如何在JSP中调用自己定义的类

[周靖峰]

我的文件目录为：

test/
----WEB-INF/
--------lib/
------------mysql-connector-java-5.1.7-bin.jar
--------classes/
------------ConnMysql.class
----index.jsp

ConnMysql.java

import java.sql.*;

public class ConnMysql {
	public static Connection getConnection(){
		try {
			Class.forName("com.mysql.jdbc.Driver");                     
			String url ="jdbc:mysql://localhost:3306/test?user=root&password=root&useUnicode=true&characterEncoding=GBK";
			return DriverManager.getConnection(url);
		} catch (Exception e) {
			System.out.print(e.getMessage());
			return null;
		}
    }
}

index.jsp

<%@page import="java.sql.*" %>
<%@page import="ConnMysql" %>
<%
	Connection conn = null;
	Statement stat = null;
	ResultSet rs = null ;

	try{
		conn = ConnMysql.getConnection();
		stat = conn.createStatement();
		rs = stat.executeQuery("SELECT * FROM test");
		while (rs.next()){
			out.print(rs.getString("id") + ":");  
			out.println(rs.getString("name"));
		}
	}catch(Exception e){
		out.println(e.getMessage());
	}
%>

但是打开index.jsp时显示错误

HTTP Status 500 - 

--------------------------------------------------------------------------------

type Exception report

message 

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

exception 

org.apache.jasper.JasperException: Unable to compile class for JSP: 

An error occurred at line: 7 in the generated java file
The import ConnMysql cannot be resolved

An error occurred at line: 9 in the jsp file: /index.jsp
ConnMysql cannot be resolved
6: 	ResultSet rs = null ;
7: 
8: 	try{
9: 		conn = ConnMysql.getConnection();
10: 		stat = conn.createStatement();
11: 		rs = stat.executeQuery("SELECT * FROM test");
12: 		while (rs.next()){


Stacktrace:
	org.apache.jasper.compiler.DefaultErrorHandler.javacError(DefaultErrorHandler.java:92)
	org.apache.jasper.compiler.ErrorDispatcher.javacError(ErrorDispatcher.java:330)
	org.apache.jasper.compiler.JDTCompiler.generateClass(JDTCompiler.java:439)
	org.apache.jasper.compiler.Compiler.compile(Compiler.java:349)
	org.apache.jasper.compiler.Compiler.compile(Compiler.java:327)
	org.apache.jasper.compiler.Compiler.compile(Compiler.java:314)
	org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:592)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:317)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:717)


note The full stack trace of the root cause is available in the Apache Tomcat/6.0.29 logs.


--------------------------------------------------------------------------------

Apache Tomcat/6.0.29

应该是说找不到ConnMysql这个类吧，请问各位高手们应该怎么做才能解决这个问题

[不止鱼]

stat = conn.createStatement();
这句话出错，不应该这么写
1.既然你用封装，应该在ConnMysql里定义好自己的conn方法，然后直接用ConnMysql.getStatement(conn)里调用，
2.而且你在index里没有关闭相关的资源，这些关闭的方法应该放在ConnMysql类里，然后直接传参调用之
3.catch里最好把各个Exception写明，如SQLException，等等，不应图省事只写个Exception，否则程序执行效率很低，特别在大型的程序之下，跟别人把Exception写清楚的那些程序相比，这种差距就会体现出来了

[coolbamboo2008]

ConnMysql初始化了么？

[周靖峰]

[Quote=引用 2 楼 coolbamboo2008 的回复:]
ConnMysql初始化了么？
[/Quote]
怎么初始化？

[周靖峰]

我照着1楼的说法改写了两个源文件

<%@page import="java.sql.*" %>
<%@page import="ConnMysql" %>
<%
	Connection conn = null;
	Statement stat = null;
	ResultSet rs = null ;

	try{
		stat = ConnMysql.getStatement();
		rs = stat.executeQuery("SELECT * FROM test");
		while (rs.next()){
			out.print(rs.getString("id") + ":");  
			out.println(rs.getString("name"));
		}
		rs.close();
		stat.close();
		conn.close();
	}catch(Exception e){
		out.println(e.getMessage());
	}
%>

import java.sql.*;

public class ConnMysql {
	public static Statement getStatement(){
		try {
			Class.forName("com.mysql.jdbc.Driver");                     
			String url ="jdbc:mysql://localhost:3306/test?user=root&password=root&useUnicode=true&characterEncoding=GBK";
			Connection conn = DriverManager.getConnection(url);
			return conn.createStatement();
		} catch (Exception e) {
			System.out.print(e.getMessage());
			return null;
		}
    }
}

对了，jsp的错误好像有两种
第一种：

HTTP Status 500 - 

--------------------------------------------------------------------------------

type Exception report

message 

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

exception 

org.apache.jasper.JasperException: An exception occurred processing JSP page /index.jsp at line 7

4: 	Connection conn = null;
5: 	Statement stat = null;
6: 	ResultSet rs = null ;
7: 
8: 	try{
9: 		stat = ConnMysql.getStatement();
10: 		rs = stat.executeQuery("SELECT * FROM test");


Stacktrace:
	org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:510)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:419)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:717)


root cause 

java.lang.NullPointerException
	org.apache.jsp.index_jsp._jspService(index_jsp.java:61)
	org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:377)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:717)


note The full stack trace of the root cause is available in the Apache Tomcat/6.0.29 logs.


--------------------------------------------------------------------------------

Apache Tomcat/6.0.29

第二种：

HTTP Status 500 - 

--------------------------------------------------------------------------------

type Exception report

message 

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

exception 

org.apache.jasper.JasperException: Unable to compile class for JSP: 

An error occurred at line: 7 in the generated java file
The import ConnMysql cannot be resolved

An error occurred at line: 9 in the jsp file: /index.jsp
ConnMysql cannot be resolved
6: 	ResultSet rs = null ;
7: 
8: 	try{
9: 		stat = ConnMysql.getStatement();
10: 		rs = stat.executeQuery("SELECT * FROM test");
11: 		while (rs.next()){
12: 			out.print(rs.getString("id") + ":");  


Stacktrace:
	org.apache.jasper.compiler.DefaultErrorHandler.javacError(DefaultErrorHandler.java:92)
	org.apache.jasper.compiler.ErrorDispatcher.javacError(ErrorDispatcher.java:330)
	org.apache.jasper.compiler.JDTCompiler.generateClass(JDTCompiler.java:439)
	org.apache.jasper.compiler.Compiler.compile(Compiler.java:349)
	org.apache.jasper.compiler.Compiler.compile(Compiler.java:327)
	org.apache.jasper.compiler.Compiler.compile(Compiler.java:314)
	org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:592)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:317)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:717)


note The full stack trace of the root cause is available in the Apache Tomcat/6.0.29 logs.


--------------------------------------------------------------------------------

Apache Tomcat/6.0.29

怎么错误还会变来变去？

[叮咚客]

试试新建一目录，将你的类考到目录下进行调用

[周靖峰]

算了，我还是自己看书的了，情况太复杂，不好讲了