java http post请求代码

下面是使用Java发送POST请求的示例代码：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;

public class HttpPostExample {

    public static void main(String[] args) {
        try {
            String url = "http://example.com/api";
            String payload = "key1=value1&key2=value2";

            URL obj = new URL(url);
            HttpURLConnection con = (HttpURLConnection) obj.openConnection();

            // 设置HTTP请求方法为POST
            con.setRequestMethod("POST");

            // 设置请求头信息
            con.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");

            // 启用输出流
            con.setDoOutput(true);

            // 创建输出流写入POST请求参数
            OutputStreamWriter writer = new OutputStreamWriter(con.getOutputStream());
            writer.write(payload);
            writer.flush();
            writer.close();

            // 获取服务器响应
            int responseCode = con.getResponseCode();
            BufferedReader in = new BufferedReader(new InputStreamReader(con.getInputStream()));
            String inputLine;
            StringBuffer response = new StringBuffer();
            while ((inputLine = in.readLine()) != null) {
                response.append(inputLine);
            }
            in.close();

            // 输出响应结果
            System.out.println(response.toString());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

其中，`url`为请求的URL地址，`payload`为POST请求的参数。在示例中，POST请求的参数是以`key1=value1&key2=value2`的形式拼接的，如果需要传递JSON数据，则可以将JSON序列化为字符串作为POST请求参数。