JAVA Annotation

待我称王封你为后i 2022-03-11 14:12 162阅读 0赞

介绍

尤其是我们在学习Spring时，都绕不开Annotation，而且使用的非常频繁，并且给我们带来很大的便利， 所以我们有必要了解JAVA Annotation。

在此，我们自定义两个Annotation：一个是Class相关的Annotation；另一个是Method相关的Annotation。然后，我们写一个类来使用那两个Annotation，这样我们就能比较快速的了解Annotation。

自定义一个Class相关的Annotation：Message.java

package shuai.study.annotation;
    
    import java.lang.annotation.Documented;
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    public @interface Message {
    	String mess();
    }

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对于上面的Annotation，你可能不清楚@Target(ElementType.TYPE) 和@Retention(RetentionPolicy.RUNTIME) 是什么意思，那么我们就可以看一下JDK的源码：

public enum ElementType {
        /** Class, interface (including annotation type), or enum declaration */
        TYPE,
    
        /** Field declaration (includes enum constants) */
        FIELD,
    
        /** Method declaration */
        METHOD,
    
        /** Parameter declaration */
        PARAMETER,
    
        /** Constructor declaration */
        CONSTRUCTOR,
    
        /** Local variable declaration */
        LOCAL_VARIABLE,
    
        /** Annotation type declaration */
        ANNOTATION_TYPE,
    
        /** Package declaration */
        PACKAGE
    }

public enum RetentionPolicy {
        /**
         * Annotations are to be discarded by the compiler.
         */
        SOURCE,
    
        /**
         * Annotations are to be recorded in the class file by the compiler
         * but need not be retained by the VM at run time.  This is the default
         * behavior.
         */
        CLASS,
    
        /**
         * Annotations are to be recorded in the class file by the compiler and
         * retained by the VM at run time, so they may be read reflectively.
         *
         * @see java.lang.reflect.AnnotatedElement
         */
        RUNTIME
    }

JDK的源码就是NB，写得非常清楚！

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另一个Method相关的Annotation：Employee.java

package shuai.study.annotation;
    
    import java.lang.annotation.Documented;
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target(ElementType.METHOD)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    public @interface Employee {
    	String name() default "XXX";
    
    	int budget() default 0;
    }

再写一个类Company.java，来使用上面的两个Annotation：

package shuai.study.annotation;
    
    @Message(mess = "NB company publish month start, please pay attention to this message")
    public class Company {
    	@Employee(name = "SB", budget = 6666)
    	public void monthStar() {
    		System.out.println("向钱看，向厚赚！");
    	}
    }

最后再来一个测试启动类AnnotationTest.java：

package shuai.study.annotation;
    
    import java.lang.reflect.Method;
    
    public class AnnotationTest {
    
    	public static void main(String[] args) throws Exception {
    		Class<?> companyClass = Class.forName("shuai.study.annotation.Company");
    
    		if (companyClass.isAnnotationPresent(Message.class)) {
    			Message message = companyClass.getAnnotation(Message.class);
    			System.out.println("=== " + message.mess() + " ===");
    		}
    
    		Method[] methods = companyClass.getMethods();
    		int length = methods.length;
    
    		for (int index = 0; index < length; index++) {
    			if (methods[index].isAnnotationPresent(Employee.class)) {
    				Employee employee = methods[index].getAnnotation(Employee.class);
    				System.out.println("Employee Name: " + employee.name());
    				System.out.println("Employee Budget: " + employee.budget());
    			}
    		}
    	}
    }