java 无法在 JSF 中实例化类错误，由 NullPointerException 引起

Can't instantiate class error in JSF, caused by NullPointerException

提问by user1705260

Here is my managed Bean class,

这是我的托管 Bean 类，

public class ChartBean implements Serializable {

    private PieChartModel pieModel;

    public ChartBean(){
        createPieModel(); 
    }

    public PieChartModel getPieModel() {  
        return pieModel;  
    }  

    private void createPieModel(){  
        try {
            pieModel = new PieChartModel();  

            String query = "SELECT b.countryname,count(b.countryname) FROM info.summery a,info.countrymcc b;";
            Connector conn = new Connector();
            Statement str = (Statement) conn.getConn().createStatement();
            ResultSet res = str.executeQuery(query);


            while(res.next()){

                  pieModel.set(res.getString(1), Integer.parseInt(res.getString(2)));
            }
        } catch (SQLException ex) {
            Logger.getLogger(ChartBean.class.getName()).log(Level.SEVERE, null, ex);
        }
    } 
}

But the problem is when it is compiled it gives an error like this "Cant instantiate class: org.primefaces.examples.view.ChartBean". What is the reason??

但问题是当它被编译时，它会给出这样的错误“无法实例化类：org.primefaces.examples.view.ChartBean”。是什么原因？？

StackTrace:

堆栈跟踪：

Caused by: java.lang.NullPointerException at  

org.primefaces.examples.view.ChartBean.createPieModel(ChartBean.java:45) at     
org.primefaces.examples.view.ChartBean.<init>(ChartBean.java:32) at     
sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at     
sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorI??    
mpl.java:39) at     
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorA??    
ccessorImpl.java:27) at java.lang.reflect.Constructor.newInstance(Constructor.java:513) at    
java.lang.Class.newInstance0(Class.java:355) at java.lang.Class

回答by Stephen C

By a process of elimination, the problem is happening because conn.getConn()is returning null. You should be able to simply confirm that the exception occurs at that line (by checking the line number!), and we know that conncannot be null, so it must be result of getConn()that is null.

通过消除过程，问题正在发生，因为conn.getConn()正在返回null。您应该能够简单地确认异常发生在该行（通过检查行号！），我们知道conn不可能是null，所以它必须是结果getConn()是null。

That's about as far as I can go without knowing what the Connectorclass is and how its getConn()method works.

在不知道Connector类是什么以及它的getConn()方法如何工作的情况下，我可以做到这一点。

---

---

For the record, here's how I eliminated other possibilities.

为了记录，以下是我消除其他可能性的方法。

The NPE is being thrown in the createPieModelcall ... and not in some method called from createPieModel:

NPE 在createPieModel调用中被抛出......而不是在调用的某些方法中createPieModel：

1. private void createPieModel(){  
2.     try {
3.         pieModel = new PieChartModel();  
4.         String query = "SELECT b.countryname,count(b.countryname) FROM info.summery a,info.countrymcc b;";
5.         Connector conn = new Connector();
6.         Statement str = (Statement) conn.getConn().createStatement();
7.         ResultSet res = str.executeQuery(query);
8.         while(res.next()){
9.             pieModel.set(res.getString(1), Integer.parseInt(res.getString(2)));
10.        }
11.    } catch (SQLException ex) {
12.        Logger.getLogger(ChartBean.class.getName()).log(Level.SEVERE, null, ex);
13.    }
14. }

• It cannot be line 3 because any NPE would be thrown in the constructor.
• It cannot be line 4
• It cannot be line 5 - see line 3
• It couldbe line 6
• It cannot be line 7 - because strmust be non-null (if we get that far)
• It cannot be line 8 - because executeQuerynever returns null
• It cannot be line 9 - because resand pieModelmust be non-null.
• It cannot be line 12 - because nothing there can return a null.

• 它不能是第 3 行，因为任何 NPE 都会在构造函数中抛出。
• 它不能是第 4 行
• 它不能是第 5 行 - 见第 3 行
• 这可能是6号线
• 它不能是第 7 行 - 因为str必须是非空的（如果我们得到那么远）
• 它不能是第 8 行 - 因为executeQuery永远不会返回null
• 它不能是第 9 行 - 因为res并且pieModel必须是非空的。
• 它不能是第 12 行 - 因为没有任何东西可以返回null.

Hence it can onlyhappen on line 6.

因此它只能发生在第 6 行。