primary coverage :

• annotation (Annotation) summary
• common Annotation Example
• Customize Annotation
• JDK Meta annotations in
• Using reflection to get annotation information （ In the reflection part )
• JDK 8 New features of annotations in

Catalog

Common annotation examples

Example 1 : Generate documentation related annotations

Example 2 : Format check at compile time (JDK Built in three basic annotations )

Example 3 : Trace code dependencies , Implement alternative profile functions

Custom annotation ：

Custom annotation examples

Yuan notes

@Retention

@Target

@Documented

@Inherited

jdk 8 New features of annotations in ：

Repeatable notes

stay jdk8 before

stay jdk8 in the future

Type notes

---

Common annotation examples

Use Annotation Add... In front of it @ Symbol , and hold Annotation As a modifier Use . Used to decorate the program elements it supports

Example 1 : Generate documentation related annotations

@author： Indicate the author who developed the module , Use... Between multiple authors , Division

@version： Indicate the version of the module
@see： Refer to steering , That is, related topics

@since： From which version to add
@param： Description of a parameter in the method , You can't write without parameters
@return Description of the return value of the method , If the return value type of the method is void Can't write

@exception： Describe the exceptions that may be thrown by the method , If the method doesn't work throws An exception thrown explicitly cannot be written in it

Be careful ：

@param @return and @exception All three tags are method only .

@param The format of :@param Parameter name parameter type parameter description
@return The format of :@return Return value type return value description

@exception The format of :exception Exception type description

@param and @exception You can have more than one side by side
 

Example 2 : Format check at compile time (JDK Built in three basic annotations )

@Override: Qualify override parent method , This annotation can only be used for methods
@Deprecated: Used to represent the decorated element ( class , Such method ) Deprecated . It's usually because of the danger of the decorated structure or the existence of a better choice
@SuppressWarnings: Suppress compiler warnings
 

Example 3 : Trace code dependencies , Implement alternative profile functions

Servlet3.o Notes are provided (annotation), Make it no longer necessary to web.xml In file Servlet Deployment of .
 

Custom annotation ：

reference @SupperWarning

1. The annotation states that :@interface
2. Internally defined members , Usually use value Express
3. You can specify default values for members , Use default Definition
4. If the custom annotation has no members , It's a sign .
    

Define a new Annotation Type used @interface keyword

Custom annotations automatically inherit java.lang.annotation.Annotation Interface

Annotation The member variable of is in Annotation To declare in the form of a parameterless method in a definition . Its method name and return value define the name and type of the member . We call it configuration parameters . Types can only be eight basic data types 、String type 、class type 、enum type 、Annotation type 、 All the above types of arrays .

You can define Annotation The initial value is specified for the member variable of , To specify the initial value of a member variable, you can use default keyword

If there is only one parameter member , It is recommended to use The parameter name is value

If the defined annotation contains configuration parameters , Then the parameter value must be specified when using , Unless it has a default value . The format is “ Parameter name = Parameter values ”, If there is only one parameter member , And the name is value, It can be omitted “value=”

No member defined Annotation be called Mark ; Containing member variables Annotation It's called metadata Annotation

Be careful : User defined annotation must be accompanied by annotation information processing flow to make sense .
 

Two meta annotations will be indicated by custom annotation : Retention（ Life cycle ）. Target（ Scope ）
 

Custom annotation examples

public @interface MyAnnotation {
    String [] value();
}

Specify default

public @interface MyAnnotation {
    String [] value() default "hello";
}

@Override  There are no members 

public @interface Override {
}

Yuan notes

Annotations that annotate existing annotations  

JDK Yuan Annotation Used to embellish other things Annotation Definition
 

JDK5.0 Provides 4 A standard meta-annotation type , Namely :

• Retention： Specify the decorated Annotation Life cycle of :SOURCE\CLASS（ Default behavior )\RUNTIME                          Only declared as RUNTIME Life cycle notes , Only by reflection can we get .
• Target： Used to specify decorated Annotation Which program elements can be used to decorate
• The following two are less used
  • Documented： Indicates that the modified annotation is being javadoc When parsing , Keep it .
  • Inherited： Decorated with it Annotation Will be inherited .
     

