通用分页

1. PageBean
   分页三要素
   page 页码 视图层传递过来
   rows 页大小 视图层传递过来
   total 总记录数 后台查出来
   pagination 是否分页 视图层传递过来
   select * from t_mvc_book where bname like ‘%斗破%’;
   拿到数据库所对应的值
   
   连接数据库的帮助类

package util;

import java.io.InputStream;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Properties;

/**
 * 提供了一组获得或关闭数据库对象的方法
 * 
 */
public class DBAccess {
	private static String driver;
	private static String url;
	private static String user;
	private static String password;

	static {// 静态块执行一次，加载 驱动一次
		try {
			InputStream is = DBAccess.class
					.getResourceAsStream("config.properties");

			Properties properties = new Properties();
			properties.load(is);

			driver = properties.getProperty("driver");
			url = properties.getProperty("url");
			user = properties.getProperty("user");
			password = properties.getProperty("pwd");

			Class.forName(driver);
		} catch (Exception e) {
			e.printStackTrace();
			throw new RuntimeException(e);
		}
	}

	/**
	 * 获得数据连接对象
	 * 
	 * @return
	 */
	public static Connection getConnection() {
		try {
			Connection conn = DriverManager.getConnection(url, user, password);
			return conn;
		} catch (SQLException e) {
			e.printStackTrace();
			throw new RuntimeException(e);
		}
	}

	public static void close(ResultSet rs) {
		if (null != rs) {
			try {
				rs.close();
			} catch (SQLException e) {
				e.printStackTrace();
				throw new RuntimeException(e);
			}
		}
	}

	public static void close(Statement stmt) {
		if (null != stmt) {
			try {
				stmt.close();
			} catch (SQLException e) {
				e.printStackTrace();
				throw new RuntimeException(e);
			}
		}
	}

	public static void close(Connection conn) {
		if (null != conn) {
			try {
				conn.close();
			} catch (SQLException e) {
				e.printStackTrace();
				throw new RuntimeException(e);
			}
		}
	}

	public static void close(Connection conn, Statement stmt, ResultSet rs) {
		close(rs);
		close(stmt);
		close(conn);
	}

	public static boolean isOracle() {
		return "oracle.jdbc.driver.OracleDriver".equals(driver);
	}

	public static boolean isSQLServer() {
		return "com.microsoft.sqlserver.jdbc.SQLServerDriver".equals(driver);
	}
	
	public static boolean isMysql() {
		return "com.mysql.jdbc.Driver".equals(driver);
	}

	public static void main(String[] args) {
		Connection conn = DBAccess.getConnection();
		DBAccess.close(conn);
		System.out.println("isOracle：" + isOracle());
		System.out.println("isSQLServer：" + isSQLServer());
		System.out.println("isMysql：" + isMysql());
		System.out.println("数据库连接(关闭)成功");
	}
}

package util;
/**
 * 分页工具类
 */
public class PageBean {
	private int page = 1;// 页码
	private int rows = 10;// 页大小
	private int total = 0;// 总记录数
	private boolean pagination = true;// 是否分页
	public PageBean() {
		super();
	}
	public int getPage() {
		return page;
	}
	public void setPage(int page) {
		this.page = page;
	}
	public int getRows() {
		return rows;
	}
	public void setRows(int rows) {
		this.rows = rows;
	}
	public int getTotal() {
		return total;
	}
	public void setTotal(int total) {
		this.total = total;
	}
	public void setTotal(String total) {
		this.total = Integer.parseInt(total);
	}
	public boolean isPagination() {
		return pagination;
	}
	public void setPagination(boolean pagination) {
		this.pagination = pagination;
	}
	public int getStartIndex() {
		return (this.page - 1) * this.rows;
	}
	@Override
	public String toString() {
		return "PageBean [page=" + page + ", rows=" + rows + ", total=" + total + ", pagination=" + pagination + "]";
	}
}

– 符合条件的记录数

select count(1) from (select * from t_mvc_book where bname like '%斗破%') t;

