首先，会用到 函数式接口 Consumer, 通过这个可以解耦回调方法，下面先写一个抽象的监听接口

监听接口类 Listenable

java
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import java.util.*;
import java.util.function.Consumer;

public interface Listenable {

    WeakHashMap LISTENERS_WEAK_MAP = new WeakHashMap<>();

    default void registerListener(Listener listener) {
        Objects.requireNonNull(listener);
        Set listeners;
        synchronized (LISTENERS_WEAK_MAP) {
            listeners = LISTENERS_WEAK_MAP.get(this);
            if (listeners == null) {
                listeners = new HashSet<>();
                LISTENERS_WEAK_MAP.put(this, listeners);
            }
        }
        synchronized (listeners) {
            listeners.add(listener);
        }
    }

    default void unregisterListener(Listener listener) {
        Objects.requireNonNull(listener);
        Set listeners;
        synchronized (LISTENERS_WEAK_MAP) {
            listeners = LISTENERS_WEAK_MAP.get(this);
            if (listeners == null) {
                return;
            }
        }
        synchronized (listeners) {
            listeners.remove(listener);
        }
    }

    default Collection getListeners() {
        synchronized (LISTENERS_WEAK_MAP) {
            Set listeners = LISTENERS_WEAK_MAP.get(this);
            if (listeners == null) {
                return new HashSet<>();
            }
            return new HashSet<>(listeners);
        }
    }

    default boolean isListenerRegistered(Listener listener) {
        synchronized (LISTENERS_WEAK_MAP) {
            Set listeners = LISTENERS_WEAK_MAP.get(this);
            if (listeners == null) {
                return false;
            }
            return listeners.contains(listener);
        }
    }

    default void forEachListener(Consumer action) {
        forEachListener(action, true);
    }

    default void forEachListener(Consumer action, boolean ignoreException) {
        Objects.requireNonNull(action);
        Iterable listeners;
        synchronized (LISTENERS_WEAK_MAP) {
            Set values = LISTENERS_WEAK_MAP.get(this);
            if (values == null) {
                return;
            }
            listeners = new ArrayList<>(values);
        }
        for (Listener listener : listeners) {
            try {
                action.accept(listener);
            } catch (Exception e) {
                if (!ignoreException) {
                    throw e;
                }
            }
        }
    }


}

实际用法

用法也比较简单，在自己的实现类 MessageManager 中，写上自定义的回调 OnEventListener，然后再实现通知方法，这样就可以很方便的写各种需要一对多通知的不同类型的回调了

java
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public class Main {

    public static class MessageManager implements Listenable {

        private MessageManager() {}

        private static final MessageManager instance = new MessageManager();

        public static MessageManager getInstance() {
            return instance;
        }

        public void onSuccess(String json) {
            forEachListener(it->it.onSuccess(json));
        }

        public void onError(int code, String error) {
            forEachListener(it->it.onError(code, error));
        }

        public interface OnEventListener {
            void onSuccess(String json);
            void onError(int code, String error);
        }
    }

    public static void main(String[] args) {
        MessageManager.getInstance().registerListener(new MessageManager.OnEventListener() {

            @Override
            public void onSuccess(String json) {
                System.out.println("onSuccess " + json);
            }

            @Override
            public void onError(int code, String error) {
                System.out.println("onError code:" + code + " error:" + error);
            }
        });

        MessageManager.getInstance().onSuccess("My json");
        MessageManager.getInstance().onError(-1, "Error");
    }
}

打印结果

vbnet
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onSuccess My json
onError code:-1 error:Error