Java -- paging, java Paging

Java -- paging, java Paging
1.Why use paging?

A) when the data volume is large, one-time query of all data may cause some problems.

B) database pressure will be high

C) data display will be slow, resulting in low user experience

D) In order to display data more conveniently and reduce the pressure on the database, the paging technology is used.

2. How to Implement paging Query

A) implementation of paging query in MySql

-- Limit is used for paging query, followed by two numbers -- The first number indicates the index at the start of the query (starting from 0) -- The second number indicates the number of data queried each time. Select * from emp limit 10, 10;   -- The current page number (size) must be known) -- Select * from emp limit (page-1) * size, size;

3. Code implementation by PAGE

A) defines the paging object class, which is used to encapsulate paging-related attributes for subsequent paging implementation. The Code is as follows (the Getter and Setter methods are omitted)

PackageCom. bjsxt. pojo;   ImportJava. util. List;   Public ClassPagination <T> {   // Current page number Private IntPage; // How many data entries are displayed on each page Private IntSize; // Total number of records Private IntTotalCount; // Total number of pages Private IntTotalPage; // Start position of each query Private IntStart; // Whether the previous page exists Private BooleanHasPrev; // Previous Page Private IntPrevPage; // Whether the next page exists Private BooleanHasNext; // Next page Private IntNextPage; // Homepage Private IntFirstPage; // Last page Private IntLastPage; // Number of paging navigation Blocks Private IntNavCount; // Start position of the navigation Private IntNavBegin; // End position of the navigation Private IntNavEnd; // List set corresponding to the query result PrivateList <T> list;   PublicPagination (String page, String size,IntTotalCount ){ // Current page number This. Page =Null! = Page? Integer.ParseInt(Page): 1; // How many data entries are displayed on each page This. Size =Null! = Size? Integer.ParseInt(Size): 10; // Total number of records This. TotalCount = totalCount; // Total number of pages This. TotalPage = (Int) Math.Ceil(This. TotalCount * 1.0/This. Size ); // The current page number cannot exceed the total page number If(This. Page>This. TotalPage &&This. TotalPage> 0 ){ This. Page =This. TotalPage; } // Start position of each query This. Start = (This. Page-1 )*This. Size; If(This. Page> 1 ){ // Whether the previous page exists This. HasPrev =True; // Previous Page This. PrevPage =This. Page-1; } If(This. Page <This. TotalPage ){ // Whether the next page exists This. HasNext =True; // Next page This. NextPage =This. Page + 1; } // Homepage This. FirstPage = 1; // Last page This. LastPage =This. TotalPage; // Number of paging navigation Blocks This. NavCount = 10; // Start position of the navigation // If (this. page> this. totalPage-this. navCount ){ // This. navBegin = this. totalPage-this. navCount + 1; //} Else { This. NavBegin =This. Page-This. NavCount/2 <1? 1:This. Page-This. NavCount/2; //} // End position of the navigation This. NavEnd =This. NavBegin +This. NavCount-1>This. TotalPage?This. TotalPage :This. NavBegin +This. NavCount-1; } // The getter and setter methods are omitted. }

B) In the object class, a constructor is provided, which has three parameters: the current page, the number of lines displayed on each page, size, and total number of records totalCount. These three parameters are provided, the reason is that all the data related to the page can be determined as long as you know the three data items.

C) Use of the paging object class. The Servlet receives relevant parameters and encapsulates them as object class objects. Then, the service is called and dao is called. The Code is as follows:

Public VoidQueryAll (HttpServletRequest req, HttpServletResponse resp)ThrowsServletException, IOException { String page = req. getParameter ("page "); String size = req. getParameter ("size "); IntTotalCount = es. queryCount (); // Create a paging object Pagination <Emp> p =NewPagination <Emp> (page, size, totalCount ); // Call the method in the service. After the query is completed, the query result is automatically set to the p object. Es. queryAll (p ); // Store the p object to the scope for use in jsp Req. setAttribute ("p", p ); // Request forwarding Req. getRequestDispatcher ("/emplist. jsp"). forward (req, resp ); }

