Java注解

基础注解
Override WebServlet

元注解

@Target 注解的作用目标
@Retention 注解的生命周期
@Documented 注解是否应当被包含在javadoc中
@Inherited 是否允许子类继承该注解

• @Target 注解的使用范围
  packages types （类 接口 枚举）
  类型成员（方法 构造方法 成员变量 枚举值）
  方法参数和本地变量

  /** Class, interface (including annotation type), or enum declaration */
    TYPE,

    /** Field declaration (includes enum constants) */
    FIELD,

    /** Method declaration */
    METHOD,

    /** Formal parameter declaration */
    PARAMETER,

    /** Constructor declaration */
    CONSTRUCTOR,

    /** Local variable declaration */
    LOCAL_VARIABLE,

    /** Annotation type declaration */
    ANNOTATION_TYPE,

    /** Package declaration */
    PACKAGE,

    /**
     * Type parameter declaration
     *
     * @since 1.8
     */
    TYPE_PARAMETER,

    /**
     * Use of a type
     *
     * @since 1.8
     */
    TYPE_USE

• Retention

 /**
     * Annotations are to be discarded by the compiler.
        只能在源文件.java 文件中保留。.class 文件中不能保留
     */
    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