如何解決java.lang.NoClassDefFoundError

Question

我正在開發一個應用程序，其中用戶以表格形式輸入他/她的信息，並將該表格的信息插入到Google Cloud MySQL數據庫中。我已經編寫了實現上述任務的代碼，如下所示：

package com.example.dao;

import java.sql.Connection;
import java.sql.DriverManager;

public class dao {
    private static Connection con;
    public static Connection getcon(){
        try{
            String instanceConnectionName = "edu-vitae1";
            String databaseName = "eduvitae";
             String username = "root";
             String password = "root";
             Class.forName("com.mysql.jdbc.Driver");
             String jdbcUrl = String.format(
             "jdbc:mysql://google/eduvitae?cloudSqlInstance=edu-vitae-211606:asia-south1:edu-vitae1&socketFactory=com.google.cloud.sql.mysql.SocketFactory&user=root&password=root&useSSL=false",
                  databaseName,
                  instanceConnectionName);
             //Connection con=DriverManager.getConnection("jdbc:mysql://google/eduvitae?cloudSqlInstance=edu-vitae-211606:asia-south1:edu-vitae1&socketFactory=com.google.cloud.sql.mysql.SocketFactory&useSSL=false&user=root&password=root");
             Connection con = DriverManager.getConnection(jdbcUrl, username, password);
             //con=DriverManager.getConnection("jdbc:mysql://35.200.134.221/edu","root","root");
        }
        catch (Exception e) {
            System.out.println(e);
        }
        return con;
    }

}

執行上述代碼后，我得到以下錯誤：

java.sql.SQLException: java.lang.NoClassDefFoundError: com/mysql/cj/protocol/SocketFactory
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1055)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:956)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:926)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:430)
at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:302)
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:282)
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
at com.example.dao.dao.getcon(dao.java:20)
at com.example.model.register.doPost(register.java:105)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:660)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at com.googlecode.objectify.ObjectifyFilter.doFilter(ObjectifyFilter.java:48)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:491)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:668)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:764)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1388)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)

我正在使用Google Cloud Engine，並且我的應用程序正在JDK 1.8.0_131上運行。我包括“ mysql-socket-factory-connector-j-8-1.0.10.jar”，“ mysql-connector-java-8.0”。應用程序的構建路徑中的“ 11.jar”和“ jetty-unixsocket-9.4.8.v20171121.jar”。
請幫助我了解並解決此問題。
先感謝您。

Answer 1

有關該錯誤的一些信息為什么我會在Java中收到NoClassDefFoundError？

對於我來說，我已經通過將mylib.jar添加到服務器上的“ tomcat 9 dir / lib”文件夾以及本地的類路徑中來解決了該問題（在Eclipse Properties> Java Build Path> Libraries-> Add External JARs中... ）。 您能描述一下如何添加罐子嗎？

Answer 2

我通過在WEB-INF / lib文件夾中插入指定的jar文件來解決此錯誤。我已將jar文件添加到項目的構建路徑中，但由於它不在WEB-INF / lib文件夾中，因此jvm無法加載類。
因此，概括起來，無論何時在項目的構建路徑中添加文件，都請記住將相同的文件復制到WEB-INF / lib文件夾中。