Using composition over inheritance is a common design pattern that is often discussed in terms of designing business logic components. However, composition can solve a number of problems in domain object modeling that are created by relying on inheritance to share interface or functionality. Composition is used to delegate implementation in logical units by enlisting the help of a reference to an object that implements the required functionality instead of inheriting from it. This reference can be changed to different implementations depending on the needs at the time making for a more flexible design. This same design can be used in domain modeling to overcome some of the problems caused by inheritance. The typical flawed example of using inheritance in object modeling is the Person class which is often subclassed into Employee, User , Customer and Vendor classes.
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Posts Tagged java
Weld 1.0.1-CR2 is available
Feb 14
Dan Allen posted that the latest CR version of Weld is available. This should contain a number of bug fixes from the initial release of Weld, including the two problems I had with the request scope being available in EJB timeouts and problems with the ability to proxy stateless beans. This last bug was for me rather crucial since there was no easy way to implement DAO (just data management) type components with transactional annotations that could be injected into business logic beans. Without that, you end up having to write your own transaction handling code.
Also in the comments of the announcement, Max Anderson notes that the nightly builds of JBoss Tools 3.1 now supports CDI auto completion and JSF 2.0. I had a very quick look at it yesterday and it looks promising. I also tried it out with the latest JBoss 6 snapshot and am very pleased to say that the redeployment times on JBoss 6 are much faster and more in line with the performance on Glassfish which is something I have raved about.
I’ll be looking at it some more and probably write up a couple of tutorial posts.
Last time we looked more in depth at CDI and how we can define beans and inject them into other beans. This time we are going to look at how we can use events to decouple the handling of actions in the system.
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I had a bit of epiphany on the subject of nested conversations the other day when I was thinking about them and thought I’d share. I think nested conversations have been a little misunderstood with people unsure of how to use them, myself included, but I think I have found the best way to think of them.
In summary, nested conversations do for regular conversations what conversations do for session scope. With session scope, you cannot have mutliple instances of a named variable, you have to put each variable instance in its own conversation where it will be unique. However, if you want to have multiple instances of a named variable within the conversation, again, you cannot and you have the same problem you have with the session scope, that variables must be unique. Therefore you have to have each variable in its own nested conversation under the main conversation the same way we had the top level conversation under the session scope.
In some weird web app which lets you pick a person and then put costumes on them, you might have a main page where you select the person, and then in separate browser windows you can pick different outfits for that person. You put the selected person value in a different conversation so the value of #{selectedPerson} is local to the conversation allowing multiple selected people in different browser windows. This overcomes the limitations of the session which allows only one value for #{selectedPerson}
However, if you had that conversation open in multiple windows or tabs so you can compare different costumes on that person, there would only be one value of #{selectedCostume} for the conversation shared between all windows. As you select a costume in one window, it would affect all the other windows as they share the variable in that conversation. Using nested conversations would allow the conversation to have different values for the selected costume under the same parent conversation with the same selected person.
Taking it further you could select the person in the top level conversation, select the costume in the nested conversation, and then you could have multiple windows open with further nested conversation letting you pick different shoes to go with that costume. Also, if you change the person in the top level conversation, it will change the selected person for all windows using that conversation or any of its nested conversations.
I’m not sure there is a great need for nested conversations, I’ve never really used them or found the need and I don’t think users open that many browser windows or tabs to create different logic paths within a conversation. I think it is acceptable to limit the data isolation to a single conversation level.
In part 1, we looked at creating a JEE 6 application with Netbeans using JSF and CDI running on Glassfish. Now we’ll take a closer look at using CDI for managing dependencies in a Java EE 6 environment.
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With Weld 1.0, the reference implementation of JSR 299 – Java Contexts and Dependency Injection now released, attention at JBoss has no doubt turned to Seam 3 which is going to be built on top of Weld. Red Hat and JBoss are committed to returning innovations back the JCP as is the case with Seam which not only resulted in JSR 299, but has also influenced a number of other JSRs especially JSF 2. With JSR 299 standardizing the Seam ’style’ of development it also brings about a some fundamental game changes for Seam 3 (hence the title) as much of the strength of Seam becomes part of the JEE standards.
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Conversational Pitfalls
Nov 17
Seam conversations have certain rules that you need to be aware of when using them. This article came about because for the last couple of years, the same questions have been asked on the Seam forums regarding conversations. It is also a couple of issues that cropped up while I was working on the Seam vs. Spring Web Flow articles. Some of the problems are uncannily similar with similar solutions, so parts of this series may be of interest to non-Seam users. Additionally, it seems like a lot of this stuff will also apply to the conversational pieces of JSR 299 – Contexts and Dependency Injection which will be a part of JEE 6.
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I’ve spent some time in the last couple of weeks playing around with Glassfish, Netbeans 6.8 Beta (and milestone 2 before it) and JSF 2.0, and I have to say that this is turning out to be a really good set of development libraries and tools.
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One way to see where your conversations start and end is to use the Seam event model to observe the conversation start and ends.
@Name("conversationListener")
@Scope(STATELESS)
public class ConversationListenerBean implements ConversationListener {
@Logger
private Log log;
@In
private Conversation conversation;
@Observer(value="org.jboss.seam.beginConversation")
public void observeConversationStart() {
log.debug("Conversation #0 Started",conversation.getId());
}
@Observer(value="org.jboss.seam.endConversation")
public void observeConversationEnd() {
log.debug("Conversation #0 Ended",conversation.getId());
}
}
Just add this bean into your project and it will automatically log when you start and end conversations.
I’m looking at starting a new project and once again find myself choosing between frameworks. Having spent some time evaluating different ones I wrote up some notes to share and get some feedback that might alter my thoughts or opinions. Here’s the criteria I’m using to choose a framework in no particular order.
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