网络编程时，经常遇到很多timeout异常，下面是java URLConnection 中经典的2种 timeout参数，这些参数设置不当的话，就会遇到timeout 异常。
1. ConnectTimeout , java 是这样解释的。 意思是用来建立连接的时间。如果到了指定的时间，还没建立连接，则报异常。
Sets a specified timeout value, in milliseconds, to be used when opening a communications link to the resource referenced by this URLConnection. If the timeout expires before the connection can be established, a java.net.SocketTimeoutException is raised. A timeout of zero is interpreted as an infinite timeout. Some non-standard implmentation of this method may ignore the specified timeout. To see the connect timeout set, please call getConnectTimeout().
经典异常如下所示：
java.net.SocketTimeoutException: connect timed out at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27) at java.lang.reflect.Constructor.newInstance(Constructor.java:513) at sun.net.www.protocol.http.HttpURLConnection$6.run(HttpURLConnection.java:1491) at java.security.AccessController.doPrivileged(Native Method) at sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1485) at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1139) at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:234) at 。。。。。。(Unknown Source) Caused by: java.net.SocketTimeoutException: connect timed out at java.net.PlainSocketImpl.socketConnect(Native Method) at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:351) at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:213) at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:200) at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366) at java.net.Socket.connect(Socket.java:529) at com.sun.net.ssl.internal.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:564) at sun.net.NetworkClient.doConnect(NetworkClient.java:158) at sun.net.www.http.HttpClient.openServer(HttpClient.java:395) at sun.net.www.http.HttpClient.openServer(HttpClient.java:530) at sun.net.www.protocol.https.HttpsClient.<init>(HttpsClient.java:272) at sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:329) at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:172) at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:911) at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:158) at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1172) at sun.net.www.protocol.http.HttpURLConnection.getHeaderField(HttpURLConnection.java:2300) at java.net.URLConnection.getHeaderFieldInt(URLConnection.java:579) at java.net.URLConnection.getContentLength(URLConnection.java:474) at sun.net.www.protocol.https.HttpsURLConnectionImpl.getContentLength(HttpsURLConnectionImpl.java:378) at 。。。。。。(Unknown Source)
通过 URLConnection setConnectTimeout 可以设置指定的值。0则指极大值。
conn = url.openConnection();
conn.setConnectTimeout(5000);
2. ReadTimeout , java 是这样解释的。 意思是已经建立连接，并开始读取服务端资源。如果到了指定的时间，没有可能的数据被客户端读取，则报异常。
Sets the read timeout to a specified timeout, in milliseconds. A non-zero value specifies the timeout when reading from Input stream when a connection is established to a resource. If the timeout expires before there is data available for read, a java.net.SocketTimeoutException is raised. A timeout of zero is interpreted as an infinite timeout. Some non-standard implementation of this method ignores the specified timeout. To see the read timeout set, please call getReadTimeout().
类似报错信息如下：
java.net.SocketTimeoutException: Read timed out at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27) at java.lang.reflect.Constructor.newInstance(Constructor.java:513) at sun.net.www.protocol.http.HttpURLConnection$6.run(HttpURLConnection.java:1491) at java.security.AccessController.doPrivileged(Native Method) at sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1485) at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1139) at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:234) at 。。。。。。(Unknown Source) Caused by: java.net.SocketTimeoutException: Read timed out at java.net.SocketInputStream.socketRead0(Native Method) at java.net.SocketInputStream.read(SocketInputStream.java:129) at com.sun.net.ssl.internal.ssl.InputRecord.readFully(InputRecord.java:293) at com.sun.net.ssl.internal.ssl.InputRecord.read(InputRecord.java:331) at com.sun.net.ssl.internal.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:830) at com.sun.net.ssl.internal.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1170) at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1197) at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1181) at sun.net.www.protocol.https.HttpsClient.afterConnect(HttpsClient.java:434) at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:166) at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1172) at sun.net.www.protocol.http.HttpURLConnection.getHeaderField(HttpURLConnection.java:2300) at java.net.URLConnection.getHeaderFieldInt(URLConnection.java:579) at java.net.URLConnection.getContentLength(URLConnection.java:474) at sun.net.www.protocol.https.HttpsURLConnectionImpl.getContentLength(HttpsURLConnectionImpl.java:378) at 。。。。。。(Unknown Source)
通过 URLConnection setReadTimeout 可以设置指定的值。0则指极大值。
conn = url.openConnection();
conn.setReadTimeout(20000);
类似的设置在很多软件都基本相同的，比如 wget.
-T, --timeout=SECONDS set all timeout values to SECONDS.
--dns-timeout=SECS set the DNS lookup timeout to SECS.
--connect-timeout=SECS set the connect timeout to SECS.
--read-timeout=SECS set the read timeout to SECS.
，但是实际的情况是当时专网的时候（不能访问百度）根本不是1.5秒抛出超时的异常，这个问题困扰2天，
也在网上查询很多资料，结果有人说还要设置读取超时conn.setReadTimeout(1500);结果一试还是不行，
就再找发现有人在使用的情况下查看控制台发现访问1500毫秒之后系统会自动的尝试很多次（集体多少次没有算）
导致很长时间才抛出访问超时异常，这样导致我们看到的现象就是我设置了1500毫秒超时结果几分钟才给我超时异常，感觉设置的超时时间没有效果。
那么说一下我的解决方法，我个人测试通过，如果有好的方法请指出：
因为系统会自动的尝试很多次超时之后才给你抛出异常，所以我把这个异常超时时间设置短一点如我设置500毫秒，公网的情况下能正常的访问，专网也能很快的抛出异常达到了我的目的。
另外判断公网和专网的另一种方法就是用ping ，记得要起一个服务区处理ping，关闭之后要杀死改进程，否则第二次ping可能就阻塞了。