org.apache.commons.httpclient.HttpClient 出现 sun.security.provider.certpath.SunCertPathBuilderException

这个异常通常表示 SSL 证书验证失败，可能是因为证书已过期、证书颁发机构不受信任、证书根证书不匹配等原因。

解决这个问题的方法是使用 HttpClient 的 setHostConfiguration 方法设置 SSL 相关参数，例如：

HttpClient httpClient = new HttpClient();
Protocol myhttps = new Protocol("https", new MySSLSocketFactory(), 443);
HostConfiguration config = httpClient.getHostConfiguration();
config.setHost("example.com", 443, myhttps);

其中，MySSLSocketFactory 是自定义的 SSL Socket 工厂类，可以根据具体情况实现其 createSocket 方法，例如：

public class MySSLSocketFactory extends org.apache.commons.httpclient.contrib.ssl.AuthSSLProtocolSocketFactory {

    @Override
    public Socket createSocket(String host, int port, InetAddress localAddress, int localPort, HttpConnectionParams params) throws IOException {
        Socket socket = super.createSocket(host, port, localAddress, localPort, params);
        if (socket instanceof SSLSocket) {
            SSLSocket sslSocket = (SSLSocket) socket;
            SSLContext sslContext = SSLContext.getInstance("TLS");
            sslContext.init(null, new TrustManager[] { new MyX509TrustManager() }, null);
            sslSocket.setEnabledProtocols(sslSocket.getSupportedProtocols());
            sslSocket.setEnabledCipherSuites(sslSocket.getSupportedCipherSuites());
            sslSocket.setUseClientMode(true);
            sslSocket.setNeedClientAuth(false);
            sslSocket.setWantClientAuth(false);
            sslSocket.setEnableSessionCreation(true);
            sslSocket.startHandshake();
        }
        return socket;
    }

}

MyX509TrustManager 是自定义的 X509 Trust Manager 类，可以根据具体情况实现其 checkServerTrusted 方法，例如：

public class MyX509TrustManager implements X509TrustManager {

    @Override
    public void checkClientTrusted(X509Certificate[] chain, String authType) throws CertificateException {
        // do nothing
    }

    @Override
    public void checkServerTrusted(X509Certificate[] chain, String authType) throws CertificateException {
        // do nothing
    }

    @Override
    public X509Certificate[] getAcceptedIssuers() {
        return null;
    }

}

这样就可以忽略 SSL 证书验证了，但是需要注意安全风险。如果需要进行 SSL 证书验证，可以参考 HttpClient 的 SSL Guide 进行配置。