Create Java8 function reference programmatically

Question

Just a theoretic question, I do not have practical use-case currently.

Assuming some my API accepts function reference as an argument and I would like to both feed it directly from code via '::' syntax or collect matching functions via reflection, store in some Map and invoke conditionally.

It is possible to programmatically convert method into Consumer<String>?

Map<String, Consumer<String>> consumers = new HashMap<>();
consumers.put("println", System.out::println);

Method method = PrintStream.class.getMethod("println", String.class);
consumers.put("println", makeFunctionReference(method));
...
myapi.feedInto(consumers.get(someInput.getConsumerId()));

---

Update:

Though not satisfied by solutions in currently provided answers, but after getting the hint about LambdaMetaFactory I tried to compile this code

public class TestImpl {
    public static void FnForString(String arg) {}
}

public class Test {
    void test() {
        List<String> strings = new ArrayList<>();
        Consumer<String> stringConsumer = TestImpl::FnForString;

        strings.stream().forEach(stringConsumer);
        strings.stream().forEach(TestImpl::FnForString);
        stringConsumer.accept("test");
    }
}

and after feeding only Test class into CFR decompiler I'm getting following back:

public class Test {
    void test() {
        ArrayList strings = new ArrayList();
        Consumer<String> stringConsumer = 
            (Consumer<String>)LambdaMetafactory.metafactory(
                null, null, null, 
                (Ljava/lang/Object;)V, 
                FnForString(java.lang.String), 
                (Ljava/lang/String;)V)();
        strings.stream().forEach(stringConsumer);
        strings.stream().forEach(
            (Consumer<String>)LambdaMetafactory.metafactory(
                null, null, null, 
                (Ljava/lang/Object;)V, 
                FnForString(java.lang.String ), 
                (Ljava/lang/String;)V)());
        stringConsumer.accept("test");
    }
}

Out of that I see that:

• This is somehow possible to do in '1-liner' manner
• No exception handling is required
• I have no idea what is (Ljava/lang/Object;)V (and others) in decompiler's output. It should match to MethodType in metafactory() arguments. Additionally - either decompiler 'eats/hides' something, but there seems to be now invocations of methods during getting of function reference.
• (offtop) Obtaining function reference even in compiled code is at least one function call - in general this may be not unnoticeably cheap operation in performance critical code.

And ... with both Test and TestImpl classes provided, CFR reconstructs absolutely same code that I've compiled.

Answer 1

You could do this with reflection like this:

consumers.put("println", s -> {
    try {
        method.invoke(System.out, s);
    } catch (InvocationTargetException | IllegalAccessException e) {
        throw new RuntimeException(e);
    }
});

But it you want your code to compile to the same code using a method-reference (i.e. using invokedynamic instructions), you can use a MethodHandle. This does not have the overhead of reflection and so it will perform a lot better.

MethodHandles.Lookup lookup = MethodHandles.lookup();
MethodType methodType = MethodType.methodType(void.class, String.class);
MethodHandle handle = lookup.findVirtual(PrintStream.class, "println", methodType);

consumers.put("println", s -> {
    try {
        handle.invokeExact(System.out, s);
    } catch (Throwable e) {
        throw new RuntimeException(e);
    }
});

consumers.get("println").accept("foo");

In this code, first a MethodHandles.Lookup object is retrieved. This class is reponsible for creating MethodHandle objects. Then a MethodType object, which represents the arguments and return type accepted and returned by a method handle is created: in this case, it is a method that returns void (hence void.class) and takes a String (hence String.class). Finally, the handle is obtained by finding the println method on the PrintStream class.

You can refer to this question (and this one) for more information about what MethodHandles are.