How to declare Java meta-annotation Retention

1. Annotation declaration: An annotation can be declared through @interface.

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface BindView {
    int value();
}

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Get {
    String value() default "";
}

@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
public @interface Queue {
    String value() ;
}

2. @Target meta-annotation, annotation of annotation, its value is defined in the ElementType enumeration class.

@Target annotation is used to define the location of our custom annotation code.

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.ANNOTATION_TYPE})
public @interface Target {
    ElementType[] value();
}

1) ElementType.FIELD is used on member variables.

2) ElementType.METHOD is used on member methods.

3) ElementType.PARAMETER is used on method parameters.

4) ElementType.TYPE is used on classes and interfaces.

5) ElementType.ANNOTATION_TYPE is used in annotations.

3.@Retention meta-annotation, the value is defined in the RetentionPolicy enumeration class.

is used to define the stage in which annotations take effect:

1) SOURCE: Annotations are only valid in the source code stage and will not be compiled into bytecode.

2) CLASS: Annotations are valid in the source code and bytecode stages, but do not exist in the running stage.

3) RUNTIME: Annotations are valid in source code, bytecode, and runtime phases, and are also the most commonly used.

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.ANNOTATION_TYPE})
public @interface Retention {
    RetentionPolicy value();
}

public enum RetentionPolicy {
    SOURCE,
    CLASS,
    RUNTIME;
    private RetentionPolicy() {
    }
}

2. Use of annotations

    @BindView(R.id.start_activity)
    TextView startTextView;

    @Get("http://www.baidu.com")
    Call getPerson(@Queue("name") String name,@Queue("200")int price);
    @Get("http://www.baidu.com")
    Call getPerson();

The use of annotations is very simple.

Annotations alone have no meaning and must be used in conjunction with other technologies.

Application:

1) Annotate Apt annotation processor, produce java code, databinding, butterknife, dagger2 hilt

2) Annotation code burying point

3) Annotate reflection dynamic proxy retrofit xUtils lifecycle

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