Question

我需要将JSON响应转换为Java Object，但我却是nullpointerException。这是我的模型课：

public class Cheque {

private String payeeName;
private String accountNumber;
private String ifsCode;
private String micr;
private String bankName;

public Cheque() {
    super();
    // TODO Auto-generated constructor stub
}

public Cheque(String payeeName, String accountNumber, String ifsCode, String micr, String bankName) {
    super();
    this.payeeName = payeeName;
    this.accountNumber = accountNumber;
    this.ifsCode = ifsCode;
    this.micr = micr;
    this.bankName = bankName;
}


public String getPayeeName() {
    return payeeName;
}

public void setPayeeName(String payeeName) {
    this.payeeName = payeeName;
}

public String getAccountNumber() {
    return accountNumber;
}

public void setAccountNumber(String accountNumber) {
    this.accountNumber = accountNumber;
}

public String getIfsCode() {
    return ifsCode;
}

public void setIfscCode(String ifsCode) {
    this.ifsCode = ifsCode;
}

public String getMicr() {
    return micr;
}

public void setMicr(String micr) {
    this.micr = micr;
}

public String getBankName() {
    return bankName;
}

public void setBankName(String bankName) {
    this.bankName = bankName;
}

下面，我将发布一种调用Python程序并从中获取Json响应的方法：

public class RunPython {

public static void main(String[] args) throws IOException,ScriptException,NullPointerException{
    // TODO Auto-generated method stub
    Process p = Runtime.getRuntime().exec("python <path to file>/reg.py");
    BufferedReader in = new BufferedReader(new InputStreamReader(p.getInputStream()));
    String line ;
    try {
        while((line =in.readLine()) != null) {
            System.out.println(line);
        }
   } finally {
     in.close();
   }

    ObjectMapper mapper = new ObjectMapper();
    try { 

        Cheque cheque = mapper.readValue(line, Cheque.class); 
        System.out.println("Java object :"); 
        System.out.println(cheque); 
        }
    catch (JsonGenerationException ex) { 
        ex.printStackTrace(); 
        } 
    catch (JsonMappingException ex) {
        ex.printStackTrace(); 
        } 
    catch (IOException ex) { 
        ex.printStackTrace(); 
        }}}

我得到的JSON响应是：

{
"bankName": [
    "Bank"
],
"accountNumber": [
    "989898989898"
],
"ifsCode": [
    "0000000"
],
"micr": [
    "0000000"
],
"payeeName": [
    "name"
]

}

运行该程序后，我得到了预期的JSON响应，但是在将其转换为Java对象时，它在主线程中显示了nullPointerException。帮助我找出错误所在。

Answer 1

在打印时，您在这里消耗/耗尽所有过程输入流：

 try {
        while((line =in.readLine()) != null) {
            System.out.println(line);
        }

后来，当您调用下面的内容时，它已经是null了：

Cheque cheque = mapper.readValue(line, Cheque.class);

您必须在上述while循环中对其进行处理，而不是仅将其打印出来

Answer 2

当您到达该语句时：

    Cheque cheque = mapper.readValue(line, Cheque.class);

变量line为空。

因此，您可以重建JSON字符串（使用StringBuilder），或者删除打印响应的代码，然后直接从p.getInputStream()解析JSON。

Answer 3

您的Java对象具有String值，但是JSON具有数组类型（[]]

将JSON响应转换为Java对象

3 个答案: