用httpclient 写一个post请求，请求发送json返回json

以下是使用Apache HttpComponents的HttpClient库发送POST请求并获取响应的示例代码：

import java.io.IOException;
import java.nio.charset.StandardCharsets;

import org.apache.http.HttpEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClientBuilder;

public class HttpClientExample {

    public static void main(String[] args) {
        String url = "http://example.com/api";

        // 构造POST请求
        HttpPost post = new HttpPost(url);
        post.setHeader("Content-Type", "application/json");

        // 构造JSON请求体
        String requestBody = "{\"username\":\"johndoe\",\"password\":\"secret\"}";
        StringEntity entity = new StringEntity(requestBody, ContentType.APPLICATION_JSON);
        post.setEntity(entity);

        // 发送POST请求
        try (CloseableHttpClient httpClient = HttpClientBuilder.create().build();
                CloseableHttpResponse response = httpClient.execute(post)) {
            // 获取响应实体
            HttpEntity responseEntity = response.getEntity();
            String responseBody = responseEntity != null ? EntityUtils.toString(responseEntity, StandardCharsets.UTF_8) : "";

            // 输出响应信息
            System.out.println("HTTP Status Code: " + response.getStatusLine().getStatusCode());
            System.out.println("Response Body:\n" + responseBody);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

在以上示例代码中，我们使用了HttpPost类来构造POST请求，设置了请求头Content-Type为application/json，并且构造了一个JSON请求体。然后使用HttpClient发送POST请求，并获取响应实体，最后输出响应信息。

需要注意的是，以上示例代码中使用了try-with-resources语句，确保HttpClient和HttpResponse对象在使用完毕后自动关闭。