day17 9.关闭资源与异常处理

Java程序跟任何外部设备进行连接之后,都要把连接断开,把资源释放掉。Connection是一个重量级资源,Connecton占内存,Connection的获取是比较消耗资源和内存的。finally是一定会被执行,刚才玩的JDBC代码,异常都没处理直接抛出异常了，这是不负责任的行为。

package cn.itcast.jdbc;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

//对异常进行try-catch
public class JdbcDemo5 {
   public static void main(String[] args) {
	   Connection con = null;
	   Statement st = null;
	   ResultSet rs = null;

	   try {

		//1.注册驱动  

		Class.forName("com.mysql.jdbc.Driver");

		//2.获取连接
		try {
			 con = DriverManager.getConnection("jdbc:mysql:///day17", "root", "");

			//3.获取操作sql语句对象Statement
			 st =   con.createStatement();

		   //4.执行sql
		     rs = st.executeQuery("select * from user");

		   //5.遍历结果集
		    while(rs.next()){
		    	int id = rs.getInt("id");
		    	//String id = rs.getString("id");//虽然用getString()行,但是用getInt()比较合适
		    	String username = rs.getString("username");
		    	String password = rs.getString("password");
		    	String email = rs.getString("email");
		    	System.out.println(id+"   "+username+"   "+password+"   "+email);
		    }

		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

	} catch (ClassNotFoundException e) {
		// TODO Auto-generated catch block
		e.printStackTrace();
	}finally{
		  //6.释放资源
		  try {
			  if(rs !=null ){
			rs.close();
			  }
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		  try {
			  if(st!=null){
			st.close();
			  }
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
          try {
        	  if(con!=null){
			con.close();
        	  }
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	 }
}
}