Java向微信群发送消息的教程

要实现Java向微信群发送消息，需要使用微信开放平台提供的API接口。以下是简要的步骤：

1. 注册微信开放平台账号，创建应用并获取应用的AppID和AppSecret。
2. 在微信开放平台中，开通“网页授权”和“公众号开发”。
3. 在微信公众平台中，创建微信公众号，并获取公众号的Token。
4. 在Java项目中，通过HTTP请求向微信API接口发送请求并获取返回结果。具体实现可以使用第三方库，如Apache HttpClient或OkHttp。
5. 使用微信API接口的“发送消息”功能，向指定的微信群发送消息。

下面是一个简单的Java代码示例：

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.util.EntityUtils;

public class WeChatSender {
    private static final String API_URL = "https://api.weixin.qq.com/cgi-bin/message/send?access_token=%s";
    private static final String APP_ID = "your_app_id";
    private static final String APP_SECRET = "your_app_secret";
    private static final String GROUP_ID = "your_group_id";

    public static void main(String[] args) throws IOException {
        String accessToken = getAccessToken();
        String message = "Hello, WeChat Group!";
        sendMessage(accessToken, GROUP_ID, message);
    }

    private static String getAccessToken() throws IOException {
        String apiUrl = String.format("https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid=%s&secret=%s",
                APP_ID, APP_SECRET);
        HttpClient httpClient = HttpClientBuilder.create().build();
        HttpPost httpPost = new HttpPost(apiUrl);
        HttpResponse httpResponse = httpClient.execute(httpPost);
        HttpEntity responseEntity = httpResponse.getEntity();
        String responseString = EntityUtils.toString(responseEntity, "UTF-8");
        Map<String, Object> responseJson = new Gson().fromJson(responseString, HashMap.class);
        return (String) responseJson.get("access_token");
    }

    private static void sendMessage(String accessToken, String groupId, String message) throws IOException {
        String apiUrl = String.format(API_URL, accessToken);
        HttpClient httpClient = HttpClientBuilder.create().build();
        HttpPost httpPost = new HttpPost(apiUrl);
        httpPost.setHeader("Content-Type", "application/json;charset=UTF-8");
        String requestBody = String.format("{\"filter\":{\"group_id\":\"%s\"},\"text\":{\"content\":\"%s\"},\"msgtype\":\"text\"}",
                groupId, message);
        httpPost.setEntity(new StringEntity(requestBody, "UTF-8"));
        HttpResponse httpResponse = httpClient.execute(httpPost);
        HttpEntity responseEntity = httpResponse.getEntity();
        String responseString = EntityUtils.toString(responseEntity, "UTF-8");
        System.out.println(responseString);
    }
}

需要注意的是，上述代码中的APP_ID、APP_SECRET和GROUP_ID需要替换为实际的值，并且在微信公众平台中需要将Java项目的服务器IP地址添加为“服务器IP白名单”。此外，微信API接口有一定的调用频率限制，需要注意避免频繁请求。