将 java.lang.reflect.Method 转换为函数式接口 [英] Cast java.lang.reflect.Method to a functional interface

问题描述

很难找到有关该主题的任何线索.我能找到的只是关于将一​​个函数式接口转换为另一个函数式接口的问题，以及一些关于 Java 中类型转换的文章.不是我要找的.

It is difficult to find any clues for the topic. All I could find is questions about converting one functional interface to another and some articles on type casting in Java. Not what I was looking for.

这个问题是关于转换 lambda → Method ，我想要相反的，转换 Method 到任何功能接口，例如，到 Consumer.

This question is about converting lambda → Method and I want the opposite, to convert Method to any functional interface, for example, to Consumer.

我发现的方法是围绕 Method#invoke 方法创建一个 lambda 适配器:

The way I found is to create a lambda adapter around the Method#invoke method:

    public void registerCallbacks(final Object annotated) {
        Class clazz = annotated.getClass();
        for (Method method : clazz.getDeclaredMethods()) {
            if (method.isAnnotationPresent(Callback.class)) {
                Callback registration = method.getAnnotation(Callback.class);

                List<String> warnings = new ArrayList<>(3);

                if (!Modifier.isPublic(method.getModifiers()))
                    warnings.add(String.format("Method %s must be public", method));
                if (method.getParameterCount() != 1)
                    warnings.add(String.format("Method %s must consume only one argument", method));
                if (method.getParameterCount() == 1 && !method.getParameterTypes()[0].equals(Integer.class))
                    warnings.add(String.format("Method %s must consume %s", method, Integer.class));

                if (!warnings.isEmpty()) {
                    warnings.forEach(log::warn);
                    continue;
                }

                CALLBACKS_MAPPER.registerCallback((param) -> {
                    try {
                        method.invoke(annotated, param);
                    } catch (IllegalAccessException | InvocationTargetException e) {
                        // Should not happen due to checks before.
                        log.warn(String.format("Could not invoke %s on %s with %s", method, annotated, param), e);
                    }
                });
                log.info("Registered {} as a callback", method);
            }
        }
    }

但是我想避免写

    CALLBACKS_MAPPER.registerCallback((param) -> {
        try {
            method.invoke(annotated, param);
        } catch (IllegalAccessException | InvocationTargetException e) {
            // Should not happen due to checks before.
            log.warn(String.format("Could not invoke %s on %s with %s", method, annotated, param), e);
        }
    });

赞成更简单的东西，比如

in favor of something simpler, like

    CALLBACKS_MAPPER.registerCallback(SomeApacheLib.methodToFunction(annotated, method));

➥ 那么，有没有办法将旧的 Java 1.1 反射库映射到新的 Java 8 功能接口，或者是我太笨了，上面提到的 lambda 解决方案很好?

➥ So, is there a way to map old Java 1.1 reflection library to newer Java 8 functional interfaces, or it is me being stupid and the abovementioned solution with lambda is fine as it is?

推荐答案

如果你满足于在幕后使用反射，只是不喜欢 try/catch 围绕 invoke，你可以做一个简单的实用函数，如:

If you're content with using reflection under the hood, just don't like the try/catch around the invoke, you can just make a simple utility function like:

public static <T> Consumer<T> toConsumer(Object annotated, Method m) {
    return param -> {
        try {
            m.invoke(annotated, param);
        } catch (IllegalAccessException | InvocationTargetException e) {
            throw new RuntimeException(e);
        }
    };
}

这正是您想要的语法:

CALLBACKS_MAPPER.registerCallback(toConsumer(annotated, method));

但是如果你想完全避免反射，你可以使用LambdaMetafactory来创建一个Consumer:

But if you want to actually avoid reflection altogether, you can use LambdaMetafactory to create a Consumer:

static Consumer<String> toConsumer(MethodHandles.Lookup lookup, Object annotated, Method method) throws Throwable {
    MethodType consumeString = MethodType.methodType(void.class, String.class);
    MethodHandle handle = lookup.unreflect(method);
    final CallSite site = LambdaMetafactory.metafactory(lookup, "accept",
            MethodType.methodType(Consumer.class, annotated.getClass()),
            consumeString.changeParameterType(0, Object.class),
            handle,
            consumeString);
    return (Consumer<String>) site.getTarget().invoke(annotated);
}

将 String 更改为您的回调预期接受的任何内容.然后:

Change String to whatever your callbacks are expected to accept. And then:

CALLBACKS_MAPPER.registerCallback(toConsumer(MethodHandles.lookup(), annotated, method));

当然，这里唯一合适的解决方案是重构代码以使用已知的回调接口，您通常可以在该回调接口上调用定义的方法，而不是传递 Method .

Of course, the only proper solution here is that you refactor your code to use a known callback interface on which you can normally call a defined method, instead of passing Methods around.

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