用Java写一个微信扫码登录的接口

以下是使用Java实现微信扫码登录的示例代码：

首先，需要引入相关的依赖包：

<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-core</artifactId>
  <version>2.8.8</version>
</dependency>

<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
  <version>2.8.8</version>
</dependency>

<dependency>
  <groupId>org.apache.httpcomponents</groupId>
  <artifactId>httpclient</artifactId>
  <version>4.5.5</version>
</dependency>

<dependency>
  <groupId>org.apache.httpcomponents</groupId>
  <artifactId>httpcore</artifactId>
  <version>4.4.9</version>
</dependency>

然后，编写调用微信接口获取二维码的函数：

public static String getQRCodeUrl() throws IOException {
  CloseableHttpClient httpclient = HttpClients.createDefault();
  HttpPost httpPost = new HttpPost(String.format("https://open.weixin.qq.com/connect/qrconnect?appid=%s&redirect_uri=%s&response_type=code&scope=snsapi_login#wechat_redirect",
  APP_ID, REDIRECT_URI));
  CloseableHttpResponse response = httpclient.execute(httpPost);
  String html = EntityUtils.toString(response.getEntity());
  Document doc = Jsoup.parse(html);
  String qrUrl = doc.select("img[src^=\"//login.weixin.qq.com/qrcode/\"]").attr("src");
  return "https:" + qrUrl;
}

接下来，在用户扫描二维码后，可以通过以下函数获取用户信息：

public static String getUserInfo(String code) throws IOException {
  CloseableHttpClient httpclient = HttpClients.createDefault();
  HttpPost httpPost = new HttpPost(String.format("https://api.weixin.qq.com/sns/oauth2/access_token?appid=%s&secret=%s&code=%s&grant_type=authorization_code", APP_ID, APP_SECRET, code));
  CloseableHttpResponse response = httpclient.execute(httpPost);
  String jsonString = EntityUtils.toString(response.getEntity());
  ObjectMapper objectMapper = new ObjectMapper();
  JsonNode jsonNode = objectMapper.readTree(jsonString);
  String accessToken = jsonNode.get("access_token").asText();
  String openId = jsonNode.get("openid").asText();
  HttpGet httpGet = new HttpGet(String.format("https://api.weixin.qq.com/sns/userinfo?access_token=%s&openid=%s&lang=zh_CN", accessToken, openId));
  response = httpclient.execute(httpGet);
  jsonString = EntityUtils.toString(response.getEntity());
  return jsonString;
}

这个函数将返回JSON格式的用户信息。根据需要，我们还可以进一步解析JSON，获取用户昵称、头像等信息。

以上就是一个简单的微信扫码登录接口的实现示例。