Null 指针异常。 我的连接 object 如何指向 null

Question

The stacktrace

java.lang.ClassNotFoundException: com.microsoft.sqlserver.jdbc.SQLServerDriver
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1952)
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1795)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:264)
    at Store.<init>(Store.java:12)
    at RegisterServlet.doGet(RegisterServlet.java:38)
    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:492)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:165)
    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:452)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1201)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:654)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:319)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)
Sep 01, 2020 9:06:08 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [RegisterServlet] in context with path [/Registration] threw exception
java.lang.NullPointerException
    at Store.insertuser(Store.java:31)
    at RegisterServlet.doGet(RegisterServlet.java:40)
    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:492)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:165)
    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:452)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1201)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:654)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:319)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)

import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.SQLException;


public class Store {
    Connection connection;

    Store() {
        try {
            Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");

            String username = "sa";
            String password = "123456abcd";
            String url = "jdbc:sqlserver://localhost;databaseName=Register";



            connection = DriverManager.getConnection(url, username, password);

        } catch (Exception e) {
            e.printStackTrace();
        }
        
    }
    public boolean insertuser(String string1, String string2, String string3, String string4, String string5) throws SQLException{
        
        String sql = "insert into dbo.Usertable values(?,?,?,?,?)";
        
        PreparedStatement pstmt = connection.prepareStatement(sql);//this is the place where connection is null

        pstmt.setString(1, string1);
        pstmt.setString(2, string2);
        pstmt.setString(3, string3);
        pstmt.setString(4, string4);
        pstmt.setString(5, string5);
        int i = pstmt.executeUpdate();
        if (i > 0) {
            return true;
        }
        
        return false;
    
    
        
    }

    

}

i have a class store . I connect jdbc in init. I insert values in usertable in a method insertuser . My connection object is pointing to null. My usertable has five varchar values in a database called Register . Why my connection object is pointing to null.I have jdbc jar connected to my classpath.

Answer 1

If the Store constructor encounters any exception, it will print a stacktrace and return a partially constructed Store object with a null in the connection field.

That would explain what you are seeing. But we can't tell you the cause of the exception you are catching without seeing the (complete) stacktrace. Typical explanations for this kind of thing include:

• Database driver JAR not the classpath (correctly)
• Incorrect database driver JAR
• Incorrect database credentials
• Incorrect database host or port information
• Network problems

Things you could fix:

• As commenters pointed out, the exception handling in your constructor is broken. If you get an exception while setting up the connection, the constructor should terminate with an exception. The caller needs to deal with it, or allow it to propagate.
• If you use DriverManager.getConnection you shouldn't need to use Class.forName to load the JDBC driver. The DriverManager infrastructure will find the driver for itself, based on what the jdbc URL says.

Answer 2

You need to add the jar file containing the class com.microsoft.sqlserver.jdbc.SQLServerDriver to your war file's WEB-INF/lib folder.