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.

Answer 2

The simplest, albeit not necessarily most performant, approach would be just wrapping the Method into a Consumer.

final Method m = ...
final T target = ...

Consumer<String> c = (arg1) => m.invoke(t, arg1);

Using the LambdaMetaFactory may yield more optimal code, but considering that you're dispatching through a Map it's probably not worth it.

---

This is somehow possible to do in '1-liner' manner

If you really want to emulate what the bytecode does that's only true for for sufficiently tortured definitions of one-liner. Your decompiler lies to you to some extent.

No exception handling is required

That is because the concept of checked exceptions does not exist on the bytecode level. This can be emulated with static helper methods that do a sneaky rethrow for you.

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.

It looks like pseudocode for invokedynamic calls. What the JVM really does is more complicated and can't be expressed concisely in java since it involves lazy initialization. It's best to read the java.lang.invoke package description to get an idea what really happens.

The java-level equivalent to the linking stage would be putting the CalleSite's dynamicInvoker MH into a static final MethodHandle field and calling its invokeExact method.

(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.

as mentioned above, the linking stage is equivalent to putting the methodhandle in a static field once and then calling that in the future instead of attempting to resolve the method a second time.

Answer 3

The decompiler failed badly on your code, however, there is no correct decompiling anyway, besides recreating the original Java 8 method reference, which is not what you were interested in.

The lambda expressions and method references are compiled using an invokedynamic byte code instruction which has no equivalent in the Java programming language. The equivalent code would be something like:

public static void main(String... arg) {
    Consumer<String> consumer=getConsumer();
    consumer.accept("hello world");
}

static Consumer<String> getConsumer() {
    try {
        MethodHandles.Lookup lookup=MethodHandles.lookup();
        MethodType consumeString = MethodType.methodType(void.class, String.class);
        return (Consumer<String>)LambdaMetafactory.metafactory(lookup, "accept",
            MethodType.methodType(Consumer.class, PrintStream.class),
            consumeString.changeParameterType(0, Object.class),
            lookup.findVirtual(PrintStream.class, "println", consumeString), consumeString)
        .getTarget().invokeExact(System.out);
    }
    catch(RuntimeException | Error e) { throw e; }
    catch(Throwable t) { throw new BootstrapMethodError(t); }
}

except, that everything that is done within getConsumer() is originally handled by a single invokedynamic instruction which will treat all involved MethodHandles and MethodType instances like constants and whose result of the first-time evaluation gets an intrinsic caching facility. You can’t model that using ordinary Java source code.

Still, the Consumer<String> returned by the getConsumer() method above is the exact equivalent of the expression System.out::println (when assigned to a Consumer<String>) bearing the same behavior and performance characteristics.

You may study “Translation of Lambda Expressions” by Brian Goetz for getting a deeper understanding of how it works. Also, the API documentation of LambdaMetafactory is quite exhaustive.