问题
Why does it say null URL and gives a empty \' \' class in the exception when I have provided the database URL?
I am trying to connect to a derby database via a servlet while using Tomcat. When the servlet gets run, I get the following exceptions:
org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot create JDBC driver of class \'\' for connect URL \'null\'
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createConnectionFactory(BasicDataSource.java:1452)
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:1371)
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.getConnection(BasicDataSource.java:1044)
at servlets.servlet_1.doGet(servlet_1.java:23) // ---> Marked the statement in servlet
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
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:100)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:405)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:964)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:515)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:885)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.NullPointerException
at sun.jdbc.odbc.JdbcOdbcDriver.getProtocol(JdbcOdbcDriver.java:507)
at sun.jdbc.odbc.JdbcOdbcDriver.knownURL(JdbcOdbcDriver.java:476)
at sun.jdbc.odbc.JdbcOdbcDriver.acceptsURL(JdbcOdbcDriver.java:307)
at java.sql.DriverManager.getDriver(DriverManager.java:253)
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createConnectionFactory(BasicDataSource.java:1437)
... 24 more
Servlet :
package servlets;
import java.io.IOException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.servlet.http.*;
import javax.servlet.*;
import javax.sql.DataSource;
public class servlet_1 extends HttpServlet {
@Override
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
try {
// String queryString = request.getQueryString();
System.out.println(\"!!!!!!!!!!!!!!!!!!!\");
Context initContext = new InitialContext();
Context envContext = (Context)initContext.lookup(\"java:comp/env\");
DataSource ds = (DataSource)envContext.lookup(\"jdbc/PollDatasource\");
Connection connection = ds.getConnection(); // -->LINE 23
String sqlQuery = \"select * from PollResult\";
PreparedStatement statement = connection.prepareStatement(sqlQuery);
ResultSet set = statement.executeQuery();
System.out.println(\"after the final statement\");
} catch (Exception exc) {
exc.printStackTrace();
}
}
}
What exception is this? Why do I get this exception?
I have added the following tag in context.xml of Tomcat :
<Resource name=\"jdbc/PollDatasource\" auth=\"Container\" type=\"javax.sql.DataSource\"
driverClassName=\"org.apache.derby.jdbc.EmbeddedDriver\"
url=\"jdbc:derby://localhost:1527/poll_database;create=true\"
username=\"suhail\" password=\"suhail\"
maxActive=\"20\" maxIdle=\"10\" maxWait=\"-1\" />
and this in web.xml :
<resource-ref>
<description>my connection</description>
<res-ref-name>jdbc/PollDatasource</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
</resource-ref>
Where am I making a mistake?
Image that shows the database URL..
![\"enter](\"https://i.stack.imgur.com/8oTLF.jpg\")
NOTE : After the answer by @Bryan Pendleton I changed the driver to org.apache.derby.jdbc.ClientDriver but I get the same exception.
回答1:
I can't see anything obviously wrong, but perhaps a different approach might help you debug it?
You could try specify your datasource in the per-application-context instead of the global tomcat one.
You can do this by creating a src/main/webapp/META-INF/context.xml (I'm assuming you're using the standard maven directory structure - if not, then the META-INF folder should be a sibling of your WEB-INF directory). The contents of the META-INF/context.xml file would look something like:
<?xml version="1.0" encoding="UTF-8"?>
<Context [optional other attributes as required]>
<Resource name="jdbc/PollDatasource" auth="Container"
type="javax.sql.DataSource" driverClassName="org.apache.derby.jdbc.ClientDriver"
url="jdbc:derby://localhost:1527/poll_database;create=true"
username="suhail" password="suhail" maxActive="20" maxIdle="10" maxWait="-1"/>
</Context>
Obviously the path and docBase would need to match your application's specific details.
Using this approach, you don't have to specify the datasource details in Tomcat's context.xml file. Although, if you have multiple applications talking to the same database, then your approach makes more sense.
At any rate, give this a whirl and see if it makes any difference. It might give us a clue as to what is going wrong with your approach.
回答2:
Several fixes:
Use the right driver class name for your environment: if you are using an out-of-process Derby server, then you want ClientDriver (and need to use derbyclient.jar), the hostname and port, etc. If you want an in-process Derby server, then you want derby.jar, EmbeddedDriver, and a URL that is appropriate for an embedded database.
Put your driver JAR file only in Tomcat's
lib/directory.Don't put anything in Tomcat's
conf/context.xml: there's really no reason for it. Instead, use your webapp'sMETA-INF/context.xmlto define your<Resource>.
The error "Cannot create JDBC driver of class '' for connect URL 'null' usually occurs because the JDBC driver is not in the right place (or in too many places, like Tomcat's lib/ directory but also in the webapp's WEB-INF/lib/ directory). Please verify that you have the right driver JAR file in the right place.
回答3:
These two things don't match:
driverClassName="org.apache.derby.jdbc.EmbeddedDriver"
url="jdbc:derby://localhost:1527/poll database;create=true"
If you are using the EmbeddedDriver, your URL should not contain network syntax.
Conversely, if you are using network syntax, you need to use the ClientDriver.
http://db.apache.org/derby/docs/10.8/getstart/rgsquck35368.html
回答4:
I was getting this problem because I put context.xml into the wrong path:
./src/main/resources/META-INF/context.xml
The correct path was:
./src/main/webapp/META-INF/context.xml
回答5:
If you are using eclipse, you should modify the context.xml, from the server project created in your eclipse package explorer. When using tomcat in eclipse it is the only one valid, the others are ignored or overwriten
回答6:
Context envContext = (Context)initContext.lookup("java:comp/env");
not:Context envContext = (Context)initContext.lookup("java:/comp/env");
回答7:
Did you try to specify resource only in context.xml
<Resource name="jdbc/PollDatasource" auth="Container" type="javax.sql.DataSource"
driverClassName="org.apache.derby.jdbc.EmbeddedDriver"
url="jdbc:derby://localhost:1527/poll_database;create=true"
username="suhail" password="suhail"
maxActive="20" maxIdle="10" maxWait="-1" />
and remove <resource-ref> section from web.xml?
In one project I've seen configuration without <resource-ref> section in web.xml and it worked.
It's an educated guess, but I think <resource-ref> declaration of JNDI resource named jdbc/PollDatasource in web.xml may override declaration of resource with same name in context.xml and the declaration in web.xml is missing both driverClassName and url hence the NPEs for that properties.
回答8:
If you're using an embedded driver, the connectString is just
jdbc:derby:databaseName
(whith options like;create=true;user=xxx etc).
If you're using client driver, the connect string can be left as is, but if changing the driver gives no result... excuse the question, but are you 100% sure you have started the Derby Network Server as per the Derby Tutorial?
回答9:
In my case I solved the problem editing [tomcat]/Catalina/localhost/[mywebapp_name].xml instead of META-INF/context.xml.
回答10:
I had a similar problem using Tomcat against Oracle. I DID have the context.xml in the META-INF directory, on the disc. This file was not showing in the eclipse project though. A simple hit on the F5 refresh and the context.xml file appeared and eclipse published it. Everything worked past that. Hope this helps someone.
Try hitting F5 in eclipse
回答11:
The problem could also come because of a lack of SQL driver in your Tomcat installation directory.
I had to had mysql-connector-java-5.1.23-bin.jar in apache-tomcat-9.0.12/lib/ folder.
https://dev.mysql.com/downloads/connector/j/
来源：https://stackoverflow.com/questions/11516747/cannot-create-jdbc-driver-of-class-for-connect-url-null-i-do-not-underst