Spring 版MediatR--中介者模式實現庫

背景

C# 版本庫 MediatR 是一箇中介者模式實現類庫，其核心是一箇中介 者模式的.NET實現，其目的是訊息傳送和訊息處理的解耦。它支援單播和多播形式使用同步或非同步的模式來發布訊息，建立和幀聽事件。
java中沒有找到類似類庫，在對MediatR原始碼閱讀中，發現其主要思路是藉助IOC獲取Request與Handler對應關係並進行處理。

中介者模式

中介者模式：用一箇中介物件封裝一系列的物件互動，中介者使各物件不需要顯示地相互作用，從而使耦合鬆散，而且可以獨立地改變他們之間的互動。

使用中介模式，物件之間的互動將封裝在中介物件中，物件不再直接互動(解耦),而是通過中介進行互動，這減少了物件之間的依賴性，從而減少了耦合。

應用

單播訊息傳輸

單播訊息傳輸，也就是一對第一的訊息傳遞，一個訊息對應一個訊息處理，通過 IReust 抽象單播訊息，使用 IRequestHandler  進行訊息處理

@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";
        }
    }

}

多播訊息傳輸

多播訊息傳輸，是一對多的訊息傳遞，一個訊息對應多個訊息處理，通過 INotification 抽象多播訊息，使用 INotificationHanlder 進行訊息處理

@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";
        }
    }

}

實現

核心實現

其主要點是從Spring的ApplicationContext中獲取相關介面bean，然會執行bean方法。
核心方法有兩個：public(多播)和send(單播)。
藉助ResolvableType型別構造解析bean資訊，得到資訊後從spring中獲取物件例項。

/**
 * 中介者實現類
 * <p>
 * 依賴 ApplicationContext
 */
@Component
public class Mediator implements IMediator, ApplicationContextAware {

    private ApplicationContext context;

    /**
     * 釋出同步
     * <p>
     * 根據通知型別和INotificationHandler,從ApplicationContext獲取Handler的BeanNames,
     * 將 BeanNames 轉化為 INotificationHandler 的例項，每個例項呼叫一次handler
     *
     * @param notification    通知內容
     * @param <TNotification> 通知型別
     */
    @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));
    }

    /**
     * 傳送求取
     * <p>
     * 根據request型別，獲取到response型別，
     * 根據IRequestHandler、request型別、response型別從ApplicationContext獲取
     * IRequestHandler例項列表，取第一個例項執行handler方法。
     * <p>
     * <p>
     * 如果為找到handler例項，丟擲NoRequestHandlerException異常
     *
     * @param request     請求
     * @param <TResponse> 響應型別
     * @return 響應結果
     */
    @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) {
            // 丟擲異常
            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 {
}

多播介面

public interface INotification {
}

單播介面

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 {
}

多播處理介面

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

單播處理介面

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;
    }
}

應用場景

mediatr 是一種程式內訊息傳遞機制，使用泛型支援訊息的只能排程，其核心是 訊息解耦 ，基於MediatR可以實現CQRS/EventBus等。

解除建構函式的依賴注入

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

        }

藉助 Mediator，僅需構造注入ImediatR即可

public class DashboardController(
                IMediator
                ) {

        }

service 迴圈依賴，使用mediatr 進行依賴解耦，並使用mediatr進行訊息傳遞

兩個service類和介面如下

    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";
        }
    }

此時，如果通過建構函式或屬性注入(@Autowird)，程式在執行時會報一下錯誤, 提示檢測是否包括迴圈引用

使用 mediatr 解耦迴圈依賴

使用mediatr的service如下，在service建構函式注 IMediator ，並實現 IRequestHandler 介面

    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> {
    }

測試程式碼如下

@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");
    }
}

測試結果

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