如何解决JEE中的NullPointerException? [英] how to resolve the NullPointerException in JEE?

问题描述

我是JEE的初学者，我正在尝试从数据库中获取对象，但是遇到了nullpointerexception错误，我试图通过System查找nullpointerexception的起源(我知道该连接以null传递) .out.println()但失败了，我忘了告诉你我有用于连接的类singletonconnection，我通过.getconnection()在函数中调用它，我也向项目构建中添加了jdbc驱动程序(版本8.0.18)路径(我问这个错误是否可以归因于jdbc驱动程序版本?)，
如果有人可以提供帮助，请先感谢.

I'm beginner in JEE and i'm trying to fetch object from my db but i has encountered an error of nullpointerexception, i tried to locate the origin of the nullpointerexception(i understood that the connection passed as null) by System.out.println() but i failed, i forgot to tell you that i have class singletonconnection for connection and i call it in my function by .getconnection() also i add a jdbc driver (version 8.0.18) to the project build path(i ask if this error can be due to the jdbc driver version??),,
please if someone can help and thanks in advance.

这是连接数据库的函数，并将对象带到我在PreparedStatement ps = connection.prepareStatement("req")中告诉过您的错误中

here is the function that connects to the db and brings objects where i have the error that i told you about in line PreparedStatement ps=connection.prepareStatement("req")

System.out.println("connection");
        Connection connection=SingletonConnection.getConnection();

        System.out.println(connection);
        try {



            PreparedStatement ps=connection.prepareStatement("SELECT * FROM PRODUITS WHERE DESIGNATION LIKE ?");
            ps.setString(1, mc);
            ResultSet rs=ps.executeQuery();

这是错误日志

HTTP Status 500 – Internal Server Error
Type Exception Report

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

java.lang.NullPointerException
    dao.ProduitDaoImpl.produitsParMC(ProduitDaoImpl.java:48)
    web.ControleurServelet.doGet(ControleurServelet.java:49)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
Note The full stack trace of the root cause is available in the server logs.

这是我的单身人士联系

public class SingletonConnection {
    private static Connection connection;
    //le block static charge la classe en mémoire lors de son appel
    static{
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            try {
                connection=DriverManager.getConnection("jdbc:mysql://localhost:3306/db-natal","root","");
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }
    public static Connection getConnection() {
        return connection;
    }

这是服务器日志中的堆栈

here is the stack in the server log

Nov 27, 2019 6:26:37 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [cs] in context with path [/essai_web] threw exception
java.lang.NullPointerException
    at dao.ProduitDaoImpl.produitsParMC(ProduitDaoImpl.java:48)
    at web.ControleurServelet.doGet(ControleurServelet.java:49)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:110)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:444)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:1025)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:445)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1137)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:637)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:319)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:748)

这是第48行

public Produit update(Produit p) {

推荐答案

@marouu我认为NullPointerException不是由Connection引起的，也许是ProduitDaoImpl，您是否在此类中添加@Service

@marouu I think the NullPointerException is not caused by Connection，maybe the ProduitDaoImpl，Do you add @Service in this class

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