我是学习JSP和Servlet的新手.我有一个Java类,它调用存储过程并从数据库返回状态列表(这是我学习的测试用例,其他更复杂的操作将在以后出现).应该将状态列表返回到JSP以便在表单等中显示/使用...现在,我很高兴通过JSP页面获取数据打印以了解它是否存在.
但是,当我从JSP调用Java代码时,似乎结果对象返回为null,因此无法访问任何数据.我也尝试将数据作为ResultSet对象返回,但结果是一样的.我知道从SQL调用时存储过程是有效的.另外,我有Java代码作为响应doPost请求的servlet,并且工作正常...
我可能会缺少什么/无法理解？这可能吗？
package edu.XXXX.ais.userapp;
import java.sql.*;
import java.util.*;
public class DBQueries {
public static Vector<String> getStatesList() {
Vector<String> results = new Vector<String>();
Connection con = null;
try {
// Example of executing a stored procedure
Class.forName("com.mysql.jdbc.Driver");
con = DriverManager.getConnection ("jdbc:mysql://hostname:3306/dbName","username","password");
CallableStatement cs = con.prepareCall("{call getStatesList()}");
ResultSet rs = cs.getResultSet();
while (rs.next()) {
results.add(rs.getString(1));
}
} catch (SQLException e) {
System.err.println("Servlet could not display records." + e);
} catch (ClassNotFoundException e) {
System.err.println("JDBC driver not found." + e);
} finally {
try {
if (con != null) {
con.close();
con = null;
}
} catch (SQLException e) {
System.err.println(e);
}
}
return results;
}
}
调用上述Java代码的JSP页面(希望能够接收结果......)
<html>
<head>
<title>Foo</title>
</head>
<body>
<h3>List of states via a servlet call to the database</h3>
<%@ page import="java.util.*" %>
<%@ page import="edu.XXXX.ais.userapp.DBQueries" %>
<%
Vector rs = DBQueries.getStatesList();
if (rs != null)
for (int index = 0; index < rs.size(); index++) {
out.println("Name : " + (String) rs.get(index));
}
%>
</body>
</html>
产生的Tomcat错误如下:
org.apache.jasper.JasperException: An exception occurred processing JSP page /states.jsp at line 12
9: <%@ page import="edu.XXXX.ais.userapp.DBQueries" %>
10:
11: <%
12: Vector rs = DBQueries.getStatesList();
13: if (rs != null)
14: for (int index = 0; index < rs.size(); index++) {
15: out.println("Name : " + (String) rs.get(index));
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:568)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:470)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
root cause
java.lang.NullPointerException
edu.XXXX.ais.userapp.DBQueries.getStatesList(Unknown Source)
org.apache.jsp.states_jsp._jspService(states_jsp.java:75)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
该异常会告诉您在哪里查看:
edu.XXXX.ais.userapp.DBQueries.getStatesList(Unknown Source)
该方法中有一个空引用.我建议在没有JSP的情况下测试它并使用调试器.
你没有关闭你的Statement或ResultSet.这将导致悲伤.按照与创建相反的顺序关闭它们Connection.
首先,请停止在JSP中添加scriptlet代码.这是1999年的风格; 不要这样做.
正确的做法是学习JSTL,只让你的JSP显示任务.
让JSP与代表数据库的servlet进行通信.
使用应用服务器设置数据源.您的代码中不应包含连接信息.
暂时忘掉servlet和JSP.从数据层开始.
使用POJO写下它,测试它,让它工作,并把它放在一边.
然后编写一个接收HTTP请求的servlet,验证并绑定参数,使用数据访问层与数据库通信,然后将响应放在请求或会话范围内以显示页面.
它被称为Model-2 MVC.这是编写Java Web应用程序的方法.
我写的DBUtils类更像是这样的:
package edu.XXXX.ais.userapp;
import java.sql.*;
import java.util.*;
public class DBQueries {
public static List<String> getStates(Connection connection) throws SQLException {
List<String> states = new ArrayList<String>();
CallableStatement cs = null;
ResultSet rs = null;
try {
cs = connection.prepareCall("{call getStatesList()}");
boolean hasResultSet = cs.execute();
rs = cs.getResultSet();
while (rs.next()) {
states.add(rs.getString(1));
}
} finally {
close(rs);
close(cs);
}
return states;
}
public static void close(ResultSet rs) {
try {
if (rs != null) {
rs.close();
}
} catch (SQLException e) {
e.printStackTrace();
}
}
public static void close(Statement st) {
try {
if (st != null) {
st.close();
}
} catch (SQLException e) {
e.printStackTrace();
}
}
public static void close(Connection conn) {
try {
if (conn != null) {
conn.close();
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}
编写JUnit测试以证明您的DBQueries正常工作:
package edu.XXX.ais.userapp;
public class DBQueriesTest {
// You have to initialize this; I'm deliberately leaving out some details.
private Connection connection;
@Test
public void testGetStates() {
List<String> actual = DbQueries.getStates(this.connection);
List<String> expected = { "CT", "NY" }; // whatever
Assert.assertEquals(expected, actual);
}
}
| 归档时间： |
|
| 查看次数： |
1885 次 |
| 最近记录： |