DBUtils

上次学到dbcp或者c3p0可创建连接池，但是Connection创建了，用什么来访问数据呢，答案是：DBUtils进行CRUD操作即可。

QueryRunner类

顾名思义，该类是用来执行Query语句的。其中构造方法有

QueryRunner() //不带事务的话，用这个就可以啦。

QueryRunner(DataSource ds) //如果用到事务，则需要带datasource的构造器。

删除、追加、修改

Connection conn = DBUtils.getConnection();
String delete = "delete from student where name=?";
String insert = "insert into student (name) values(?)";
String modify = "update student set name=? where id=?";

//执行sql语句
QueryRunner queryRunner = new QueryRunner();
queryRunner.update(conn, insert, "hello");
queryRunner.update(conn, delete, "hello");

原始方式查询

String sql=“select name from student where id=？”;

DBUtils将PreparedStatement进行封装，不需要用户自己进行繁琐的处理。

在查询中只需要通过实现ResultSetHandler接口，实现handle方法，处理ResultSet即可。

query(Connection conn, String sql, ResultSetHandler<T> rsh, param1, param2)

public class DBUtilsTest {
public static void main(String[] args) throws Exception{
Connection conn = DAO.getConnection();
QueryRunner queryRunner = new QueryRunner();

String sql = "select name, sex, age from student where id=";
class MyResultSetHandler implements ResultSetHandler<List<Student>>{
@Override
public List<Student> handle(ResultSet arg0) throws SQLException {
List<Student> list = new ArrayList<Student>();
while(arg0.next())
{
Student s = new Student(arg0.getString(1), arg0.getString(2), arg0.getInt(3));
list.add(s);
}
return list;
}
}

List<Student> rs= queryRunner.query(conn, sql, new MyResultSetHandler());
for(Student s:rs){
System.out.println(s);
}
}
}

高级的BeanHandler举例

上面说的实现ResultSetHanlder，其实在DBUtils中已经有很多默认的实现方法，用起来更方便。

BeanHandler,返回记录中的第一条记录，并且转为指定的Bean对象。

※注意：由于BeanHandler是通过反射来创建对应类的，所以指定的类必须有一个public的无参构造器。

public class <span style="font-family: Arial, Helvetica, sans-serif;">DBUtilsTest </span>
{
public static void main(String[] args) throws Exception{
Connection conn = DAO.getConnection();
QueryRunner queryRunner = new QueryRunner();

String sql = "select name, sex, age from student where id=?";
Student rs= queryRunner.query(conn, sql, new BeanHandler<Student>(Student.class), 3);
System.out.println(rs);
}
}

BeanListHandler举例：

public class DBUtilsTest  {
public static void main(String[] args) throws Exception{
Connection conn = DAO.getConnection();
QueryRunner queryRunner = new QueryRunner();

String sql = "select name, sex, age from student";
List<Student> rs= queryRunner.query(conn, sql, new BeanListHandler<Student>(Student.class));
System.out.println(rs);
}
}

MapHandler,MapListHandler

也可以不用集合，用MapHandler,MapListHandler返回键值对的形式。代码和上面一样不再重复。

{name=student1, sex=Female, age=20}

ScalarHandler,返回一个单个值。

public class DBUtilsTest  {
public static void main(String[] args) throws Exception{
Connection conn = DAO.getConnection();
QueryRunner queryRunner = new QueryRunner();

String sql = "select count(id) from student";
Long rs= queryRunner.query(conn, sql, new ScalarHandler<Long>(), 3);
System.out.println(rs);
}
}

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