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JSF 中处理分页问题

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JSF Bean Seam JSP EJB

WorkingWithLargeTables

本文转自 http://wiki.apache.org/myfaces/WorkingWithLargeTables，由于该文最后一部分没有jsp代码部分，所有又加上了一些jsp代码。

Components t:dataModel and t:dataScroller work together nicely to allow a user to "page" through a set of a few dozen to a few hundred records. However the implementation does assume that the entire set of available records are in memory and wrapped up inside a ListDataModel or ArrayDataModel.

When the available dataset is quite large, and the application can have many users, this can lead to memory usage problems.

This page contains discussions on how to handle this scenario.

On-demand loading

A custom DataModel can be used to allow data to be loaded "on demand".

First, a class needs to be defined which your "business methods" (eg EJBs) can use to pass "pages" of data back to the UI. This class needs to be defined in your project, as the "business" level shouldn't be extending MyFaces classes:

package example;
import java.util.List;
/**
* A simple class that represents a "page" of data out of a longer set, ie
* a list of objects together with info to indicate the starting row and
* the full size of the dataset. EJBs can return instances of this type
* when returning subsets of available data.
*/
public class DataPage<t></t> {
private int datasetSize;
private int startRow;
private List<t></t> data;
/**
* Create an object representing a sublist of a dataset.
*
* @param datasetSize is the total number of matching rows
* available.
*
* @param startRow is the index within the complete dataset
* of the first element in the data list.
*
* @param data is a list of consecutive objects from the
* dataset.
*/
public DataPage(int datasetSize, int startRow, List<t></t> data) {
this.datasetSize = datasetSize;
this.startRow = startRow;
this.data = data;
}
/**
* Return the number of items in the full dataset.
*/
public int getDatasetSize() {
return datasetSize;
}
/**
* Return the offset within the full dataset of the first
* element in the list held by this object.
*/
public int getStartRow() {
return startRow;
}
/**
* Return the list of objects held by this object, which
* is a continuous subset of the full dataset.
*/
public List<t></t> getData() {
return data;
}
}

Now a custom DataModel can use this DataPage stuff. Again, it is recommended that you copy this code into your project and change the package name appropriately. This class can't go in the MyFaces libraries as it depends on DataPage, and as noted above, DataPage is accessed by your business level code so it really can't be in the MyFaces libs:

package example;
import javax.faces.model.DataModel;
import example.DataPage;
/**
* A special type of JSF DataModel to allow a datatable and datascroller
* to page through a large set of data without having to hold the entire
* set of data in memory at once.
*
* Any time a managed bean wants to avoid holding an entire dataset,
* the managed bean should declare an inner class which extends this
* class and implements the fetchData method. This method is called
* as needed when the table requires data that isn't available in the
* current data page held by this object.
*
* This does require the managed bean (and in general the business
* method that the managed bean uses) to provide the data wrapped in
* a DataPage object that provides info on the full size of the dataset.
*/
public abstract class PagedListDataModel<t></t> extends DataModel {
int pageSize;
int rowIndex;
DataPage<t></t> page;
/*
* Create a datamodel that pages through the data showing the specified
* number of rows on each page.
*/
public PagedListDataModel(int pageSize) {
super();
this.pageSize = pageSize;
this.rowIndex = -1;
this.page = null;
}
/**
* Not used in this class; data is fetched via a callback to the
* fetchData method rather than by explicitly assigning a list.
*/
@Override
public void setWrappedData(Object o) {
throw new UnsupportedOperationException("setWrappedData");
}
@Override
public int getRowIndex() {
return rowIndex;
}
/**
* Specify what the "current row" within the dataset is. Note that
* the UIData component will repeatedly call this method followed
* by getRowData to obtain the objects to render in the table.
*/
@Override
public void setRowIndex(int index) {
rowIndex = index;
}
/**
* Return the total number of rows of data available (not just the
* number of rows in the current page!).
*/
@Override
public int getRowCount() {
return getPage().getDatasetSize();
}
/**
* Return a DataPage object; if one is not currently available then
* fetch one. Note that this doesn't ensure that the datapage
* returned includes the current rowIndex row; see getRowData.
*/
private DataPage<t></t> getPage() {
if (page != null)
return page;
int rowIndex = getRowIndex();
int startRow = rowIndex;
if (rowIndex == -1) {
// even when no row is selected, we still need a page
// object so that we know the amount of data available.
startRow = 0;
}
// invoke method on enclosing class
page = fetchPage(startRow, pageSize);
return page;
}
/**
* Return the object corresponding to the current rowIndex.
* If the DataPage object currently cached doesn't include that
* index then fetchPage is called to retrieve the appropriate page.
*/
@Override
public Object getRowData(){
if (rowIndex < 0) {
throw new IllegalArgumentException(
"Invalid rowIndex for PagedListDataModel; not within page");
}
// ensure page exists; if rowIndex is beyond dataset size, then
// we should still get back a DataPage object with the dataset size
// in it...
if (page == null) {
page = fetchPage(rowIndex, pageSize);
}
// Check if rowIndex is equal to startRow,
// useful for dynamic sorting on pages
if (rowIndex == page.getStartRow()){
page = fetchPage(rowIndex, pageSize);
}
int datasetSize = page.getDatasetSize();
int startRow = page.getStartRow();
int nRows = page.getData().size();
int endRow = startRow + nRows;
if (rowIndex >= datasetSize) {
throw new IllegalArgumentException("Invalid rowIndex");
}
if (rowIndex < startRow) {
page = fetchPage(rowIndex, pageSize);
startRow = page.getStartRow();
} else if (rowIndex >= endRow) {
page = fetchPage(rowIndex, pageSize);
startRow = page.getStartRow();
}
return page.getData().get(rowIndex - startRow);
}
@Override
public Object getWrappedData() {
return page.getData();
}
/**
* Return true if the rowIndex value is currently set to a
* value that matches some element in the dataset. Note that
* it may match a row that is not in the currently cached
* DataPage; if so then when getRowData is called the
* required DataPage will be fetched by calling fetchData.
*/
@Override
public boolean isRowAvailable() {
DataPage<t></t> page = getPage();
if (page == null)
return false;
int rowIndex = getRowIndex();
if (rowIndex < 0) {
return false;
} else if (rowIndex >= page.getDatasetSize()) {
return false;
} else {
return true;
}
}
/**
* Method which must be implemented in cooperation with the
* managed bean class to fetch data on demand.
*/
public abstract DataPage<t></t> fetchPage(int startRow, int pageSize);
}

