使用 Web 服务时出现 UnKnowHost 异常

我正在尝试使用 Web 服务并在该 api 上执行 POST。我试过邮递员，一切都很好。但是，一旦我编写了一个 Java 客户端来使用该 Web 服务，我就会收到以下错误。

java.net.UnknownHostException: www.api.com
    at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
    at java.net.InetAddress$2.lookupAllHostAddr(InetAddress.java:928)
    at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1323)
    at java.net.InetAddress.getAllByName0(InetAddress.java:1276)
    at java.net.InetAddress.getAllByName(InetAddress.java:1192)
    at java.net.InetAddress.getAllByName(InetAddress.java:1126)
    at org.apache.http.impl.conn.SystemDefaultDnsResolver.resolve(SystemDefaultDnsResolver.java:44)
    at org.apache.http.impl.conn.DefaultClientConnectionOperator.resolveHostname(DefaultClientConnectionOperator.java:259)
    at org.apache.http.impl.conn.DefaultClientConnectionOperator.openConnection(DefaultClientConnectionOperator.java:159)
    at org.apache.http.impl.conn.ManagedClientConnectionImpl.open(ManagedClientConnectionImpl.java:304)
    at org.apache.http.impl.client.DefaultRequestDirector.tryConnect(DefaultRequestDirector.java:611)
    at org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:446)
    at org.apache.http.impl.client.AbstractHttpClient.doExecute(AbstractHttpClient.java:863)
    at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:82)
    at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:106)

Java客户端代码：

public static void sendPost() throws ClientProtocolException, IOException {

    try {
        HttpClient client = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost(_api.trim());
        List<NameValuePair> postParameters = new ArrayList<NameValuePair>();
        postParameters.add(new BasicNameValuePair("parm_1", "dgs"));
        postParameters.add(new BasicNameValuePair("parm_2", "ab"));
        postParameters.add(new BasicNameValuePair("parm_3", "4hdiin"));
        httpPost.setHeader("Content-type", "application/x-www-form-urlencoded");

        httpPost.setEntity(new UrlEncodedFormEntity(postParameters));

        HttpResponse response = client.execute(httpPost);
    } catch (Exception e) {
        e.printStackTrace();
    }