类的说明：
- Address、Customer、Order类是普通的bean类
- Config类可以理解为Spring中添加了@Configuration的类
- Container类可以理解为Spring中的容器
- 重点在于Container类，其中的属性和方法在代码块中添加了注释
在Spring中，对bean的注入基本方式有两种，一是我们自己项目中的类，添加了如@Component注解的方式；二是添加了@Configuration搭配@Bean的方式
这个小示例就演示了这两种方式注入的基本原理。其中Container类中的getServiceInstanceByClass()演示了方法二的基本实现；createInstance()演示了方法一的基本实现，且有一个Autowired注入的演示。
很多反射的使用，注解本质上就是一个标记，除此之外没有任何意义，有意义的在于处理该标记的方法。
bilibili视频地址
public class Address {private String street;private String postCode;public void printStreet() {System.out.println("Address street: " + street);}public void printPostCode() {System.out.println("Address postCode: " + postCode);}
}
public class Customer {private String name;private String email;public void printName() {System.out.println("Address street: " + name);}public void printEmail() {System.out.println("Address postCode: " + email);}
}
public class Order {private Customer customer;private Address address;public Order() {}@Autowiredpublic Order(Customer customer, Address address) {this.customer = customer;this.address = address;}
}
/*** 配置类，类比Spring中添加了Configuration注解的类*/
public class Config {@Beanpublic Customer customer(){return new Customer();}@Beanpublic Address address(){return new Address();}public Message message(){return new Message();}
}
/*** 模拟Spring的容器*/
public class Container {//该map存储的是配置类中的所有方法，作用是通过method创建实例对象，见getServiceInstanceByClass方法private Map<Class<?>, Method> methods;private Object config;// 该map是存储已经实例化成功的beanprivate Map<Class<?>, Object> service;/**该方法的作用是初始化配置类，以及将配置类中的所有method存储到容器的methods map中*/public void init() throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {this.methods = new HashMap<>();this.service = new HashMap<>();Class<?> clazz = Class.forName("Reflection.Config");Method[] methods = clazz.getDeclaredMethods();for (Method method : methods) {if (method.isAnnotationPresent(Bean.class)) {this.methods.put(method.getReturnType(), method);}}this.config = clazz.getConstructor().newInstance();}/*** 该方法的作用是通过class获得容器中已经实例化成功的对象* 双重锁检测*/public Object getServiceInstanceByClass(Class<?> clazz) throws InvocationTargetException, IllegalAccessException {if (this.service.containsKey(clazz)) {return this.service.get(clazz);}synchronized (Container.class){if (this.service.containsKey(clazz)) {return this.service.get(clazz);}if (this.methods.containsKey(clazz)) {Method method = this.methods.get(clazz);Object obj = method.invoke(this.config);this.service.put(clazz,obj);return obj;}}return null;}/***该方法的作用是通过class参数创建一个该类型的对象*/public Object createInstance(Class<?> clazz) throws InvocationTargetException, IllegalAccessException, InstantiationException, NoSuchMethodException {Constructor<?>[] constructors = clazz.getDeclaredConstructors();for(Constructor<?> constructor : constructors){if(constructor.getDeclaredAnnotation(Autowired.class) != null){Class<?>[] parameterTypes = constructor.getParameterTypes();Object[] arguments = new Object[parameterTypes.length];for(int i = 0; i < parameterTypes.length; i++){arguments[i] = getServiceInstanceByClass(parameterTypes[i]);}return constructor.newInstance(arguments);}}// 如果没有@Autowired注解，就创建一个无参构造函数的实例return clazz.getConstructor().newInstance();}
}
public class Main {public static void main(String[] args) throws Exception {Container container = new Container();container.init();Class<?> clazz = Class.forName("Reflection.Order");Object obj = container.createInstance(clazz);Field filed = clazz.getDeclaredField("customer");filed.setAccessible(true);Object fieldValue = filed.get(obj);Method[] declaredMethods = fieldValue.getClass().getDeclaredMethods();for (Method method : declaredMethods){System.out.println(method.getName());method.invoke(fieldValue);}}
}
