Annotations


  • They provide information that you need to fully describe your program, but that cannot be expressed in Java.
  • In general the kind of annotations you add and what you do with them are entirely up to you.
  • Java SE5 contains three generalpurpose built-in annotations.
  • Four additional annotation types support the creation of new annotations.
  • Annotations are true language constructs and hence are structured, and are type-checked at compile time.

Basic syntax

  • Syntactically, annotations are used in much the same way as modifiers.

Defining annotations

  • Annotation definitions look a lot like interface definitions. In fact, they compile to class files like any other Java interface.
  • **@Target** defines where you can apply this annotation (a method or a field).
  • **@Retention** defines whether the annotations are available in the source code (SOURCE), in the class files (CLASS), or at run time (RUNTIME).
  • Elements look like interface methods, except that you can declare default values.

Meta-annotations

  • The meta-annotations are for annotating annotations.
  • Most of the time, you will be defining your own annotations and writing your own processors to deal with them.

Writing annotation processors

  • An important part of the process of using annotations is to create and use annotation processors.
  • Java SE5 provides extensions to the reflection API to help you create these tools.

Annotation elements

  • Note that you are not allowed to use any of the wrapper classes, but because of autoboxing this isn’t really a limitation.
  • You can also have elements that are themselves annotations.

Default value constraints

  • No element can have an unspecified value. This means that elements must either have default values or values provided by the class that uses the annotation.
  • non-primitive type elements are allowed to take null as a value, either when declared in the source code or when defined as a default value in the annotation interface.

Generating external files

  • Annotations are especially useful when working with frameworks that require some sort of additional information to accompany your source code.
  • After defining a Java class, the programmer must undergo the tedium of respecifying information like the name, package and so on—information that already exists in the original class.
  • If you define an element on an annotation with the name value, then as long as it is the only element type specified you don’t need to use the name-value pair syntax; you can just specify the value in parentheses.

Annotations don’t support inheritance

  • You cannot use the extends keyword with **@interfaces**. There doesn’t seem to be any suggestion of annotations supporting inheritance in future releases.

Implementing the processor

  • There is no inheritance of annotation interfaces, using getDeclaredAnnotations( ) is the only way you can approximate polymorphic behavior.

Using apt to process annotations

  • apt is designed to be run on Java source files rather than compiled classes. By default, apt compiles the source files when it has finished processing them.
  • The apt tool can easily group several annotation processors together. It allows you to specify multiple classes to be processed, which is a lot easier than having to iterate through File classes yourself.
  • apt works by using an AnnotationProcessorFactory to create the right kind of annotation processor for each annotation it finds.
  • When you run apt, you specify either a factory class or a classpath where it can find the factories it needs.
  • When you create an annotation processor for use with apt, you can’t use the reflection features in Java because you are working with source code, not compiled classes.

Using the Visitor pattern with apt

  • Processing annotations can become complex.
  • To prevent the complexity from scaling up badly when you have more annotations and more processors, the mirror API provides classes to support the Visitor design pattern.
  • A Visitor traverses a data structure or collection of objects, performing an operation on each one.
  • The data structure need not be ordered, and the operation that you perform on each object will be specific to its type.
  • This decouples the operations from the objects themselves, meaning that you can add new operations without adding methods to the class definitions.
  • When you use the apt tool with the Visitor pattern, you provide a Visitor class which has a method for handling each type of declaration that you visit.

Annotation-based unit testing

  • With annotations we can include the unit tests inside the class to be tested, and thus reduce the time and trouble of unit testing to a minimum.
  • This approach has the additional benefit of being able to test private methods as easily as public ones.

Thinking in Java——笔记(20)的更多相关文章

  1. Java笔记20:迭代器模式

    迭代器模式 所谓Iterator模式,即是Iterator为不同的容器提供一个统一的访问方式.本文以Java中的容器为例,模拟Iterator的原理. 1 定义一个容器Collection接口 pub ...

  2. JAVA自学笔记20

    JAVA自学笔记20 1.递归: 1)方法定义中定义中调用方法本身的现象 2)要有出口,否则就是死递归 次数不能太多.否则内存将溢出 构造方法不能递归使用 //斐波那契数列:1,1,2,3,5,8,1 ...

