总共就一个超链接连了一个servlet
servlet里的代码是:
package com.simple.mvc;
import java.io.IOException;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/getAllProject")
public class GetAllProjectServlet extends HttpServlet {
private static final long serialVersionUID = 2L;
protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
DAO dao=new DAO();
List<Project> prs=dao.getAllProject();
System.out.println(prs.size());
}
}
DAO里面的代码是:
package com.simple.mvc;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.ArrayList;
import java.util.List;
import org.junit.Test;
public class DAO {
@Test
public void test(){
DAO dao=new DAO();
List<Project> prs=dao.getAllProject();
System.out.println(prs.size());
}
public List<Project> getAllProject(){
List<Project> prs=new ArrayList<>();
Connection connection =null;
PreparedStatement preparedStatement=null;
ResultSet resultSet=null;
try {
connection=JDBCTools.getConnection();
String sql="SELECT p_id,p_name,t_name "+
"FROM project p,teacher t "+
"WHERE p.`t_id`=t.`t_id` AND p_if=2";
preparedStatement = connection.prepareStatement(sql);
resultSet = preparedStatement.executeQuery();
while(resultSet.next()){
int p_id=resultSet.getInt(1);
String p_name=resultSet.getString(2);
String t_name=resultSet.getString(3);
Project project = new Project(p_id, p_name,t_name);
prs.add(project);
}
} catch (Exception e) {
e.printStackTrace();
}finally{
JDBCTools.release(resultSet, preparedStatement, connection);
}
return prs;
}
}
在超链接访问servlet时就报java.lang.NullPointerException
我看了是因为List prs=dao.getAllProject();返回为null
但是为什么会返回为null
我在DAO写了一个测试单元,测试返回不为null呀,求大神解答
报错内容为
java.lang.NullPointerException
at java.util.Properties$LineReader.readLine(Properties.java:434)
at java.util.Properties.load0(Properties.java:353)
at java.util.Properties.load(Properties.java:341)
at com.simple.mvc.JDBCTools.getConnection(JDBCTools.java:44)
at com.simple.mvc.DAO.getAllProject(DAO.java:28)
at com.simple.mvc.GetAllProjectServlet.doGet(GetAllProjectServlet.java:22)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:624)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:110)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:962)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:445)0
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1115)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:637)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:745)