General paging - background

Catalog

One 、 generic class BaseDao

Two 、 concrete class

3、 ... and 、 test

---

The focus of general paging is general , Today I'd like to share my understanding of Ge , Paging is divided into foreground and background code , Today, we bring the background code

One 、 generic class BaseDao

1. Get all the methods  executeQuery

Three parameters are defined in this method , Respectively, get the original sql Statements and encapsulated paging classes pageBean And a return function interface for processing the result set callBack

In determining whether to page , If you want to page, call this class to get the total number of records CountSQL And how many pieces of data per page PageSQL, Execute the original without paging sql sentence

public List<T> executeQuery( String sql,PageBean pagebean,CallBack<T> callBack) throws Exception{
		
		/**
		 * 1. Get the database connection 
		 * 2. Get preparstatement
		 * 3. perform sql sentence 
		 */
		
		Connection con = DBAccess.getConnection();// Duplicate code 1
		
		PreparedStatement ps = con.prepareStatement(sql);// Duplicate code 2
		ResultSet rs = ps.executeQuery();// Duplicate code 3
		
		if(pagebean!=null&&pagebean.isPagination()) {
			String countsql = getCountsql(sql);
			 con = DBAccess.getConnection();
			 ps = con.prepareStatement(countsql);
			 rs = ps.executeQuery();
			 if(rs.next()) {
				 
				 pagebean.setTotal(rs.getString("n"));
				 
			 }
			 String pageSql= getpageSql(sql,pagebean);
			 con = DBAccess.getConnection();
			 ps = con.prepareStatement(pageSql);
			 rs = ps.executeQuery();
		}else {
			 con = DBAccess.getConnection();
			 ps = con.prepareStatement(sql);
			 rs = ps.executeQuery();
		}
		// Querying different tables must deal with different result sets 
		return callBack.foreach(rs);
	}

The reason why it is universal ：

In the collection, replace the specific entity class with letters T It shows that this is a general-purpose entity class that can accommodate any entity class we need , The second is to return the function interface class Collback How to process the result set , Whoever calls this interface class will process the result set

2、 Get the total number of records CountSQL

This is a general method to obtain the total data in the database

/**
	 *  The total number of records that meet the conditions is Sql
	 * @param sql
	 * @return
	 */
	private String getCountsql(String sql) {
		// TODO Auto-generated method stub
		return "select count(1) as n from ("+sql+") t";
	}

3、 How many pieces of data per page PageSQL

Get the original sql The method of obtaining subscript and total row number in the page of statement splicing gives a general method of displaying data on the page

/**
	 *  Splice section N Of page data sql
	 * @param sql
	 * @param pagebean
	 * @return
	 */
	private String getpageSql(String sql, PageBean pagebean) {
	
		return sql +"limit"+pagebean.getStartIndex()+","+pagebean.getRows();
	}

Two 、 concrete class

Take books as an example , We are reading books Dao in Inherit BaseDao, We just need to return return BaseDao Get all the methods in  executeQuery, At the same time, we get three parameters sql,pagebean, as well as CallBack, We just need to put CallBack Replace with the result set of the book , Then traverse the result set of the book

public List<Book> list2(Book book,PageBean pagebean) throws Exception{
		List<Book> ls = new ArrayList<Book>();
		/**
		 * 1. Get the database connection 
		 * 2. Get preparstatement
		 * 3. perform sql sentence 
		 */
		
		
		String sql = "select * from t_mvc_book where 1=1";
		String bname = book.getBname();
		if(StringUtils.isNotBlank(bname)) {
			sql +="and bname like'%"+bname+"%'";
		}
		int bid = book.getBid();
		if(bid!=0) {
			sql +=" and bid = " +bid;
		}
		
		return super.executeQuery(sql, pagebean, rs->{
			List<Book> list = new ArrayList<>();
			try {
				while(rs.next()) {
					ls.add(new Book(rs.getInt("bid"), rs.getString("bname"), rs.getFloat("price")));
				}
			} catch (SQLException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			
			return list;
		});
	}
	

Next, I will share a method that does not call general methods , Compare it with the general method we call

public List<Book> list(Book book,PageBean pagebean) throws Exception{
		List<Book> ls = new ArrayList<Book>();
		/**
		 * 1. Get the database connection 
		 * 2. Get preparstatement
		 * 3. perform sql sentence 
		 */
		
		Connection con = DBAccess.getConnection();// Duplicate code 1
		String sql = "select * from t_mvc_book where 1=1";
		String bname = book.getBname();
		if(StringUtils.isNotBlank(bname)) {
			sql +="and bname like'%"+bname+"%'";
		}
		int bid = book.getBid();
		if(bid!=0) {
			sql +=" and bid = " +bid;
		}
		PreparedStatement ps = con.prepareStatement(sql);// Duplicate code 2
		ResultSet rs = ps.executeQuery();// Duplicate code 3
		while(rs.next()) {
			ls.add(new Book(rs.getInt("bid"), rs.getString("bname"), rs.getFloat("price")));
		}
		return ls;
	}

It can be seen with the naked eye that there are a lot of duplicate code , And we call general methods to greatly reduce the duplication of code , It improves our development speed and efficiency , This is the benefit of GM

3、 ... and 、 test

  It uses Junit, It can be tested according to a single method , be relative to main In terms of method , The coupling is reduced

There are three ways , Before testing SetUP, After testing TearDown, And one we tested inside TestList Method

Here we only need to instantiate entity classes and paging classes , You can also set the number of pages and total rows by yourself , Then call our method , It's so convenient ！ There are three ways to distinguish by marking notes

package com.zjy.dao;

import static org.junit.Assert.*;

import java.util.ArrayList;
import java.util.List;
/**
 * junit  Be able to test according to a single method 
 *  Compare with main In terms of method , The coupling is reduced 
 * 
 */
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import com.zjy.entity.Book;
import com.zjy.util.BaseDao;
import com.zjy.util.PageBean;

public class BookDaoTest {

	@Before
	public void setUp() throws Exception {
		System.out.println(" Call before the tested method executes ");
	}

	@After
	public void tearDown() throws Exception {
		System.out.println(" Call after the tested method is executed ");
	}

	/**
	 *  Do not use general methods or paging 
	 */
	@Test
	public void testList() {
		List<Book> list;
		
		try {
			list = new BookDao().list(new Book(), null);
			for (Book book : list) {
				System.out.println(book);
			}
		
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
	}
	
	/**
	 *  Use common methods but not paging 
	 */
	@Test
	public void testList2() {
		List<Book> list;
		
		try {
			list = new BookDao().list2(new Book(), null);
			for (Book book : list) {
				System.out.println(book);
			}
		
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
	}
	
	/**
	 *  Use general methods and paging  
	 */
	@Test
	public void testList3() {
		List<Book> list;
		
		try {
			Book b = new Book();
			PageBean pageBean = new PageBean();
			pageBean.setPage(2);
			pageBean.setRows(20);
			list = new BookDao().list2(b, pageBean);
			for (Book book : list) {
				System.out.println(book);
			}
		
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
	}

}

That's all for today. Bye ~