Implementing Keyword Auto-Reply for a WeChat Public Account with Java Servlets

Introduction

Most developers have seen a WeChat public account that replies with a download link when a user sends a keyword such as “python”. This article explains how to implement that keyword‑reply feature.

Implementation Flow

The process is: the WeChat client sends a message to the WeChat server, the server forwards it to our backend, the backend processes the keyword, formats the response in the XML structure required by WeChat, returns it to the WeChat server, and finally the server pushes the reply back to the client.

WeChat Backend Configuration

In the WeChat public‑account console, go to “Settings & Development → Basic Configuration”. Fill in the server URL, Token, EncodingAESKey and choose the encryption mode. After saving, two HTTP endpoints must be provided at the URL.

GET Verification Endpoint

The GET endpoint validates that the request comes from WeChat. The servlet extracts the parameters signature, timestamp, nonce and echostr, checks the signature with SignUtil.checkSignature, and returns echostr if the check succeeds.

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    String signature = req.getParameter("signature");
    String timestamp = req.getParameter("timestamp");
    String nonce = req.getParameter("nonce");
    String echostr = req.getParameter("echostr");
    PrintWriter out = resp.getWriter();
    if (SignUtil.checkSignature(signature, timestamp, nonce)) {
        out.print(echostr);
    }
    if (out != null) {
        out.close();
    }
    out = null;
}

POST Message-Handling Endpoint

The POST endpoint receives the XML message sent by the WeChat server, decodes it as UTF‑8, passes it to a core service that builds the reply, and writes the reply back to the response.

protected void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    req.setCharacterEncoding("UTF-8");
    resp.setCharacterEncoding("UTF-8");
    String respMessage = CoreService.processRequest(req);
    PrintWriter out = resp.getWriter();
    out.print(respMessage);
    out.close();
}

Conclusion

After configuring the server settings and deploying the two endpoints, the public account can automatically respond to keyword messages such as “python” with the desired download link.