【java】对数据库操作的那些事(包含数据库中的预处理)
一、连接问题
前面刚介绍了怎么连接数据库,也写了对应的模板。可是它的可维护性很差。那么怎么解决问题呢?
首先写一个配置文件jdbc.properties
<span style="font-size:18px;">## MySQL driver=com.mysql.jdbc.Driver url=jdbc:mysql://localhost:3306/hncu?useUnicode=true&characterEncoding=UTF-8 username=root password=1234 ## Oracle #driver=oracle.jdbc.driver.OracleDriver #url=jdbc:oracle:thin:@192.168.31.12:1521:orcl #username=scott #password=tiger </span>
然后创建一个生成连接的工厂ConnFactory .java
<span style="font-size:18px;">package cn.hncu.hibernate0; import java.io.IOException; import java.sql.Connection; import java.sql.DriverManager; import java.util.Properties; public class ConnFactory { private static Connection conn; static { try { //读取配置文件 Properties p = new Properties(); p.load(ConnFactory.class.getClassLoader().getResourceAsStream("jdbc.properties")); String driver = p.getProperty("driver"); String url = p.getProperty("url"); String username = p.getProperty("username"); String pwd = p.getProperty("password"); Class.forName(driver); conn = DriverManager.getConnection(url,username,pwd); System.out.println("已连接到数据库..."+conn); } catch (Exception e) { throw new RuntimeException("读取配置文件失败", e); } } public static Connection getConn(){ return conn; } public static void main(String[] args) { getConn(); } } </span>最后直接通过ConnFactory.getConn()获得。
这要做的优点,当改变所要连接的数据库类型时,仅仅须要改动配置文件里的内容就可以。
二、解说getXXX()方法
<span style="font-size:18px;"><span style="white-space:pre"> </span>@Test public void getXXXDemo() throws Exception{ Statement st = ConnFactory.getConn().createStatement(); String sql = "select * from book"; ResultSet rs = st.executeQuery(sql); while(rs.next()){ Integer id = rs.getInt(1);//这里的1表示数据表中的第一列,以下同理 String name = rs.getString(2); //Double price = (Double)rs.getObject(3);//出异常,由于内部是採用BigDecimal来处理 Double price = rs.getDouble(3); Object dateTime = rs.getObject(4);//把日期和时间作为一个总体读取出来 System.out.println(id+","+name+","+price+","+dateTime); String strDateTime = dateTime.toString(); System.out.println(strDateTime); strDateTime = rs.getDate(4)+"--"+rs.getTime(4);//日期和时间能够单独获取 System.out.println(":::"+strDateTime); } ConnFactory.getConn().close(); }</span>注:对于自己主动增长列。删除之后再插入新记录,序号不会回头,继续往前增长。即中间会出现空号
三、解说Statement中的三个executeXXX()方法
1、executeQuery: 仅仅能运行select语句
2、executeUpdate: 能够运行insert、delete和update语句,但不能运行select
3、execute:增删改查的4种(随意)语句都能运行。该方法若运行非select语句时返回false,运行select语句时返回true,且st对象会缓存该次查询的结果。我
们可通过ResultSet rs = st.getResultSet()来获得结果集
<span style="font-size:18px;"><span style="white-space:pre"> </span>@Test public void executeXXXDemo() throws Exception{ Statement st = ConnFactory.getConn().createStatement(); String sql = "select * from book"; //String sql = "insert into book(name,price,pub) values('软件project',22.35,'2015-12-05 22:12:23')"; //String sql = "update book set price=38.88 where name='软件project'"; //String sql = "delete from book where name='软件project'"; //st.executeQuery(sql); //st.executeUpdate(sql); boolean boo = st.execute(sql); if(boo){ ResultSet rs = st.getResultSet(); while(rs.next()){ System.out.println(rs.getObject(2)); } } }</span>
四、数据库查询时防黑技术(预处理语句)
案例、用户登录(通过用户输入信息来拼接sql语句----非常危急)
<span style="font-size:18px;"><span style="white-space:pre"> </span>@Test//用户登录 public void login() throws Exception{ Connection con = ConnFactory.getConn(); Statement st = con.createStatement(); Scanner sc = new Scanner(System.in); int id = sc.nextInt(); sc.nextLine(); String name = sc.nextLine(); String sql = "select count(*) from stud where id="+id+" and sname='"+name+"'"; System.out.println("sql:"+sql); ResultSet rs = st.executeQuery(sql); rs.next(); int a = rs.getInt(1); if(a<=0){ System.out.println("登录不成功"); }else{ System.out.println("登录成功"); } con.close(); }</span>黑的方法。输入:1002(回车) 1' or '1'='1
因此:假设须要用 用户输入的信息 来拼接 sql语句,就不能用statement。否则用户能够通过构建where子句中的一个true条件来突破防护对于上面的情
况,应该用PreparedStatement来解决!
