JSF与数据库的交互

以下bean的数据库相关代码在简单的Java应用程序中工作，但当我在JSF页面中使用它时，它会给Java空指针异常错误。提前谢谢。

Bean类:-

import java.io.Serializable;
import javax.faces.bean.ManagedBean;
import javax.faces.bean.SessionScoped;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
@ManagedBean
@SessionScoped     
public class Db implements Serializable{
    int eId;
    public int geteId() {
        return eId;
    }
    public void seteId(int eId) {
        this.eId = eId;
    }
    public static Connection getConnection() throws Exception {
        String driver = "oracle.jdbc.driver.OracleDriver";
        String url = "jdbc:oracle:thin:@localhost:1521:globldb3";
        String username = "scott";
        String password = "tiger";
        Class.forName(driver);
        Connection conn = DriverManager.getConnection(url, username, password);
        return conn;
      }
    public String addEmployee() throws Exception{
        Connection conn = null;
        PreparedStatement pstmt = null;
            try {
                    int a = this.eId;
                    conn = getConnection();
                    String query = "INSERT INTO c(n) VALUES(?)";
                    pstmt = conn.prepareStatement(query); 
                    pstmt.setInt(1,a); 
                    pstmt.executeUpdate(); // execute insert statement
                    return "success";
                 } catch (Exception e) {
                        e.printStackTrace();
                        return "failure";
                 } finally {
                        pstmt.close();
                        conn.close();
                 }
     }     
}

下面是我的JSF页面

<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:ui="http://java.sun.com/jsf/facelets"
xmlns:h="http://java.sun.com/jsf/html"
xmlns:f="http://java.sun.com/jsf/core">
<body>
     <h:form>
  <p>Enter value  <h:inputText value="#{db.eId}"/> </p>
  <p> <h:commandButton value="Add record" action="#{db.addEmployee}"/> </p>
  </h:form>
 </body>
 </html>

下面是堆栈跟踪

javax.faces.el.EvaluationException: java.lang.NullPointerException
    at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:102)
    at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
    at javax.faces.component.UICommand.broadcast(UICommand.java:315)
    at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:787)
    at javax.faces.component.UIViewRoot.processApplication(UIViewRoot.java:1252)
    at com.sun.faces.lifecycle.InvokeApplicationPhase.execute(InvokeApplicationPhase.java:81)
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
    at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:312)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:306)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:161)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:108)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:558)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:379)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:243)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:259)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:281)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.NullPointerException
    at erpJavaFiles.Employee.addEmployee(Employee.java:118)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.apache.el.parser.AstValue.invoke(AstValue.java:262)
    at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:278)
    at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:105)
    at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:88)
    ... 23 more

这里，

} finally {
    pstmt.close();
    conn.close();
}

在关闭它们之前没有检查它们是否为null。如果在连接时抛出异常，它们将保持null。修改如下:

} finally {
    if (pstmt != null) pstmt.close();
    if (conn != null) conn.close();
}

在addEmployee()法中没有其他可能导致NPE的原因。我看到你在catch块中有一个e.printStackTrace()调用。阅读这个真正的问题原因。我猜测是JDBC驱动程序类上的ClassNotFoundException，因为您忘记将JDBC驱动程序放在webapp的/WEB-INF/lib文件夹中。

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