Java: reflection, annotations, generics

1. Reflection

Role: to do things that can't be done normally

Usage scenarios: plug-in skinning, plug-in development

All reflection functions are based on class bytecode, which consists of three parts

• Filed: Properties
• Constructor: constructor
• method: method

public class TestBean {
    private String name="xiaoming";//属性

    public TestBean(String name,int age){// 构造函数
        this.name = name;
    }

    public void sysName(){//方法
        Log.e("TAG",name);
    }
    
    private void say(String desc){
        Log.e("TAG",name + " : "+desc);
    }
}

constructor call

   try {
        Constructor<TestBean> constructor = TestBean.class.getDeclaredConstructor(String.class, int.class);
        constructor.setAccessible(true);
        TestBean testBean = constructor.newInstance("名称", 18);
        testBean.sysName();
    } catch (Exception e) {
        e.printStackTrace();
    }

TestBean testBean = TestBean.class.newInstance(parameter type);

getDeclaredConstructor finds from all constructors
getConstructor finds from public constructors

constructor.setAccessible(true);//Set permissions

method call

Method method = TestBean.class.getDeclaredMethod("say",String.class);
method.setAccessible(true);
method.invoke(constructor,"描述信息");

attribute call

Field field = TestBean.class.getDeclaredField("name");
field.setAccessible(true);
String name = (String) field.get(testBean);

2. Notes

Annotation is just an identifier and has no specific function

@Target(ElementType.FIELD)  
@Retention(RetentionPolicy.RUNTIME)
public @interface ViewById {// @interface 代表注解
    int value();
}

@Target: where to put it

• ElementType.METHOD: acts on the method
• ElementType.Type: acts on the class
• ElementType.Field: acting on attributes

@Retention：

• RetentionPolicy.RUNTIME: runtime, such as: xUtils
• RetentionPolicy.CLASS: compile time (package time), such as: ButterKnife
• RetentionPolicy.SOURCE: While programming

Annotation combined with reflection solves findViewById injection

public class ViewUtils {
    public static void inject(Activity activity) {
        // 1.获取所有的属性
        Field[] fields = activity.getClass().getDeclaredFields();
        // 2.过滤关于 ViewById 属性
        for (Field field : fields) {
            ViewById viewById =  field.getAnnotation(ViewById.class);
            if(viewById != null){
                // 3.findViewById
                View view = activity.findViewById(viewById.value());
                // 4.反射注入
                field.setAccessible(true);
                try {
                    // activity 属性所在类，view 代表的是属性的值
                    field.set(activity,view);
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

3. Generics

Class generic: appearing anywhere, representing the same type

Method Generics: acting on methods

Upper limit: class name <? extends class> object name class and its subclasses

Lower limit: class name <? super class> object name class and its parent class