Java 5.0 基础 - Annotation

Reference Doc,

Official Site: http://java.sun.com/j2se/1.5.0/docs/guide/language/annotations.html

优快云 Site: http://blog.youkuaiyun.com/numberpig/archive/2006/08/18/1095144.aspx

 

Quick Start

 Example: we want to map the JavaBean user with the DB2 table USER

 1) TableInfo Annotation

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface TableInfo {
	String value(); // Element declaration
}

2)  FieldInfo Annotation

// set to Runtime, then we can load those Annotation info by jave reflection
@Retention(RetentionPolicy.RUNTIME)  
@Documented  
public @interface FieldInfo {  
    String FieldName();  
    Class<?> FieldType();
    String FieldOper();
}  

3) JavaBean User.java

@TableInfo("USER")
public class User {
	private String name = null;
	private Date birth = null;
	
	@FieldInfo(FieldName="USERNAME", FieldType=String.class, FieldOper="SET")
	public void setName(String name){
		this.name = name;
	}
	@FieldInfo(FieldName="USERNAME", FieldType=String.class, FieldOper="GET")
	public String getName(){
		return this.name;
	}
	
	@FieldInfo(FieldName="BIRTHDAY", FieldType=Date.class, FieldOper="SET")
	public void setBirth(Date date){
		this.birth = date;
	}
	
	@FieldInfo(FieldName="BIRTHDAY", FieldType=Date.class, FieldOper="GET")
	public Date getBirth(Date date){
		return this.birth;
	}
}

3) Test get Annotation Info  

@Test
public void UserInfoTest() throws ClassNotFoundException, IllegalAccessException, InstantiationException{
  String className = "annotation.User";		
  Class<User> urClass = User.class;
  // for type safe
  User u = urClass.cast( Class.forName(className).newInstance() );
	
  if ( u.getClass().isAnnotationPresent( TableInfo.class ) ){
  	TableInfo tf = urClass.getAnnotation( TableInfo.class );
  	assertTrue("DB table is USER", "USER".equalsIgnoreCase(tf.value()));
  }
		
  Method[] methods = urClass.getMethods();
  for(Method m : methods){
  	FieldInfo finfo = null;
	if(m.isAnnotationPresent( FieldInfo.class ) && m.getName().indexOf("Name") >= 0 ){
	  finfo = m.getAnnotation( FieldInfo.class );
	  assertTrue("DB Field Name is USERNAME", "USERNAME".equals(finfo.FieldName()));
	  assertTrue("DB varchar mapping with String", String.class == finfo.FieldType() );
	  if(m.getName().equals("getName")) 
	     assertTrue("Operation is Set", "GET".equals(finfo.FieldOper()));
	  if(m.getName().equals("setName")) 
	     assertTrue("Operation is Get", "SET".equals(finfo.FieldOper()));
	}
		
	if(m.isAnnotationPresent( FieldInfo.class ) && m.getName().indexOf("Birth") >= 0 ){
	  finfo = m.getAnnotation( FieldInfo.class );
	  assertTrue("DB Field Name is BIRTHDAY", "BIRTHDAY".equals(finfo.FieldName()));
	  assertTrue("DB Date mapping with Date", Date.class == finfo.FieldType() );
	  if(m.getName().equals("getBirth")) 
	      assertTrue("Operation is Set", "GET".equals(finfo.FieldOper()));
	  if(m.getName().equals("SetBirth")) 
	      assertTrue("Operation is Get", "SET".equals(finfo.FieldOper()));
	}
    }
}

 

Annotation Defination,

Annotation Type

/**
 * Describes the Request-For-Enhancement(RFE) that led
 * to the presence of the annotated API element.
 */
public @interface RequestForEnhancement {
    int    id(); // annotation element declearation, id is the name, int is type
    String synopsis();
    String engineer() default "[unassigned]"; 
    String date();    default "[unimplemented]"; 
}

Annotation type declarations are similar to normal interface declarations

a. Each method declaration defines an element of the annotation type.

b. Method declarations must not have any parameters or a throws clause

c. Return types are restricted to primitives, String, Class, enums, annotations, and arrays of the preceding types 

d. Methods can have default values

Annotation

@RequestForEnhancement(
    id = 2868724 // annotate the element declaration
    synopsis = "Enable time-travel",
    engineer = "Mr. Peabody",
    date     = "4/1/3007"
)
public static void travelThroughTime(Date destination) { ... }

An annotation is a special kind of modifier, and can be used anywhere that other modifiers (such as public, static, or final) can be used, and precede other modifiers

a.  Annotations consist of an at-sign (@) followed by an annotation type and a parenthesized list of element-value pairs

b. The values must be compile-time constants.

Marker_Annotation_Type

The Annotation Type with no elements

/**
 * Indicates that the specification of the annotated API element
 * is preliminary and subject to change.
 */
public @interface Preliminary { }
 

 Meta-Annotation

 Meta-Annotation is a special kind of Annotation to declare the Annotation Type

 1. Target Meta-Annotation

     Target Annotation Type Source Code 

@Documented  // Copy the message of this Annotation Type into Java API
@Retention(RetentionPolicy.RUNTIME)  
@Target(ElementType.ANNOTATION_TYPE)  
public @interface Target {  
       ElementType[] value();  
}  

      ElementType Source code     

 public enum ElementType {  
  TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR,  
  LOCAL_VARIABLE, ANNOTATION_TYPE,PACKAGE  
 }  

      it defines the following,

         a. Defined the effective scale(On Method, FIELD......) of the annotation declared  by Target.

      Example     

 @Target(ElementType.METHOD)  
 @Retention(RetentionPolicy.RUNTIME)  
 @Documented  
 public @interface Name {  
     String originate();  
     String community();  
 }  

     The Annotation "Name" can only effect on Method level

     Focus that, if the Annotation didn't specify the Target, the Annotation effect on any level declare in ElementType

 2. Rentation Meta-Annotation

     Rentation Source Code

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

     RetationPolicy Source Code  

 public enum RetentionPolicy {  
  SOURCE,  
  CLASS,  
  RUNTIME  
 }  

     It defines the annotation active scare,

     a.  SOURCE - the annotation only retained in Source code, after complie, it will lost

     b.  CLASS    - the annotation retained in both Souce code and Classes

     c.   RUNTIM - the annotaion reatined in both above, and it will be loaded into JVM. Then we can use the Reflection to retrieve the annotation info.