出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;
}
这是一个接口类，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，目前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，不清楚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;
}
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