为了实现Java程序发送POST请求，需要使用Java API中的HttpURLConnection类。具体的步骤如下：

1.获取HttpURLConnection对象

HttpURLConnection是Java中实现HTTP协议的常用类。利用URL.openConnection()方法可以获取HttpURLConnection对象。

URL url = new URL("http://www.example.com/resource");
HttpURLConnection connection = (HttpURLConnection)url.openConnection();

2.设置请求方法和头部参数

利用HttpURLConnection.setRequestMethod()可以设置HTTP请求的方法，如POST、GET、PUT等。同时可以设置头部参数，如Content-Type、Authorization等。

connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Type", "application/json");

3.设置请求体内容

针对POST请求方法，需要设置请求体内容。

String requestBody = "{\"name\":\"John\", \"age\":30}";
OutputStream os = connection.getOutputStream();
os.write(requestBody.getBytes());
os.flush();
os.close();

4.获取响应

利用HttpURLConnection.getResponseCode()方法可以获取HTTP响应的状态码。如果状态码等于HttpURLConnection.HTTP_OK，则可以用InputStream获取服务器返回的响应内容。

if (connection.getResponseCode() == HttpURLConnection.HTTP_OK) {
    InputStream is = connection.getInputStream();
    BufferedReader reader = new BufferedReader(new InputStreamReader(is));
    String line;
    StringBuilder response = new StringBuilder();
    while ((line = reader.readLine()) != null) {
        response.append(line);
    }
    reader.close();
    is.close();
    System.out.println(response.toString());
}

下面是两个不同的使用HTTP POST请求发送JSON数据的示例：

示例1：使用java.net.HttpURLConnection类

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class Example1 {

    public static void main(String[] args) throws IOException {
        URL url = new URL("http://www.example.com/resource");
        HttpURLConnection connection = (HttpURLConnection)url.openConnection();
        connection.setRequestMethod("POST");
        connection.setRequestProperty("Content-Type", "application/json");

        String requestBody = "{\"name\":\"John\", \"age\":30}";
        OutputStream os = connection.getOutputStream();
        os.write(requestBody.getBytes());
        os.flush();
        os.close();

        if (connection.getResponseCode() == HttpURLConnection.HTTP_OK) {
            InputStream is = connection.getInputStream();
            BufferedReader reader = new BufferedReader(new InputStreamReader(is));
            String line;
            StringBuilder response = new StringBuilder();
            while ((line = reader.readLine()) != null) {
                response.append(line);
            }
            reader.close();
            is.close();
            System.out.println(response.toString());
        }
    }
}

示例2：使用Apache HttpClient库

import java.io.IOException;

import org.apache.http.HttpEntity;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

public class Example2 {

    public static void main(String[] args) throws ClientProtocolException, IOException {
        CloseableHttpClient httpClient = HttpClients.createDefault();
        HttpPost httpPost = new HttpPost("http://www.example.com/resource");
        httpPost.setHeader("Content-Type", "application/json");

        String requestBody = "{\"name\":\"John\", \"age\":30}";
        HttpEntity httpEntity = new UrlEncodedFormEntity(requestBody);
        httpPost.setEntity(httpEntity);

        try (CloseableHttpResponse response = httpClient.execute(httpPost)) {
            HttpEntity responseEntity = response.getEntity();
            String responseString = EntityUtils.toString(responseEntity);
            System.out.println(responseString);
        }
    }
}

以上就是Java发送POST请求的详细攻略，希望对您有所帮助。