Java调用Exchange Rates API传全参数仍获403响应码的原因

可能的问题点及解决步骤： #

1. 未捕获错误响应详情
   你当前只打印了响应码，但没获取非200状态下API返回的错误信息，这是排查问题的核心。比如API key无效、参数格式错误、接口权限不足等，API都会返回具体的错误提示。需要在else分支里读取错误流：

else {
    System.out.println("GET request failed, response code: " + responseCode);
    // 读取错误响应内容
    BufferedReader errorIn = new BufferedReader(new InputStreamReader(httpURLConnection.getErrorStream()));
    String errorLine;
    StringBuffer errorResponse = new StringBuffer();
    while((errorLine = errorIn.readLine()) != null){
        errorResponse.append(errorLine);
    }
    errorIn.close();
    System.out.println("Error message: " + errorResponse.toString());
}

2. JSON解析逻辑错误
   根据Exchange Rates API的convert接口返回格式，rates字段下的键是目标货币代码（toCode），而非源货币代码（fromCode）。你现在用fromCode去取值会抛出JSON异常，修正如下：

// 原代码（错误）
Double exchangeRate = obj.getJSONObject("rates").getDouble(fromCode);
// 修改后
Double exchangeRate = obj.getJSONObject("rates").getDouble(toCode);

3. 参数拼接格式问题
   直接将double类型的amount拼接到URL中，可能因地区本地化格式（比如用逗号代替小数点）导致参数无效。建议用String.format标准化格式：

// 原代码
String GET_URL = "https://api.apilayer.com/exchangerates_data/convert?to="+toCode+"&from="+fromCode+"&amount="+amount;
// 修改后
String GET_URL = String.format("https://api.apilayer.com/exchangerates_data/convert?to=%s&from=%s&amount=%.2f", toCode, fromCode, amount);

4. API Key有效性验证
   确认你的API Key是否正确、未过期，且拥有exchangerates_data/convert接口的访问权限。可以先通过Postman等工具直接调用接口，验证Key的有效性，排除代码外的问题。

修正后的完整代码示例： #

private static void sendHttpGETRequest(String fromCode, String toCode, double amount) throws IOException, JSONException{
        
    String GET_URL = String.format("https://api.apilayer.com/exchangerates_data/convert?to=%s&from=%s&amount=%.2f", toCode, fromCode, amount);

    URL url = new URL(GET_URL);
    HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
    httpURLConnection.setReadTimeout(200 * 1000);
    httpURLConnection.setConnectTimeout(200 * 1000);                
    
    httpURLConnection.setRequestMethod("GET");
    httpURLConnection.setRequestProperty("apikey", "MyApiKeY");
    int responseCode = httpURLConnection.getResponseCode();
    
    System.out.println("Response code: " + responseCode);
            
    if(responseCode == HttpURLConnection.HTTP_OK){//SUCESS
        BufferedReader in = new BufferedReader(new InputStreamReader(httpURLConnection.getInputStream()));
        String inputLine;
        StringBuffer response = new StringBuffer();
        
        while((inputLine = in.readLine()) != null){
            response.append(inputLine);
        }
        in.close();             
        
        JSONObject obj = new JSONObject(response.toString());
        // 修正：用目标货币代码获取汇率
        Double exchangeRate = obj.getJSONObject("rates").getDouble(toCode);
        System.out.println(obj.getJSONObject("rates"));
        System.out.println(exchangeRate); //keep for debugging
        System.out.println();
        
    } 
    else {
        System.out.println("GET request failed, response code: " + responseCode);
        // 读取错误响应内容
        BufferedReader errorIn = new BufferedReader(new InputStreamReader(httpURLConnection.getErrorStream()));
        String errorLine;
        StringBuffer errorResponse = new StringBuffer();
        while((errorLine = errorIn.readLine()) != null){
            errorResponse.append(errorLine);
        }
        errorIn.close();
        System.out.println("Error message: " + errorResponse.toString());
    }                                           
}

内容的提问来源于stack exchange，提问作者BluesGo