在Java中，我們經常需要從后端服務器獲取Json數據。對于Java來說，可以使用HttpURLConnection或者HttpClient等框架來發送Http請求，獲取數據。

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
public class GetJsonData {
public static void main(String[] args) {
String url = "http://localhost:8080/getData";
try {
URL obj = new URL(url);
HttpURLConnection conn = (HttpURLConnection) obj.openConnection();
conn.setRequestMethod("GET");
BufferedReader in = new BufferedReader(new InputStreamReader(conn.getInputStream()));
String inputLine;
StringBuffer response = new StringBuffer();
while ((inputLine = in.readLine()) != null) {
response.append(inputLine);
}
in.close();
System.out.println(response.toString());
} catch (Exception e) {
e.printStackTrace();
}
}
}

上述代碼使用HttpURLConnection發送一個GET請求，請求url為"http://localhost:8080/getData"，然后讀取返回的json數據。

如果使用HttpClient框架，代碼如下：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;
public class GetJsonDataWithHttpClient {
public static void main(String[] args) {
String url = "http://localhost:8080/getData";
try {
HttpClient client = new DefaultHttpClient();
HttpGet get = new HttpGet(url);
HttpResponse response = client.execute(get);
String json = EntityUtils.toString(response.getEntity());
System.out.println(json);
} catch (Exception e) {
e.printStackTrace();
}
}
}

HttpClient框架使用更加靈活，代碼量也更少，但是需要導入相應的jar包。

無論是使用HttpURLConnection還是HttpClient，我們都需要注意響應的狀態碼，以及json數據的格式。