04 jdbc中关闭连接的方式

之前在02jdbc web项目使用jdbc示例及不同版本调用jdbc的差异https://blog.csdn.net/sinat_41132860/article/details/84981142

中调用jdbc的方式是有问题的,当时的代码如下

import java.sql.Connection;
 
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
 
public class JdbcTest01 {
	public static void main(String[]args) throws Exception {
        //注册驱动,以反射方式加载驱动
		Class.forName("com.mysql.cj.jdbc.Driver");
        //获取连接Connection
		Connection conn=DriverManager.getConnection("jdbc:mysql://localhost:3306/study1?useSSL=false&serverTimezone=UTC", "root", "root");
        ///得到执行sequel语句的对象Statement
	    Statement stmt=conn.createStatement();
        //执行sql语句，并返回结果
	    ResultSet rs=stmt.executeQuery("select * from employee");
        ////处理结果
	    while(rs.next()) {
	    	System.out.println(rs.getObject("employeenumber"));
	    	System.out.println(rs.getObject("employeename"));
	    	System.out.println(rs.getObject("departmentnumber"));
	    	System.out.println(rs.getObject("salary"));
	    	System.out.println(rs.getObject("hiredate"));
	    	System.out.println("-================");
	    }
        //关闭Connection
	    rs.close();
	    stmt.close();
	    conn.close();
	}
}

 如果在关闭连接之前代码抛出异常,那么显然关闭连接的代码不会再执行

解决此问题的两种方式

1 为了保证一定能关闭连接,可以讲关闭连接的代码放到finally代码块中,如下:

package jdbc;

import java.sql.Connection;

import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
 
public class JdbcTest01 {
	public static void main(String[]args)  {
		Connection conn=null;
		Statement stmt=null;
		ResultSet rs=null;
        //注册驱动,以反射方式加载驱动
		try {
			Class.forName("com.mysql.cj.jdbc.Driver");
			//获取连接Connection
			conn=DriverManager.getConnection("jdbc:mysql://localhost:3306/study1?useSSL=false&serverTimezone=UTC", "root", "root");
	        ///得到执行sequel语句的对象Statement
		    stmt=conn.createStatement();
	        //执行sql语句，并返回结果
		    rs=stmt.executeQuery("select * from employee");
	        ////处理结果
		    while(rs.next()) {
		    	System.out.println(rs.getObject("employeenumber"));
		    	System.out.println(rs.getObject("employeename"));
		    	System.out.println(rs.getObject("departmentnumber"));
		    	System.out.println(rs.getObject("salary"));
		    	System.out.println(rs.getObject("hiredate"));
		    	System.out.println("-================");
		    }
		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}finally {
			 //关闭Connection
		    try {
		    	if (rs!=null) {
		    		rs.close();
				}
				if (stmt!=null) {
					stmt.close();
				}
				if (conn!=null) {
					conn.close();
				}
			    
			} catch (SQLException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		    
		}
        
       
	}
}

说明

1 try语句块内部声明的变量,try语句块访问不到,所以为了保证finally语句块中能访问到rs,stmt,conn要在try语句块外部声明

2 关闭资源前检查资源是否为空,若不为空才需要关闭,如果直接关闭资源,当上面某一步出现异常而资源为null时,会出现空指针异常

2 jdk7和 JDBC4.1之后的正确关闭资源

Connection、Statement、ResultSet都继承了AutoCloseable接口，因此可以使用try-with-resources的方式关闭这些资源,即之前IO流时用的jdk新写法,不用显示关闭资源：

package jdbc;

import java.sql.Connection;

import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
 
public class JdbcTest01 {
	public static void main(String[]args) {
        //注册驱动,以反射方式加载驱动
		try {
			Class.forName("com.mysql.cj.jdbc.Driver");
		} catch (ClassNotFoundException e1) {
			e1.printStackTrace();
		}
		
		try (
				//获取连接Connection
				Connection conn=DriverManager.getConnection("jdbc:mysql://localhost:3306/study1?useSSL=false&serverTimezone=UTC", "root", "root");
		        ///得到执行sequel语句的对象Statement
			    Statement stmt=conn.createStatement();
		        //执行sql语句，并返回结果
			    ResultSet rs=stmt.executeQuery("select * from employee");
		        //处理结果
								){
			  while(rs.next()) {
			    	System.out.println(rs.getObject("employeenumber"));
			    	System.out.println(rs.getObject("employeename"));
			    	System.out.println(rs.getObject("departmentnumber"));
			    	System.out.println(rs.getObject("salary"));
			    	System.out.println(rs.getObject("hiredate"));
			    	System.out.println("-================");
			    }
		} catch (SQLException e1) {
			e1.printStackTrace();
		}
            
	}
}

说明:try后面小括号中最后一行代码的分号加不加都可以