ReflectionAndAnnotation

本文講解java的 Reflect、Annotation 反射 和 註解的使用
文中的測試程式碼請前往 ReflectAndAnnotation

反射

本文主要從應用的角度講解反射。

先寫一個Bean類,定義int型的age, String name, List beans。

package com.example.dj.reflectandannotation;

import java.util.List;

/**
 * @author DragonJiang
 * @date 2015/11/16
 * @Description:
 */
public class Bean {
    private int age;

    public String name;

    List<Bean> beans;

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<Bean> getBeans() {
        return beans;
    }

    public void setBeans(List<Bean> beans) {
        this.beans = beans;
    }

    @Override
    public String toString() {
        return "name = " + name + ", age = " + age;
    }
}
 

然後在BeanTest類進行測試：

package com.example.dj.reflectandannotation;

import android.util.Log;

import java.lang.reflect.Field;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

/**
 * @author DragonJiang
 * @date 2015/11/16
 * @Description:
 */
public class BeanTest {

    private static final String TAG = "BeanTest";

    public static void testBean(){
        //定義一個bean的實體，然後初始化
        Bean bean = new Bean();
        List<Bean> beans = new ArrayList<>();
        beans.add(bean);
        bean.setAge(12);
        bean.setName("dragon");
        bean.setBeans(beans);

        Class clazz = bean.getClass();//取得類的物件

        Field[]  fields = clazz.getDeclaredFields();//取得類中所有屬性的集合

        try {
            for (Field field : fields) {
                if (!field.isAccessible()) {//如果這些屬性是不可訪問的，設為可訪問
                    field.setAccessible(true);
                }

                Class fieldType = field.getType();//取得該屬性的型別
                String strType = fieldType.toString();

                if(int.class.isAssignableFrom(fieldType)){//判斷該屬性是否int型別，取得屬性值
                    int i = (int) field.get(bean);
                    Log.d(TAG, strType + ", " + field.getName() + " = "+ i);

                } else if(String.class.isAssignableFrom(fieldType)){//判斷該屬性是否String型別，取得屬性值
                    String s = (String) field.get(bean);
                    Log.d(TAG, strType + ", " + field.getName() + " = "+ s);

                } else if(List.class.isAssignableFrom(fieldType)){//判斷該屬性是否List型別,如果是，取得List的引數型別
                    Type type = field.getGenericType();//取得該屬性的引數型別

                    if(type instanceof ParameterizedType){
                        Class itemClazz = (Class) ((ParameterizedType) type).getActualTypeArguments()[0];//取得list的實際引數型別

                        if(Bean.class.isAssignableFrom(itemClazz)){//判斷型別是否Bean型別
                            List list = (List) field.get(bean);//取得List
                            Iterator it = list.iterator();
                            while (it.hasNext()){
                                Bean b = (Bean) it.next();//取得Bean
                                Log.d(TAG, strType + ", " + field.getName() + "; itemType = "
                                        + itemClazz.getSimpleName() + "， value : " + b.toString());
                            }
                        }
                    }

                }
            }

        } catch (Exception e){
            e.printStackTrace();
        }
    }
}
 

執行BeanTest.testBean();
打印出來的log如下：

11-20 11:23:34.750 7652-7652/? D/BeanTest: class java.lang.String, name = dragon
11-20 11:23:34.750 7652-7652/? D/BeanTest: interface java.util.List, beans; itemType = Bean， value : name = dragon, age = 12
11-20 11:23:34.750 7652-7652/? D/BeanTest: int, age = 12

通過反射分別取到了Bean定義的三個屬性值。舉一反三可以同樣的用反射呼叫類內部的方法。

註解

註解在安卓的開發中可以提供很大的便利。

定義三個註解類，分別是類註解、方法註解、屬性註解

類註解：

package com.example.dj.reflectandannotation.annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * @author DragonJiang
 * @date 2015/11/16
 * @Description:
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface ClassAnnotation {
}

方法註解：

package com.example.dj.reflectandannotation.annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * @author DragonJiang
 * @date 2015/11/16
 * @Description:
 */
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface MethodAnnotation {
}

屬性註解：（註解可以帶有引數，相比前兩個註解，屬性註解增加了一個引數，用於測試帶引數的註解）

package com.example.dj.reflectandannotation.annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * @author DragonJiang
 * @date 2015/11/16
 * @Description:
 */

