Annotation，注解或元注释

Annotation 的本意即注释，更精确一点，是指与正文关联紧密的边注或旁注。在 Java 的语境中，用来指代一种特殊的、用于修饰上下文代码的注释，我们称其为注解或元注释。与普通的注释不同，Annotation 虽然不是代码，但事实上影响程序的运行逻辑，其用途包含但不限于：

• Information for the compiler — Annotations can be used by the compiler to detect errors or suppress warnings.
• Compile-time and deployment-time processing — Software tools can process annotation information to generate code, XML files, and so forth.
• Runtime processing — Some annotations are available to be examined at runtime.

@cite "http://docs.oracle.com/javase/tutorial/java/annotations/"

自 Java 5 开始，允许开发人员定义自己的 annotation。

Annotation vs. Doc Tag

元注释与注释中的标签（tag）是两回事，虽然它们之前可能存在某种联系，比如：

// Javadoc comment follows
/**
 * @deprecated
 * explanation of why it was deprecated
 */
@Deprecated
static void deprecatedMethod() { }

@cite http://docs.oracle.com/javase/tutorial/java/annotations/predefined.html

DEMO

import java.lang.annotation.Documented;

@Documented
@interface ClassPreamble {
    String author();
    String date();
    int currentRevision() default 1;
    String lastModified() default "N/A";
    String lastModifiedBy() default "N/A";
    // Note use of array
    String[] reviewers();
}

@ClassPreamble (
        author = "John Doe",
        date = "3/17/2002",
        currentRevision = 6,
        lastModified = "4/12/2004",
        lastModifiedBy = "Jane Doe",
        // Note array notation
        reviewers = {"Alice", "Bob", "Cindy"}
)
public class AnnotationTest {
}

@cite http://docs.oracle.com/javase/tutorial/java/annotations/declaring.html

Java SE 预定义元注释

@cite http://docs.oracle.com/javase/tutorial/java/annotations/predefined.html

• @Deprecated

  @Deprecated annotation indicates that the marked element is deprecated and should no longer be used. The compiler generates a warning whenever a program uses a method, class, or field with the @Deprecated annotation.

• @Override

  While it is not required to use this annotation when overriding a method, it helps to prevent errors. If a method marked with @Override fails to correctly override a method in one of its superclasses, the compiler generates an error.

• @SuppressWarnings

  @SuppressWarnings annotation tells the compiler to suppress specific warnings that it would otherwise generate.

SEE

• The Java™ Tutorials, Lesson: Annotations
  http://docs.oracle.com/javase/tutorial/java/annotations/index.html

• Java 8 Annotation 新特性在软件质量和开发效率方面的提升
  http://www.ibm.com/developerworks/cn/java/j-lo-java8annotation/index.html