javaweb开发问题：java.lang.NullPointerException

求助问答
,
1 
tacktrace:
java.lang.NullPointerException
	at com.util.DBO.close(DBO.java:57)
	at com.bean.SystemBean.getSiteInfo(SystemBean.java:74)
	at org.apache.jsp.index_jsp._jspService(index_jsp.java:148)
	at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
	at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:466)
	at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:379)
	at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:327)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:197)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
	at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:687)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:360)
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:399)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:890)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1743)
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
	at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
	at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:748)



代码：
package com.util;

import java.sql.*;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;

import javax.activation.DataSource;
import java.sql.*;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import javax.naming.Context;
import javax.naming.InitialContext;

public class DBO {

	private Connection conn;
	private Statement stmt;
  	private DataSource ds;
	
	public DBO()
	{
	}

	/**
		打开数据库
	*/
	public void open() 
	{
		try 
		{
			Class.forName("com.mysql.cj.jdbc.Driver"); 
			conn=DriverManager.getConnection("jdbc:mysql://localhost:3306/jf?useSSL=false&serverTimezone=UTC","root","wangbo321");
			//Class.forName("com.microsoft.jdbc.sqlserver.SQLServerDriver");           
			//conn = DriverManager.getConnection("jdbc:microsoft:sqlserver://localhost:1433;databasename=xswz","sa","123");
			stmt=conn.createStatement();
			System.out.println("打开数据库连接");
		} 
		catch (Exception ex) 
		{
		System.err.println("打开数据库时出错: " + ex.getMessage());
		}
	}

	/**
		关闭数据库，将连接返还给连接池
	*/
	public void close() 
	{
		try 
		{
		
				
		//	connMgr.freeConnection("java", conn);
			conn.close();
			System.out.println ("释放连接");
		} 
		catch (SQLException ex) 
		{
			System.err.println("返还连接池出错: " + ex.getMessage());
		}
	}

	/**
		执行查询
	*/
	public ResultSet executeQuery(String sql) throws SQLException
	{
		ResultSet rs = null;
		

		rs = stmt.executeQuery(sql);
		System.out.println ("执行查询");
		return rs;
	}

	/**
		执行增删改
	*/
	public int executeUpdate(String sql) throws SQLException
	{
		int ret = 0;
		
	
		ret = stmt.executeUpdate(sql);
	
		System.out.println ("执行增删改");
		return ret;
	}

	/**
		将SQL语句加入到批处理
	*/
	public void addBatch(String sql) throws SQLException 
	{
		stmt.addBatch(sql);
	}

	/**
		执行批处理
	*/
	public int [] executeBatch() throws SQLException 
	{
		boolean isAuto=conn.getAutoCommit();
		
		conn.setAutoCommit(false);
		int [] updateCounts = stmt.executeBatch();
		
//		conn.commit();
		
//		conn.setAutoCommit(isAuto);
		//conn.setAutoCommit(true);
		return updateCounts;
	}
	public boolean getAutoCommit() throws SQLException
	{
		return conn.getAutoCommit();
	}
	public void setAutoCommit(boolean auto)  throws SQLException 
	{
		conn.setAutoCommit(auto);
	}
	
	public void commit() throws SQLException 
	{
		conn.commit();
//		this.close();
	}
	public void rollBack() throws SQLException 
	{
		conn.rollback();
//		this.close();
	}
	
}
2

这个方法里面空指针异常，57行。至于为什么空指针，没人知道。

3

有可能是Open的时候，发生了异常。导致conn对象是null。