@ Override Public VoidQueryAll (Pagination <Emp> p ){ // Call dao Dao. queryAll (p ); }

@ Override Public VoidQueryAll (Pagination <Emp> p ){ P. setList (query (Emp.Class, "Select * from emp limit ?,? ", P. getStart (), p. getSize ())); }

D) implementation code of pagination in jsp

<Font color ="Blue">$ {P. totalCount} </font> record, current <font color ="Blue">$ {P. page} </font> page, total <font color ="Blue">$ {P. totalPage} </font> page <Table> <Tr> <Td> NO. </td> <Td> name </td> <Td> salary </td> <Td> date of employment </td> <Td> operation </td> </Tr> <C: forEach items ="$ {P. list}"Var ="E"> <Tr> <Td >$ {e. empno} </td> <Td >$ {e. ename} </td> <Td >$ {e. sal} </td> <Td >$ {e. hiredate} </td> <Td> <a href ="Query. sxt? Op = queryByEmpno & empno =$ {E. empno}& Page =$ {P. page}& Amp; size =$ {P. size}"> Modify </a>/<a href ="Query. sxt? Op = deleteEmp & empno =$ {E. empno}& Page =$ {P. page}& Amp; size =$ {P. size}"Onclick = "return confirm ('Are you sure you want to delete it? '); "> Delete </a> </td> </Tr> </C: forEach> </Table> <A href ="Query. sxt? Op = queryAll & page =$ {P. firstPage}& Amp; size =$ {P. size}"> Homepage </a> | <A href ="Query. sxt? Op = queryAll & page =$ {P. prevPage}& Amp; size =$ {P. size}"Onclick ="Return$ {P. hasPrev?True:False}"> Previous page </a> | <C: forEach begin ="$ {P. navBegin}"End ="$ {P. navEnd}"Var ="Each"> <C: choose> <C: when test ="$ {P. page = each}"> <A href ="Query. sxt? Op = queryAll & page =$ {Each}& Amp; size =$ {P. size}"> <Font color ="Red"Size ="5">$ {Each} </font> </a> </C: when> <C: otherwise> <A href ="Query. sxt? Op = queryAll & page =$ {Each}& Amp; size =$ {P. size}">$ {Each} </a> </C: otherwise> </C: choose> </C: forEach> | <A href ="Query. sxt? Op = queryAll & page =$ {P. nextPage}& Amp; size =$ {P. size}"Onclick ="Return$ {P. hasNext?True:False}"> Next page </a> | <A href ="Query. sxt? Op = queryAll & page =$ {P. lastPage}& Amp; size =$ {P. size}"> Last page </a>

4. Deleting,   Paging Problems

A) after the deletion is successful, how can I continue to stay on the current page? The current page number and the number of records per page can be carried during the deletion? The Code is as follows:

<A href ="Query. sxt? Op = deleteEmp & empno =$ {E. empno}& Page =$ {P. page}& Amp; size =$ {P. size}"Onclick = "return confirm ('Are you sure you want to delete it? '); "> Delete </a>

B) after deleting the last data record of the last page, make sure to return to the previous page. In fact, the current page number cannot exceed the total page number and the total page number must be greater than 0.

// The current page number cannot exceed the total page number If(This. Page>This. TotalPage &&This. TotalPage> 0 ){ This. Page =This. TotalPage; }

5. Paging during Modification

A) Ensure that the modification is left on the current page. Because you want to jump to two pages, you need to pass the value twice.

<A href ="Query. sxt? Op = queryByEmpno & empno =$ {E. empno}& Page =$ {P. page}& Amp; size =$ {P. size}"> Modify </a>

<Input type ="Hidden"Name ="Page"Value ="$ {Param. page}"/> <Input type ="Hidden"Name ="Size"Value ="$ {Param. size}"/>