–分页的查询语句

select * from t_mvc_book where bname like '%斗破%' limit start,offset

start 查询数据集的起始下标
offset：查询展示在当前页的记录

2. 后台
   2.1 entity
   2.2 dao
   第一次查满足条件的总记录数
   第二次查指定页码并满足条件的记录
   二次查询的条件要一致
   普通查询方法：

	public List<Book> list(Book book,PageBean pageBean) throws SQLException{
		String sql="select * from t_mvc_book where true";
		String bname=book.getBname();
		if (StringUtils.isNotBlank(bname)) {
			//为空就拼接下面的代码，不为空就拼
			sql +=" and bname like '%"+bname+"%'";
		}
		Connection con=DBAccess.getConnection();
		PreparedStatement ps = con.prepareStatement(sql);
		ResultSet rs =ps.executeQuery();
		List<Book> list=new ArrayList<>();
		while(rs.next()) {
			list.add(new Book(rs.getInt("bid"),rs.getString("bname"),rs.getFloat("price")));
		}
		关流
		DBAccess.close(con, ps, rs);
		return list;
	}

建立一个通用分页的BaseDao将查询方法进行反射优化，使其变成一个可以被所有实体类dao层所继承的通用查询方法

package Dao;
import java.lang.reflect.Field;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import entity.Book;
import util.DBAccess;
import util.PageBean;
import util.StringUtils;

public class BaseDao<T>{

	public List<T> executeQuery(String sql,Class clz,PageBean pageBean) throws SQLException, InstantiationException, IllegalAccessException{
		Connection con=DBAccess.getConnection();
		PreparedStatement ps=null;
		ResultSet rs=null;
		if(pageBean!=null && pageBean.isPagination()) {
			//需要分页 总记录数
			String countSql=getCountSql(sql);
			ps=con.prepareStatement(countSql);
			rs=ps.executeQuery();
			if (rs.next()) {
				pageBean.setTotal(rs.getLong(1)+"");
			}
			//查询符合条件的结果集
			  String pageSql=getPageSql(sql,pageBean);
			  ps = con.prepareStatement(pageSql);
			  
			  rs =ps.executeQuery();
		}else {
			ps = con.prepareStatement(sql);
			rs =ps.executeQuery();
		}
		List<T> list=new ArrayList<>();
		T t;
		while(rs.next()) {
			//实例化book对象，对象是空的
			t = (T) clz.newInstance();
			Field[] fields = clz.getDeclaredFields();
			for (Field field : fields) {
				field.setAccessible(true);
				field.set(t,rs.getObject(field.getName()));
			}
			list.add(t);
//			list.add(new Book(rs.getInt("bid"),rs.getString("bname"),rs.getFloat("price")));
		}
		//关流
		DBAccess.close(con, ps, rs);
		return list;
}
	private String getPageSql(String sql, PageBean pageBean) {
		// TODO Auto-generated method stub
		return sql+" limit "+pageBean.getStartIndex()+","+pageBean.getRows();
	}
	//获取符合条件的总记录数的sql语句
	private String getCountSql(String sql) {
		// TODO Auto-generated method stub
		return "select count(1) from ("+sql+") t";
	}
}

反射优化后的查询方法

package Dao;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import entity.Book;
import util.DBAccess;
import util.PageBean;
import util.StringUtils;
public class BookDao extends BaseDao<Book>{
	public List<Book> list(Book book,PageBean pageBean) throws SQLException, InstantiationException, IllegalAccessException{
	String sql="select * from t_mvc_book where true";
	String bname=book.getBname();
	if (StringUtils.isNotBlank(bname)) {
		//为空就拼接下面的代码，不为空就拼
		sql +=" and bname like '%"+bname+"%'";
	}
	return super.executeQuery(sql,Book.class, pageBean);
}	
	public static void main(String[] args) throws InstantiationException, IllegalAccessException {
		BookDao bookDao=new BookDao();
		Book book=new Book();
		PageBean pageBean2=new PageBean();
		pageBean2.setPage(5);
		book.setBname("圣墟");
		try {
			List<Book> list = bookDao.list(book, pageBean2);
			for (Book b : list) {
				System.out.println(b);
			}
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

运行结果

不分页