Java SE 三个基本注解(JDK内置)+四个元注解

使用注解(Annotation)时要在前面加@符号，注解可以当作一个修饰符来修饰他支持的修饰的元素
@Override - 重写，该注解只能用于方法
@Deprecated - 已弃用，用在程序元素上，如某个类上或者某个方法上
@SuppressWarnings - 抑制编译器警告

@Target - 指定注解可以在哪些地方使用

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
    /**
     * Returns an array of the kinds of elements an annotation interface
     * can be applied to.
     * @return an array of the kinds of elements an annotation interface
     * can be applied to
     */
    ElementType[] value();
}

@Retention - 指定注解作用范围，三种范围SOURCE,CLASS,RUNTIME

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    /**
     * Returns the retention policy.
     * @return the retention policy
     */
    RetentionPolicy value();
}

public enum RetentionPolicy {
    /**
     * Annotations are to be discarded by the compiler.
     * 作用在源码层面(.java文件层面),会被编译器丢弃,即不会编译进.class文件中
     */
    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.
     * 作用在字节码层面(.class文件层面),会被编译器记录,即会编译进.class文件中,但不会被JVM保留,即运行
     * 时被丢弃
     */
    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.
     * 作用在JVM层面(.class文件层面),会被编译器记录,即会编译进.class文件中,也会被JVM保留,即在JVM上运行时也能被反射
     * 机制读取到
     * @see java.lang.reflect.AnnotatedElement
     */
    RUNTIME
}

@Documented - 是否在JavaDoc里面体现

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

@Inherited - 子类会继承父类注解