第一天：java与mysql的链接工具类

　　　　　　　　　　　　　　　　　　　　　　　　　　第一天：java与mysql的链接工具类

java最新版立刻就要收费，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#，可是毕业设计已经选好用java来写一个动态网站，               

这已是一个事实，仍是得学，好在一法通万法通，不至于一无所得。

首先咱们要把链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，咱们须要导包，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

而后把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

接下来就能够写链接类了，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"); //与数据库创建链接
            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("操做成功！"); } } 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"); //与数据库创建链接
            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(); } } } }

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基本上链接类中用到的就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，理由以下：

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.提升性能(由于 有预编译操做，预编译只须要执行一次)

须要重复增长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（能够有效防止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+"' 

用户有心捣乱就能够进行sql注入，输入恶意用户名就变成了如下后果：

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，由于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

除了以上直接输入链接信息外还能够写在一个/项目名/src/a.properties文件，再从a.properties取出来用，代码以下：

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文件语句以下：

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链接工具类，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; //须要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，相似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(); } } }

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