Scenarios for annotation ：
Yes web We all know about it , Many development frameworks use annotations , such as Spring The code is as follows ：
@RequestMapping(value="/inputPerson") public String inputPerson(){
System.out.println("..........inputPerson"); return "inputPerson"; }
@RequestMapping(value="/updatePerson") public String updatePerson(){
System.out.println("..........updatePerson"); return "updatePerson"; }
In this code RequestMapping It's just a note , Note with @ Symbolic modification , This annotation applies to methods , Inside value For annotation assignment .
annotation type ：
1： Standard notes ：
Override effect ： Guaranteed compile time Override The correctness of function declaration .
Deprecated effect ： Guaranteed compile time Override The correctness of function declaration .
SuppressWarnings Turn off specific warnings , His main parameters are ：
1)：deprecation： Warning when obsolete classes or methods are used
2)：unchecked Warning when an uncalibrated conversion is performed
3)：path In classpath , There is a warning that there is a non-existent path such as the source file path
4)：serial When a class in a sequenceable speech is missing
5)：serialVersionUID Warning when defining
6)：finally whatever finally Warning when clause cannot complete properly
7)：all Warning on all of the above
Meta annotation ：( Responsible for annotation and other notes )
@Retention ( main parameter ：SOURCE CLASS RUNTIME)
@Target
( main parameter ：CONSTRUCTOR,FIELD,LOCAL_VARIABLE,METHOD,PACKAGE,PARAMETER,TYPE)
@Documented (jiang Include comments in Javadoc in )
@Inhertied( Allow subclasses to inherit comments from parent classes )
Look at the following example ：
We define an annotation , This annotation applies to all fields Target Of value The value is ElementType.FIELD
import java.lang.annotation.ElementType; import
java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target; @Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME) public @interface FiledAnnotation { String
userName() default "zhangming"; String passWord() default "123456"; String
unit() default "android"; }
definition Person Using annotations , And by annotating to Person Object's field assignment ：
public class Person { @FiledAnnotation(userName=" Xiao Ming ") public String userName;
@FiledAnnotation(passWord="abc123456") public String passWord;
@FiledAnnotation(unit="java") public String unit; public String getUserName() {
return userName; } public void setUserName(String userName) { this.userName =
userName; } public String getPassWord() { return passWord; } public void
setPassWord(String passWord) { this.passWord = passWord; } public String
getUnit() { return unit; } public void setUnit(String unit) { this.unit = unit;
} @Override public String toString() { return "Person [userName=" + userName +
", passWord=" + passWord + ", unit=" + unit + "]"; } }
Test results to see if the annotation works correctly ：
public static void main(String[] args) { // Create a Person object Person person = new
Person(); // obtain person The field array corresponding to the object Field[] fields = person.getClass().getFields();
for(int i = 0;i < fields.length;i++){
// Judge whether it is currently userName field , also userName Is the annotation in the field yes FiledAnnotation
if(fields[i].isAnnotationPresent(FiledAnnotation.class) &&
fields[i].getName().equals("userName")){ FiledAnnotation filedAnnotation =
fields[i].getAnnotation(FiledAnnotation.class);
System.out.println(filedAnnotation.userName());
// Judge whether it is currently passWord field , also userName Is the annotation in the field yes FiledAnnotation }else
if(fields[i].isAnnotationPresent(FiledAnnotation.class) &&
fields[i].getName().equals("passWord")){ FiledAnnotation filedAnnotation =
fields[i].getAnnotation(FiledAnnotation.class);
System.out.println(filedAnnotation.passWord());
// Judge whether it is currently unit field , also userName Is the annotation in the field yes FiledAnnotation }else
if(fields[i].isAnnotationPresent(FiledAnnotation.class) &&
fields[i].getName().equals("unit")){ FiledAnnotation filedAnnotation =
fields[i].getAnnotation(FiledAnnotation.class);
System.out.println(filedAnnotation.unit()); } } }
Through the above example, the console print results are as follows ：
Xiao Ming
abc123456
java
Here we are Person Object to assign the current object field with annotation , Get the field name of the current object by reflection method , Then call isAnnotationPresent Determine whether the current field contains FiledAnnotation annotation , If it is , Finally, it was approved getAnnotation（） Method FiledAnnotation example , Last output current person The value assigned by the object annotation .
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