在Java中，要打印HTTP請求報文，你可以使用Java的網絡編程庫，例如java.net.HttpURLConnection。以下是一個簡單的示例，展示了如何使用HttpURLConnection發送GET請求并打印請求報文：

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

public class HttpRequestExample {
    public static void main(String[] args) {
        try {
            // 創建URL對象
            URL url = new URL("https://api.example.com/data");

            // 打開連接
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();

            // 設置請求方法
            connection.setRequestMethod("GET");

            // 設置請求頭
            connection.setRequestProperty("User-Agent", "Mozilla/5.0");

            // 打印請求報文
            System.out.println("Request Method: " + connection.getRequestMethod());
            System.out.println("Request URL: " + connection.getURL());
            System.out.println("Request Headers: ");
            for (String header : connection.getRequestProperties().keySet()) {
                System.out.println(header + ": " + connection.getRequestProperty(header));
            }

            // 獲取響應狀態碼
            int responseCode = connection.getResponseCode();
            System.out.println("Response Code: " + responseCode);

            // 讀取響應內容
            BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String inputLine;
            StringBuilder response = new StringBuilder();

            while ((inputLine = in.readLine()) != null) {
                response.append(inputLine);
            }
            in.close();

            // 輸出響應內容
            System.out.println("Response Content: " + response.toString());

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

這個示例向https://api.example.com/data發送一個GET請求，并打印請求報文。請注意，這個示例僅適用于簡單的GET請求。對于更復雜的請求（如POST請求、包含請求體的請求等），你需要根據需要調整代碼。