Java Learning Note 11--annotation

Java Learning Note 11--annotation

Annotation: A new feature added after JDK1.5, known as the metadata feature, is called a comment after JDK1.5, that is, the use of annotations to add information about some programs.

The Java.lang.annotation annotation interface is an interface that all annotation must implement.

System built-in annotation

After JDK1.5, the system has established the following three built-in annotation types, users can directly use.

@Override: Overwrite the annotation

@Deprecated: annotation used in disapproval

@SuppressWarnings: annotation to suppress safety warnings

Custom annotation

Annotation Definition Format:

"Public" @interface annotation name {

Data type variable name ();

}

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1. Public @interface meaning {
2. String value ();
3. }

After that, you can use the @meaning format directly in your program.

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1. @Meaning (value="Itmyhome")
2. Class demo{
4. }

You can set a parameter in annotation to receive the contents of the variable, such as value above, and when using annotation, you must assign a value to the parameter such as: Value= "Itmyhome"

Since you can set a parameter, you can also set multiple parameters.

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1. Public @interface meaning {
2. String key ();
3. String value ();
4. }

This annotation need to set two parameters when using, a key one value

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1. @Meaning (key="Hi", value="Itmyhome")
2. Class demo{
4. }

You can also set an array in

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1. Public @interface meaning {
2. String[] Value ();
3. }

The received content to pass the array

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1. @Meaning (value={"Hello","World"})
2. Class demo{
4. }

There is a feature in all the annotation defined above, all the parameter contents need to be set up when using the annotation, then you can set the default content for a parameter, use default when declaring.

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1. Public @interface meaning {
2. String value () default ""; //default is empty
3. }

You can not set a value when you use it

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1. @Meaning
2. Class demo{
4. }

In the operation, for a annotation, sometimes the fixed period of the value range, only a fixed number of values, this time actually need to rely on enumeration.

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1. Public enum Formitemtype { //define enum type
2. Hidden,text,select,date
3. }

define Annotation

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1. Public @interface meaning {
2. Formitemtype value (); //Set to enum type
3. }

The value of the annotation can only be a value in the enumeration type

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1. @Meaning (Value=formitemtype.date)
2. Class demo{
4. }

Retention and Retentionpolicy

In annotation, you can use retention to define the save scope of a annotation, which is defined as follows:

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1. @Retention (Retentionpolicy.runtime)
2. @Target (Elementtype.field)
3. Public @interface meaning {
4. Formitemtype value (); //Set to enum type
5. }

In the retetion definition above, there is a retentionpolicy variable, which is used to specify annotation's save range, Retentionpolicy contains three ranges

Of the three ranges, the most important one is the runtime range, because it works at the time of execution.

Built-in annotation retentionpolicy

Three built-in definitions of annotation:

The override definition uses @retention (retentionpolicy.source) to appear only in the source file

The deprecated definition uses @retention (retentionpolicy.runtime), which can occur at execution time.

The suppresswarnings definition is @retention (retentionpolicy.source) and can only appear in the source file

A annotation if you want to make it meaningful, you must combine the reflection mechanism to get all the content set in the annotaion.

There are several methods in class that are related to annotation operations

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2. Package com.itmyhome;
6. Import java.lang.annotation.Annotation;
8. Import Java.lang.reflect.Method;
12. Class demo{
14. @SuppressWarnings ("unchecked")
16. @Deprecated
18. @Override
20. Public String toString () {
22. return "Hello";
24. }
26. }
30. Public class T {
32. public static void Main (string[] args) throws exception{
34. class<?> C = class.forname ("Com.itmyhome.Demo");
36. Method MT = C.getmethod ("toString"); //Find the ToString method
38. Annotation an[] = mt.getannotations (); //Get all the annotation
40. For (Annotation a:an) {
42. System.out.println (a);
44. }
48. }
50. }

At this point a annota has been made. The above operations are actually done through the built-in annotation of three systems, or you can customize a annotation

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1. @Retention (Retentionpolicy.runtime)
2. @Target (Elementtype.method)
3. Public @interface meaning {
4. Formitemtype value (); //Set to enum type
5. }

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2. Package com.itmyhome;
6. Import Java.lang.reflect.Method;
10. Class demo{
12. @Meaning (Value=formitemtype.select) //Custom annotation
14. @SuppressWarnings ("unchecked")
16. @Deprecated
18. @Override
20. Public String toString () {
22. return "Hello";
24. }
26. }
30. Public class T {
32. public static void Main (string[] args) throws exception{
34. class<?> C = class.forname ("Com.itmyhome.Demo");
36. Method MT = C.getmethod ("toString"); //Find the ToString method
40. //The specified comment is present on this element
42. if (mt.isannotationpresent (meaning. Class)) {
44. meaning M = mt.getannotation (meaning.   Class); //Get the specified annotation
46. System.out.println (M.value ()); //Get the value of annotation
48. }
52. }
54. }

@Target

Indicates the kind of program element that the annotation type applies to. If a Target meta comment does not exist in the annotation type declaration, the declared type can be used on either program element. If such meta-annotations exist, the compiler enforces the specified usage restrictions, such as: @Target (Elementtype.annotation_type)

ElementType Range of storage

Java Learning Note 11--annotation