Java：JsonPareser不包含任何數據

Question

我正在一個Java應用程序中嘗試從基於Spring-MVC的服務器獲取數據，並且當前是system.out它，但由於標記爲空而得到輸出。我打印出正在從服務器發送的代碼，這是正確的。我對REST沒有太多經驗，所以如果有人告訴我我做錯了什麼，我將不勝感激。

客戶機代碼：

public void getCanvas(){ 
      JsonFactory jsonFactory = new JsonFactory(); 
      String canvas = ""; 
      try { 
       JsonParser jsonParser = jsonFactory.createJsonParser(new URL(canvasURL)); 
       JsonToken token = jsonParser.nextToken(); 
       while (token!=JsonToken.START_ARRAY){ 
        token = jsonParser.nextToken(); 
        System.out.println("Token is "+token); 
       } 
       while (token != JsonToken.END_ARRAY){ 
        token = jsonParser.nextToken(); 
        if(token == JsonToken.START_OBJECT){ 
         canvas = jsonParser.toString(); 
    // I also tried token.toString 
         System.out.println("Canvas is "+canvas); 
        } 

       } 

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

    } 

客戶機輸出：

Token is FIELD_NAME 
Token is VALUE_NUMBER_INT 
Token is FIELD_NAME 
Token is VALUE_STRING 
Token is FIELD_NAME 
Token is VALUE_NUMBER_FLOAT 
Token is FIELD_NAME 
Token is VALUE_NULL 
Token is FIELD_NAME 
Token is VALUE_STRING 
Token is FIELD_NAME 
Token is VALUE_STRING 
Token is FIELD_NAME 
Token is VALUE_STRING 
Token is FIELD_NAME 
Token is VALUE_NULL 
Token is FIELD_NAME 
Token is VALUE_FALSE 
Token is FIELD_NAME 
Token is VALUE_NULL 
Token is FIELD_NAME 
Token is VALUE_NUMBER_INT 
Token is END_OBJECT 

即必須是已印刷ATLEAST 100倍。

Server代碼：

@RequestMapping(value = "/getcanvas",method = RequestMethod.GET) 
    public @ResponseBody String getCanvasforFX(){ 
     System.out.println("Canvas was requested"); 
     Canvas canvas = this.canvasService.getCanvasById(10650); 

     ObjectMapper objectMapper = new ObjectMapper(); 
     try { 
      System.out.println("Canvas value is "+objectMapper.writeValueAsString(canvas)); 
      return objectMapper.writeValueAsString(canvas); 
     } catch (IOException e) { 
      e.printStackTrace(); 
      return null; 
     } 
    } 

服務器輸出：

Canvas was requested 
Canvas value is {"canvasid":10650,"canvasname":"Aks","canvasnumber":1.0,"canvastitle":null,"canvasdate":"","canvascreator":"","canvasiteration":"","canvasimage":"iVBORw0K5CYII=","canvasDisabled":false,"imageData":null,"person1id":7402} 

什麼我做錯了，我想打印在客戶端受此畫布價值。

Answer 1

嗯，JsonToken.nextToken的JsonParser API文檔（）說，這非常好（重點煤礦）：

主要迭代法，這將推進流足以確定下一個令牌的類型，如果有的話。如果沒有剩餘（流在結束之前沒有除可能的空白之外的內容），空將被返回。

換句話說，如果沒有剩餘的標記，nextToken（）將返回null。因此，因爲您的而聲明總是評估爲true，它會繼續打印令牌爲空在無限循環中。

我要提到的另一點是，如果你想令牌值而非JsonToken.type，你應該改變

System.out.println("Token is "+ token); 

到

System.out.println("Token is "+ jsonParser.getText()); 

否則，代碼將打印以下內容：

Token is FIELD_NAME 
Token is VALUE_NUMBER_INT 
Token is FIELD_NAME 
Token is VALUE_STRING 
Token is FIELD_NAME 
Token is VALUE_NUMBER_FLOAT 
Token is FIELD_NAME 
Token is VALUE_NULL 
Token is FIELD_NAME 
Token is VALUE_STRING 
Token is FIELD_NAME 
Token is VALUE_STRING 
Token is FIELD_NAME 
Token is VALUE_STRING 
Token is FIELD_NAME 
Token is VALUE_STRING 
Token is FIELD_NAME 
Token is VALUE_FALSE 
Token is FIELD_NAME 
Token is VALUE_NULL 
Token is FIELD_NAME 
Token is VALUE_NUMBER_INT 
Token is END_OBJECT 

因此，您的代碼應作如下修改：

public void getCanvas(){ 
     JsonFactory jsonFactory = new JsonFactory(); 
     String canvas = ""; 
     try { 
      JsonParser jsonParser = jsonFactory.createJsonParser(new URL(canvasURL)); 
      JsonToken token = jsonParser.nextToken(); 
      while (token!=JsonToken.START_ARRAY && token != null){ 
       token = jsonParser.nextToken(); 

       // if we already passed last token, 
       // break, don't print it out unnecessarily 
       if (token == null) break; 

       System.out.println("Token is "+ token); 
      } 
      while (token != JsonToken.END_ARRAY){ 
       token = jsonParser.nextToken(); 
       if(token == JsonToken.START_OBJECT){ 
        canvas = jsonParser.toString(); 
// I also tried token.toString 
        System.out.println("Canvas is "+canvas); 
       } 

      } 

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

這將產生以下輸出：

Token is canvasid 
Token is 10650 
Token is canvasname 
Token is Aks 
Token is canvasnumber 
Token is 1.0 
Token is canvastitle 
Token is null 
Token is canvasdate 
Token is 
Token is canvascreator 
Token is 
Token is canvasiteration 
Token is 
Token is canvasimage 
Token is iVBORw0K5CYII= 
Token is canvasDisabled 
Token is false 
Token is imageData 
Token is null 
Token is person1id 
Token is 7402 
Token is } 

祝你好運！