  3. java笔记整理

    Java 笔记整理 包含内容     Unix Java 基础, 数据库(Oracle jdbc Hibernate pl/sql), web, JSP, Struts, Ajax Spring, E ...

  4. 转 Java笔记:Java内存模型

    Java笔记:Java内存模型 2014.04.09 | Comments 1. 基本概念 <深入理解Java内存模型>详细讲解了java的内存模型,这里对其中的一些基本概念做个简单的笔记 ...

  5. Java笔记 —— 继承

    Java笔记 -- 继承 h2{ color: #4ABCDE; } a{ text-decoration: none!important; } a:hover{ color: red !import ...

  6. Java笔记 —— this 关键字

    Java笔记 -- this 关键字 h2{ color: #4ABCDE; } a{ color: blue; text-decoration: none; } a:hover{ color: re ...

  7. Effective Java笔记一 创建和销毁对象

    Effective Java笔记一 创建和销毁对象 第1条 考虑用静态工厂方法代替构造器 第2条 遇到多个构造器参数时要考虑用构建器 第3条 用私有构造器或者枚举类型强化Singleton属性 第4条 ...

  8. java笔记00-目录

    --2013年7月26日17:49:59 学习java已久,趁最近有空,写一个总结: java笔记01-反射:

  9. Ext.Net学习笔记20:Ext.Net FormPanel 复杂用法

    Ext.Net学习笔记20:Ext.Net FormPanel 复杂用法 在上一篇笔记中我们介绍了Ext.Net的简单用法,并创建了一个简单的登录表单.今天我们将看一下如何更好是使用FormPanel ...

随机推荐

  1. 介绍两个Ubuntu上的桌面小工具

    经常使用Windows10,Sticky Notes和壁纸自动切换功能挺好用的.我经常会使用Sticky Notes来记录一些信息,内容是实时保存的,而且启动的时候会自动显示在桌面上.其实Ubuntu ...

  2. 《Entity Framework 6 Recipes》中文翻译系列 (43) ------ 第八章 POCO之使用POCO加载实体

    翻译的初衷以及为什么选择<Entity Framework 6 Recipes>来学习,请看本系列开篇 8-2  使用POCO加载关联实体 问题 你想使用POCO预先加载关联实体. 解决方 ...

  3. Qt And MFC Mouse Over Tips

    Qt鼠标提示分析说明 关于鼠标停留在控件上面,显示提示内容的方法. 对于Qt来说, Qt的某一个控件类, 如果属于GUI的, 那么这个控件类会有一个setToolTip(QString text)的方 ...

  4. 微信小程序DEMO初体验

    小程序虽然被炒的很热,但是绝大部分人却从未亲自体验过,在2017年的上班第一天,献上一个小程序DEMO,您可以体验! 注意:由于微信限制,只能使用扫一扫来体验下方小程序DEMO. DEMO首页截图如下 ...

  5. salesforce 零基础学习(五十)自定义View或者List以及查看系统原来的View或者List

    salesforce给我们提供了标准的页面,比如标准的页面包括标准的列表和标准的详细页视图.有的时候我们想要自定义视图,比如做一个项目的时候不希望使用者直接通过ID查看到标准的详细页,而是跳转到指定处 ...

  6. iOS常见用户头像的圆形图片裁剪常见的几种方法

    在开发中,基本上APP的用户头像的处理都需要把用户所上传的方形图片,处理为圆形图片.在这里就总结三种常见的处理圆形图片的方法. 1.使用位图上下文 2.使用UIView的layer进行处理 3.使用r ...

  7. C#实现约瑟夫环问题

    using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace orde ...

  8. VS中行号对齐的辅助线(虚线)去除

    3张图 2 3

  9. 【Win10应用开发】自定义磁贴通知的排版

    前面老周用了两篇烂文,向大家介绍了Adaptive磁贴的模板使用.那些XML模板已经很强大了,不过,如果你觉得那些排版还不足以满足需求,不妨试试自己来定义磁贴的内容. 其实,Runtime App支持 ...

  10. Python补充06 Python之道

    作者:Vamei 出处:http://www.cnblogs.com/vamei 欢迎转载,也请保留这段声明.谢谢! Python有一个彩蛋,用下面语句调出: import this 该彩蛋的文档记录 ...