背景

C# 版本库 MediatR 是一个中介者模式实现类库，其核心是一个中介 者模式的.NET实现，其目的是消息发送和消息处理的解耦。它支持单播和多播形式使用同步或异步的模式来发布消息，创建和帧听事件。
java中没有找到类似类库，在对MediatR源码阅读中，发现其主要思路是借助IOC获取Request与Handler对应关系并进行处理。

中介者模式

中介者模式：用一个中介对象封装一系列的对象交互，中介者使各对象不需要显示地相互作用，从而使耦合松散，而且可以独立地改变他们之间的交互。

image.png

使用中介模式，对象之间的交互将封装在中介对象中，对象不再直接交互(解耦),而是通过中介进行交互，这减少了对象之间的依赖性，从而减少了耦合。

image.png

应用

单播消息传输

单播消息传输，也就是一对第一的消息传递，一个消息对应一个消息处理，通过 IReust 抽象单播消息，使用 IRequestHandler 进行消息处理

@ExtendWith(SpringExtension.class)
@Import(
        value = {
                Mediator.class,
                PingPongTest.PingHandler.class,
        }
)
public class PingPongTest {

    @Autowired
    IMediator mediator;

    @Test
    public void should() {
        String send = mediator.send(new Ping());

        assertThat(send).isNotNull();
        assertThat(send).isEqualTo("Pong");
    }

    public static class Ping implements IRequest<String> {
    }

    public static class PingHandler implements IRequestHandler<Ping, String> {
        @Override
        public String handle(Ping request) {
            return "Pong";
        }
    }

}

多播消息传输

多播消息传输，是一对多的消息传递，一个消息对应多个消息处理，通过 INotification 抽象多播消息，使用 INotificationHanlder 进行消息处理

@ExtendWith(SpringExtension.class)
@Import(
        value = {
                Mediator.class,
                PingNoticeTests.Pong1.class,
                PingNoticeTests.Pong2.class,
        }
)
public class PingNoticeTests {

    @Autowired
    IMediator mediator;

    @Autowired
    Pong1 pong1;

    @Autowired
    Pong2 pong2;

    @Test
    public void should() {
        mediator.publish(new Ping());

        assertThat(pong1.getCode()).isEqualTo("Pon1");
        assertThat(pong2.getCode()).isEqualTo("Pon2");
    }

    public static class Ping implements INotification {
    }

    public static class Pong1 implements INotificationHandler<Ping> {

        private String code;

        public String getCode() {
            return code;
        }

        @Override
        public void handle(Ping notification) {
            this.code = "Pon1";
        }
    }

    public static class Pong2 implements INotificationHandler<Ping> {

        private String code;

        public String getCode() {
            return code;
        }

        @Override
        public void handle(Ping notification) {
            this.code = "Pon2";
        }
    }

}

实现

核心实现

其主要点是从Spring的ApplicationContext中获取相关接口bean，然会执行bean方法。
核心方法有两个：public(多播)和send(单播)。
借助ResolvableType类型构造解析bean信息，得到信息后从spring中获取对象实例。

/**
 * 中介者实现类
 * <p>
 * 依赖 ApplicationContext
 */
@Component
public class Mediator implements IMediator, ApplicationContextAware {

    private ApplicationContext context;

    /**
     * 发布同步
     * <p>
     * 根据通知类型和INotificationHandler,从ApplicationContext获取Handler的BeanNames,
     * 将 BeanNames 转化为 INotificationHandler 的实例，每个实例调用一次handler
     *
     * @param notification    通知内容
     * @param <TNotification> 通知类型
     */
    @Override
    public <TNotification extends INotification> void publish(TNotification notification) {

        ResolvableType handlerType = ResolvableType.forClassWithGenerics(
                INotificationHandler.class, notification.getClass());

        String[] beanNamesForType = this.context.getBeanNamesForType(handlerType);
        List<INotificationHandler<TNotification>> list = new ArrayList<>();
        for (String beanBane :
                beanNamesForType) {
            list.add((INotificationHandler<TNotification>) this.context.getBean(beanBane));
        }
        list.forEach(h -> h.handle(notification));
    }

    /**
     * 发送求取
     * <p>
     * 根据request类型，获取到response类型，
     * 根据IRequestHandler、request类型、response类型从ApplicationContext获取
     * IRequestHandler实例列表，取第一个实例执行handler方法。
     * <p>
     * <p>
     * 如果为找到handler实例，抛出NoRequestHandlerException异常
     *
     * @param request     请求
     * @param <TResponse> 响应类型
     * @return 响应结果
     */
    @Override
    public <TResponse> TResponse send(IRequest<TResponse> request) {
        Type[] genericInterfaces = request.getClass().getGenericInterfaces();

        Type responseType = null;

        for (Type type : genericInterfaces) {
            if ((type instanceof ParameterizedType)) {
                ParameterizedType parameterizedType = (ParameterizedType) type;
                if (!parameterizedType.getRawType().equals(IRequest.class)) {
                    continue;
                }
                responseType = parameterizedType.getActualTypeArguments()[0];
                break;
            }
        }

        if (responseType == null) {
            // 抛出异常
            throw new NoRequestHandlerException(request.getClass());
        }


        Class<?> requestClass = request.getClass();
        Class<?> responseClass = (Class<?>) responseType;

        ResolvableType handlerType = ResolvableType.forClassWithGenerics(
                IRequestHandler.class,
                requestClass,
                responseClass);

