chapter6.htm

使用eclipse 开发 j2ee应用的教程

<P><B>After adding all these fields, press Finish.</B></P>
<P><B>This will create a package named au.com.tusc.cmp under src, and ItemBean 
will be be created within that package as shown below.</B></P>
<P><IMG height=560 src="chapter6.files/ItemsBean.png" width=522 align=left 
border=0 name=Graphic3><BR clear=left><FONT color=#800000><I>Note: In comparison 
with our earlier BMP Entity Beans (Customer &amp; Manager), more tags have been 
generated at class level. Note also that CMP doesn't require a Data Access 
Object (DAO) interface, as communication between database and bean is controlled 
by the container.</I></FONT></P>
<P align=left>Let's first the generate EJB classes and then we will examine 
these tags.</P>
<P><B>Go to node ItemBean under au.com.tusc.cmp &gt; LombozJ2EE &gt; Add EJB to 
Module &gt; Select MyStoreMgr &gt; Ok.</B></P>
<P><B>Go to MyStoreMgr node &gt; LombozJ2EE &gt; Generate EJB classes.</B></P>
<P><FONT color=#800000><I>Note: All these steps are covered in previous chapters 
(chapters 3 and 1) in detail, so please refer to these if you have any queries. 
</I></FONT></P>
<P>Now, let's see what files have been generated by Xdoclet.</P>
<P><B>As shown below, the files generated are nearly the same as for BMP, except 
there are no Primary Key or DAO classes, and there is now ItemCMP which extends 
the ItemBean class. The remainder are the same as for BMP entity beans</B>.</P>
<P><IMG height=306 src="chapter6.files/ItemsBeanGeneratedFiles.png" width=216 
align=left border=0 name=Graphic4><BR clear=left><B>Now, let's examine these new 
tags, some of which have been covered in previous chapters. </B></P>
<OL>
  <LI>
  <P style="MARGIN-BOTTOM: 0in"><B>@ejb.bean </B>tag provides information about 
  the EJB. It is the one compulsory tag for all EJBs<B>.</B></P>
  <LI>
  <P style="MARGIN-BOTTOM: 0in"><B>@ejb.persistence tag is being used at two 
  levels, at class level and method level. </B>At class level it provides 
  information about the persistence of a CMP entity bean, that is which database 
  table this bean is going to interact with, which will provide that 
  persistence. At method level it provides information about the mapping of the 
  bean's persistent attributes to columns in that database table.</P>
  <LI>
  <P><B>@ejb.finder</B> tag defines a finder method for the home interface. This 
  requires the EJB QL query to fetch the data and a signature for the method. 
  This tag can be used for multiple finder methods.</P>
  <LI>
  <P style="MARGIN-BOTTOM: 0in"><B>@ejb.persistence-field</B> method level tag 
  is being deprecated in favour of @ejb.persistence tag, it provided information 
  about persistent fields.</P>
  <LI>
  <P style="MARGIN-BOTTOM: 0in"><B>@ejb.pk-field</B> tag defines the primary 
  key.</P></LI></OL>
<P style="MARGIN-BOTTOM: 0in">The code snippet below shows how persistent 
attributes are declared in a CMP Entity Bean.</P>
<P style="MARGIN-BOTTOM: 0in"><IMG height=558 
src="chapter6.files/ItemsBeanAttributes.png" width=376 align=left border=0 
name=Graphic6><BR clear=left><BR></P>
<P style="MARGIN-BOTTOM: 0in"><FONT color=#800000><I>Note: All persistent 
attributes are declared with abstract accessor and mutator methods in ItemBean. 
Note also that in the case of a composite primary key you have to specify the 
@ejb.pk-field tag on all other attributes/properties which combine to form the 
overall key as well.</I></FONT></P>
<P style="MARGIN-BOTTOM: 0in"><BR></P>
<H3>Implement ejbCreate Method :</H3>
<P>The ejbCreate method is created by method is created by the Lomboz Bean 
wizard, but we still have to add some code to complete it. Modify the signature 
of ejbCreate, passing all the attributes as parameters and then setting all 
these attributes using their associated mutator methods, as shown below.</P>
<P><FONT color=#800000><I>Errata note :- There is an error in the figure shown 
below. The Integer attribute 'qunatity' should be called 'quantity'. 
</I></FONT></P>
<P><IMG height=300 src="chapter6.files/ItemsBeanEjbCreate.png" width=526 
align=left border=0 name=Graphic9><BR clear=left><FONT color=#800000><I>Note : 
The other interesting aspect of ejbCreate here is its return type, as its return 
type has to be same as the primary key type (e.g. it has to of type String, 
Integer, Float, or whatever). In this case it is String - the type of itemID, 
and when implemented it should return null.(Refer to the EJB Spec 10.5.1). 
