Question

我目前正在处理一个Web应用程序，但它无法运行。下面是我到目前为止servlet和Java类的代码。我检查了提供数据库连接的代码，我知道这是正确的。我相信servlet或Java类有问题。

以下是我收到的错误：

HTTP Status 500 -

type Exception report

message

description The server encountered an internal error that prevented it from fulfilling this request.

exception

java.lang.NullPointerException
    music.data.ConnectionPool.freeConnection(ConnectionPool.java:41)
    music.data.ProductDB.selectProducts(ProductDB.java:159)
    music.admin.ProductAdminController.displayProducts(ProductAdminController.java:74)
    music.admin.ProductAdminController.doGet(ProductAdminController.java:36)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:618)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)

这是Java Servlet：

package music.admin;

import java.io.IOException;
import java.util.ArrayList;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import music.business.Product;
import music.data.ProductDB;

public class ProductAdminController extends HttpServlet {

    /* Comment this method out when using this class with a database
     * instead of a text file.
     */
    //@Override
    //public void init() {
        //ProductIO.init(getServletContext()
               // .getRealPath("/WEB-INF/products.txt"));
    //}

    @Override
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        // get current action
        String action = request.getParameter("action");
        if (action == null) {
            action = "displayProducts";  // default action
        }

        // perform action and set URL to appropriate page
        String url = "/index.jsp";
        if (action.equals("displayProducts")) {
            url = displayProducts(request, response);
        } else if (action.equals("displayProduct")) {
            url = displayProduct(request, response);
        } else if (action.equals("addProduct")) {
            url = "/product.jsp";
        } else if (action.equals("deleteProduct")) {
            url = deleteProduct(request, response);
        }
        getServletContext()
                .getRequestDispatcher(url)
                .forward(request, response);
    }

    @Override
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        // get current action
        String action = request.getParameter("action");
        if (action == null) {
            action = "displayProducts";  // default action
        }

        // perform action and set URL to appropriate page
        String url = "/index.jsp";
        if (action.equals("updateProduct")) {
            url = updateProduct(request, response);
        } else if (action.equals("deleteProduct")) {
            url = deleteProduct(request, response);
        }
        getServletContext()
                .getRequestDispatcher(url)
                .forward(request, response);
    }

    private String displayProducts(HttpServletRequest request,
            HttpServletResponse response) {

        ArrayList<Product> products = (ArrayList) ProductDB.selectProducts();
        request.setAttribute("products", products);
        return "/products.jsp";
    }

    private String displayProduct(HttpServletRequest request,
            HttpServletResponse response) {

        String productCode = request.getParameter("productCode");
        Product product;
        if (productCode == null || productCode.isEmpty()) {
            product = new Product();
        } else {
            product = ProductDB.selectProduct(productCode);
        }

        request.setAttribute("product", product);
        return "/product.jsp";
    }

    private String addProduct(HttpServletRequest request,
            HttpServletResponse response) {

        return "/product.jsp";
    }

    private String updateProduct(HttpServletRequest request,
            HttpServletResponse response) {

        String productCode = (String) request.getParameter("productCode");
        String description = (String) request.getParameter("description");
        String priceString = (String) request.getParameter("price");

        double price;
        try {
            price = Double.parseDouble(priceString);
        } catch (NumberFormatException e) {
            price = 0;
        }

        Product product = (Product) request.getAttribute("product");
        if (product == null) {
            product = new Product();
        }
        product.setCode(productCode);
        product.setDescription(description);
        product.setPrice(price);
        request.setAttribute("product", product);

        String message = "";
        if (product.getPrice() <= 0) {
            message = "You must enter a positive number for the price without "
                    + "any currency symbols.";
        }
        if (product.getDescription().length() == 0) {
            message = "You must enter a description for the product.";
        }
        if (product.getCode().length() == 0) {
            message = "You must enter a code for the product.";
        }
        request.setAttribute("message", message);

        String url;
        if (message.isEmpty()) {
            if (ProductDB.exists(product.getCode())) {
                ProductDB.updateProduct(product);
            } else {
                ProductDB.insertProducts(product);
            }
            url = displayProducts(request, response);
        } else {
            url = "/product.jsp";
        }
        return url;
    }

    private String deleteProduct(HttpServletRequest request,
            HttpServletResponse response) {

        String productCode = request.getParameter("productCode");
        Product product = ProductDB.selectProduct(productCode);
        request.setAttribute("product", product);

        String url;
        String yesButton = request.getParameter("yesButton");
        if (yesButton != null) {
            ProductDB.removeProduct(product);
            url = displayProducts(request, response);
        } else {
            url = "/confirm_product_delete.jsp";
        }
        return url;
    }    
}

这是Java类：

package music.data;

import java.sql.*;
import java.util.*;

import music.business.Product;

public class ProductDB
{
    //This method returns null if a product isn't found.
    public static Product selectProduct(String productCode)
    {
        ConnectionPool pool = ConnectionPool.getInstance();
        Connection connection = pool.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;

