java.net.UnknownHostException

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问题

快递鸟查询物流的时候，在Window平台下没有问题，在Linux下会出现以下的问题：

java.net.UnknownHostException: api.kdniao.com
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:184)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at java.net.Socket.connect(Socket.java:538)
at sun.net.NetworkClient.doConnect(NetworkClient.java:180)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:432)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:527)
at sun.net.www.http.HttpClient.(HttpClient.java:211)
at sun.net.www.http.HttpClient.New(HttpClient.java:308)
at sun.net.www.http.HttpClient.New(HttpClient.java:326)
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1167)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1103)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:997)
at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:931)
at com.genersoft.ypzt.yjys.controller.YjysController.sendPost(YjysController.java:444)
at com.genersoft.ypzt.yjys.controller.YjysController.getOrderTracesByJson(YjysController.java:334)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

方案

修改hosts文件
vi /etc/hosts
134.175.75.136 api.kdniao.com

原因

在大多数Linux操作系统中，都是以/etc/hosts中的配置查找主机名的，但是 Detian based system用/etc/Hostname文件中的配置做主机名。

而Java 的InetAddress.java 调用 InetAddressImpl.java 的 public native String getLocalHostName() throws UnknownHostException; 来获取本地主机名, Java 的这个方法是native的，是本地系统的一个实现，所以在本地配置出现问题的情况下，Java 代码出现问题。
分析下代码：

 /**
     * Creates a socket and connects it to the specified address on
     * the specified port.
     * @param address the address
     * @param timeout the timeout value in milliseconds, or zero for no timeout.
     * @throws IOException if connection fails
     * @throws  IllegalArgumentException if address is null or is a
     *          SocketAddress subclass not supported by this socket
     * @since 1.4
     */
    protected void connect(SocketAddress address, int timeout)
            throws IOException {
        boolean connected = false;
        try {
            if (address == null || !(address instanceof InetSocketAddress))
                throw new IllegalArgumentException("unsupported address type");
            InetSocketAddress addr = (InetSocketAddress) address;
            if (addr.isUnresolved())
                throw new UnknownHostException(addr.getHostName());  //这里出的错误，应该是需要读取host文件
            this.port = addr.getPort();
            this.address = addr.getAddress();

            connectToAddress(this.address, port, timeout);
            connected = true;
        } finally {
            if (!connected) {
                try {
                    close();
                } catch (IOException ioe) {
                    /* Do nothing. If connect threw an exception then
                       it will be passed up the call stack */
                }
            }
        }
    }