Java註解

JavaSe5
開始內置了三種註解,定義在Java.lang中

@Override 表示覆蓋超類的方法(父類)
@Deprecated 編譯器發出警告
@SuppressWarnings　關閉不當的編譯器警告信息

另外還有四種元註解

@Target　表示該註解可以用在什麼地方
@Retention　表示在什麼級別保存該註解信息
@Documented 將此註解包含在Javadoc中
@Inherited 允許子類繼承父類的註解

一般來說用到註解的地方很大可能會用到反射以及枚舉
因此我舉例的時候直接把三者融合在一起

定義註解

@Inherited
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface XXXX{
    public int id();
    public String name()default "zhou";
}

那麼既然定義了註解必然要使用註解的

public class Testable(){
    @XXXX(id = 56,name = "blake")
    public void isDemo(){
        System.out.println("..............getAddress()");
    }
}

同樣的道理如果用到了註解,那麼必須有註解處理器呀!

 Class<Testable> testAble = Testable.class;
 Method m = testAble.getDeclaredMethod("isDemo", null);
 //因爲沒有參數嘛!假如有參數並且恰好是String,則null改   爲String.class
 //重點來了
 XXXX xx = m.getAnnotation(XXXX.class);
 //調用即可
 System.out.println("name:" + xx.name());

下面我把編好的例子copy上來
四個自定義註解
FruitAddress

@Inherited
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface FruitAddress {
    public enum Address {
        A1("越南"), A2("緬甸"), A3("菲律賓"), A4("中國海南"), A5("柬埔寨"), A6("文萊"), A7("老撾"), A8("泰國"), A9("新加坡"), A10("馬來西亞"), A11("東帝汶"), A12("印度尼西亞");
        private String nameAddress;

        Address(String nameAddress) {
            this.nameAddress = nameAddress;
        }

        /**
         * 獲取地址
         * @return
         */
        public String getNameAddress() {
            return nameAddress;
        }
    }

    public Address fruitAddress() default Address.A2;
}

FruitColor

@Documented
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface FruitColor {
    /**
     * 顏色枚舉
     */
    public enum Color {
        BLUE, GREEN, RED
    }

    /**
     * 顏色屬性
     *
     * @return
     */
    public Color fruitColor() default Color.BLUE;
}

FruitName

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FruitName {
    public String value() default "";
}

FruitProvider

@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface FruitProvider {
    /**
     * 供應商編號
     *
     * @return
     */
    public int id() default 01;

    /**
     * 供應商名稱
     *
     * @return
     */
    public String name() default "";

    /**
     * 供應商地址
     *
     * @return
     */
    public String address() default "";
}

使用四個註解的class

@FruitProvider(id = 23, name = "blake", address = "china")
public class Apple {

    @FruitName("apple")
    private String appleName;

    @FruitColor(fruitColor = FruitColor.Color.RED)
    private String appleColor;

    @FruitAddress(fruitAddress = FruitAddress.Address.A5)
    public void getAddress() {
        System.out.println("..............getAddress()");
    }


    public Apple() {

    }

    public void displayName() {
        System.out.println("水果的名字是:蘋果!");
    }
}

註解處理器的測試 (測試和註解處理器並沒有分開都在這個例子中)

public class Demo {
    public static void main(String[] args) {
        try {
            isRun();
            isStart();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void isStart() {
        Class<Apple> appleClass = Apple.class;
        Thread thread = new Thread() {
            @Override
            public void run() {
                try {
                    //獲取指定成員變量(字段)
                    Field f1 = appleClass.getDeclaredField("appleName");
                    Field f2 = appleClass.getDeclaredField("appleColor");

                    System.out.println(f1.getName() + " " + f2.getName());

                    //獲取成員變量上面的註解
                    FruitName fruitName = f1.getAnnotation(FruitName.class);
                    FruitColor fruitColor = f2.getAnnotation(FruitColor.class);
                    System.out.println(fruitName.value() + "---->");
                    System.out.println(fruitColor.fruitColor().name() + "<----");

                    //獲取指定方法
                    Method m1;
                    m1 = appleClass.getDeclaredMethod("getAddress", null);
                    m1 = appleClass.getMethod("getAddress",null); //ps 因爲沒有參數,如果有傳入參數的class
                    System.out.println("method Name:" + m1.getName());
                    //獲取方法上的註解
                    FruitAddress fruitAddress = m1.getAnnotation(FruitAddress.class);
                    System.out.println(".." + fruitAddress);
                    if (fruitAddress != null) {
                        System.out.println("address:" + fruitAddress.fruitAddress());
                    } else {
                        System.out.println("point exception");
                    }
                    ;

                    //獲取類或者說接口上的註解
                    FruitProvider fruitProvider = appleClass.getAnnotation(FruitProvider.class);
                    if (fruitProvider != null)
                        System.out.println(fruitProvider.address() + " " + fruitProvider.name() + " " + fruitProvider.id());
                } catch (NoSuchFieldException e) {
                    System.out.println("exception:" + e.getMessage());
                    e.printStackTrace();
                } catch (NoSuchMethodException ex) {
                    System.out.println("exception:" + ex.getMessage());
                    ex.printStackTrace();
                }

            }
        };
        thread.start();
    }

    public static void isRun() {
        Class<Apple> appleClass = Apple.class;
        Field[] fields = appleClass.getDeclaredFields();
        for (Field f : fields) {//遍歷
            FruitColor fruitColor = f.getAnnotation(FruitColor.class);//獲取註解在字段(成員變量)上的註解對象
            FruitName fruitName = f.getAnnotation(FruitName.class);
            if (fruitColor != null) {
                System.out.println("color:" + fruitColor.fruitColor());
            }
            if (fruitName != null) {
                System.out.println("name:" + fruitName.value());
            }
        }
    }
}

基本上用到了Java的線程,註解,反射,枚舉這四個基礎知識.如有問題請留言!