@Retention

Can only be used to decorate one Annotation Definition , Used to specify the Annotation Life cycle of , @Rentention Contains a RetentionPolicy A member variable of type , Use
@Rentention When you have to be value The member variable specifies the value :

• RetentionPolicy.SOURCE: Valid in source file （ That is, source file retention ）, The compiler simply discards comments for this strategy
• RetentionPolicy.CLASS: stay class Valid in file ( namely class Retain ), When running Java The program ,JVM Comments will not be retained . This is the default
• RetentionPolicy.RUNTIME: Valid at run time （ That is, keep it at run time ）, When running Java The program ,JVM The comments will be kept . The program can get this comment through reflection .
   

for example @SuppressWarnings() Source code

@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.SOURCE)
public @interface SuppressWarnings {
    /**
     * The set of warnings that are to be suppressed by the compiler in the
     * annotated element.  Duplicate names are permitted.  The second and
     * successive occurrences of a name are ignored.  The presence of
     * unrecognized warning names is <i>not</i> an error: Compilers must
     * ignore any warning names they do not recognize.  They are, however,
     * free to emit a warning if an annotation contains an unrecognized
     * warning name.
     *
     * <p> The string {@code "unchecked"} is used to suppress
     * unchecked warnings. Compiler vendors should document the
     * additional warning names they support in conjunction with this
     * annotation type. They are encouraged to cooperate to ensure
     * that the same names work across multiple compilers.
     * @return the set of warnings to be suppressed
     */
    String[] value();
}

public enum RetentionPolicy {
    /**
     * Annotations are to be discarded by the compiler.
     */
    SOURCE,

    /**
     * Annotations are to be recorded in the class file by the compiler
     * but need not be retained by the VM at run time.  This is the default
     * behavior.
     */
    CLASS,

    /**
     * Annotations are to be recorded in the class file by the compiler and
     * retained by the VM at run time, so they may be read reflectively.
     *
     * @see java.lang.reflect.AnnotatedElement
     */
    RUNTIME
}

@Target

Used to modify Annotation Definition , Used to specify decorated Annotation Which program elements can be used to decorate .@Target It also contains a name called value Member variables of .
 

Value		Value
CONSTRUCTOR	Used to describe a constructor	PACKAGE	Used to describe a package
FLELD	Used to describe domain	PARAMETRE	Used to describe parameters
LOCAL_VARIABLE	Used to describe local variables	TYPE	Used to describe a class 、 Interface （ Including annotation types ） or enum Statement
METHOD	Used to describe a method

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
    /**
     * Returns an array of the kinds of elements an annotation type
     * can be applied to.
     * @return an array of the kinds of elements an annotation type
     * can be applied to
     */
    ElementType[] value();
}

public enum ElementType {
    /** Class, interface (including annotation type), or enum declaration */
    TYPE,

    /** Field declaration (includes enum constants) */
    FIELD,

    /** Method declaration */
    METHOD,

    /** Formal parameter declaration */
    PARAMETER,

    /** Constructor declaration */
    CONSTRUCTOR,

    /** Local variable declaration */
    LOCAL_VARIABLE,

    /** Annotation type declaration */
    ANNOTATION_TYPE,

    /** Package declaration */
    PACKAGE,

    /**
     * Type parameter declaration
     *
     * @since 1.8
     */
    TYPE_PARAMETER,

    /**
     * Use of a type
     *
     * @since 1.8
     */
    TYPE_USE
}

@Documented

Used to specify the Annotation Embellished Annotation Class will be javadoc Tools to extract documents . By default ,javadoc It doesn't include comments .

Defined as Documented The annotation for must be set Retention The value is RUNTIME.

@Inherited

Decorated with it Annotation Will be inherited . If a class uses the @lnherited Embellished Annotation, The subclass will automatically have the annotation .

• such as : If you mark it with @Inherited Custom annotations for annotations are annotated at the class level , Subclasses can inherit annotations at the parent class level
• Practical application , Use less
   

jdk 8 New features of annotations in ：

Repeatable notes

@MyAnnotation(value = "hello")
@MyAnnotation(value = "hi")
class Person {
    private String name;
    private int  age;}

Writing this directly will report an error

terms of settlement ：

stay jdk8 before

1. Declare the value in the annotation as an array of annotation types

public @interface MyAnnotations {
        MyAnnotation[] value();
}

2. Put two comments in

@MyAnnotations({@MyAnnotation(value = "hello"),@MyAnnotation(value = "hi")})
class Person {
    private String name;
    private int  age;}

stay jdk8 in the future

stay MyAnnotation Statement above @Repeatable, The member value is MyAnnotations.class

MyAnnotation Of @Target and @Retention@Inherited It has to be with MyAnnotations identical .

@Repeatable(MyAnnotations.class)
public @interface MyAnnotation {
    String [] value() default "hello";
}
public @interface MyAnnotations {
        MyAnnotation[] value();
}
@MyAnnotation(value = "hello")
@MyAnnotation(value = "hi")
class Person {
    private String name;
    private int  age;
}

Type notes
 

JDK1.8 after , About meta annotations @Target Parameter type ElementType Enumeration value is two more individual

• TYPE_PARAMETER
• TYPE_USE

stay Java 8 Before , Annotations can only be used where they are declared ,Java8 Start , Annotations can be applied anywhere .

• ElementType.TYPE_PARAMETER Indicates that the annotation can be written in the declaration statement of type variable （ Such as : Generic declaration ）
• ElementType.TYPE_USE Indicates that the annotation can be used in any statement using the type .