Java Programming Thoughts Learning Class (7) Chapter 20 - Notes

 Annotations (also known as metadata) provide us with adding information to our code A formal method is developed that allows us to use these data very conveniently at a later time.

1 Basic syntax

 The annotated method is no different from other methods. Annotations can be used with any modifier on a method, for example, public, static or void. From a syntax perspective, annotations are used almost exactly like modifiers.

1.1 Definition of annotations

The definition of an annotation looks a lot like the definition of an interface. In fact, like any other Java interface, the annotations will also be compiled into class files .

When defining annotations, you will need some meta-annotations , such as #@Target, @Retention .

In annotations, generally contain some elements to represent certain values ​​. Programs or tools can make use of these values ​​when parsing and processing annotations. Annotated elements look just like interface methods, the only difference is that you can specify a default value for them. There are restrictions on the types of elements.

An annotation without elements is called a marker annotation.

 All annotations inherit annotation# ##.

package net.mrliuli.annotations;import java.lang.annotation.*;@Target(ElementType.METHOD)@Retention(RetentionPolicy.RUNTIME)public @interface Test {} //(marker annotation)

1.2 Three standard annotations and four meta-annotations

The three standard annotations defined in

java.lang:

• @Overrided

• @Deprecated

• @SuppressWarnings

 Four meta-annotations:

• @Target indicates where the annotation can be used.

• @Retension indicates at what level the annotation information needs to be saved.

• @Documented Include this annotation in Javadoc.

• @Inherited allows subclasses to inherit annotations from parent classes.

1.3 About annotations

• The type of annotation elements is limited and cannot be any type. The compiler will Report an error.

• Default value restrictions:

  @Target(ElementType.METHOD)@Retention(RetentionPolicy.RUNTIME)public @interface SimulationNull{
      public int id() default -1;    public String description() default "";
  }

• First, elements cannot have undefined values. That is, the element must either have a default value or provide the element's value when using the annotation.

• Secondly, for elements of non-basic types, whether they are declared in source code or when defining default values ​​in annotation interfaces, they cannot be

  null as its value. This constraint makes it difficult for an annotation processor to represent the presence or absence of an element, because in each annotation declaration, all elements are present and have corresponding values. In order to get around this constraint, we can only define some special values ​​such as empty strings or negative numbers to indicate that an element does not exist:

• Generate external files. Some frameworks require some additional information to work with your source code, and in this case annotations are best suited to express their value. Technologies like Enterprise JavaBeans require deployment description files. Web services, custom tag libraries, and object/relational mapping tools (such as Toplink and Hibernate) generally require XML description files, and these description files are separated from the source code.

2 Writing annotation processor

 In the process of using annotations, a very important part is to create and use

Annotation processor, are used to read the annotation .

• The API of the reflection mechanism can construct an annotation processor

• The external tool apt can parse Java source code with annotations

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