注解和反射的demo

Posted 2022-01-07 hans774882968

showPrivateField、callPrivateMethod分别展示如何获取私有属性和调用私有方法。

showMethods展示如何得到一个类的所有成员方法。

showAnno展示如何得到一个类的所有注解。

checkClassesThatHasAnno展示如何获取具有某个特定注解的所有类。

代码

import java.lang.annotation.Annotation;
import java.lang.annotation.Retention;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

@Retention(RUNTIME)
@interface Best 
    String name();

    int age();


@Retention(RUNTIME)
@interface Better 
    String name();

    int age();


@Best(name = "best", age = 18)
@Better(name = "better", age = 22)
class A 
    private String name;

    A() 
        this.name = "wsw";
    

    A(String name) 
        this.name = name;
    

    public String publicMethod() 
        return this.name + "说：就勉强让你调用人家一次吧！说好了就这一次哦！";
    

    private String privateMethod(int v) 
        return this.name + "说：球球你别调用我啊！" + v;
    


@Best(age = 24, name = "B也是best的！")
class B 

@Better(age = 26, name = "我只是better而已。")
class C 

@Best(age = 28, name = "D也是best的！")
class D 

public class RetentionDemo 
    private static void showPrivateField(Class<?> targetClass, A a1, A a2) throws
        IllegalAccessException, NoSuchFieldException 
        Field nameField = targetClass.getDeclaredField("name");
        nameField.setAccessible(true);
        System.out.println(nameField.get(a1) + " , " + nameField.get(a2));
    

    private static void callPrivateMethod(Class<?> targetClass, A a1, A a2) throws
        IllegalAccessException, NoSuchMethodException, InvocationTargetException 
        Method privateMethod = targetClass.getDeclaredMethod("privateMethod", int.class);
        privateMethod.setAccessible(true);
        System.out.println(privateMethod.invoke(a1, 1) + " , " +
            privateMethod.invoke(a2, 2));
    

    private static void showMethods(Class targetClass) 
        Method[] methods = targetClass.getDeclaredMethods();
        for (Method m : methods) 
            System.out.println(m.getName() + "的参数类型表：");
            Class[] ptypes = m.getParameterTypes();
            for (Class p : ptypes) System.out.println(p.getName());
        
    

    private static void showAnno(Class targetClass) 
        System.out.println(targetClass.getName() + "的所有已定义注解：");
        Annotation[] anns = targetClass.getDeclaredAnnotations();
        for (Annotation ann : anns) System.out.println(ann);
    

    private static void show(Class[] classes, Class wantedAnno) 
        List<Class> bestClasses = Arrays.stream(classes).filter(c -> 
            List<Annotation> anns = Arrays.asList(c.getDeclaredAnnotations());
            return anns.stream().anyMatch(ann -> ann.annotationType() == wantedAnno);
        ).collect(Collectors.toList());
        System.out.println("所有具有" + wantedAnno.getName() + "注解的类：");
        for (Class c : bestClasses) 
            Annotation ann = c.getAnnotation(wantedAnno);
            System.out.println(c.getName() + ": " + ann);
            Class typ = c.getAnnotation(wantedAnno).annotationType();
            if (typ == Best.class) 
                Best b = (Best) ann;
                System.out.println(b.name() + " ; " + b.age());
            
            if (typ == Better.class) 
                Better b = (Better) ann;
                System.out.println(b.name() + " ; " + b.age());
            
        
    

    private static void checkClassesThatHasAnno() 
        Class[] classes = A.class, B.class, C.class, D.class;
        show(classes, Best.class);
        show(classes, Better.class);
    

    public static void main(String[] args) throws
        ClassNotFoundException, InstantiationException, IllegalAccessException,
        NoSuchMethodException, InvocationTargetException, NoSuchFieldException 
        A a1 = new A("hans");
        Class targetClass = Class.forName("A");
        A a2 = (A) targetClass.newInstance();
        showPrivateField(targetClass, a1, a2);
        callPrivateMethod(targetClass, a1, a2);
        showMethods(targetClass);
        showAnno(targetClass);
        checkClassesThatHasAnno();
    

输出

hans , wsw
hans说：球球你别调用我啊！1 , wsw说：球球你别调用我啊！2
publicMethod的参数类型表：
privateMethod的参数类型表：
int
A的所有已定义注解：
@Best(name=best, age=18)
@Better(name=better, age=22)
所有具有Best注解的类：
A: @Best(name=best, age=18)
best ; 18
B: @Best(age=24, name=B也是best的！)
B也是best的！ ; 24
D: @Best(age=28, name=D也是best的！)
D也是best的！ ; 28
所有具有Better注解的类：
A: @Better(name=better, age=22)
better ; 22
C: @Better(age=26, name=我只是better而已。)
我只是better而已。 ; 26