java - 如何使用反射来获取对可以传递的 lambda 的引用？

Question

我可以使用反射来动态获取方法并调用它。我怎样才能用 lambda 做同样的事情？ IE 之类的东西

class Foo{
    public static String mapper(Object o){
        return "dummy val";
    }
}

class Bar {
    public static void main(String args[]){
        String[] arr = {"a", "b", "c"};
        Lambda reflectedMapper = ReflectionUtils.getLambda("Foo.mapper");//How do I actually write this line?
        List<String> mapped = arr.stream().map(reflectedMapper).collect(Collectors.toList();
    }
}

Answer 1

如果我理解正确的话，例如:

static class Test {
    public static boolean transform(String s) {
        return s.toUpperCase().equals("SOME");
    }
}

您可以获得您正在寻找的Function，如下所示:

MethodType methodType = MethodType.methodType(boolean.class, String.class);
MethodHandles.Lookup lookup = MethodHandles.lookup();
MethodHandle handle = lookup.findStatic(Test.class, "transform", methodType);
Function<String, Boolean> f = (Function<String, Boolean>) LambdaMetafactory.metafactory(
                 lookup, 
                 "apply",
                 MethodType.methodType(Function.class),
                 methodType.generic(),
                 handle,
                 methodType)
            .getTarget()
            .invokeExact();

另请注意，同样的方法也符合 Predicate 的资格；因为它的签名是 String, boolean 并且可以这样使用:

  MethodType predicateMT = MethodType.methodType(boolean.class, Object.class);
    MethodType stringPredicateMT = MethodType.methodType(boolean.class, String.class);

    MethodType methodType = MethodType.methodType(boolean.class, String.class);

    MethodHandles.Lookup lookup = MethodHandles.lookup();
    MethodHandle handle = lookup.findStatic(Test.class, "transform", methodType);
    Predicate<String> p = (Predicate<String>) LambdaMetafactory.metafactory(
            lookup, "test",
            MethodType.methodType(Predicate.class),
            predicateMT,
            handle,
            stringPredicateMT)
            .getTarget().invokeExact();


 System.out.println(p.test("Some"));