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Let's talk about exceptions ...
How do you handle exceptions? Do you think upfront about the type of exceptions that you want to catch or do you just let
the outside world handle it?
-- Jeroen van Bergen in JW Blogs
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BeanLint: A JavaBeans troubleshooting tool, Part 1
Use this new tool to avoid JavaBeans loading and runtime problems
Every couple of months, I receive panicked or bewildered e-mail from a JavaBeans neophyte who is trying to create a JavaBean containing anImage and who can't figure out why the BeanBox won't load the bean. The problem is that java.awt.Image isn't Serializable, therefore neither is anything that contains a java.awt.Image, at least without custom serialization.I myself have spent countless hours putting println() statements into the BeanBox code then recompiling it, trying to figure out why my beans won't load. Sometimes it's due to
some simple, stupid thing -- like forgetting to define the zero-argument constructor, or even the class, as public. Other times, it turns out to be something more obscure.
The case of the missing bean
While the requirements to write a Java class as a JavaBean are simple and straightforward, there are some hidden implications
that many bean builder tools don't address. These little gotchas can easily eat up an afternoon, as you hunt through your code, searching for the reason your builder tool can't find your
bean. If you're lucky, you'll get a pop-up dialog box with a cryptic error message -- something along the lines of "NoSuchMethodException caught in FoolTool Introspection." If you're unlucky, the JavaBean you've poured so much sweat into will refuse to appear in your builder tool, and you'll
spend the afternoon rehearsing the vocabulary your mother tried so hard to cure you of. The BeanBox has long been an egregious
offender in this regard, and though it has been improving, it will still drop properties and even whole beans without providing
the developer with a single clue as to why.
This month, I'll be leading you out of the "land of the missing bean" by introducing a new tool called, oddly, BeanLint, which analyzes classes within jar files, looking for possible problems that would make the classes unusable as beans. While
this tool doesn't cover every possible bean problem, it does identify some of the major common problems that make beans unloadable.
In order to understand how BeanLint works its magic, this month and next we'll delve into some of the lesser-known corners of the standard Java API:
- We'll create a custom class loader, which loads new Java classes from a jar file
- We'll use the reflection mechanism, which lets Java programs analyze Java classes, to identify what's inside our class files
- We'll use the
Introspectorto produce a report of all of the class's beanlike properties for any class in the jar file that passes all tests (and is, therefore, a potential bean)
By the time we're done, you'll have a useful tool for debugging your beans, you'll better understand bean requirements, and you'll learn about some of Java's cool new features at the same time.
Bean basics
For a class file to be a JavaBean, there are two simple requirements:
- The class must have a public constructor with no arguments (a zero-arg constructor)
- The class must implement the empty tag interface
java.io.Serializable
That's it. Follow those two simple rules, and your class will be a JavaBean. The simplest JavaBean, then, looks something like this:
- The actual specification for what makes a JavaBean is the JavaBeans Specification. Read the spec yourself and extend
BeanLintto do even more. The spec is available for download at http://java.sun.com/beans/spec.html - The complete source code for the
class BeanLintcan be downloaded at http://www.javaworld.com/jw-12-1998/beans/BeanLint.java - Download the sources for the examples in this article in Unix tar format http://www.javaworld.com/jw-12-1998/beans/BadBeans.tar
- Or in zip format http://www.javaworld.com/jw-12-1998/beans/BadBeans.zip
- You can read up on manifest files at http://java.sun.com/docs/books/tutorial/jar/basics/manifest.html
- To see how a hashtable works, go to http://java.sun.com/products/jdk/1.1/docs/api/java.util.Hashtable.html
- To learn about the
resolveflag injava.lang.ClassLoader, see http://java.sun.com/products/jdk/1.1/docs/api/java.lang.ClassLoader.html