About Annotation

Java 5引入了Annotation, 這極大減輕了開發的負擔，不用寫很多的代碼，只需要在代碼中加入一些"tag"就可以了，這個很符合聲明式編程(declarative programming)的思想。

當然有個問題還是要問的，雖然作為使用annotation的開發者來說，不用考慮Annotation最后究竟會被怎樣執行，只要在需要使用Annotation的地方tag一下就可以，但是很顯然這個只是把問題處理地點轉移了而已，終歸是需要有人來處理的，不然，這個Annotation就沒有任何意義了。

可以想見，很多JDK提供的annotation, JVM或者編譯器肯定是要負責解釋處理這些anntation的。如果使用一些framework提供的annotation, 自然這些framework會提供解釋執行這些annotation的代碼實現，否則JVM怎么可能知道如何去解釋這些tag呢。那么究竟如何去識別代碼中哪些地方使用了annotation呢，很容易想到可以運用reflection來實現。

所以，annotation看起來很神秘，其實只是借助了reflection，將相關的代碼實現處理邏輯封閉在一個地方，從而使得運用這些annoation的代碼不需要重復實現這些功能，從而簡化了工作量。

The Rules of Defining Java Annotation Types

// MyAnnotation.java
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface MyAnnotation {
public String doSomething() default "do something";
public String date();
}

上面是一個簡單的Annotation定義，Annotation的定義很特別，很像interface,不過需要在interface前面加個@符號。另外annotation中定義的method (attribute) 不能是private的，不能接受參數，不能有throws語句，返回值的類型必須是如下幾種之一：

1）primitives

2) ?String

3) ?Class

4) Enum

5) Array of the above types

Java Annotation Types

Java 支持兩種Annotation類型：

1） 簡單類型：這種annotation只能用于其它code，不能用于annotation 類型。

2）Meta Annotation: 這種annotation是用于annotation type的，也就是annotation of annotation.

Out-of-the-box Simple Annotation Types in JDK

JDK提供了3個簡單類型的annotations, 包括：

1) Override

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}

　　

可以看到Override只能用于method, 而且retention級別是SOURCE。這個Annotation的好處在于可以幫助我們快速發現問題，如果把toString寫成了toString1，編譯器會給出錯誤提示，這樣就不要等到運行的時候發現出錯了?！　?/p>

2) Deprecated

@Documented
@Retention(RetentionPolicy.RUNTIME)
public @interface Deprecated {
}

　如果一個方法或者變量被標示成Deprecated，那就意味著該方法或者變量是不應該被使用的，否則的話，編譯器會報出警告信息?！?/p>

3) Suppresswarnings

@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.SOURCE)
public @interface SuppressWarnings {
/**
* The set of warnings that are to be suppressed by the compiler in the
* annotated element. Duplicate names are permitted. The second and
* successive occurrences of a name are ignored. The presence of
* unrecognized warning names is <i>not</i> an error: Compilers must
* ignore any warning names they do not recognize. They are, however,
* free to emit a warning if an annotation contains an unrecognized
* warning name.
*
* <p>Compiler vendors should document the warning names they support in
* conjunction with this annotation type. They are encouraged to cooperate
* to ensure that the same names work across multiple compilers.
*/
String[] value();
}

這個Annotation是讓編譯器忽略某些warning信息?！　?/p>

Out-of-the-box Meta Annotation Types in JDK

JDK提供了如下4種Meta Annotation Types:

1) Target

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

這個Annotation用來標示定義的annotation類型可以用在哪些類型的元素上，包括如下幾種....

-- @Target(ElementType.TYPE)

-- @Target(ElementType.FIELD) : public attributes of the class

-- @Target(ElementType.METHOD)

-- @Target(ElementType.PARAMETER)

-- @Target(ElementType.CONSTRUCTOR)

-- @Target(ElementType.LOCAL_VARIABLE)

-- @Target(ElementType.ANNOTATION_TYPE)

　

2) Retention

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

這個Annotation表示定義的annotation會retention的時間，有如下三種值：

-- RententionPolicy.SOURCE: retained at the source level, ignored by the compiler

-- RententionPolicy.CLASS: retained by the compiler, ignored by the VM

-- RententionPolicy.RUNTIME: retained by the VM, can be read only at run-time.

3) Documented

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

表示定義的annotation type應該被javadoc加入到生成的document中的?！　?/p>

4) Inherited

/*** Indicates that an annotation type is automatically inherited. If* an Inherited meta-annotation is present on an annotation type* declaration, and the user queries the annotation type on a class* declaration, and the class declaration has no annotation for this type,* then the class's superclass will automatically be queried for the* annotation type. This process will be repeated until an annotation for this* type is found, or the top of the class hierarchy (Object)* is reached. If no superclass has an annotation for this type, then* the query will indicate that the class in question has no such annotation.** <p>Note that this meta-annotation type has no effect if the annotated* type is used to annotate anything other than a class. Note also* that this meta-annotation only causes annotations to be inherited* from superclasses; annotations on implemented interfaces have no* effect.** @author Joshua Bloch* @since 1.5*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Inherited {
}

A Simple Example

定義一個簡單的Annotation Type -- MyAnnoation

// MyAnnotation.java
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface MyAnnotation {
public String doSomething() default "do something";
public String date();
}

　　

通過Reflection來獲取annotation定義的field...

// MainTest.java
public class MainTest {

@MyAnnotation(doSomething = "Test Annotation", date = "2011-08-01")
public String annotationTest;

public void testAnnotation() {
Class aClass = MainTest.class;
try {
Field aField = aClass.getField("annotationTest");
MyAnnotation myAnnotation = aField.getAnnotation(MyAnnotation.class);

annotationTest = myAnnotation.doSomething() + " on " + myAnnotation.date();
System.out.println(this.annotationTest);
} catch (SecurityException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (NoSuchFieldException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}


}
/**
* @param args
*/
public static void main(String[] args) {
new MainTest().testAnnotation();
}

}

　　

　　

轉載于:https://www.cnblogs.com/fangwenyu/archive/2011/08/01/2124310.html

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