始终无法为连接 URL 'null 创建类 '' 的 JDBC 驱动程序

Question

您好，我有以下问题： 每当我尝试连接到 MySQL DB 时，我都会收到此错误：

com.pbs.web.jdbc.ParkingBookSystem.BookControllerServlet doGet
SEVERE: null
**java.sql.SQLException: Cannot create JDBC driver of class '' for connect URL 'null'**
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createConnectionFactory(BasicDataSource.java:2224)
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createDataSource(BasicDataSource.java:2104)
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.getConnection(BasicDataSource.java:1563)
    at com.pbs.web.jdbc.ParkingBookSystem.BookDbUtil.doesBookExist(BookDbUtil.java:91)
    at com.pbs.web.jdbc.ParkingBookSystem.BookControllerServlet.addBook(BookControllerServlet.java:117)
    at com.pbs.web.jdbc.ParkingBookSystem.BookControllerServlet.doGet(BookControllerServlet.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
    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 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:490)
    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:74)
    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$ConnectionHandler.process(AbstractProtocol.java:770)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1415)
    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)

**Caused by: java.sql.SQLException: No suitable driver
    at java.sql.DriverManager.getDriver(Unknown Source)**
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createConnectionFactory(BasicDataSource.java:2209)
    ... 29 more

我将 context.xml 放置在：C:\Users\userName\eclipse-workspace\WebAppName\src\main\resources\META-INF 代码如下：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xml>
<Context>
<Resource 
    name="ParkingBookSystem" 
    auth="Container"
    type="javax.sql.DataSource"
    maxTotal="20"
    maxIdle="5"
    maxWaitMillis="10000"
    username="sql2226123"
    password="cccccccc"
    driverClassName="com.mysql.jdbc.Driver"
    url="jdbc:mysql://sql2.freemysqlhosting.net:3306/sql2226123"
/>
</Context>

和我的 web.xml（放在 C:\Users\userName\eclipse-workspace\WebAppName\src\main\webapp\WEB-INF ）：

<web-app xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
version="2.4">
<description>ParkingBookSystem</description>
<resource-ref>
    <description>DB Connection</description>
    <res-ref-name>/ParkingBookSystem</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
</resource-ref>
<welcome-file-list>
    <welcome-file>index.html</welcome-file>
</welcome-file-list>
</web-app>

这是我用来建立数据库连接的类构造函数：

// constructor
public BookDbUtil() {
    try {
        Context initContext = new InitialContext();
        Context envContext = (Context) initContext.lookup("java:/comp/env");
        dataSource = (DataSource) envContext.lookup("ParkingBookSystem");
    } catch (NamingException e) {
        throw new IllegalStateException("Could not initialize database", e);
    }
}

我正在使用 Maven、Eclipse 和 Tomcat，这是我迄今为止尝试过的：

• 我把mysql-connector-java-5.1.38-bin放到tomcat lib中
• 手动将 context.xml 放到 C:\Program Files\Apache Software Foundation\Tomcat 9.0\conf\Catalina\localhost
• 同上，但将名称从 context.xml 更改为 projectName.xml
• 将所有内容放入 server.xml 中（并注释掉此处的默认资源）
• 我还发现，tomcat 文件中的每一次更改都需要管理员权限，因此我为每个用户添加了完全控制，以确保不是这种情况。

我没有更多的想法，有没有人可以帮忙？

Answer 1

请；

将mysql连接器jar文件放到WEB-INF/lib目录下

并在 context.xml 中更改

<Context>
    <Resource name="jdbc/ParkingBookSystem" ...

并在 web.xml 中进行更改

<res-ref-name>jdbc/ParkingBookSystem</res-ref-name>

并更改查找

Context initContext = new InitialContext();
Context envContext  = (Context)initContext.lookup("java:/comp/env");
dataSource = (DataSource)envContext.lookup("jdbc/ParkingBookSystem");

Answer 2

好的，我找到了解决方案...问题是我的带有 context.xml 的 META-INF 位于 C:\Users\lukas\eclipse-workspace\ParkingBookSystem\src\main\resources\META-INF\context.xml 时我把它放到： C:\Users\lukas\eclipse-workspace\ParkingBookSystem\src\main\webapp 它开始起作用了。