JAVA之Annotation(注释)

要使用注解，那么必须会自定义注解，在学自定义注解之前， 需要知道元注解 meta-annotation(四个),他们被用来对其他注解提供说明；  注意自定义注解时需要用到反射相关内容

元注解：

     元注解就是负责注解其他注解

     1、@Target

     2、@Retention

     3、@Documented

     4、@Inherited

   目前主要使用的是前两个

   @Target  说明了Annotation所修饰的对象范围       

            取值(ElementType)的取值范围有：

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

@Retention   用于描述注释的生命周期

  取值(RetentionPolicy)的取值范围有

 

public enum RetentionPolicy {
    /**
     * Annotation is only available in the source code.
     */
    SOURCE,
    /**
     * Annotation is available in the source code and in the class file, but not
     * at runtime. This is the default policy.
     */
    CLASS,
    /**
     * Annotation is available in the source code, the class file and is
     * available at runtime.
     */
    RUNTIME
}

  @Documented  将注释包含在javadoc中

   @Inhertied  允许子类继承父类的注释

 自定义注解

   定义注解格式：

    public @interface  注解名{  定义体 }

  定义体内每个方法的名称就是参数的名称，返回值类型就是参数的类型

返回值类型只能是  基本数据类型、Enum、String、Class、Annotation以及这一些类型的数组

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FruitColor {
    /**
     * 颜色枚举
     * @author peida
     *
     */
    public enum Color{ BULE,RED,GREEN};
    
    /**
     * 颜色属性
     * @return
     */
    Color fruitColor() default Color.GREEN;

}

 一个自定义注解的案例

     

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)

//用来注解布局文件
public @interface LayoutInject {
    int value()default 0;  //布局的id
}

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface ViewInject {
    int value()default 0;  //控件的id
    boolean isClick() default false; //控件上点击事件
}

public class AnnotationUtil {
    
    //设置布局文件，通过在类上面设置注解
    public static void setContentView(Activity activity){
        if(activity.getClass().isAnnotationPresent(LayoutInject.class)){
            LayoutInject inject = activity.getClass().getAnnotation(LayoutInject.class);
            activity.setContentView(inject.value());
        }
    }
    
    public  static  void findView(Activity activity){
        Field[] fields = activity.getClass().getDeclaredFields();
        if(fields.length==0 && fields==null){
            return;
        }
        try {
        for (Field field:fields){
            if(field.isAnnotationPresent(ViewInject.class)){
                ViewInject viewInject = field.getAnnotation(ViewInject.class);
                int id = viewInject.value();
                if(id>0){
                    View view = activity.findViewById(id);
                    //强制反射私有变量
                    field.setAccessible(true);
                    field.set(activity,view);
                    if(viewInject.isClick()){
                        view.setOnClickListener((View.OnClickListener) activity);
                    }
                    }
                }
            }
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}

public class BaseAcitvity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
        AnnotationUtil.setContentView(this);
        AnnotationUtil.findView(this);
    }
}

@LayoutInject(value = R.layout.activity_main)
public class MainActivity extends BaseAcitvity implements View.OnClickListener {
    
    @ViewInject(value = R.id.tv_onclick,isClick = true)
    TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.e("xiaoma","你好啊a");
    }

    @Override
    public void onClick(View v) {
        Log.e("xiaoma","点击");
        switch (v.getId()){
            case R.id.tv_onclick:
                Toast.makeText(this,"点击事件",Toast.LENGTH_SHORT).show();
                break;
        }
    }
}