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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>XDoclet 和 EJB 技术：ejb.relation 标记</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr> 第 12 页（共16 页）</nobr></font></td>
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<p>
<code>ejb.relation</code> 标记使您能够定义 CMR 关系。这个标记相当简单，因为属性名非常接近地匹配 EJB 部署描述符中的相应的元素名。</p>
<p>以下示例设置了一（<code>DeptBean</code>）对多（<code>EmployeeBean</code>）的关系。
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
...
public abstract class DeptBean implements EntityBean {
...
/**
* @return return employees in this department
*
* @ejb.interface-method view-type="local"
*
* @ejb.transaction type="Required"
*
* @ejb.relation
* name="EmployeesInADepartmentRelation"
* role-name="DeptHasEmployees"
* target-role-name="EmployeeInADept"
* target-cascade-delete="no"
*/
public abstract Collection getEmployees();
/** @ejb.interface-method view-type="local" */
public abstract void setEmployees(Collection collection);
...
</code>
</pre>
</p>
<p>
请注意，该关系被命名。然后您描述该关系中 DeptBean 的角色。您还可以用 <code>target-cascade-delete</code> 属性来指定在删除这个 bean 的实例的时候也删除该关系中它的所有的子元素。请注意，您还指定 target-role-name。实际上以 target 开头的属性仅适用于单向关系。由于这个示例是双向的，您可以不包括它。关系的多重性是基于 cmr 字段中 getter 方法的返回类型。因为 <code>getEmployees</code> 返回 Collection，所以 XDoclet 推断 <code>Employee</code> 有较大的多重性。</p>
<p>
无论薄煎饼是多么地平，它总有两面，这类似于 CMR 关系。以下是 CMR 关系的另一面。</p>
<p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
public abstract class EmployeeBean implements EntityBean {
...
/**
* @return Return the group this user is in.
*
* @ejb.interface-method view-type="local"
*
* @ejb.transaction type="Required"
*
* @ejb.relation
* name="EmployeesInADeptartmentRelation"
* role-name="EmployeeInADept"
* target-role-name="DeptHasEmployees"
*
*/
public abstract Dept getDept();
/** @ejb.interface-method view-type="local" */
public abstract void setDept(Dept dept);
</code>
</pre>
</p>
<p>
同样，目标属性是一点多余的信息，实际上这点信息仅适用于单向关系，但是它在这里是附加的。请注意，关系名与关系的 Dept 一侧的关系名相同。这被 XDoclet 用来使关系的两个成员相关。</p>
<p>以上 ejb.relation 标记将生成部署描述符中的以下条目。</p>
<p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
<!-- Relationships -->
<relationships >
<ejb-relation >
<ejb-relation-name>EmployeesInADeptartmentRelation</ejb-relation-name>
<ejb-relationship-role >
<ejb-relationship-role-name>EmployeeInADept</ejb-relationship-role-name>
<multiplicity>Many</multiplicity>
<relationship-role-source >
<ejb-name>EmployeeBean</ejb-name>
</relationship-role-source>
<cmr-field >
<cmr-field-name>dept</cmr-field-name>
</cmr-field>
</ejb-relationship-role>
<ejb-relationship-role >
<ejb-relationship-role-name>DeptHasEmployees</ejb-relationship-role-name>
<multiplicity>One</multiplicity>
<relationship-role-source >
<ejb-name>DeptBean</ejb-name>
</relationship-role-source>
<cmr-field >
<cmr-field-name>employees</cmr-field-name>
<cmr-field-type>java.util.Collection</cmr-field-type>
</cmr-field>
</ejb-relationship-role>
</ejb-relation>
</relationships>
</code>
</pre>
</p>
<p>
使用 XDoclet 时，您必须为关系的每一侧编写一个带有两个属性的标记。请比较这个和您不使用 XDoclet 而必须编写的 26 行 XML。我不明白为什么有人不想把 XDoclet 用于 EJB 开发。</p>
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