26: Making Your Application Fault-Tolerant in Java

Integrate EAN13 in Java 26: Making Your Application Fault-Tolerant
26: Making Your Application Fault-Tolerant
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In addition, keep your eyes open to the kinds of errors you get as you are actively developing your program. An error that occurs to you once in a freak accident can point you to a weakness in your error-handling strategy.
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Categorizing Failures
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After identifying the kinds of errors that might occur in your program, you can determine how to react to them. For each potential error, there might be steps you can take to alleviate the problem. For some errors, however, there may be no way to recover. In determining how to handle an error, you must rst decide if it is possible to recover from it. These kinds of decisions affect how the application works for the end user, and they should be made with the end user s experience in mind. For example, if an image fails to load, you can draw a broken image icon or placeholder image in its place. But think about how it will make your program behave. In some cases, drawing a placeholder image might not be appropriate, for example if the image is a CAPTCHA image. CAPTCHA images are simple queries that theoretically prove that a human user is interacting with the computer by requiring some trivial task that is dif cult for a computer, typically recognizing a distorted word, as demonstrated in Figure 26-1. If this image fails to load, showing a replacement image won t be an acceptable solution, as Figure 26-2 makes evident. This example illustrates the need to react to errors based on the user s experience. A good strategy in one case can be unacceptable in another case.
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In general, your intent in handling an error should depend on the severity of the failure. If the error is nonfatal, your intent should be to return the program to a state in which everything else can continue normally. If the error is fatal, your intent should be to try again, or gracefully terminate the program. Returning the program to a normal state can be dif cult, especially if other areas of the program depend on something that failed. You must also ensure that even if a runtime error disrupts the ow of control of your application, you can resume and perform necessary cleanup code. In other words, make sure you utilize the catch and finally blocks when handling runtime errors. In addition to your primary goal of returning the program to normalcy, two actions are useful to any error-handling strategy: logging errors and reporting failures to the user.
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Part V: Error Handling
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Why should you log errors When developing an application, these logs can be vital to your own development and bug- xing process. Once the application is deployed, the logs can help you collect and x bugs that appear in the wild. For instance, if the person who paid for the software you developed has some problem with it on her computer at home, the hope is that you will still be able to investigate the causes of that failure. Logging, although it technically doesn t help you handle an error once the error occurs, is an essential part of any long-term error-handling strategy. Without logging, you won t be able to tell the difference between a program that is handling lots of faults correctly and a program that is having no trouble at all. Error logs help you start identifying problems without a debugger (remember that a debugger only works with SWFs compiled for debugging, which the public versions of your application probably won t be), and they let you capture error information in a way that you can control and retain. Logging can be tricky because there are many ways to get information out of a running SWF. Your approach can greatly depend on how your program is deployed, as well. Therefore, the implementation details of different logging methods are beyond the scope of this book. Some ways you can log information, in brief, are Using trace() output This method can be viewed only if you have the debugging version of Flash Player and have it con gured correctly for logging. This approach takes no effort to write code for, but it can be complicated to set up properly, especially if you need to capture logs from a SWF running in a browser. Mark Walters explains how in his post at http://bit.ly/capture-trace. To a separate application through sockets or a LocalConnection This approach doesn t require special setup of the Flash Player but does require an external application to capture the logs. Several third-party products provide the external log viewer and the framework to log to it. Popular loggers (that often provide further debugging features) include SOS (http://fdt.powerflasher.de/developer-tools/sosmax/home/), XRay (http://osflash.org/xray), and Arthropod (http://arthropod.stopp.se/). To the browser or enclosing web page using JavaScript One example of this is logging to FireBug, a debugging extension to the Firefox browser. This approach requires that speci c browser and extension, but it is easy to set up. One implementation of this idea is hosted at http://bit.ly/logger-marumushi, another at http://bit.ly/logger-grumblecode. To a le, if the program is running on the desktop with Adobe AIR One approach is described at http://dispatchevent.org/roger/logging-to-a-file-in-apollo. To a server You should be careful of both network overhead and privacy concerns if you choose this method. You should let the user know exactly what is being transferred and avoid logging personal or sensitive information at any cost. In all these cases, you should log information that will help you debug problems if they arise. A message in the log that says, Something went wrong! is useless. Log a description of what went wrong and where. The following snippet shows speci c messages depending on the kind of error that happened:
public function displayText():void { try { var tf:TextField = TextField(getChildAt(0)); tf.htmlText = "Hello World!"; } catch (error:RangeError) { Console.log("displayText(): No display child found at position 0!");
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