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第一天：java与mysql的连接工具类

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07/24
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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();
}
}}