</I></FONT></P>
<P style="MARGIN-BOTTOM: 0in"><BR></P>
<H3 style="MARGIN-TOP: 0in; MARGIN-BOTTOM: 0in">Add Finder Method :</H3>
<P style="MARGIN-BOTTOM: 0in"><BR></P>
<P><FONT color=#800000><I>Note: An entity bean's home interface defines one or 
more finder methods, to find an entity object or collection of entity objects. 
The name of each finder method starts with the prefix 'find', such as findPrice 
or findQuantity in our case. Every finder method except findByPrimaryKey(key) 
must be associated with a query element in the deployment descriptor. The entity 
bean provider declares the EJB QL finder query and associates it with the finder 
method in the deployment descriptor. A finder method is normally characterized 
by an EJB QL query string specified via the query element. Refer to the EJB Spec 
10.5.6. This is covered in the Exercise section of this chapter. </I></FONT></P>
<P style="MARGIN-BOTTOM: 0in"><B>Now let's add a finder method to our bean class 
to find items supplied by a particular supplier</B>. </P>
<P style="MARGIN-BOTTOM: 0in"><B>In order to add this finder method we have to 
declare a class level tag as discussed above, that is @ejb.finder.</B></P>
<P style="MARGIN-BOTTOM: 0in"><B>So, add this tag:</B></P><PRE><FONT face=Sans><FONT size=3><FONT color=#3f5fbf>@ejb.finder</FONT> </FONT></FONT>
<FONT face=Sans><FONT size=3><FONT color=#3f5fbf>query="SELECT</FONT> <FONT color=#3f5fbf>OBJECT(a)</FONT> <FONT color=#3f5fbf>FROM</FONT> <FONT color=#3f5fbf>MyStoreItem</FONT> <FONT color=#3f5fbf>a</FONT> <FONT color=#3f5fbf>where</FONT> <FONT color=#3f5fbf>a.supplierID</FONT> <FONT color=#3f5fbf>=</FONT> <FONT color=#3f5fbf>?1"</FONT>  </FONT></FONT>
<FONT face=Sans><FONT size=3><FONT color=#3f5fbf>signature="java.util.Collection</FONT> <FONT color=#3f5fbf>findBySupplierID(java.lang.String</FONT> <FONT color=#3f5fbf>supplierID)"</FONT>  </FONT></FONT></PRE>
<P><FONT color=#800000><I>Note : In EJB QL, instead of the name of the table, 
the schema name is used (in this case it is MyStoreItem, rather then specifying 
Item as you would for an SQL query). Similarly the column names that would be 
used in SQL are replaced by their corresponding attributes as declared in the 
bean.</I></FONT></P>
<P style="MARGIN-BOTTOM: 0in"><B>Generate your EJB classes to see what this tag 
and the previous finder tag have created in the Home Interface as shown below in 
this code snippet from the ItemLocalHome interface.</B></P>
<P style="MARGIN-BOTTOM: 0in"><IMG height=442 
src="chapter6.files/ItemsLocalHome.png" width=582 align=left border=0 
name=Graphic7><BR clear=left><B>It has four methods, including the two finder 
methods generated by the finder tags declared at class level. The other two, 
create and findByPrimaryKey are created by Xdoclet(</B>because of the 
&lt;entitycmp/&gt; tag in ejbGenerate.xml<B>).</B></P>
<P style="MARGIN-BOTTOM: 0in"><BR></P>
<P style="MARGIN-BOTTOM: 0in"><B>Add another finder method to find out-of-stock 
items in MyStore.</B></P>
<P style="MARGIN-BOTTOM: 0in"><B>Add the following tag.</B></P><PRE><FONT color=#3f5fbf><FONT face=Sans><FONT size=3>@ejb.finder </FONT></FONT></FONT>
<FONT face=Sans><FONT size=3><FONT color=#3f5fbf>query="SELECT</FONT> <FONT color=#3f5fbf>OBJECT(c)</FONT> <FONT color=#3f5fbf>FROM</FONT> <FONT color=#3f5fbf>MyStoreItem</FONT> <FONT color=#3f5fbf>c</FONT> <FONT color=#3f5fbf>where</FONT> <FONT color=#3f5fbf>c.quantity</FONT> <FONT color=#3f5fbf>=</FONT> <FONT color=#3f5fbf>0"</FONT>  </FONT></FONT>
<FONT face=Sans><FONT size=3><FONT color=#3f5fbf>signature="java.util.Collection</FONT> <FONT color=#3f5fbf>findByOutOfStock()"</FONT></FONT></FONT></PRE>
<P><B>Code snippet from ItemLocalHome after generating EJB classes for these 
finder methods.</B></P>
<P><IMG height=490 src="chapter6.files/ItemsLocalHomeByStock.png" width=450 
align=left border=0 name=Graphic24><BR clear=left><BR>Now, generate your EJB 
classes and analyze the relevant classes. All the finder methods are complete. 
