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align=left border=0> <SPAN
class=tpc_title> </SPAN><BR><SPAN class=tpc_content
id=text1><BR><BR>在JSF中实现<B
style="COLOR: red; BACKGROUND-COLOR: #ffff66">分页</B></FONT>（二）<BR><BR><BR>
前面一篇直接使用了Myfaces中的两个Component完成了一个简单的<B
style="COLOR: red; BACKGROUND-COLOR: #ffff66">分页</B></FONT>，这里将会介绍一种On-demand
loading的方法来进行<B
style="COLOR: red; BACKGROUND-COLOR: #ffff66">分页</B></FONT>，仅仅在需要数据的时候加载。<BR><BR>
先来说一些题外话，为了实现这种方式的<B
style="COLOR: red; BACKGROUND-COLOR: #ffff66">分页</B></FONT>，公司里大约5-6个人做了半个多月的工作，扩展了dataTable，修改了dataScrollor，以及各种其他的方法，但是都不是很优雅。在上个月底的时候，在Myfaces的Mail
List中也针对这个问题展开了一系列的讨论，最后有人总结了讨论中提出的比较好的方法，提出了以下的<B
style="COLOR: red; BACKGROUND-COLOR: #ffff66">分页</B></FONT>方法，也是目前实现的最为优雅的方法，也就是不对dataTable和dataScrollor做任何修改，仅仅通过扩展DataModel来实现<B
style="COLOR: red; BACKGROUND-COLOR: #ffff66">分页</B></FONT>。<BR><BR>
DataModel是一个抽象类，用于封装各种类型的数据源和数据对象的访问，JSF中dataTable中绑定的数据实际上被包装成了一个DataModel，以消除各种不同数据源和数据类型的复杂性，在前面一篇中我们访问数据库并拿到了一个List，交给dataTable，这时候，JSF会将这个List包装成ListDataModel，dataTable访问数据都是通过这个DataModel进行的，而不是直接使用List。<BR><BR>
接下来我们要将需要的页的数据封装到一个DataPage中去，这个类表示了我们需要的一页的数据，里面包含有三个元素：datasetSize，startRow，和一个用于表示具体数据的List。datasetSize表示了这个记录集的总条数，查询数据的时候，使用同样的条件取count即可，startRow表示该页的起始行在数据库中所有记录集中的位置。<BR><BR><BR><BR><FONT
color=red>以下是代码:</FONT><BR><TEXTAREA class=java name=code rows=15 cols=100>/**
* 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
{
private int datasetSize;
private int startRow;
private List 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 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 getData()
{
return data;
}
}
</TEXTAREA><BR><BR>接下来，我们要对DataModel进行封装，达到我们<B
style="COLOR: red; BACKGROUND-COLOR: #ffff66">分页</B></FONT>的要求。该DataModel仅仅持有了一页的数据DataPage，并在适当的时候加载数据，读取我们需要页的数据。<BR><BR><FONT
color=red>以下是代码:</FONT><BR><TEXTAREA class=java name=code rows=15 cols=100>/**
* 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.
* <p>
* 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.
* <p>
* 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 extends DataModel
{
int pageSize;
int rowIndex;
DataPage 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.
*/
public void setWrappedData(Object o)
{
if(o instanceof DataPage)
{
this.page = (DataPage) o;
}
else
{
throw new UnsupportedOperationException("setWrappedData");
}
}
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.
*/
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!).
*/
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 getPage()
{
if (page != null)
{
return page;
}
int rowIndex = getRowIndex();
int startRow = rowIndex;
if (rowIndex == -1)
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