Annotation简介

对于Annotation，是Java5的新特性，下面是Sun的Tutorial的描述，因为是英文，这里我翻译下，希望能够比较清晰的描述一下Annotation的语法以及思想。
Annotation:
Release 5.0 of the JDK introduced a metadata facility called annotations. Annotations provide data about a program that is not part of the program, such as naming the author of a piece of code or instructing the compiler to suppress specific errors. An annotation has no effect on how the code performs.
Annotations use the form @annotation and may be applied to a program‘s declarations: its classes, fields, methods, and so on. The annotation appears first and often (by convention) on its own line, and may include optional arguments:
JDK5引入了Metedata(元数据)很容易的就能够调用Annotations.Annotations提供一些本来不属于程序的数据，比如:一段代码的作者或者告诉编译器禁止一些特殊的错误。An annotation 对代码的执行没有什么影响。Annotations使用@annotation的形势应用于代码：类(class),属性(field),方法(method)等等。一个Annotation出现在上面提到的开始位置，而且一般只有一行,也可以包含有任意的参数。
@Author("MyName")
class myClass() { }
or
@SuppressWarnings("unchecked")
void MyMethod() { }
Defining your own annotation is an advanced technique that won‘t be described here, but there are three built-in annotations that every Java programmer should know: @Deprecated, @Override, and @SuppressWarnings. The following example illustrates all three annotation types, applied to methods:
定义自己的Annotation是一个比较高级的技巧，这里我们不做讨论，这里我们仅仅讨论每一个Java programer都应该知道的内置的annotations:@Deprecated, @Override, and @SuppressWarnings。下面的程序阐述了这三种annotation如何应用于methods。
import java.util.List;
class Food {}
class Hay extends Food {}
class Animal {
Food getPreferredFood() {
return null;
}
/**
* @deprecated document why the method was deprecated
*/
@Deprecated
static void deprecatedMethod() { }
}
class Horse extends Animal {
Horse() {
return;
}
@Override
Hay getPreferredFood() {
return new Hay();
}
@SuppressWarnings("deprecation")
void useDeprecatedMethod() {
Animal.deprecateMethod(); //deprecation warning - suppressed
}
}
@Deprecated
The @Deprecated annotation indicates that the marked method should no longer be used. The compiler generates a warning whenever a program uses a deprecated method, class, or variable. When an element is deprecated, it should be documented using the corresponding @deprecated tag, as shown in the preceding example. Notice that the tag starts with a lowercase "d" and the annotation starts with an uppercase "D". In general, you should avoid using deprecated methods — consult the documentation to see what to use instead.
@Deprecated
@Deprecated annotation标注一个method不再被使用。编译器在一个program(程序？)使用了不赞成的方法，类，变量的时候会产生警告(warning)。如果一个元素(element:method, class, or variable)不赞成被使用，应该像前面的例子里使用相应的@deprecated 标签，并且注意标签的首字母是小写的"d",而annotation时大写的"D"。一般情况下，我们应该避免使用不赞成使用的方法(deprecated methods)，而应该考虑替代的方法。
@Override
The @Override annotation informs the compiler that the element is meant to override an element declared in a superclass. In the preceding example, the override annotation is used to indicate that the getPreferredFood method in the Horse class overrides the same method in the Animal class. If a method marked with @Override fails to override a method in one of its superclasses, the compiler generates an error.
While it‘s not required to use this annotation when overriding a method, it can be useful to call the fact out explicitly, especially when the method returns a subtype of the return type of the overridden method. This practice, called covariant return types, is used in the previous example: Animal.getPreferredFood returns a Food instance. Horse.getPreferredFood (Horse is a subclass of Animal) returns an instance of Hay (a subclass of Food). For more information, see Overriding and Hiding Methods.
@Override
@Override annotation 告诉编译器当前元素是重写(override)自父类的一个元素。在前面的例子中，override annotation用来说明Horse类中的getPreferredFood这个方法重写(override)自Animal类中相同的方法。如果一个方法被标注了@Override，但是其父类中没有这个方法时，编译器将会报错。
但是并不是说我们一定要使用这个annotation，但是它能够很明显的给出实际行为，尤其是在方法返回一个被重写的方法返回类型的子类型的时候。上面的例子中，Animal.getPreferredFood 返回一个 Food实例，Horse.getPreferredFood 返回一个Hay实例，这里Horse是Animal的子类，Hay是Food的子类。
@SuppressWarnings
The @SuppressWarnings annotation tells the compiler to suppress specific warnings that it would otherwise generate. In the previous example, the useDeprecatedMethod calls a deprecated method of Animal. Normally, the compiler generates a warning but, in this case, it is suppressed.
Every compiler warning belongs to a category. The Java Language Specification lists two categories: "deprecation" and "unchecked". The "unchecked" warning can occur when interfacing with legacy code written before the advent of generics. To suppress more than one category of warnings, use the following syntax:
@SuppressWarnings
@SuppressWarnings annotation 告诉编译器禁止别的元素产生的特殊的警告(warnings)，在前面的例子里，useDeprecatedMethod调用了Animal的不赞成使用的一个方法。一般情况下，编译器会给出一个警告(warning)，但是在这种情况下，不会产生这个警告，也就是说被suppress。
每个编译器的警告都属于一个类型。Java Language Specification列出了两种类型："deprecation" 和 "unchecked"。"unchecked" warning 发生在使用非generic的旧代码交互的generic collection类时。为了禁止不止一种的警告时，使用下面的语法:
@SuppressWarnings({"unchecked", "deprecation"})
关于Annotation还可以参考下面的文章:
作者:cleverpig(作者的Blog:http://blog.matrix.org.cn/page/cleverpig)
原文:http://www.matrix.org.cn/resource/article/44/44048_Java+Annotation.html
http://www.matrix.org.cn/resource/article/44/44055_Java+Annotation+Reflect.html