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Java注解详解

Aliven888 2022-04-06 阅读 86


注解相当于一个特殊的类。

定义一个简单的注解并使用,细节在注释中:

MyFirstAnnotation.java

/**
* java在编译的过程中可能会把一些注解文件丢掉,使用
* @Retention(RetentionPolicy.RUNTIME)注解让该注解一直保存到运行阶段
* @Retention(RetentionPolicy.CLASS),保存到编译阶段,默认即此
* @Retention(RetentionPolicy.SOURCE)保存到源文件阶段
*
*@Target({ElementType.METHOD,ElementType.TYPE})表示
*该注解可以在类上,也可以在方法上
*
*/
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD,ElementType.TYPE})
public @interface MyFirstAnnotation {

}

AnnotationTest.java

@MyFirstAnnotation
public class AnnotationTest {

@Test
public void test3(){
if (AnnotationTest.class.isAnnotationPresent(MyFirstAnnotation.class)) {
MyFirstAnnotation mfa = AnnotationTest.class
.getAnnotation(MyFirstAnnotation.class);
System.out.println(mfa);
}
}
}

输出:

@lenve.test.MyFirstAnnotation()

为注解增加一个方法,并且调用该方法:

MyFirstAnnotation.java

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD,ElementType.TYPE})
public @interface MyFirstAnnotation {

//公共的抽象的方法返回一个字符串,前面的东西省略
String color();
}
@MyFirstAnnotation(color="red")
public class AnnotationTest {

@Test
public void test3(){
if (AnnotationTest.class.isAnnotationPresent(MyFirstAnnotation.class)) {
MyFirstAnnotation mfa = AnnotationTest.class
.getAnnotation(MyFirstAnnotation.class);
System.out.println(mfa.color());
}
}
}

输出:

red

注解返回枚举类型和注解类型:

MyFirstAnnotation.java

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD,ElementType.TYPE})
public @interface MyFirstAnnotation {

//公共的抽象的方法返回一个字符串,前面的东西省略
String color() default "blue";
String value();
int[] arrayAttr() default {1,2,3};
MyColorEnum retEnum() default MyColorEnum.BLUE;
MySecondAnnotation retAnnotation() default @MySecondAnnotation("zhangsan");
}

MyColorEnum.java

public enum MyColorEnum {
RED,BLUE,GREEN,YELLO,GRAY;
}

MySecondAnnotation.java

public @interface MySecondAnnotation {
String value();
}

AnnotationTest.java

@MyFirstAnnotation(retAnnotation=@MySecondAnnotation("lisi"),color = "red", value = "abc", arrayAttr = { 4, 5, 6 }, retEnum = MyColorEnum.RED)
public class AnnotationTest {

@Test
// 如果方法名为value可以省略
@MyFirstAnnotation("xyz")
public void test3() {
if (AnnotationTest.class.isAnnotationPresent(MyFirstAnnotation.class)) {
MyFirstAnnotation mfa = AnnotationTest.class
.getAnnotation(MyFirstAnnotation.class);
System.out.println(mfa.color() + "," + mfa.value());
System.out.println(mfa.retEnum());
System.out.println(mfa.retAnnotation().value());
}
}
}



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