Let's add some business methods now.</P>
<H3>Add Business Methods :</H3>
<P><B>Now, add a business method with this signature:</B></P>
<P><FONT color=#000000><FONT face="Nimbus Roman No9 L"><B>public ItemData 
getItemData()</B></FONT></FONT></P>
<P><FONT color=#000000><FONT face="Nimbus Roman No9 L"><B>.. with Interface type 
as local.</B></FONT></FONT></P>
<P><FONT color=#800000><I><FONT face="Nimbus Roman No9 L">Note : The steps to 
add a business method are covered in previous chapters (1 and 3), so please 
refer to them. Also we have chosen the</FONT> Interface type as local because 
these methods will be invoked within the same Java Virtual Machine. 
</I></FONT></P>
<P><FONT color=#000000><FONT face="Nimbus Roman No9 L"><FONT size=3><B>This will 
provide description of individual items in MyStore. Add some debug statements 
and return an instance of ItemData as shown below in this code snippet from the 
Item bean. </B></FONT></FONT></FONT></P>
<P><IMG height=168 src="chapter6.files/ItemsBeanGetItemsData.png" width=380 
align=left border=0 name=Graphic8><BR clear=left><BR><BR></P>
<P><B>Add another business method with the following signature:</B></P>
<P><B>public void fillStock (Integer quantity)</B></P>
<P><B>.. with Interface type as local. This will increment the items available 
to MyStore after the delivery of further items. Add some debug statements and 
following lines to complete the method.</B></P><PRE><FONT face=Sans><FONT size=3>Integer qty   = <FONT color=#7f0055>new</FONT> Integer(  (quantity.intValue() + getQuantity().intValue()) ) ;</FONT></FONT>
<FONT face=Sans><FONT size=3><FONT color=#000000>setQuantity</FONT> <FONT color=#000000>(</FONT> <FONT color=#000000>qty</FONT> <FONT color=#000000>);</FONT></FONT></FONT></PRE>
<P>Code snippet of fillStock in ItemBean shown below.</P>
<P><IMG height=212 src="chapter6.files/ItemsBeanDeliverItem.png" width=584 
align=left border=0 name=Graphic10><BR clear=left><BR><BR></P>
<H3>Add Callback Method</H3>
<P>Unlike BMP (where they were generated) we have to add callback methods which 
will be overridden in the ItemCMP class.</P>
<P>First import the following package: <B><FONT face=Sans><FONT 
color=#000000>javax.ejb.EntityContext</FONT></FONT></B></P>
<P><B>Add a field to store the entity context.</B></P>
<P>protected EntityContext eContext;</P>
<P><B>Add a method setEntityContext with entityContext as its parameter and 
assign that to your entityContext variable.</B></P>
<P><B>Similarly add a method unsetEntityContext, which sets the entityContext 
variable to null.</B></P>
<P><B>Code snippet for both methods is shown below.</B></P>
<P><IMG height=318 src="chapter6.files/ItemsBeanEnityContext.png" width=394 
align=left border=0 name=Graphic11><BR clear=left><FONT color=#000000><FONT 
face="Nimbus Roman No9 L"><B>Now all business and finder methods are complete, 
so it's time to generate EJB classes.</B></FONT></FONT></P>
<P><B><FONT face="Nimbus Roman No9 L"><FONT color=#000000>Let's examine the 
generated</FONT></FONT></B> <B>ItemCMP class, which is of most interest</B> 
.</P>
<P>Unlike our BMP bean all persistent attribute behavior is being overridden by 
abstract methods. This is because the EJB container is responsible for 
maintaining their persistence. </P>
<P><IMG height=586 src="chapter6.files/ItemCMPAbstractMethods.png" width=444 
align=left border=0 name=Graphic22><BR clear=left><BR><BR></P>
<P>All the callback methods we have implemented are being overridden as shown 
below. </P>
<P><IMG height=166 src="chapter6.files/ItemCMPCallBackMethods.png" width=378 
align=left border=0 name=Graphic23><BR clear=left><FONT color=#800000><I>Note : 
There are no ejbFinder methods in this class as with BMP beans, as all this is 
controlled by the container. Also as pointed out before, there is no PrimaryKey 
class generated and there is no need for a DAO class, as this too is controlled 
by the container.</I></FONT></P>
<P><FONT color=#000000><FONT face="Nimbus Roman No9 L"><B>Now, before we deploy 
our bean, we will just have a look at ejb-jar.xml and jboss.xml to see what 
descriptors are generated.</B></FONT></FONT></P>
<P><FONT color=#800000><FONT face="Nimbus Roman No9 L"><I>Note : We don't have