<span style="font-size:18px;"><span style="white-space:pre"> </span>@Test//用户登录 黑:1002(回车) 1' or '1'='1 public void login2() throws Exception{ Scanner sc = new Scanner(System.in); Connection con = ConnFactory.getConn(); String sql = "select count(*) from stud where id=? and sname=?";//须要用户输入的地方,用占位符('?')来取代。然后在兴许通过设參来给占位符赋值 PreparedStatement pst = con.prepareStatement(sql); //设置參数 int id = sc.nextInt(); sc.nextLine(); pst.setInt(1, id); //參数1----代表第1个占位符 String name = sc.nextLine(); pst.setString(2, name);//參数2 ResultSet rs = pst.executeQuery(); rs.next(); int a = rs.getInt(1); if(a<=0){ System.out.println("登录不成功"); }else{ System.out.println("登录成功"); } con.close(); }</span>
五、获取自己主动增长列(如id)
<span style="font-size:18px;"><span style="white-space:pre"> </span>@Test //演示获取自己主动增长列如id public void saveAuto() throws Exception{ Connection con = ConnFactory.getConn(); String sql = "insert into book(name,price,pub) values('JavaEE',100.8,'2013-06-12 08:30:30')"; Statement st = con.createStatement(); //st.executeUpdate(sql); st.executeUpdate(sql,Statement.RETURN_GENERATED_KEYS); ResultSet rs = st.getGeneratedKeys();//里面封装了自己主动生成的全部值 if(rs.next()){ int id = rs.getInt(1);//获取第1个自己主动增长列 System.out.println("自己主动增长的id:"+id); } System.out.println("-----------------"); //预处理语句 sql = "insert into book(name,price) values(?,?)"; PreparedStatement pst = con.prepareStatement(sql,Statement.RETURN_GENERATED_KEYS); pst.setString(1, "计算机基础"); pst.setDouble(2, 28); pst.executeUpdate(); ResultSet rs2 = pst.getGeneratedKeys();//里面封装了自己主动生成的全部值 if(rs2.next()){ int id = rs2.getInt(1);//获取第1个自己主动增长列 System.out.println("自己主动增长的id:"+id); } }</span>
六、演示批处理语句
public void batch() throws Exception{ Connection con = ConnFactory.getConn(); String sql = "insert into book(name,price,pub) values('JavaEE',100.8,'2015-06-12 08:30:30')"; Statement st = con.createStatement(); for(int i=0;i<10;i++){ st.addBatch(sql); } sql = "update book set price=price*1.1 where price>100"; st.addBatch(sql); int[] a = st.executeBatch(); for(int r:a){ System.out.println(r);//r为每条sql语句所影响的记录数 } }
预处理
public void preparedBatch() throws Exception{ Connection con = ConnFactory.getConn(); String sql = "insert into book(name,price,pub) values(?,?,?)"; PreparedStatement pst = con.prepareStatement(sql); for(int i=0;i<5;i++){ pst.setString(1, "Java"+i); pst.setDouble(2, 55.85+i); pst.setString(3, "2016-12-10 07:07:08"); pst.addBatch(); //pst的方式不能带參数sql } //pst.executeBatch(); int[] a = pst.executeBatch(); for(int r:a){ System.out.println(r);//r为每条sql语句所影响的记录数 } }
注意:预处理的方式不能带參数sql,普通的须要