Java Annotations @retention @interface meta-data

Original http://www.cnblogs.com/coolgun/p/3802535.html

Java annotations

That is, the comment, Baidu explained: Also called meta-data. A description of the code level. Personal understanding: is a comment that the content can be understood by the code, which is generally a class.

Meta data

Also called meta-annotations, is placed in front of a defined annotation class, is a restriction on annotations. Only two: @Retention and @Target (I only found these two).

@Retention: Used to describe the life cycle of the annotation class. It has the following three parameters:

Retentionpolicy.source: Annotations remain in the source file only

Retentionpolicy.class: Annotations remain in the CLASS file and discarded when loaded into the JVM virtual machine

Retentionpolicy.runtime: Annotations remain during a program run, at which point all annotations defined on a class can be obtained through reflection.

@Target: Used to indicate that the annotation can be declared before those elements.

Elementtype.type: Note that the note can only be declared before a class.

Elementtype.field: Note that the note can only be declared in front of a field in a class.

Elementtype.method: Indicates that the annotation can only be declared before a method of a class.

Elementtype.parameter: Note that the note can only be declared before a method parameter.

Elementtype.constructor: Note that the annotation can only be declared before the constructor of a class.

Elementtype.local_variable: Note that the note can only be declared before a local variable.

Elementtype.annotation_type: Note that the note can only be declared before an annotation type.

Elementtype.package: Note that the note can only be declared before a package name.

Definition of annotations:

The definition of the annotation uses the keyword @interface, and @rentention (ARG) or @target (args) in the line above, as in the following example:

@Rentention (Retentionpolicy.runtime)

Public @interface Annotation01 {

Defining a public final static property

.....

public abstract methods.

......

}

Common basic built-in annotations

@Override When we want to rewrite a method, add @override to the method, and when the name of our method goes wrong, the compiler will error. Defined as follows:

@Retention (Retentionpolicy.source)

Public @interface Override

@Deprecated used to indicate that a property or method of a class is obsolete and is not intended to be used by others, and is decorated with @deprecated on properties and methods. Defined as follows:

@Retention (Retentionpolicy.source)

Public @interface Deprecated

@SuppressWarnings used to suppress warnings from the program. Defined as follows:

@Retention (Retentionpolicy.source)

Public @interface suppresswarnings

Use of annotations

Define annotations--note--get annotations;

The obtained annotations are obtained through the Getannotation or Getannotations method of the modified object.

Java Annotations @retention @interface meta-data