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池不在servlet中，但在独立应用程序中工作。

池类常见于两种情况：

public class Pool {
private static DataSource ds;

static {
    DriverAdapterCPDS cpds = new DriverAdapterCPDS();
    try {
        cpds.setDriver("com.mysql.jdbc.Driver");
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
    cpds.setUrl("jdbc:mysql://localhost:3306/collage");
    cpds.setUser("root");
    cpds.setPassword("r00t");

    SharedPoolDataSource tds = new SharedPoolDataSource();
    tds.setConnectionPoolDataSource(cpds);
    tds.setMaxTotal(10);
    /*tds.setMaxWait(50);
    tds.setMaxTotal();*/

    tds.setMaxTotal(50);

    ds = tds;
}

public static Connection getConnection() throws SQLException {
    return ds.getConnection();
}

对于独立类，它的工作方式如下

public class MainClass {
public static void main(String[] args) {
    Connection connection = null;
    Statement statement = null;
    ResultSet resultSet = null;

    try {
        connection = Pool.getConnection();
        // Do work with connection
        statement = connection.createStatement();
        String selectEmployeesSQL = "select * from students";
        resultSet = statement.executeQuery(selectEmployeesSQL);

        while (resultSet.next()) {
            printTestTable(resultSet);
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finnaly {
        ...
    }
}

来自servlet 的部分如下：

@WebServlet("/Demo.do")
public class DemoController extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ResultSet rsObj = null;
        Connection connObj = null;
        PreparedStatement pstmtObj = null;

        try {
            connObj = Pool.getConnection();
        ...

例外在这里”

    HTTP Status 500 – Internal Server Error

Type Exception Report

Message Servlet execution threw an exception

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

javax.servlet.ServletException: Servlet execution threw an exception
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

Root Cause

java.lang.NoClassDefFoundError: Could not initialize class am.ak.dao.jdbc.connections.Pool
    test.DemoController.doGet(DemoController.java:27)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

Note The full stack trace of the root cause is available in the server logs.

谢谢。

添加：

项目设置中的工件：在此处输入图像描述

解决：

所以这两个库被错过了）

在此处输入图像描述

有趣： 在 POM 中，我只添加了一个依赖机器人，它支持两个：

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-dbcp2</artifactId>
    <version>2.5.0</version>
</dependency>
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