[英]Null pointer exception. How my connection object is pointing to null
堆栈跟踪
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;
}
}
我有一家 class商店。 我在 init 中连接 jdbc。 我使用 insertuser 方法在usertable中插入值。 我的连接 object 指向 null。我的用户表在名为Register的数据库中有五个 varchar 值。 为什么我的连接 object 指向 null。我有 jdbc jar 连接到我的类路径。
如果Store
构造函数遇到任何异常,它将打印堆栈跟踪并返回部分构造的Store
object, connection
字段中为null
。
那可以解释你所看到的。 但是我们不能在没有看到(完整的)堆栈跟踪的情况下告诉您您正在捕获的异常的原因。 对这种事情的典型解释包括:
您可以解决的问题:
DriverManager.getConnection
,则不需要使用Class.forName
来加载 JDBC 驱动程序。 DriverManager
基础结构将根据 jdbc URL 的内容为自己找到驱动程序。您需要将包含 class com.microsoft.sqlserver.jdbc.SQLServerDriver 的 jar 文件添加到 war 文件的 WEB-INF/lib 文件夹中。
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