JSON（JavaScript Object Notation）是一種輕量級的數據交換格式。Java通過使用類庫可以方便地操作JSON，同時發送JSON格式的請求也成為了Web開發必不可少的技能之一。

在Java中發送JSON請求，需要使用HttpURLConnection或者HttpClient這兩種方式。

（一）使用HttpURLConnection發送JSON請求

try {
URL url = new URL("http://www.example.com/api");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Type", "application/json");
connection.setRequestProperty("Accept", "application/json");
connection.setDoOutput(true);
String jsonInputString = "{\"name\": \"John\", \"age\": 30, \"city\": \"New York\"}";
try (OutputStream os = connection.getOutputStream()) {
byte[] input = jsonInputString.getBytes("utf-8");
os.write(input, 0, input.length);
}
try (BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream(), "utf-8"))) {
StringBuilder response = new StringBuilder();
String responseLine;
while ((responseLine = br.readLine()) != null) {
response.append(responseLine.trim());
}
System.out.println(response.toString());
}
connection.disconnect();
} catch (IOException e) {
e.printStackTrace();
}

以上代碼中，首先創建一個URL對象，設置請求方法為POST，添加請求頭中的Content-Type和Accept，同時設置允許輸出請求正文內容。接著，將JSON字符串寫入輸出流中，再通過輸入流獲取響應代碼，最后斷開連接。

（二）使用HttpClient發送JSON請求

CloseableHttpClient client = HttpClients.createDefault();
HttpPost httpPost = new HttpPost("http://www.example.com/api");
httpPost.setHeader("Content-Type", "application/json");
httpPost.setHeader("Accept", "application/json");
String jsonInputString = "{\"name\": \"John\", \"age\": 30, \"city\": \"New York\"}";
StringEntity entity = new StringEntity(jsonInputString, ContentType.APPLICATION_JSON);
httpPost.setEntity(entity);
HttpResponse httpResponse = client.execute(httpPost);
HttpEntity httpEntity = httpResponse.getEntity();
String responseString = EntityUtils.toString(httpEntity, "UTF-8");
System.out.println(responseString);
client.close();

以上代碼中，首先創建一個CloseableHttpClient對象，然后創建HttpPost對象并設置請求URL、Content-Type和Accept。接著，將JSON字符串寫入實體中，并將實體設置到HttpPost對象中，最后執行HttpPost請求獲取響應內容并關閉HttpClient對象。