        String query = "SELECT * FROM Product " +
                "WHERE ProductCode = ?";
        try
        {
            ps = connection.prepareStatement(query);
            ps.setString(1, productCode);
            rs = ps.executeQuery();
            if (rs.next())
            {
                Product p = new Product();

                p.setCode(rs.getString("ProductCode"));
                p.setDescription(rs.getString("ProductDescription"));
                p.setPrice(rs.getDouble("ProductPrice"));
                return p;
            }
            else
            {
                return null;
            }
        }
        catch(SQLException e)
        {
            System.out.println(e);
            return null;
        }
        finally
        {
            DBUtil.closeResultSet(rs);
            DBUtil.closePreparedStatement(ps);
            pool.freeConnection(connection);
        }
    }

    //This method will return 0 if productID isn't found.
    public static int selectProductID(Product product)
    {
        ConnectionPool pool = ConnectionPool.getInstance();
        Connection connection = pool.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;

        String query = "SELECT ProductID FROM Product " +
                "WHERE ProductCode = ?";
        try
        {
            ps = connection.prepareStatement(query);
            ps.setString(1, product.getCode());
            rs = ps.executeQuery();
            rs.next();
            int productID = rs.getInt("ProductID");
            return productID;
        }
        catch(SQLException e)
        {
            System.out.println(e);
            return 0;
        }
        finally
        {
            DBUtil.closeResultSet(rs);
            DBUtil.closePreparedStatement(ps);
            pool.freeConnection(connection);
        }
    }

    //This method returns null if a product isn't found.
    public static Product selectProduct(int productID)
    {
        ConnectionPool pool = ConnectionPool.getInstance();
        Connection connection = pool.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;

        String query = "SELECT * FROM Product " +
                "WHERE ProductID = ?";
        try
        {
            ps = connection.prepareStatement(query);
            ps.setInt(1, productID);
            rs = ps.executeQuery();
            if (rs.next())
            {
                Product p = new Product();
                p.setCode(rs.getString("ProductCode"));
                p.setDescription(rs.getString("ProductDescription"));
                p.setPrice(rs.getDouble("ProductPrice"));
                return p;
            }
            else
            {
                return null;
            }
        }
        catch(SQLException e)
        {

            System.out.println(e);
            return null;
        }
        finally
        {
            DBUtil.closeResultSet(rs);
            DBUtil.closePreparedStatement(ps);
            pool.freeConnection(connection);
        }
    }

    //This method returns null if a product isn't found.
    public static ArrayList<Product> selectProducts()
    {
        ConnectionPool pool = ConnectionPool.getInstance();
        Connection connection = pool.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;

        String query = "SELECT * FROM Product";
        try
        {
            ps = connection.prepareStatement(query);
            rs = ps.executeQuery();
            ArrayList<Product> products = new ArrayList<Product>();
            while (rs.next())
            {
                Product p = new Product();
                p.setCode(rs.getString("ProductCode"));
                p.setDescription(rs.getString("ProductDescription"));
                p.setPrice(rs.getDouble("ProductPrice"));
                products.add(p);
            }
            return products;
        }
        catch(SQLException e)
        {
           System.out.println(e);
            return null;
        }
        finally
        {
            DBUtil.closeResultSet(rs);
            DBUtil.closePreparedStatement(ps);
            pool.freeConnection(connection);
        }
    }    



public static void insertProducts(Product product)
    {
        ConnectionPool pool = ConnectionPool.getInstance();
        Connection connection = pool.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;

        String query = "INSERT INTO product (ProductCode, ProductDescription, ProductPrice)"
                + "VALUES"
                + "('"+product.getCode()+"','"+product.getDescription()+"','"+product.getPriceNumberFormat()+"');";
        try
        {
            ps = connection.prepareStatement(query);
           ps.execute(query);


        }
        catch(SQLException e)
        {
            System.out.println(e);

        }
        finally
        {
            DBUtil.closeResultSet(rs);
            DBUtil.closePreparedStatement(ps);
            pool.freeConnection(connection);
        }
    } 

public static void updateProduct(Product product)
    {
        ConnectionPool pool = ConnectionPool.getInstance();
        Connection connection = pool.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;



        String query = "UPDATE product SET productDescription ='"+product.getDescription()+"',"+"ProductPrice='"+product.getPriceNumberFormat()+"' WHERE ProductCode='"+product.getCode()+"';";

        try
        {
            ps = connection.prepareStatement(query);
            ps.execute(query);
            //rs = ps.executeQuery();


        }
        catch(SQLException e)
        {
            System.out.println(e);

