注解

注解(Annotation):对程序的解释，但是和注释(comment)的区别是可以被编译器读取

1.常用内置注解：

　　@Override：用于方法重写，被重写的方法必须被声明在父类或Object中

　　@Deprecated:废弃一个方法，表示可以使用但是不推荐使用，该注解已经被淘汰，知道就行

       @SuppressWarning：抑制警告，一般不用，除非看见警告实在是太烦了

              看一下这个注解的源码：

@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.SOURCE)
public @interface SuppressWarnings {
    /**
     * The set of warnings that are to be suppressed by the compiler in the
     * annotated element.  Duplicate names are permitted.  The second and
     * successive occurrences of a name are ignored.  The presence of
     * unrecognized warning names is <i>not</i> an error: Compilers must
     * ignore any warning names they do not recognize.  They are, however,
     * free to emit a warning if an annotation contains an unrecognized
     * warning name.
     *
     * <p> The string {@code "unchecked"} is used to suppress
     * unchecked warnings. Compiler vendors should document the
     * additional warning names they support in conjunction with this
     * annotation type. They are encouraged to cooperate to ensure
     * that the same names work across multiple compilers.
     * @return the set of warnings to be suppressed
     */
    String[] value();
}

　　　　　　能看到它是需要参数的（String[] value();不是方法是参数），一般参数写all

    @SuppressWarnings("all")
    public static  void test2(){
        System.out.println("不推荐");
    }

2.元注解：负责注解其他注解，有@Target、@Retention、@Documented、@Inherited，最重要的是前两个

　　@Target:描述注解适用范围，源码如下：

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

 　　可以看到有ElementType类型的参数，再看ElementType的定义：

public enum ElementType {
    /** 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
}

　　是一个枚举类型，规定了注解的使用范围有类，方法等

　　@Retention：表示注解的声明周期，因为RUNTIME>CLASS>SOURCE,所以参数一般写RUNTIME就行

　　@Documented：表示是否将注解生成在JAVAdoc中

　　@Inherited：子类可以继承父类注解

3.自定义一个注解

public class Annotationtest {
    @MyAnnotation(name = "Bob",age = 18)
    public static void main(String[] args){
        System.out.println("hello world");
    }
    @Target({ElementType.METHOD,ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @interface MyAnnotation{
        String name();     //用方法证明注解的参数，方法名就是参数名，方法的返回类型就是参数类型
        int age();
    }
}