JDBC 连接的 Java 空指针异常

Question

我正在尝试运行一个 Web 应用程序，该应用程序从用户那里获取特定长度的数字，并根据数据库进行检查(因此，如果用户打字错误，它会就用户的实际含义提出建议)。我遇到的问题是应用程序无法连接到数据库，我收到此错误:

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java.lang.NullPointerException
at Validator.Validate.checkIDs(Validate.java:410)
at Validator.Validate.getAllValidIds(Validate.java:203)
at Python.WebServlet.processRequest(WebServlet.java:70)
at Python.WebServlet.doPost(WebServlet.java:247)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:224)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:169)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:927) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:987)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:579)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:1805)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)

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Validator.Validate 中的第 410 行是:

Statement stmt = Conn.createStatement();

其中 Conn 是一个 java.sql.Connection 变量

第 203 行是对执行 ^ 语句的方法的调用

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java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1701)
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1546)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:169)
at Validator.Validate.dbConnect(Validate.java:394)
at Validator.Validate.getAllValidIds(Validate.java:193)
at Python.WebServlet.processRequest(WebServlet.java:70)
at Python.WebServlet.doPost(WebServlet.java:247)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:224)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:169)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:927)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:987)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:579)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:1805)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)

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这是我在我的服务器上遇到的错误，如果我在我的本地机器上运行它，它工作正常(它验证 ID)。这里可能是什么问题？它不能定位 com.mysql.jdbc.Driver 类吗？是那个类(class)的问题吗？

提前感谢您的建议/帮助!

注意:

Validator 类是一个位于我的类路径中的外部 jar。它在其类路径中调用另一个包含 com.mysql.jdbc 类的 jar，如下图所示

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BugValidator jar 包含具有 Drivers 类的 mysql-connector jar

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Answer 1

java.lang.ClassNotFoundException: com.mysql.jdbc.Driver

您必须在服务器的类路径中包含 MySQL JDBC 驱动程序。
通过将其与您的应用程序一起发布或将其放入服务器的 lib 目录。

希望这对您有所帮助。玩得开心!