注释是添加到Java源文件中的元数据(关于数据的数据)的一种形式。框架主要使用它们来简化客户端代码的集成。下面是我脑海中浮现的几个真实的例子:

JUnit 4 - you add the @Test annotation to each test method you want the JUnit runner to run. There are also additional annotations to do with setting up testing (like @Before and @BeforeClass). All these are processed by the JUnit runner, which runs the tests accordingly. You could say it's an replacement for XML configuration, but annotations are sometimes more powerful (they can use reflection, for example) and also they are closer to the code they are referencing to (the @Test annotation is right before the test method, so the purpose of that method is clear - serves as documentation as well). XML configuration on the other hand can be more complex and can include much more data than annotations can. Terracotta - uses both annotations and XML configuration files. For example, the @Root annotation tells the Terracotta runtime that the annotated field is a root and its memory should be shared between VM instances. The XML configuration file is used to configure the server and tell it which classes to instrument. Google Guice - an example would be the @Inject annotation, which when applied to a constructor makes the Guice runtime look for values for each parameter, based on the defined injectors. The @Inject annotation would be quite hard to replicate using XML configuration files, and its proximity to the constructor it references to is quite useful (imagine having to search to a huge XML file to find all the dependency injections you have set up).

希望我已经让您了解了在不同框架中如何使用注释。

下面是一些可以使用注释的地方。

a. Annotations can be used by compiler to detect errors and suppress warnings
b. Software tools can use annotations to generate code, xml files, documentation etc., For example, Javadoc use annotations while generating java documentation for your class.
c. Runtime processing of the application can be possible via annotations.
d. You can use annotations to describe the constraints (Ex: @Null, @NotNull, @Max, @Min, @Email).
e. Annotations can be used to describe type of an element. Ex: @Entity, @Repository, @Service, @Controller, @RestController, @Resource etc.,
f. Annotation can be used to specify the behaviour. Ex: @Transactional, @Stateful
g. Annotation are used to specify how to process an element. Ex: @Column, @Embeddable, @EmbeddedId
h. Test frameworks like junit and testing use annotations to define test cases (@Test), define test suites (@Suite) etc.,
i. AOP (Aspect Oriented programming) use annotations (@Before, @After, @Around etc.,)
j. ORM tools like Hibernate, Eclipselink use annotations

有关注释的更多详细信息，请参考此链接。

您可以参考此链接，了解如何使用注释构建简单的测试套件。

JPA(来自Java EE 5)是(过度)使用注释的一个很好的例子。Java EE 6还将在很多新领域引入注释，比如RESTful webservices和每个老Servlet API下的新注释。

这里有一些资源:

Sun - Java持久性API Java EE 5教程- JPA 介绍Java EE 6平台(查看所有三个页面)。

注释不仅可以接管配置细节，而且还可以用来控制行为。您可以在Java EE 6的JAX-RS示例中看到这一点。

它对于注释类非常有用，无论是在方法、类或字段级别，都是关于类的一些与类不太相关的东西。

您可以有自己的注释，用于将某些类标记为仅测试使用。它可能只是为了文档的目的，或者您可以在编译产品候选发行版时通过过滤它来强制执行它。

你可以使用注释来存储一些元数据，就像在插件框架中一样，例如插件的名称。

它只是另一种工具，它有很多用途。

Java中的注释提供了一种描述类、字段和方法的方法。本质上，它们是添加到Java源文件中的一种元数据形式，它们不能直接影响程序的语义。但是，注释可以在运行时使用反射读取&这个过程被称为内省。然后，它可以用来修改类、字段或方法。

这个特性，经常被库和sdk (hibernate, JUnit, Spring Framework)利用来简化或减少程序员在使用这些库或sdk时需要做的代码量。因此，可以公平地说，注释和反射在Java中是齐头并进的。

我们还将注释的可用性限制在编译时或运行时。下面是一个创建自定义注释的简单示例

Driver.java

package io.hamzeen;

import java.lang.annotation.Annotation;

public class Driver {

    public static void main(String[] args) {
        Class<TestAlpha> obj = TestAlpha.class;
        if (obj.isAnnotationPresent(IssueInfo.class)) {

            Annotation annotation = obj.getAnnotation(IssueInfo.class);
            IssueInfo testerInfo = (IssueInfo) annotation;

            System.out.printf("%nType: %s", testerInfo.type());
            System.out.printf("%nReporter: %s", testerInfo.reporter());
            System.out.printf("%nCreated On: %s%n%n",
                    testerInfo.created());
        }
    }
}

TestAlpha.java

package io.hamzeen;

import io.hamzeen.IssueInfo;
import io.hamzeen.IssueInfo.Type;

@IssueInfo(type = Type.IMPROVEMENT, reporter = "Hamzeen. H.")
public class TestAlpha {

}

IssueInfo.java

package io.hamzeen;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * @author Hamzeen. H.
 * @created 10/01/2015
 * 
 * IssueInfo annotation definition
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface IssueInfo {

    public enum Type {
        BUG, IMPROVEMENT, FEATURE
    }

    Type type() default Type.BUG;

    String reporter() default "Vimesh";

    String created() default "10/01/2015";
}

像Hibernate这样的框架需要大量的配置/映射，大量使用annotation。

看一看Hibernate注解