java 注解 1

Java 注解1

比较好的简单说明，菜鸟教程

@Retention

@Retention - 标识这个注解怎么保存，是只在代码中，还是编入class文件中，或者是在运行时可以通过反射访问。

/**
 * Indicates how long annotations with the annotated type are to
 * be retained.  If no Retention annotation is present on
 * an annotation type declaration, the retention policy defaults to
 * {@code RetentionPolicy.CLASS}.
 *
 * 
A Retention meta-annotation has effect only if the
 * meta-annotated type is used directly for annotation.  It has no
 * effect if the meta-annotated type is used as a member type in
 * another annotation type.
 *
 * @author  Joshua Bloch
 * @since 1.5
 * @jls 9.6.3.2 @Retention
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    /**
     * Returns the retention policy.
     * @return the retention policy
     */
    RetentionPolicy value();
}

其中 RetationPolicy 是个enum，记录retation的几种情况，运行时，源码和类。

/**
 * Annotation retention policy.  The constants of this enumerated type
 * describe the various policies for retaining annotations.  They are used
 * in conjunction with the {@link Retention} meta-annotation type to specify
 * how long annotations are to be retained.
 *
 * @author  Joshua Bloch
 * @since 1.5
 */
public enum RetentionPolicy {
    /**
     * Annotations are to be discarded by the compiler.
     * 被编译器抛弃，即编译时候就没了
     */
    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.
     * 编译时仍然存在
     */
    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.
     *运行时仍然存在
     * @see java.lang.reflect.AnnotatedElement
     */
    RUNTIME
}

Example 运行时仍然存在的注解

注解的定义：

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
// 用于修饰方法
@Target(ElementType.METHOD)
// 运行时 依然会被记住
@Retention(RetentionPolicy.RUNTIME)
public @interface runtime {
    // name  weight两个属性
    String name();
    String weight();
}

测试类，里面有个函数，使用这个注解，然后在运行时，调用这个注解，看下注解的信息是否还存在。

public class test1 {
   // 被测试的方法，使用注解 runtime，并且对name和weight 进行赋值
    @runtime(name="I am Peter", weight = "Weight 80kg")
    public static void runtime_func(String name, String weight){
        System.out.println("name is " +name);
        System.out.println("weight is "+ weight);
    }
    // 无注解的方法
    public void func(){
        System.out.println("ordinate methods");
    }

    public static void main(String[] args) throws ClassNotFoundException {
        //反射 获得类 对象
        Class<?> annotation =  Class.forName("test1");
        System.out.println("class name:   "+annotation.toString());
        // 获得类中的所有方法
        Method[] ms  =  annotation.getMethods();
        for (Method m:ms
             ) {
            System.out.println("method name:  "+m.getName());
            // 实例化 ： 类中 类型时 runtime.class 的 annotation
            runtime runtiem_an = m.getAnnotation(runtime.class);
            if(runtiem_an !=null ) {
                try {
                    // 创建一个对象，执行这个函数，并且使用注解中的属性值
                    m.invoke(new test1(),runtiem_an.name(),runtiem_an.weight());
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                } catch (InvocationTargetException e) {
                    e.printStackTrace();
                }

            }
        }
    }
}

打印输出结果

class name:   class test1
method name:  main
method name:  runtime_func
 annotaoton nameI am Peter
name is I am Peter
weight is Weight 80kg
method name:  func
method name:  wait
method name:  wait
method name:  wait
method name:  equals
method name:  toString
method name:  hashCode
method name:  getClass
method name:  notify
method name:  notifyAll

可以看到，运行时，注解仍然是存在的。

如果将runtime换成source或者class,run程序，得到的结果是：

class name:   class test1
method name:  main
method name:  func
method name:  runtime_func
method name:  wait
method name:  wait
method name:  wait
method name:  equals
method name:  toString
method name:  hashCode
method name:  getClass
method name:  notify
method name:  notifyAll