从 servlet 连接到 mysql 数据库

Question

我正在尝试连接到我的数据库以检查用户名和密码。 我正在学习教程，但我的连接不起作用。我的问题是：

String url = "jdbc:mysql://localhost:3306/"; 
String dbName = "LoginExample";
String driver = "com.mysql.jdbc.Driver";  
String userName = "root";  
String password = "password"; 

所有这些值的含义是什么？那些是标准的吗？如果不是，我怎样才能找到哪些对应于我的数据库。此外，我在创建数据库时也没有被要求输入用户名和密码。

public class Login extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public Login() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
    }

    public static boolean validate(String name, String pass) {
        boolean status = false;
        Connection conn = null;
        PreparedStatement pst = null;
        ResultSet rs = null;

        String url = "jdbc:mysql://localhost:3306/"; 
        String dbName = "LoginExample";
        String driver = "com.mysql.jdbc.Driver";  
        String userName = "root";  
        String password = ""; 

        try {  
            Class.forName(driver).newInstance();  
            conn = DriverManager  
                    .getConnection(url + dbName, userName, password);  

            pst = conn  
                    .prepareStatement("select * from Users where username=? and password=?");  
            pst.setString(1, name);  
            pst.setString(2, pass);  

            rs = pst.executeQuery();  
            status = rs.next();  

        } catch (Exception e) {  
            System.out.println(e);  
        } finally {  
            if (conn != null) {  
                try {  
                    conn.close();  
                } catch (SQLException e) {  
                    e.printStackTrace();  
                }  
            }  
            if (pst != null) {  
                try {  
                    pst.close();  
                } catch (SQLException e) {  
                    e.printStackTrace();  
                }  
            }  
            if (rs != null) {  
                try {  
                    rs.close();  
                } catch (SQLException e) {  
                    e.printStackTrace();  
                }  
            }  
        }
        return status;
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        String un = request.getParameter("username");
        String pw = request.getParameter("password");

        response.setContentType("text/html");    
        PrintWriter out = response.getWriter();  

        HttpSession session = request.getSession(false);
        if (session != null) {
            session.setAttribute("name", un);
        }

        if (validate(un,pw)) {
            RequestDispatcher rd=request.getRequestDispatcher("welcome.jsp");    
            rd.forward(request,response); 
        } else {
            out.print("<p style=\"color:red\">Sorry username or password error</p>"); 
        }
    }

}

错误日志：

com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception: 

** BEGIN NESTED EXCEPTION ** 

java.net.ConnectException
MESSAGE: Connection refused: connect

STACKTRACE:

java.net.ConnectException: Connection refused: connect
    at java.net.DualStackPlainSocketImpl.connect0(Native Method)
    at java.net.DualStackPlainSocketImpl.socketConnect(Unknown Source)
    at java.net.AbstractPlainSocketImpl.doConnect(Unknown Source)
    at java.net.AbstractPlainSocketImpl.connectToAddress(Unknown Source)
    at java.net.AbstractPlainSocketImpl.connect(Unknown Source)
    at java.net.PlainSocketImpl.connect(Unknown Source)
    at java.net.SocksSocketImpl.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at java.net.Socket.<init>(Unknown Source)
    at java.net.Socket.<init>(Unknown Source)
    at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:256)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:271)
    at com.mysql.jdbc.Connection.createNewIO(Connection.java:2771)
    at com.mysql.jdbc.Connection.<init>(Connection.java:1555)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:285)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at servlets.Login.validate(Login.java:55)
    at servlets.Login.doPost(Login.java:112)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    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:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
    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)


** END NESTED EXCEPTION **

Answer 1

Url 告诉程序在哪里以及如何找到数据库。在这种情况下，您使用 JDBC 访问在您自己的计算机上运行的 MySQL 数据库 (localhost)。

dbname 是数据库的名称。 MySQL 服务器可能托管多个数据库，因此您需要指定您想要的数据库。

Driver 是用于访问数据库的 Java 类。

用户名和密码自行解释。

假设 url 中的端口和数据库名称是正确的，您的问题很可能是用户名/密码组合。默认情况下，用户名为“root”，密码为空。

Answer 2

是的，这些都是标准的，您必须使用它通过 java 连接到您的数据库。

    //since you have to connect to database you have to connect through to database server with url(where localhost signifies your current system and 3306 is a port)
    String url = "jdbc:mysql://localhost:3306/"; 

    //In order to connect to specific database specify its name
    String dbName = "LoginExample";

    //Initialize a driver so you can open a communications channel with the database.
    String driver = "com.mysql.jdbc.Driver"; 

    //credentials required to connect 
    String userName = "root";  
    String password = "password"

由于您在安装过程中输入了任何用户名和密码，因此设置了默认名称和密码。默认用户名是root，密码是无密码。您也可以reset password。

com.mysql.jdbc.CommunicationsException：通信链接失败 由于潜在的异常：

首先您不需要实例化驱动程序，只需将其称为静态初始化（例如，通过调用 Class.forName），以便在 DriverManager 中注册

所以从Class.forName(driver).newInstance() 中删除.newInstance()；

同时检查端口3306是否被其他进程获取检查这个

运行 resmon.exe 并转到网络选项卡以检查3306端口是否被其他进程获取。

Answer 3

您不应该手动尝试从您的 servlet 建立数据库连接。相反，您应该定义一个 DataSource 并从您的 servlet 中查找它；

tomcat 分步指南：

1. 把你的mysql驱动放到tomcat的/lib文件夹中

2. 在Tomcat的context.xml中定义一个数据源

   < Resource name="jdbc/DefaultDB"
         type="javax.sql.DataSource"  auth="Container"
         description="Default datasource"
         maxActive="100" maxIdle="30" maxWait="10000"
         username="" password="" 
         driverClassName="com.mysql.jdbc.Driver"
         url="com.mysql.jdbc.Driver"/>
   

3. 在您的web.xml 中引用此数据源

   < resource-ref>  
           < res-ref-name>jdbc/DefaultDB</res-ref-name>  
           < res-type>javax.sql.DataSource</res-type>  
           < res-auth>Container</res-auth>  
   < /resource-ref >
   

4. 查找数据源：

   InitialContext ctx = new InitialContext(); DataSource ds = (DataSource)ctx.lookup("java:comp/env/jdbc/DefaultDB");

   作为替代方案，您可以注入数据源：

   @Resource(name="jdbc/DefaultDB") private javax.sql.DataSource ds;

5. 获取连接：

Connection connection = ds.getConnection();