最后的语句在try/catch java中不起作用



我正在尝试访问 netbeans 中的数据库,这是我第一次这样做。当我进入finally声明时,我遇到了问题。ConnectionPrintWriter似乎没有注册,我不确定我做错了什么。在try/catch中使用变量con然后在其之后使用变量out的问题。

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;

@WebServlet(name = "DBServlet1", urlPatterns = {"/db1"})
public class DBServlet1 extends HttpServlet {
/**
 * Processes requests for both HTTP <code>GET</code> and <code>POST</code>
 * methods.
 *
 * @param request servlet request
 * @param response servlet response
 * @throws ServletException if a servlet-specific error occurs
 * @throws IOException if an I/O error occurs
 */
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    response.setContentType("text/html;charset=UTF-8");
    //Connection con = null;
    //PrintWriter out = response.getWriter();
    try (PrintWriter out = response.getWriter()) {
        Connection con = null;
        // Load the Driver class file
        Class.forName("org.apache.derby.jdbc.ClientDriver");
        // Make a connection to the ODBC datasource Movie Catalog
        // In this example we are opening a connection to the
        // database with every request.
        con = DriverManager.getConnection("jdbc:derby://localhost:1527/movies","user1", "password");
        if ( con != null ) {
            out.println("<html>n<body>n<table border="1" width="100%">");
            // Create the statement
            Statement statement = con.createStatement();
            ResultSet rs = statement.executeQuery("SELECT * FROM USER1.TMovie");
            ResultSetMetaData rsmd = rs.getMetaData();
            int columnCount = rsmd.getColumnCount();
            out.println("<tr>");
            for(int i=1; i<=columnCount; i++) {
                out.println("<td><h3>" +rsmd.getColumnName(i) + "</td>");
            }
            out.println("</tr>");
            while ( rs.next() ) {
                out.println("<tr>");
                // get the id, which is an int
                out.println("<td>" + rs.getInt("id") + "</td>");
                // get the name, which is a String
                out.println("<td>" + rs.getString("title") + "</td>");
                // get the rating, which is a String
                out.println("<td>" + rs.getString("rating") + "</td>");
                // get the price, which is a Float
                out.println("<td>" + rs.getFloat("price") + "</td>");
                // get the Quantity, which is a Integer
                out.println("<td>" + rs.getInt("quantity") + "</td>");
                // get the Category, which is a Integer
                out.println("<td>" + rs.getString("category") + "</td>");
                out.println("</tr>");
            }// end while
            // Close the ResultSet
            rs.close();
            out.println("</table>");
        }// end if
        else {
            out.println("Data Not Found");
        }
   }catch (Exception e) {
       System.err.println(e.getMessage());
    }// end try-catch
    finally {
        try{
            if ( con != null ) {
                // Close the connection no matter what
                con.close();
            }// end if
        }
        catch (SQLException sqle) {
            System.err.println(sqle.getMessage());
        }// end try-catch
    }// end finally
    out.close();

连接和PrintWriter似乎没有注册,我不确定我做错了什么。

它们都在try声明。因此,就像任何其他块范围的变量一样,它们在该块之外无法访问。如果您需要在catchfinally中访问它们,则需要在try之外声明它们。


旁注:如果你对所有自动关闭对象(不仅仅是PrintWriter)使用 try-with-resources 语句,代码会更简单,例如连接和语句;如果你正确使用它(你没有关闭你在 try-with-resources 中打开的东西,它会);教程。

下面是一个示例,请注意***注释:

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    response.setContentType("text/html;charset=UTF-8");
    // Load the Driver class file
    try {
        Class.forName("org.apache.derby.jdbc.ClientDriver");
    } catch (Exception e) {
       // *** Ideally, do something more useful with the exception or *don't* catch it
       System.err.println(e.getMessage());
       return;
    }
    try (
        // *** Note all auto-closeables are created here
        PrintWriter out = response.getWriter();
        // Make a connection to the ODBC datasource Movie Catalog
        // In this example we are opening a connection to the
        // database with every request.
        Connection con = DriverManager.getConnection("jdbc:derby://localhost:1527/movies","user1", "password");
        // Create the statement
        Statement statement = con.createStatement();
        ResultSet rs = statement.executeQuery("SELECT * FROM USER1.TMovie");
        ) {
        out.println("<html>n<body>n<table border="1" width="100%">");
        ResultSetMetaData rsmd = rs.getMetaData();
        int columnCount = rsmd.getColumnCount();
        out.println("<tr>");
        for(int i=1; i<=columnCount; i++) {
            out.println("<td><h3>" +rsmd.getColumnName(i) + "</td>");
        }
        out.println("</tr>");
        while ( rs.next() ) {
            out.println("<tr>");
            // get the id, which is an int
            out.println("<td>" + rs.getInt("id") + "</td>");
            // get the name, which is a String
            out.println("<td>" + rs.getString("title") + "</td>");
            // get the rating, which is a String
            out.println("<td>" + rs.getString("rating") + "</td>");
            // get the price, which is a Float
            out.println("<td>" + rs.getFloat("price") + "</td>");
            // get the Quantity, which is a Integer
            out.println("<td>" + rs.getInt("quantity") + "</td>");
            // get the Category, which is a Integer
            out.println("<td>" + rs.getString("category") + "</td>");
            out.println("</tr>");
        }// end while
        // *** Don't close auto-closeables like the result set
        out.println("</table>");
        /* *** This else was connected to an if (con != null), so the message doesn't really make sense
        else {
            out.println("Data Not Found");
        }
        */
    } catch (Exception e) {
        // *** Ideally, do something more useful here or don't catch the exception
        System.err.println(e.getMessage());
    }
    // *** No `finally` at all
}

我没有做过完整的代码审计或任何事情,我只是查看了资源的使用并指出了与正确处理它们相关的更改。

变量 con 和 out 的作用域不正确。您需要在尝试/捕获/最终阻止之前声明它们。

在尝试使用资源块之前声明变量:-

Connection con = null;          
PrintWriter out =null,

然后像这样编写尝试资源块:-

try ( out = response.getWriter()) {  
//here is your code

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