How Do Annotations Work? | Object Partners

How Do Annotations Work? | Object Partners

Recently, I was giving an overview of Java to a bunch of C/C++ developers to help them bridge the gap. Mostly I ended up assuring them that they knew what they were doing and filled in very few gaps. I did enlighten them on some of the fun things like Collections and some confusing things like Date. We covered lots of ground in those few hours of presentation and banter, but one question I couldn't answer was about annotations.

I had put a few code samples in my presentation that I'd pulled from a recent project. Stuff I knew worked, so I wouldn't be surprised with any hastily thrown together code with style, format, or syntax errors. One of the classes was an annotated Spring class, with some code not too much unlike this snippet:

@Autowire private SomeBean someBean;

We went down a path of queries related to how to create new ones, which is a simple @interface declaration like the following.

public @interface Foo{ }

They queried how to put them in the code, and I pointed back to the example. I also showed them some other examples related to using them on class definitions, methods, and on parameter values, as defined by the @Target annotation and ElementType enum. We looked at examples of commonly encountered annotations such as @Deprecated and @SuppressWarning among others. We even looked at the different uses of @Retention and discussed the RetentionPolicy enum; how SOURCE is used to provide hints for the compilers (and IDEs) but aren't retained in the compiled code, how CLASS is retained in the code but not necessarily available at runtime, and how RUNTIME is certain to be available at runtime.

Then they wanted to know how to use them. Not how to implement them, but how to access the @Retention(RetentionPolicy.RUNTIME) declared annotations.

I hated to admit that I didn't really know. I knew there weren't any frequently used utility classes for grabbing annotations, but that there were annotation-related methods on the reflection classes. I knew in practice that they were used, but not directly how, so I set out to learn so I could completely satisfy their questions.

To be sure, I use annotations all day long. I probably @Deprecate a method every day, sometimes permanently, sometimes just to quickly find uses as Eclipse is faster at adding them to problems than it is at searching. Any class that extends or implements another is surely fraught with @Override annotations. I probably write twice as many @Test annotated methods than anything else. I have @Autowire in nearly every function-full class of a Spring application, and every Hibernate project is filled with the @Column and all the other JPA annotations.

What I hadn't had to do, though, was write any code to find and use annotations. I can't even recall passing an annotation to a method to try to identify them. After much digging, I ascertained that indeed there aren't any utility classes that aid in finding or using annotations. There's got to be some heavy work behind the classpath scanning in frameworks like Spring and Hibernate to find annotated classes and methods. I'm not quite prepared to dive into that, but let's look at some simple cases of using annotations.

First, a simple set of annotations should be built for our various uses. Let's take one for each kind of element: class, method, and property.

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