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                 Understanding Callbacks
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               <P>The rules here are pretty hard to follow without examples.  </P>

 <P>When they say one AroundInvoke per class they mean that in the most literal sense as in one individual java class definition, not including it's parent class or classes, may exactly one AroundInvoke method.  The bean or interceptor class may have an AroundInvoke method, its parent class may have an AroundInvoke method, the parent's parent class may have an AroundInvoke method and so on.</P>

 <P>So the following is legal.</P>

 <DIV class="code panel" style="border-width: 1px;"><DIV class="codeContent panelContent">
 <PRE class="code-java">
     <SPAN class="code-keyword">public</SPAN> class Plant {
         @AroundInvoke
         <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> a(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception {
             <SPAN class="code-keyword">return</SPAN> ctx.proceed();
         }
     }

     <SPAN class="code-keyword">public</SPAN> class Fruit <SPAN class="code-keyword">extends</SPAN> Plant {
         @AroundInvoke
         <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> b(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception {
             <SPAN class="code-keyword">return</SPAN> ctx.proceed();
         }
     }

     @Stateless
     <SPAN class="code-keyword">public</SPAN> class Apple <SPAN class="code-keyword">extends</SPAN> Fruit <SPAN class="code-keyword">implements</SPAN> AppleLocal {
         @AroundInvoke
         <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> c(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception {
             <SPAN class="code-keyword">return</SPAN> ctx.proceed();
         }

         <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">String</SPAN> grow(){
             <SPAN class="code-keyword">return</SPAN> <SPAN class="code-quote">&quot;ready to pick&quot;</SPAN>;
         }
     }

     <SPAN class="code-keyword">public</SPAN> <SPAN class="code-keyword">interface</SPAN> AppleLocal {
         <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">String</SPAN> grow();
     }
 </PRE>
 </DIV></DIV>

 <P>The result is that when the &quot;grow&quot; method on AppleLocal (and consequently on Apple) is invoked, the container will first invoke the following AroundInvoke methods in this order; a, b, c.</P>

 <P>What the ejb spec doesn't do such a good job at stating is that there are ways to effectively shut off the callbacks, including AroundInvoke, of a parent class by simply overriding the method.  We can shut off the &quot;a&quot; around invoke with a slightly different version of Apple as follows:</P>

 <DIV class="code panel" style="border-width: 1px;"><DIV class="codeContent panelContent">
 <PRE class="code-java">
     @Stateless
     <SPAN class="code-keyword">public</SPAN> class Apple <SPAN class="code-keyword">extends</SPAN> Fruit <SPAN class="code-keyword">implements</SPAN> AppleLocal {

         <SPAN class="code-comment">// This will now never be called.
 </SPAN>        <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> a(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception {
             <SPAN class="code-keyword">return</SPAN> <SPAN class="code-keyword">null</SPAN>;
         }

         @AroundInvoke
         <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> c(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception {
             <SPAN class="code-keyword">return</SPAN> ctx.proceed();
         }

         <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">String</SPAN> grow(){
             <SPAN class="code-keyword">return</SPAN> <SPAN class="code-quote">&quot;ready to pick&quot;</SPAN>;
         }
     }
 </PRE>
 </DIV></DIV>

 <P>The result of this is that when the &quot;grow&quot; method on AppleLocal is invoked, the container will first invoke the AroundInvoke methods &quot;b&quot; then &quot;c&quot; skipping &quot;a&quot; completely.</P>

 <P>When they say that an AroundInvoke method cannot be a business method, they mean that they cannot be exposed to clients through a local or remote interface.  The following would be illegal.</P>

 <DIV class="code panel" style="border-width: 1px;"><DIV class="codeContent panelContent">
 <PRE class="code-java">
     <SPAN class="code-keyword">public</SPAN> <SPAN class="code-keyword">interface</SPAN> AppleLocal {
         <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">String</SPAN> grow();

         <SPAN class="code-comment">// This is an AroundInvoke method in the bean class, not a legal business method!
 </SPAN>        <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> c(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception;
     }
 </PRE>
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	Understanding Callbacks
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	<P>The rules here are pretty hard to follow without examples. </P>

