CodenameOne应用无代理时Connection refused错误的原因与解决方法

问题场景 #

运行以下CodenameOne代码实现图片下载并显示功能：

int WIDTH = Display.getInstance().getDisplayWidth();
Form f = new Form();
f.getToolbar().hideToolbar();
f.getStyle().setBgColor(0xFFFFFF);
EncodedImage placeholder = EncodedImage.createFromImage(Image.createImage(WIDTH, WIDTH), false);
URLImage puppyImage = URLImage.createToStorage(placeholder, "puppy.png", "https://images.newscientist.com/wp-content/uploads/2021/06/03141753/03-june_puppies.jpg?crop=1:1,smart&width=1200&height=1200&upscale=true");
Container imageContainer = new Container();
imageContainer.add(puppyImage);
f.add(imageContainer);
f.show();

代理环境下应用运行正常，图片可成功下载保存；但取消代理后抛出连接拒绝错误，堆栈信息如下：

java.net.ConnectException: Connection refused: connect
    at java.base/java.net.PlainSocketImpl.connect0(Native Method)
    at java.base/java.net.PlainSocketImpl.socketConnect(PlainSocketImpl.java:101)
    at java.base/java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:412)
    at java.base/java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:255)
    at java.base/java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:237)
    at java.base/java.net.Socket.connect(Socket.java:608)
    at java.base/java.net.Socket.connect(Socket.java:557)
    at java.base/sun.net.NetworkClient.doConnect(NetworkClient.java:182)
    at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:474)
    at java.base/sun.net.www.http.HttpClient$1.run(HttpClient.java:526)
    at java.base/sun.net.www.http.HttpClient$1.run(HttpClient.java:524)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at java.base/sun.net.www.http.HttpClient.privilegedOpenServer(HttpClient.java:523)
    at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:564)
    at java.base/sun.net.www.protocol.https.HttpsClient.<init>(HttpsClient.java:265)
    at java.base/sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:372)
    at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:212)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1208)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1081)
    at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:189)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1592)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1520)
    at java.base/java.net.HttpURLConnection.getResponseCode(HttpURLConnection.java:527)
    at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.getResponseCode(HttpsURLConnectionImpl.java:334)
    at com.codename1.impl.javase.JavaSEPort.getResponseCode(JavaSEPort.java:10179)
    at com.codename1.io.ConnectionRequest.performOperationComplete(ConnectionRequest.java:905)
    at com.codename1.io.NetworkManager$NetworkThread.runCurrentRequest(NetworkManager.java:314)
    at com.codename1.io.NetworkManager$NetworkThread.run(NetworkManager.java:390)
    at com.codename1.impl.CodenameOneThread.run(CodenameOneThread.java:176)

原因分析 #

1. 本地网络环境限制：所在网络（如企业/校园网）的防火墙或出站规则强制要求通过代理访问外部HTTPS资源，直接连接被拦截。
2. 代理配置残留：取消代理后，CodenameOne应用或开发环境仍保留旧的代理配置，导致尝试用无效代理地址发起连接。
3. DNS解析失败：本地DNS服务器无法正确解析目标图片的域名，导致无法建立有效连接。
4. 目标服务器IP限制：目标服务器限制了你的公网IP访问，仅允许代理IP段的请求。

解决办法 #

1. 验证本地网络可用性 #

• 直接在浏览器中打开目标图片URL，确认是否能正常加载。如果浏览器也无法访问，说明是网络环境问题，需联系网络管理员开放出站HTTPS权限，或确认是否必须使用代理。

2. 清除代理配置残留 #

• 在代码中显式重置网络代理设置：

  // 清除全局代理配置
  ConnectionRequest.setProxy(null, 0);
  NetworkManager.getInstance().clearProxy();
  

• 完全重启CodenameOne开发环境和应用，确保旧配置被彻底清除。

3. 修复DNS解析问题 #

• 在命令行执行nslookup images.newscientist.com，检查是否能返回正确的IP地址。
• 切换公共DNS服务器（如8.8.8.8或114.114.114.114），重新测试DNS解析和应用连接。

4. 验证服务器访问权限 #

• 使用手机热点等其他网络测试应用，如果能正常下载图片，说明当前网络的IP被目标服务器限制，需更换网络或联系服务器管理员。

5. 添加异常处理优化体验 #

• 在图片下载代码中添加异步回调，处理下载失败场景：

  String imageUrl = "https://images.newscientist.com/wp-content/uploads/2021/06/03141753/03-june_puppies.jpg?crop=1:1,smart&width=1200&height=1200&upscale=true";
  URLImage puppyImage = URLImage.createToStorage(placeholder, "puppy.png", imageUrl, new URLImage.ImageAdapter() {
      @Override
      public EncodedImage adaptImage(EncodedImage downloadedImage, EncodedImage placeholderImage) {
          return downloadedImage;
      }
  
      @Override
      public boolean asyncOperationComplete(boolean success) {
          if (!success) {
              Dialog.show("提示", "图片下载失败，请检查网络设置", "确定", null);
          }
          return success;
      }
  });
  

内容的提问来源于stack exchange，提问作者Alex