java - 传递参数时获取 java.lang.NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 5 年前。

我在 fieldMappingDao.postFields(theFields); 中遇到空指针异常您能帮我解决这个问题吗？

com.ri.pac.dao.FieldMappingDao.postFields 处的 java.lang.NullPointerException(FieldMappingDao.java:39)在 com.ri.pac.servlet.PostFieldsMappingServlet.doPost(PostFieldsMappingServlet.java:127)

package com.ri.pac.dao;
import java.sql.Connection;
import java.sql.Date;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;
import javax.sql.DataSource;


public class FieldMappingDao {

private DataSource dataSource;

public FieldMappingDao(DataSource theDataSource) {
    dataSource = theDataSource;
}
    public void postFields(FieldsMapping theFields) throws Exception {

        Connection myConn = null;
        PreparedStatement myStmt = null;

        try {


            // create sql for insert
            String sql =  "INSERT INTO FIELDS"
                + "( Field_Label,Version,Display_Column,Display_Order,"
                   + "Creation_Date)"
                   + "values (?,?,?,?,?)";


            myStmt = myConn.prepareStatement(sql);
            myStmt.setString(1, theFields.getFieldLabel());
            myStmt.setInt(2, theFields.getVersion());
            myStmt.setString(3, theFields.getDisplayColumn());
            myStmt.setInt(4, theFields.getDisplayOrder());
            Date sqlCreationDate= new Date( theFields.getCreationDate().getTime());
            myStmt.setDate(5,sqlCreationDate);

            myStmt.execute();
        }
        finally {
            // clean up JDBC objects
            close(myConn, myStmt, null);
        }
    }
    private void close(Connection myConn, Statement myStmt, ResultSet myRs) {

        try {
            if (myRs != null) {
                myRs.close();
            }

            if (myStmt != null) {
                myStmt.close();
            }

            if (myConn != null) {
                myConn.close();   
            }
        }
        catch (Exception exc) {
            exc.printStackTrace();
        }
    }


    }





package com.ri.pac.servlet;
import java.io.IOException;

@WebServlet("/postFieldMappings")
 public class PostFieldsMappingServlet extends HttpServlet {
private static final long serialVersionUID = 1L;


private  FieldMappingDao fieldMappingDao ;
@Resource(name="jdbc/Person")
private DataSource dataSource;

@Override
public void init() throws ServletException {
    super.init();


    try {
        fieldMappingDao= new FieldMappingDao(dataSource);
    }
    catch (Exception exc) {
        throw new ServletException(exc);
    }
}


protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
     try {

         List<MatchType> matchTypes =fieldMappingDao.getMatchTypeId();
        request.setAttribute("Type",matchTypes);            
    }   catch (Exception e) {

        e.printStackTrace();
    }
    RequestDispatcher rdispatcher=request.getRequestDispatcher("/add-fields.jsp");
    rdispatcher.forward(request, response);
}



protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    String fieldLabel=request.getParameter("fieldLabel");
    int version=Integer.parseInt(request.getParameter("version"));
    String displayColumn=request.getParameter("displayColumn");
    int displayOrder=Integer.valueOf(request.getParameter("displayOrder"));
    Date creationDate =new Date();
    Date lastUpdateDate = new Date();

    FieldsMapping theFields=new FieldsMapping();
    theFields.setFieldLabel(fieldLabel);
    theFields.setVersion(version)
    theFields.setDisplayColumn(displayColumn);
    theFields.setDisplayOrder(displayOrder);
    theFields.setCreationDate(creationDate);


    try {
        fieldMappingDao.postFields(theFields);
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    RequestDispatcher rdispatcher=request.getRequestDispatcher("/confirmation.jsp");
    rdispatcher.forward(request, response);
}

}

Answer 1

尽管问题不完整，但在 doPost 方法中，变量 fieldLabel 和 displayColumn 的值可能为 null。对于 null 检查，请添加以下代码:

if(fieldLabel == null) fieldLabel = "";

if(displayColumn == null) displayColumn = "";