	<P>When they say one AroundInvoke per class they mean that in the most literal sense as in one individual java class definition, not including it's parent class or classes, may exactly one AroundInvoke method. The bean or interceptor class may have an AroundInvoke method, its parent class may have an AroundInvoke method, the parent's parent class may have an AroundInvoke method and so on.</P>

	<P>So the following is legal.</P>

	<DIV class="code panel" style="border-width: 1px;"><DIV class="codeContent panelContent">
	<PRE class="code-java">
	<SPAN class="code-keyword">public</SPAN> class Plant {
	@AroundInvoke
	<SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> a(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception {
	<SPAN class="code-keyword">return</SPAN> ctx.proceed();
	}
	}

	<SPAN class="code-keyword">public</SPAN> class Fruit <SPAN class="code-keyword">extends</SPAN> Plant {
	@AroundInvoke
	<SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> b(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception {
	<SPAN class="code-keyword">return</SPAN> ctx.proceed();
	}
	}

	@Stateless
	<SPAN class="code-keyword">public</SPAN> class Apple <SPAN class="code-keyword">extends</SPAN> Fruit <SPAN class="code-keyword">implements</SPAN> AppleLocal {
	@AroundInvoke
	<SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> c(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception {
	<SPAN class="code-keyword">return</SPAN> ctx.proceed();
	}

	<SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">String</SPAN> grow(){
	<SPAN class="code-keyword">return</SPAN> <SPAN class="code-quote">"ready to pick"</SPAN>;
	}
	}

	<SPAN class="code-keyword">public</SPAN> <SPAN class="code-keyword">interface</SPAN> AppleLocal {
	<SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">String</SPAN> grow();
	}
	</PRE>
	</DIV></DIV>

	<P>The result is that when the "grow" method on AppleLocal (and consequently on Apple) is invoked, the container will first invoke the following AroundInvoke methods in this order; a, b, c.</P>

	<P>What the ejb spec doesn't do such a good job at stating is that there are ways to effectively shut off the callbacks, including AroundInvoke, of a parent class by simply overriding the method. We can shut off the "a" around invoke with a slightly different version of Apple as follows:</P>

	<DIV class="code panel" style="border-width: 1px;"><DIV class="codeContent panelContent">
	<PRE class="code-java">
	@Stateless
	<SPAN class="code-keyword">public</SPAN> class Apple <SPAN class="code-keyword">extends</SPAN> Fruit <SPAN class="code-keyword">implements</SPAN> AppleLocal {

	<SPAN class="code-comment">// This will now never be called.
	</SPAN> <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> a(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception {
	<SPAN class="code-keyword">return</SPAN> <SPAN class="code-keyword">null</SPAN>;
	}

	@AroundInvoke
	<SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> c(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception {
	<SPAN class="code-keyword">return</SPAN> ctx.proceed();
	}

	<SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">String</SPAN> grow(){
	<SPAN class="code-keyword">return</SPAN> <SPAN class="code-quote">"ready to pick"</SPAN>;
	}
	}
	</PRE>
	</DIV></DIV>

	<P>The result of this is that when the "grow" method on AppleLocal is invoked, the container will first invoke the AroundInvoke methods "b" then "c" skipping "a" completely.</P>

	<P>When they say that an AroundInvoke method cannot be a business method, they mean that they cannot be exposed to clients through a local or remote interface. The following would be illegal.</P>

	<DIV class="code panel" style="border-width: 1px;"><DIV class="codeContent panelContent">
	<PRE class="code-java">
	<SPAN class="code-keyword">public</SPAN> <SPAN class="code-keyword">interface</SPAN> AppleLocal {
	<SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">String</SPAN> grow();

	<SPAN class="code-comment">// This is an AroundInvoke method in the bean class, not a legal business method!
	</SPAN> <SPAN class="code-keyword">public</SPAN> <SPAN class="code-object">Object</SPAN> c(InvocationContext ctx) <SPAN class="code-keyword">throws</SPAN> Exception;
	}
	</PRE>
	</DIV></DIV>

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