隨筆分類(103)

???? 前面一篇直接使用了Myfaces中的兩個Component完成了一個簡單的分頁，這里將會介紹一種On-demand loading的方法來進行分頁，僅僅在需要數據的時候加載。

???? 先來說一些題外話，為了實現這種方式的分頁，公司里大約5-6個人做了半個多月的工作，擴展了dataTable，修改了dataScrollor，以及各種其他的方法，但是都不是很優雅。在上個月底的時候，在Myfaces的Mail List中也針對這個問題展開了一系列的討論，最后有人總結了討論中提出的比較好的方法，提出了以下的分頁方法，也是目前實現的最為優雅的方法，也就是不對dataTable和dataScrollor做任何修改，僅僅通過擴展DataModel來實現分頁。

???? DataModel 是一個抽象類，用于封裝各種類型的數據源和數據對象的訪問，JSF中dataTable中綁定的數據實際上被包裝成了一個DataModel，以消除各種不同數據源和數據類型的復雜性，在前面一篇中我們訪問數據庫并拿到了一個List，交給dataTable，這時候，JSF會將這個List包裝成 ListDataModel ，dataTable訪問數據都是通過這個DataModel進行的，而不是直接使用List。

???? 接下來我們要將需要的頁的數據封裝到一個DataPage中去，這個類表示了我們需要的一頁的數據，里面包含有三個元素：datasetSize，startRow，和一個用于表示具體數據的List。datasetSize表示了這個記錄集的總條數，查詢數據的時候，使用同樣的條件取count即可，startRow表示該頁的起始行在數據庫中所有記錄集中的位置。

/**?*/ /**
?*?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;
????}
}

???? 接下來，我們要對DataModel進行封裝，達到我們分頁的要求。該DataModel僅僅持有了一頁的數據DataPage，并在適當的時候加載數據，讀取我們需要頁的數據。

/**?*/ /**
?*?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 )
???????? {
???????????? // ?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.
????? */

???? 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);
????????}

???????? 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);
????}

???? 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.
????? */

???? public ? boolean ?isRowAvailable()
???? {
????????DataPage?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?fetchPage( int ?startRow,? int ?pageSize);
????
}

???? 最后，我們需要在Backing Bean中加一些東西，調用業務邏輯，并將數據交給PagedListDataModel，來幫我們完成最后的分頁工作。

?? public ?SomeManagedBean? {
????.


???? private ?DataPage?getDataPage( int ?startRow,? int ?pageSize)? {
?????? // ?access?database?here,?or?call?EJB?to?do?so
????}

???? public ?DataModel?getDataModel()? {
???????? if ?(dataModel? == ? null )? {
????????????dataModel? = ? new ?LocalDataModel(20);
????????}

???????? return ?dataModel;
????}

???? 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);
????????}
}

這里面有一個getDataPage的方法，只需要把所有業務邏輯的調用放在這里就可以了，最后業務邏輯調用的結果返回一個List，總條數返回一個int型的count放到DataPage中去就可以了。

為了實現復用，把上面第三段的代碼中的LocalDataModel類和getDataPage方法抽到BasePagedBackingBean中，把getDataPage方法改成：

protected abstract DataPage getDataPage(int startRow, int pageSize);

這樣我們把所有需要分頁的Backing Bean繼承自這個抽象類，并實現getDataPage方法即可很容易的實現分頁。

?

???在具體應用中可以這么寫：

???? protected ?DataPage?getDataPage( int ?startRow,? int ?pageSize)
???? {
????????List?scheduleList? = ?scheduleService.getSchedulesByDate(scheduleDate,?startRow,?pageSize);
???????? int ?dataSetSize? = ?scheduleService.getSchedulesCountByDate(scheduleDate);
???????? return ? new ?DataPage(dataSetSize,?startRow,?scheduleList);
????}

在數據訪問中，我們只需要取出我們需要行數的記錄就可以了，這在hibernate中非常容易實現。

如果使用Criteria查詢的話，只要加上：

???? criteria.setFirstResult(startRow);

???? criteria.setMaxResults(pageSize);

使用Query查詢的話，只要加上

???? query.setFirstResult(startRow);

???? query.setMaxResults(pageSize);

并把兩個參數傳入即可。

我們還需要另外寫一個Count的DAO，取出相同查詢條件的記錄條數即可。

還要修改一下Backing Bean中與dataTable綁定的property，將返回類型由List改成DataModel，而第一篇中用到的頁面不需要做任何修改就可以滿足新的需求了。

里面最重要的是 PagedListDataModel 中 fetchPage 這個方法，當滿足取數據的條件時，都會調用它取數據，因為業務邏輯不同，不便于將業務邏輯的調用放在里面實現，于是將其作為抽象方法，將具體的實現放到具體的Backing Bean中進行，在BaseBackingBean中，實現了這個方法，調用了getDataPage(startRow, pageSize)這個方法，而在BaseBackingBean中，這個方法又推遲到更具體的頁面中實現，這樣，我們在具體的頁面中只需要實現一個getDataPage(startRow, pageSize)這個方法訪問業務邏輯。

大功告成，這個實現把前面遇到的兩個問題都解決了， On-demand loading 是沒有問題了，因為只有在首次讀取和換頁的時候DataModel才會向數據庫請求數據，雖然在JSF的生命周期中多次調用與dataTable綁定的方法，但是因為每次業務邏輯請求以后，數據都會存放在DataPage中，如果里面的數據滿足需求的話，就不再請求訪問數據庫，這樣多次訪問數據庫的問題也解決了。

雖然這樣的話，dataScrollor的Tag使用起來還是很復雜，通常在同一個項目中，我們只會使用一種樣式的分頁導航，不過沒關系，我們只需要修改以下DataScrollor的Render Kit，把一些可以定義的值固定下來，再定義一個TLD文件，就可以在項目中使用簡化版的Tag了。

這個方法一開始發布在Myfaces的Wiki中，http://wiki.apache.org/myfaces/WorkingWithLargeTables，那里很少有人關注到，大家有興趣可以看看原文，本文只是對這種方法做一些簡單的介紹，并非自創，希望大家能夠多多關注開源社區，因為那里有最新最好的東西。

從Nightly Build服務器中拿到的12.27的Myfaces包，發現里面擴充了很多新的Component，只是并沒有正式發布，大家有興趣的話可以研究研究。