我今天转移到 macOS 并重新创建了一个非常简单的数据库测试项目来检查我的 mySQL 连接。但是我得到一个 SQL 异常:“无法创建到数据库服务器的连接”。这是完整的日志:https://pastebin.com/iZrktVKn

我从字面上复制了每一步，我在 Windows 上所做的。在 Windows 上一切都很好。我有一个 context.xml、一个 Servlet 和一个 connector.jar 文件。这个项目应该在 Tomcat 上运行。

context.xml:

<Context>

  <Resource name="jdbc/web_student_tracker" 
            auth="Container" type="javax.sql.DataSource"
               maxActive="20" maxIdle="5" maxWait="10000"
               username="webstudent" password="webstudent" 
               driverClassName="com.mysql.jdbc.Driver"
               url="jdbc:mysql://localhost:3306/web_student_tracker?useSSL=false"/>

</Context>

Servlet(只是重要的事情):

        @Resource(name="jdbc/web_student_tracker")
        private DataSource dataSource;

Servlet 中的 doGet 方法:

        //Printwiter + content type setting
        PrintWriter out = response.getWriter();
        response.setContentType("text/plain");

        //Get a connection to the database
        Connection myConn = null;
        Statement myStmt = null;
        ResultSet myRs = null;

        try{
            myConn = dataSource.getConnection();

            //Create SQL statements
            String sql = "select * from student";
            myStmt = myConn.createStatement();

            //Execute SQL query
            myRs = myStmt.executeQuery(sql);

            //Process the ResultSet
            while(myRs.next()) {
                String email = myRs.getString("email");
                out.println(email);
            }


        } catch(Exception ex) {
            ex.printStackTrace();
        }   
    }

我确实在数据库中添加了一个“webstudent”用户:

CREATE USER 'webstudent'@'localhost' IDENTIFIED BY 'webstudent';

GRANT ALL PRIVILEGES ON * . * TO 'webstudent'@'localhost';

一切都已启动并运行，我还缺少什么？我也是 macOS 开发的新手，也是 MySQL 的新手。但在 Windows 上，一切正常。

堆栈跟踪:

java.sql.SQLException: Cannot create PoolableConnectionFactory (Could not create connection to database server.)
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createPoolableConnectionFactory(BasicDataSource.java:2333)
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createDataSource(BasicDataSource.java:2062)
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.getConnection(BasicDataSource.java:1532)
    at com.jsptanulas.web.jdbc.TestServlet.doGet(TestServlet.java:40)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:94)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:496)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:502)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1132)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1539)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1495)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.base/java.lang.Thread.run(Thread.java:844)
Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Could not create connection to database server.
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:488)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
    at com.mysql.jdbc.Util.getInstance(Util.java:387)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:917)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:896)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:885)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:860)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2332)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2085)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:795)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:44)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:488)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:400)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:327)
    at org.apache.tomcat.dbcp.dbcp2.DriverConnectionFactory.createConnection(DriverConnectionFactory.java:38)
    at org.apache.tomcat.dbcp.dbcp2.PoolableConnectionFactory.makeObject(PoolableConnectionFactory.java:260)
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.validateConnectionFactory(BasicDataSource.java:2343)
    at org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createPoolableConnectionFactory(BasicDataSource.java:2329)
    ... 26 more
Caused by: java.lang.NullPointerException
    at com.mysql.jdbc.ConnectionImpl.getServerCharset(ConnectionImpl.java:3005)
    at com.mysql.jdbc.MysqlIO.sendConnectionAttributes(MysqlIO.java:1916)
    at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1845)
    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1215)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2255)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2286)
    ... 40 more

最佳答案

问题是连接器版本和 MySQL 版本不匹配。 MySQL 8 需要来自 JDBC 连接器 .jar 的版本 8。

关于com.mysql.jdbc.ConnectionImpl.getServerCharset 处的 java.lang.NullPointerException(ConnectionImpl.java :3005)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/49966834/