        String[] beanNamesForType = this.context.getBeanNamesForType(handlerType);
        List<IRequestHandler<IRequest<TResponse>, TResponse>> list = new ArrayList<>();
        for (String beanBane :
                beanNamesForType) {
            list.add((IRequestHandler<IRequest<TResponse>, TResponse>) this.context.getBean(beanBane));
        }

        if (list.isEmpty()) {
            throw new NoRequestHandlerException(request.getClass());
        }

        return list.stream()
                .findFirst()
                .map(h -> h.handle(request))
                .orElseThrow(() -> new NoRequestHandlerException(request.getClass()));
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.context = applicationContext;
    }
}

public interface IBaseRequest {
}

多播接口

public interface INotification {
}

单播接口

public interface IRequest<TResponse> extends IBaseRequest {
}

public interface IPublisher {
    <TNotification extends INotification> void publish(TNotification notification);
}

public interface ISender {
    <TResponse> TResponse send(IRequest<TResponse> request);
}

public interface IMediator extends ISender, IPublisher {
}

多播处理接口

public interface INotificationHandler<TNotification extends INotification> {
    void handle(TNotification notification);
}

单播处理接口

public interface IRequestHandler<TRequest extends IRequest<TResponse>, TResponse> {
    TResponse handle(TRequest request);
}

public abstract class AbsRequestHandler<TRequest extends IRequest<TResponse>, TResponse>
        implements IRequestHandler<TRequest, TResponse> {
    @Override
    public abstract TResponse handle(TRequest request);
}

public abstract class AbsNotificationHandler<TNotification extends INotification>
        implements INotificationHandler<TNotification> {
    @Override
    public abstract void handle(TNotification notification);
}

public class Unit implements Comparable<Unit> {
    public static final Unit VALUE = new Unit();

    private Unit() {
    }

    @Override
    public boolean equals(Object obj) {
        return true;
    }

    @Override
    public int hashCode() {
        return 0;
    }

    @Override
    public String toString() {
        return "()";
    }

    @Override
    public int compareTo(@NotNull Unit o) {
        return 0;
    }
}

public interface IUnitRequest extends IRequest<Unit> {
}

public class MediatorException extends RuntimeException {
}

@Getter
public class NoRequestHandlerException extends MediatorException {
    private Class<?> requestClass;

    public NoRequestHandlerException(
            Class<?> requestClass
    ) {
        this.requestClass = requestClass;
    }
}

应用场景

mediatr 是一种进程内消息传递机制，使用泛型支持消息的只能调度，其核心是 消息解耦 ，基于MediatR可以实现CQRS/EventBus等。

解除构造函数的依赖注入

public class DashboardController(
                ICustomerRepository customerRepository,
                IOrderService orderService,
                ICustomerHistoryRepository historyRepository,
                IOrderRepository orderRepository,
                IProductRespoitory productRespoitory,
                IRelatedProductsRepository relatedProductsRepository,
                ISupportService supportService,
                ILog logge
        ) {

        }

借助 Mediator，仅需构造注入ImediatR即可

public class DashboardController(
                IMediator
                ) {

        }

service 循环依赖，使用mediatr 进行依赖解耦，并使用mediatr进行消息传递

两个service类和接口如下

    public static interface IDemoAService {
        String hello();

        String helloWithB();
    }

    public static interface IDemoBService {
        String hello();

        String helloWithA();
    }

    public static class DemoAService implements IDemoAService {
        private final IDemoBService bService;

        public DemoAService(IDemoBService aService) {
            this.bService = aService;
        }

        @Override
        public String hello() {
            return this.bService.helloWithA();
        }

        @Override
        public String helloWithB() {
            return "call A in B";
        }
    }

    public static class DemoBService implements IDemoBService {
        private final IDemoAService aService;

        public DemoBService(IDemoAService aService) {
            this.aService = aService;
        }

        @Override
        public String hello() {
            return this.aService.helloWithB();
        }

        @Override
        public String helloWithA() {
            return "call B in A";
        }
    }

此时，如果通过构造函数或属性注入(@Autowird)，程序在运行时会报一下错误, 提示检测是否包括循环引用

image.png

使用 mediatr 解耦循环依赖

使用mediatr的service如下，在service构造函数注 IMediator ，并实现 IRequestHandler 接口

    public static class DemoAService implements IDemoAService, IRequestHandler<RequestAService, String> {
        //private final IDemoBService bService;

        private final IMediator mediator;

        public DemoAService(IMediator mediator) {
            this.mediator = mediator;
        }

        @Override
        public String hello() {
            return this.mediator.send(new RequestBService());
        }

        @Override
        public String helloWithB() {
            return "call A in B";
        }

        @Override
        public String handle(RequestAService request) {
            return this.helloWithB();
        }
    }

    public static class DemoBService implements IDemoBService, IRequestHandler<RequestBService, String> {
        //private final IDemoAService aService;
        private final IMediator mediator;

        public DemoBService(IMediator mediator) {
            this.mediator = mediator;
        }

        @Override
        public String hello() {
            return this.mediator.send(new RequestAService());
        }

        @Override
        public String helloWithA() {
            return "call B in A";
        }

        @Override
        public String handle(RequestBService request) {
            return this.helloWithA();
        }
    }

    public static class RequestAService implements IRequest<String> {
    }

    public static class RequestBService implements IRequest<String> {
    }

测试代码如下

@ExtendWith(SpringExtension.class)
@Import(
        value = {
                Mediator.class,
                ServiceCycTests.DemoAService.class,
                ServiceCycTests.DemoBService.class,
        }
)
public class ServiceCycTests {

    @Autowired
    IDemoAService aService;

    @Autowired
    IDemoBService bService;

    @Test
    public void should() {
        String a = aService.hello();
        assertThat(a).isEqualTo("call B in A");

        String b = bService.hello();
        assertThat(b).isEqualTo("call A in B");
    }
}

测试结果

image (1).png