第一天：java与mysql的连接工具类

　　　　　　　　　　　　　　　　　　　　　　　　　　第一天：java与mysql的连接工具类

java最新版马上就要收费，这无疑是这门语言的衰败起始，毕竟在中国收费便难发展，例如c#，但是毕业设计已经选好用java来写一个动态网站，               

这已经是一个事实，还是得学，好在一法通万法通，不至于一无所获。

首先我们要把连接数据库的工具类写好，这里面无非就那么几个固定的对象、语句，

第一步，我们需要导包，进入maven随便选择一个版本下载就是，网址是https://mvnrepository.com/artifact/mysql/mysql-connector-java

然后把jar包复制到java动态网站下的lib文件夹里就是了，位置在/项目名/WebContent/WEB-INF/lib/mysql-connector-java-5.1.24-bin.jar

接下来就可以写连接类了，来个最简单的：

public class Jdbc {
    public static final String URL = "jdbc:mysql://localhost:3306/test";
    public static final String USER = "root";
    public static final String PWD = "123456";
    
    public static void main(String[] args) {
        //update();
        query();
    }    
    public static void update() {        
        Connection conn = null;        
        Statement stmt =null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        try {
            //加载驱动类
            Class.forName("com.mysql.jdbc.Driver");
            //与数据库建立连接
            conn = (Connection) DriverManager.getConnection(URL,USER,PWD);
            //执行sql
            //1.statement
            /*stmt = conn.createStatement();
            String sql = "insert into user values(2,'qzj',123) ";
            int count = stmt.executeUpdate(sql);*/
            //2.prepareStatement
            String sql = "insert into user values(?,?,?) ";
            pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, 3);
            pstmt.setString(2, "gg");
            pstmt.setInt(3, 22);
            int count = pstmt.executeUpdate();            
            if(count > 0) {
                System.out.println("操作成功！");
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        }catch (Exception e) {
            e.printStackTrace();
        }
        finally{
            try {
                if(stmt != null) stmt.close();
                if(conn != null) conn.close();
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        
    }    
public static void query() {        
        Connection conn = null;        
        Statement stmt =null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        try {
            //加载驱动类
            Class.forName("com.mysql.jdbc.Driver");
            //与数据库建立连接
            conn = (Connection) DriverManager.getConnection(URL,USER,PWD);
            //执行sql            
            String sql = "select * from user";
            /*stmt = conn.createStatement();
            rs = stmt.executeQuery(sql);*/
            pstmt = conn.prepareStatement(sql);            
            rs = pstmt.executeQuery();
            while(rs.next()) {
                int id = rs.getInt("id");
                String name = rs.getString("uname");
                String pwd = rs.getString("upwd");
                System.out.println(id+"--"+name+"--"+pwd);
            }            
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        }catch (Exception e) {
            e.printStackTrace();
        }
        finally{
            try {
                if(stmt != null) stmt.close();
                if(conn != null) conn.close();
                if(rs != null) rs.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        
    }    
}

基本上连接类中用到的就connection、statement、preparstatement、resultset四个对象，其中preparstatement是statement的子类，功能更多更好，

推荐优先使用preparstatement，理由如下：

1.编码更加简便（避免了字符串的拼接）

String name = "zs" ;
int age = 23 ;

stmt:
String sql =" insert into student(stuno,stuname) values('"+name+"',  "+age+" )    " ;
stmt.executeUpdate(sql);

pstmt:
String sql =" insert into student(stuno,stuname) values(?,?) " ;
pstmt = connection.prepareStatement(sql);//预编译SQL
pstmt.setString(1,name);
pstmt.setInt(2,age);

2.提高性能(因为 有预编译操作，预编译只需要执行一次)

需要重复增加100条数 
stmt:
String sql =" insert into student(stuno,stuname) values('"+name+"',  "+age+" )    " ;
for(100)
stmt.executeUpdate(sql);

pstmt:
String sql =" insert into student(stuno,stuname) values(?,?) " ;
pstmt = connection.prepareStatement(sql);//预编译SQL
pstmt.setString(1,name);
pstmt.setInt(2,age);
for( 100){
pstmt.executeUpdate();
}