        }
        finally
        {
            DBUtil.closeResultSet(rs);
            DBUtil.closePreparedStatement(ps);
            pool.freeConnection(connection);
        }
    }


    public static boolean exists(String productCode) {
        ConnectionPool pool = ConnectionPool.getInstance();
        Connection connection = pool.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;

        String query = "SELECT productCode FROM Product " +
                "WHERE ProductCode = ?";
        try {
            ps = connection.prepareStatement(query);
            ps.setString(1, productCode);
            rs = ps.executeQuery();
            return rs.next();
        } catch (SQLException e) {
            System.out.println(e);
            return false;
        } finally {
            DBUtil.closeResultSet(rs);
            DBUtil.closePreparedStatement(ps);
            pool.freeConnection(connection);
        }
    }

public static void removeProduct(Product product)
    {
        ConnectionPool pool = ConnectionPool.getInstance();
        Connection connection = pool.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;

        String query = "DELETE FROM product WHERE ProductCode='"+product.getCode()+"';";
        try
        {
            ps = connection.prepareStatement(query);
           ps.execute(query);


        }
        catch(SQLException e)
        {
           System.out.println(e);

        }
        finally
        {
            DBUtil.closeResultSet(rs);
            DBUtil.closePreparedStatement(ps);
            pool.freeConnection(connection);
        }
    } 

}

以下是连接池的代码：

package music.data;


import java.sql.*;
import javax.sql.DataSource;
import javax.naming.InitialContext;
import javax.naming.NamingException;

public class ConnectionPool {

    private static ConnectionPool pool = null;
    private static DataSource dataSource = null;

    private ConnectionPool() {
        try {
            InitialContext ic = new InitialContext();
            dataSource = (DataSource) ic.lookup("java:/comp/env/jdbc/music");
        } catch (NamingException e) {
            System.out.println(e);
        }
    }

    public static synchronized ConnectionPool getInstance() {
        if (pool == null) {
            pool = new ConnectionPool();
        }
        return pool;
    }

    public Connection getConnection() {
        try {
            return dataSource.getConnection();
        } catch (SQLException e) {
            System.out.println(e);
            return null;
        }
    }

    public void freeConnection(Connection c) {
        try {
            c.close();
        } catch (SQLException e) {
            System.out.println(e);
        }
    }
}

DBUtil Class

package music.data;

import java.sql.*;

public class DBUtil
{
    public static void closeStatement(Statement s)
    {
        try
        {
            if (s != null)
                s.close();
        }
        catch(SQLException e)
        {
            e.printStackTrace();
        }
    }

    public static void closePreparedStatement(Statement ps)
    {
        try
        {
            if (ps != null)
                ps.close();
        }
        catch(SQLException e)
        {
            e.printStackTrace();
        }
    }

    public static void closeResultSet(ResultSet rs)
    {
        try
        {
            if (rs != null)
                rs.close();
        }
        catch(SQLException e)
        {
            e.printStackTrace();
        }
    }
}

Answer 1

从您的问题和提供的代码中，我认为可以重新审视几点，看看我们是否有任何问题。

您已使用Class DBUtil关闭结果集和语句对象。
- DBUtil似乎不是标准类。可能使用来自apache的DBUtils将是一个更好的选择。鉴于DBUtil似乎没有标准实现，请提供DBUtil的实现以及其他类。
- 此外，使用DButil关闭结果集和语句会丢失对称性，但还有一些其他方法可以关闭连接。我建议使用DBUtils.close（）或closequietly（）来关闭连接。
此外，在您的实现中，初始连接池大小似乎不清楚。通过使用ds.setInitialPoolSize（）显式将poolsize设置为更高的数字。
- 此外，如果需要，请设置maximumPoolsize和preferredPoolSize。

这样做应该主要解决问题，或者说明问题的清晰度。

Answer 2

既然您已经发布了代码，问题非常明确：您收到NullPointerException，因为您尝试在空连接上调用close()。

为什么连接为空？

因为getConnection（）方法返回null：

    try {
        return dataSource.getConnection();
    } catch (SQLException e) {
        System.out.println(e);
        return null;
    }

在上面的代码中，您尝试从DataSource获取连接，如果数据源通过抛出异常来表示您无法获得连接，则基本上忽略该异常，并返回null。

结果：不是得到一个明确的SQLException来解释为什么你不能从DataSource获得连接，你会在一个不相关的方法中得到一个模糊的NullPointerException，它不会告诉你关于它的任何信息。实际问题。

不要这样做。如果你真的不想在调用getConnection()时处理检查的SQLException，那么将SQLException包装成运行时异常。然后，您将有一个明确的消息和堆栈跟踪指示问题的确切位置和位置：

    try {
        return dataSource.getConnection();
    } catch (SQLException e) {
        throw new RuntimeSQLException(e);
    }

public class RuntimeSQLException extends RuntimeException {
    public RuntimeSQLException(Throwable cause) {
        super(cause);
    }
}

JSP Servlet Web应用程序

2 个答案: