網絡編程時，經常遇到很多
timeout異常，下面是java URLConnection 中經典的2種
timeout參數，這些參數設置不當的話，就會遇到
timeout 異常。
1. ConnectTimeout , java 是這樣解釋的。 意思是用來建立連接的時間。如果到了指定的時間，還沒建立連接，則報異常。
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().
經典異常如下所示：
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)
通過 URLConnection setConnectTimeout 可以設置指定的值。0則指極大值。
conn = url.openConnection();
conn.setConnectTimeout(5000);
2. ReadTimeout , java 是這樣解釋的。 意思是已經建立連接，並開始讀取服務端資源。如果到了指定的時間，沒有可能的數據被客戶端讀取，則報異常。
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().
類似報錯信息如下：
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)
通過 URLConnection setReadTimeout 可以設置指定的值。0則指極大值。
conn = url.openConnection();
conn.setReadTimeout(20000);
類似的設置在很多軟件都基本相同的，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.
，但是實際的情況是當時專網的時候（不能訪問百度）根本不是1.5秒拋出超時的異常，這個問題困擾2天，
也在網上查詢很多資料，結果有人說還要設置讀取超時conn.setReadTimeout(1500);結果一試還是不行，
就再找發現有人在使用的情況下查看控制台發現訪問1500毫秒之后系統會自動的嘗試很多次（集體多少次沒有算）
導致很長時間才拋出訪問超時異常，這樣導致我們看到的現象就是我設置了1500毫秒超時結果幾分鍾才給我超時異常，感覺設置的超時時間沒有效果。
那么說一下我的解決方法，我個人測試通過，如果有好的方法請指出：
因為系統會自動的嘗試很多次超時之后才給你拋出異常，所以我把這個異常超時時間設置短一點如我設置500毫秒，公網的情況下能正常的訪問，專網也能很快的拋出異常達到了我的目的。
另外判斷公網和專網的另一種方法就是用ping ，記得要起一個服務區處理ping，關閉之后要殺死改進程，否則第二次ping可能就阻塞了。