how to resolve the NullPointerException in JEE? [duplicate]

问题

This question already has answers here:

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

Is it safe to use a static java.sql.Connection instance in a multithreaded system? (2 answers)

How should I connect to JDBC database / datasource in a servlet based application? (2 answers)

Closed 20 days ago.

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.

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();

here is the error log

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.

here is my singleton connection

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;
    }

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)

here is the line 48

public Produit update(Produit p) {

回答1:

Do you add mysql-connector-java to the project build path

回答2:

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

来源：https://stackoverflow.com/questions/59048448/how-to-resolve-the-nullpointerexception-in-jee