如何解决 JEE 中的 NullPointerException？ [复制]

Question

我是 JEE 的初学者，我正在尝试从我的数据库中获取对象，但我遇到了 nullpointerexception 的错误，我试图定位系统的 nullpointerexception 的来源（我知道连接传递为 null） .out.println() 但我失败了，我忘了告诉你我有用于连接的类单例连接，我在我的函数中通过 .getconnection() 调用它，我还在项目构建中添加了一个 jdbc 驱动程序（版本 8.0.18）路径（我问这个错误是否可能是由于 jdbc 驱动程序版本引起的？？），，
如果有人可以提供帮助，请提前致谢。

这是连接到数据库并将对象带到我在 PreparedStatement ps=connection.prepareStatement("req") 中告诉您的错误的函数

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.

这是我的单例连接

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

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) {

Answer 1

@marouu 我觉得NullPointerException不是Connection引起的，可能是ProduitDaoImpl，这个类里加@Service吗

Answer 2

你是否将mysql-connector-java添加到项目构建路径