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What Can be Returned From an Event Handler in Java Integrated Universal Product Code version A in Java What Can be Returned From an Event Handler




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What Can be Returned From an Event Handler generate, create ucc - 12 none with java projects Specific Terms for GS1 Barcodes There are actuall UPC-A Supplement 5 for Java y a few different options of what you can return from an event handler:. Nothing: This is when the method is void or when it returns a null value. In this case the current page will render the request. That is, it will be redisplayed. String: Tapestry will expect it to be some application page"s logical name, like another (the name is case-insensitive). You will find more on logical names later in this chapter. Class: This should be a page class, like Another.class. Returning a class is a preferable a pproach, as compared to returning a logical name of the page as a string. This is because the logical name can change if application is restructured in the future, while its class will remain the same..

[ 62 ]. 3 . Page: An instance of a page can be returned exactly like we did previously. Tapestry will mak e sure that page is displayed. Link: An implementation of a Link interface will be converted into an appropriate URL and the client will be redirected to that URL. Stream: A stream or more specifically, StreamResponse object.

This option allows us to send to the user a binary stream say, a generated PDF or an Excel document.. Any other object UPCA for Java returned from an event handler will cause an error. Run the application, submit some message, and everything will seem to work properly. The messages on the console will be as expected, and the Another page will be shown, but it will not display the message that we have just passed to it:.

Why is that To u nderstand what has happened in this case, we need to discuss one subtle but important feature of Tapestry.. Tapestry Pages are Pooled Tapestry was desi gned with great scalability in mind. This means that a Tapestry application should be able to easily handle a huge amount of concurrent users, and for this, it should spend minimal efforts to serve every individual request. Say, a user requests the Start page.

Should Tapestry create an instance of this page especially for this user and discard it as soon as the user will go to another page This wouldn"t be efficient, as the next moment, twenty other users will come and request an instance of the Start page. It would be reasonable not to discard, but to reuse the instance that was just used for the first user. However, there is a potential problem.

The page might have some sensitive data put into it by the first user say a password, or anything else. To avoid this problem, Tapestry will wipe clean the instance used by the first user before giving it to any other user. It will reset all variables on the page class to their initial values.

. [ 63 ]. The Foundations of Tapestry In reality, Tapes GS1 - 12 for Java try maintains a pool of instances for every page, and when a page is requested, it takes an instance from the appropriate pool to serve that request, and then returns the instance to its pool. At the end of the request, Tapestry will wipe clean the variables of the instance of any user-specific data. This mechanism makes Tapestry very efficient.

But we have to remember that even if we think that we are dealing with the same page, in most cases the page will be represented by a different instance (may be even coming from a completely different server, in a clustered environment), and the next instance will have no idea of what we have just told the previous instance. That is, unless we take special care and make sure that the information is passed between instances properly. What happened when we tried to pass a message to the Another page is that we took an instance of it, and put our message into it.

Then we told Tapestry to show the Another page to this user. What Tapestry 5 does is that it tells the user"s browser: "Okay, thank you, I have successfully handled your submission; now go on and ask me to show you Another page". When the browser requests for the Another page, it takes a fresh instance of that page from the pool and uses it to produce the output.

Unfortunately, that fresh instance will know nothing about the message we wanted to pass. So how can we make our application work with all this instance pooling and jiggling Should there be a way to make Tapestry remember the information we have put into the page, no matter which instance of that page is being used There are actually two approaches to this: We can make a page field persistent. This is a very simple approach, but it has some downsides.

We can use page activation context. This is a superior approach, but not for every case, and it involves more coding..

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