Finally, the managed bean needs to provide a simple inner class that provides the fetchPage implementation:

java 代码

public SomeManagedBean {
....
private DataPage<somerowobject></somerowobject> getDataPage(int startRow, int pageSize) {
// access database here, or call EJB to do so
}
public DataModel getDataModel() {
if (dataModel == null) {
dataModel = new LocalDataModel(getRowsPerPage());
}
return dataModel;
}
private class LocalDataModel extends PagedListDataModel {
public LocalDataModel(int pageSize) {
super(pageSize);
}
public DataPage<somerowobject></somerowobject> fetchPage(int startRow, int pageSize) {
// call enclosing managed bean method to fetch the data
return getDataPage(startRow, pageSize);
}
}

The jsp pages should be like this:

public class PagedTableListBean {
private DataModel pagedDataModel;
private TableListBean listBean;
/**
*
*/
public PagedTableListBean() {
}
/**
* @return the itemList
*/
public DataModel getItemList() {
if (pagedDataModel == null) {
pagedDataModel = new LocalDataModel(15);
}
return pagedDataModel;
}
/**
* @return the pagedDataModel
*/
public DataModel getPagedDataModel() {
return pagedDataModel;
}
/**
* @param pagedDataModel
* the pagedDataModel to set
*/
public void setPagedDataModel(DataModel pagedDataModel) {
this.pagedDataModel = pagedDataModel;
}
/**
* @return the listBean
*/
public TableListBean getListBean() {
return listBean;
}
/**
* @param listBean
* the listBean to set
*/
public void setListBean(TableListBean listBean) {
this.listBean = listBean;
}
// local class
private class LocalDataModel extends PagedListDataModel {
public LocalDataModel(int pageSize) {
super(pageSize);
}
public DataPage fetchPage(int startRow, int pageSize) {
// call enclosing managed bean method to fetch the data
return getDataPage(startRow, pageSize);
}
}
// get datePage from DAO
private DataPage getDataPage(int startRow, int pageSize) {
// access database here, or call EJB to do so
List subList = listBean.getTableList(startRow, pageSize);
DataPage dataPage = new DataPage(listBean.getTotal(), startRow, subList);
return dataPage;
}
}

页面的绑定:

<hx:dataTableEx id="tableEx1" value="#{pagedTableListBean.itemList}"
var="vartableList" styleClass="dataTableEx" headerClass="headerClass"
footerClass="footerClass" rowClasses="rowClass1, rowClass2"
columnClasses="columnClass1" border="1" cellpadding="2"
cellspacing="0" rows="15">

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每日学口语 | 如何去除烦人的水垢

2007-09-12 16:56
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评论(5)
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5 楼 sunyson 2008-03-21

引用

DataPage getDataPage(startRow, pageSize); 中的startRow 和pageSize可以通过 <hx:dataTableEx id="tableEx1" value="#{pagedTableListBean.itemList}" binding="table"...>
中的table获取相关参数
table 在ManagerBean中定义为UIDataTable
table.getFirst(), table.getRows

4 楼 hintcnuie 2008-03-09

这个问题好像是JSF的生命周期中出问题了，具体原因忘记了，你逐步debug一下会看出。后来我们套用seam的pagescope，就把这个问题解决了，不过这一部分不是我具体做的，所以我也说不很清楚，只能给你个思路了，呵呵。

3 楼 fangbiao23 2008-03-07

我是利用起始行和每页多少行来显示一页的数据的
现在我如果把管理页面4 的支撑bean 其Scope设为request时，另用一个属性变量来获取每页的一个first行并用session保侟，在getDataPage时再来check该变量，如果有时则获取的数据将从first行开始，这样的实现效果是可以，可是总要慢一个节拍，在经过
现在在管理页面4-->>通过修改操作-->>返回管理页面按钮--返回管理页面4
后，其管理页面4显示的数据是页面1的数据，如果此时我再去单击datatable上的任可一个按钮时将显示正确的(管理页面4)，这好像就是一个响应慢一拍的感觉，请给予帮助？

现在我如果把管理页面4 的支撑bean 其Scope设为session时，另用一个属性变量来获取每页的一个first行并用session保侟，在getDataPage时再来check该变量，如果有时则获取的数据将从first行开始，这样的实现效果是可以，但是好像一旦有新的数据加进去后，不能及时显示出来，这是非常不好的地方，好像dataPage的数据被缓存了一样，真是奇怪，请给予帮助，谢谢

这两种方式都有一些小毛病，能否把你的实现思路说一下吗？谢谢

2 楼 hintcnuie 2008-03-07

这个问题遇到过，解决办法就是记住页码，等到修改完成的时候再根据这个页码分页。

1 楼 fangbiao23 2008-03-04

朋友，如果要做到
现在在管理页面4-->>通过修改操作-->>返回管理页面按钮--返回管理页面4
用了你楼上的那种方式后要怎么做到这种效果?

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