3.安全（可以有效防止sql注入），何为sql注入，就是--将客户输入的内容  和 开发人员的SQL语句 混为一体

stmt:存在被sql注入的风险  

(例如输入  用户名：任意值 ' or 1=1 --

  密码：任意值)

分析：

当源代码像select count(*) from login where uname='"+name+"' and upwd ='"+pwd+"' 

用户有心捣乱就可以进行sql注入，输入恶意用户名就变成了以下后果：

select count(*) from login where uname='任意值 ' or 1=1 --' and upwd ='任意值'  ;//--是sql中的注释，后面语句被注释了

select count(*) from login where uname='任意值 ' or 1=1 ;//结果就变成了这样，因为or1=1，最终就正确了

select count(*) from login ;

pstmt:有效防止sql注入，推荐使用pstmt

除了以上直接输入连接信息外还可以写在一个/项目名/src/a.properties文件，再从a.properties取出来用，代码如下：

private static String url = null;
    private static String user = null;
    private static String password = null;
    private static String dv = null;

    static {
        Properties prop = new Properties();
        InputStream in = JdbcUtils.class.getResourceAsStream("/a.properties");
        
        try {
            prop.load(in);
            url = prop.getProperty("url");
            user = prop.getProperty("user");
            password= prop.getProperty("password");
            dv = prop.getProperty("driver");
            
            //加载驱动类
            try {
                Class.forName(dv);
            } catch (ClassNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }                
    }

a.properties文件语句如下：

url:jdbc:mysql://localhost:3306/diary?characterEncoding=utf8
user:root
password:123456
driver:com.mysql.jdbc.Driver

上面只是最简单的也是最原始的连接工具类，再来个比较高级通用的：

package jdbc.util;

import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.Connection;
//需要WebContent/WEB-INF/lib/mysql-connector-java-5.1.24-bin.jar
public class JdbcUtil {
    public static final String URL = "jdbc:mysql://localhost:3306/test";
    public static final String USER = "root";
    public static final String PWD = "123456";
    public static PreparedStatement pstmt = null ;
    public static Connection connection = null ;
    public static ResultSet rs = null ; 
    //通用增删改
    public static boolean executeUpdate(String sql,Object[]params){
        try {
            pstmt = createPreParedStatement(sql,params);
            int count = pstmt.executeUpdate();
            if(count>0) {
                return true;
            }else {
                return false;
            }
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return false;
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return false;
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return false;
        } finally {
            closeAll(null,pstmt,connection);
        }        
    }
    //通用查
    public static ResultSet executeQuery(String sql,Object[]params)  {
        //Student student = null;        
        //List<Student> students = new ArrayList<>();    
        try {
            pstmt = createPreParedStatement(sql,params);
            rs = pstmt.executeQuery();
            return rs;
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        }catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        }        
    }
    //导入驱动，加载具体的驱动类
    public static Connection getConnection() throws ClassNotFoundException, SQLException {
        Class.forName("com.mysql.jdbc.Driver");
        return DriverManager.getConnection(URL,USER,PWD);        
    }
    public static PreparedStatement createPreParedStatement(String sql,Object[] params) throws ClassNotFoundException, SQLException {
        pstmt = getConnection().prepareStatement(sql);
        if(pstmt != null) {
            for(int i=0;i<params.length;i++) {
                //循环赋值，类似pstmt.setint(1,id)
                pstmt.setObject(i+1, params[i]);
            }
        }
        return pstmt;        
    }    
    public static void closeAll(ResultSet rs,Statement stmt,Connection connection)
    {
        try {
            if(rs!=null)rs.close();
            if(pstmt!=null)pstmt.close();
            if(connection!=null)connection.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }         
    }
    
    
    
    
}