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface FieldAnnotation {
    /**
     * 註解的引數privacyType的值。
     */
    int PRIVACY = 0;      //隱私的屬性
    int PUBLIC = 1;       //公開的屬性

    int privacyType() default PRIVACY;
}

然後對Bean類進行修改，增加註解：

package com.example.dj.reflectandannotation;

import com.example.dj.reflectandannotation.annotation.ClassAnnotation;
import com.example.dj.reflectandannotation.annotation.FieldAnnotation;
import com.example.dj.reflectandannotation.annotation.MethodAnnotation;

import java.util.List;

/**
 * @author DragonJiang
 * @date 2015/11/16
 * @Description:
 */
@ClassAnnotation
public class Bean {
    @FieldAnnotation(privacyType = FieldAnnotation.PRIVACY)
    private int age;

    @FieldAnnotation(privacyType = FieldAnnotation.PUBLIC)
    public String name;

    @FieldAnnotation(privacyType = FieldAnnotation.PRIVACY)
    List<Bean> beans;

    public int getAge() {
        return age;
    }

    @MethodAnnotation
    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<Bean> getBeans() {
        return beans;
    }

    public void setBeans(List<Bean> beans) {
        this.beans = beans;
    }

    @Override
    public String toString() {
        return "name = " + name + ", age = " + age;
    }
}

在AnnotationTest類解析註解：

package com.example.dj.reflectandannotation;

import android.util.Log;

import com.example.dj.reflectandannotation.annotation.ClassAnnotation;
import com.example.dj.reflectandannotation.annotation.FieldAnnotation;
import com.example.dj.reflectandannotation.annotation.MethodAnnotation;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.List;

/**
 * @author DragonJiang
 * @date 2015/11/16
 * @Description:
 */
public class AnnotationTest {
    private static final String TAG = "AnnotationTest";

    public static void testAnnotation(){
        Bean bean = new Bean();
        List<Bean> beans = new ArrayList<>();
        beans.add(bean);
        bean.setAge(12);
        bean.setName("dragon");
        bean.setBeans(beans);

        //類註解
        Class clazz = bean.getClass();//取得類的物件
        ClassAnnotation ca = (ClassAnnotation) clazz.getAnnotation(ClassAnnotation.class);//取類的註解
        if(ca != null) {
            Log.d(TAG, "the class "+ clazz.getSimpleName() + " has ClassAnnotation : " + ca.toString());//列印註解
        }

        //方法註解
        Method[] methods = clazz.getDeclaredMethods();
        for(Method method : methods){
            MethodAnnotation ma = method.getAnnotation(MethodAnnotation.class);
            if(ma != null){
                Log.d(TAG, "the method " + method.getName() + " has MethodAnnotation : " + ma.toString());//列印註解
            }
        }

        //屬性註解
        Field[]  fields = clazz.getDeclaredFields();//取得類中所有屬性的集合
        for (Field field : fields) {
            FieldAnnotation fa = field.getAnnotation(FieldAnnotation.class);
            if(fa == null){
                continue;
            }

            int privacyType = fa.privacyType();
            if(privacyType == FieldAnnotation.PRIVACY){
                Log.d(TAG, field.getName() + " has FieldAnnotation : " + "privacyType == FieldAnnotation.PRIVACY");
            } else {
                Log.d(TAG, field.getName() + " has FieldAnnotation : " + "privacyType == FieldAnnotation.PUBLIC");
            }
        }
    }
}

執行AnnotationTest.testAnnotation();列印log：

11-20 11:23:34.750 7652-7652/? D/AnnotationTest: the class Bean has ClassAnnotation : @com.example.dj.reflectandannotation.annotation.ClassAnnotation()
11-20 11:23:34.750 7652-7652/? D/AnnotationTest: the method setAge has MethodAnnotation : @com.example.dj.reflectandannotation.annotation.MethodAnnotation()
11-20 11:23:34.754 7652-7652/? D/AnnotationTest: name has FieldAnnotation : privacyType == FieldAnnotation.PUBLIC
11-20 11:23:34.754 7652-7652/? D/AnnotationTest: beans has FieldAnnotation : privacyType == FieldAnnotation.PRIVACY
11-20 11:23:34.754 7652-7652/? D/AnnotationTest: age has FieldAnnotation : privacyType == FieldAnnotation.PRIVACY