How to explicitly invoke default method from a dynamic Proxy?

Question

Since Java 8 interfaces could have default methods. I know how to invoke the method explicitly from the implementing method, i.e. (see Explicitly calling a default method in Java)

But how do I explicitly invoke the default method using reflection for example on a proxy?

Example:

interface ExampleMixin {

  String getText();

  default void printInfo(){
    System.out.println(getText());
  }
}

class Example {

  public static void main(String... args) throws Exception {

    Object target = new Object();

    Map<String, BiFunction<Object, Object[], Object>> behavior = new HashMap<>();

    ExampleMixin dynamic =
            (ExampleMixin) Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),new Class[]{ExampleMixin.class}, (Object proxy, Method method, Object[] arguments) -> {

                //custom mixin behavior
                if(behavior.containsKey(method.getName())) {
                    return behavior.get(method.getName()).apply(target, arguments);
                //default mixin behavior
                } else if (method.isDefault()) {
                    //this block throws java.lang.IllegalAccessException: no private access for invokespecial
                    return MethodHandles.lookup()
                                        .in(method.getDeclaringClass())
                                        .unreflectSpecial(method, method.getDeclaringClass())
                                        .bindTo(target)
                                        .invokeWithArguments();
                //no mixin behavior
                } else if (ExampleMixin.class == method.getDeclaringClass()) {
                    throw new UnsupportedOperationException(method.getName() + " is not supported");
                //base class behavior
                } else{
                    return method.invoke(target, arguments);
                }
            });

    //define behavior for abstract method getText()
    behavior.put("getText", (o, a) -> o.toString() + " myText");

    System.out.println(dynamic.getClass());
    System.out.println(dynamic.toString());
    System.out.println(dynamic.getText());

    //print info should by default implementation
    dynamic.printInfo();
  }
}

---

Edit: I know a similar question has been asked in How do I invoke Java 8 default methods refletively, but this has not solved my problem for two reasons:

• the problem described in that question aimed on how to invoked it via reflection in general - so no distinction between default and overriden method was made - and this is simple, you only need an instance.
• one of the answers - using method handles - does only work with nasty hack (imho) like changing access modifiers to fields of the lookup class, which is the same category of "solutions" like this: Change private static final field using Java reflection: it's good to know it's possible, but I wouldn't use it in production - I'm looking for an "official" way to do it.

The IllegalAccessException is thrown in unreflectSpecial

Caused by: java.lang.IllegalAccessException: no private access for invokespecial: interface example.ExampleMixin, from example.ExampleMixin/package
at java.lang.invoke.MemberName.makeAccessException(MemberName.java:852)
at java.lang.invoke.MethodHandles$Lookup.checkSpecialCaller(MethodHandles.java:1568)
at java.lang.invoke.MethodHandles$Lookup.unreflectSpecial(MethodHandles.java:1227)
at example.Example.lambda$main$0(Example.java:30)
at example.Example$$Lambda$1/1342443276.invoke(Unknown Source)

Answer 1

I've been troubled by similar issues as well when using MethodHandle.Lookup in JDK 8 - 10, which behave differently. I've blogged about the correct solution here in detail.

This approach works in Java 8

In Java 8, the ideal approach uses a hack that accesses a package-private constructor from Lookup:

import java.lang.invoke.MethodHandles.Lookup;
import java.lang.reflect.Constructor;
import java.lang.reflect.Proxy;

interface Duck {
    default void quack() {
        System.out.println("Quack");
    }
}

public class ProxyDemo {
    public static void main(String[] a) {
        Duck duck = (Duck) Proxy.newProxyInstance(
            Thread.currentThread().getContextClassLoader(),
            new Class[] { Duck.class },
            (proxy, method, args) -> {
                Constructor<Lookup> constructor = Lookup.class
                    .getDeclaredConstructor(Class.class);
                constructor.setAccessible(true);
                constructor.newInstance(Duck.class)
                    .in(Duck.class)
                    .unreflectSpecial(method, Duck.class)
                    .bindTo(proxy)
                    .invokeWithArguments(args);
                return null;
            }
        );

        duck.quack();
    }
}

This is the only approach that works with both private-accessible and private-inaccessible interfaces. However, the above approach does illegal reflective access to JDK internals, which will no longer work in a future JDK version, or if --illegal-access=deny is specified on the JVM.

This approach works on Java 9 and 10, but not 8

import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.lang.reflect.Proxy;

interface Duck {
    default void quack() {
        System.out.println("Quack");
    }
}

public class ProxyDemo {
    public static void main(String[] a) {
        Duck duck = (Duck) Proxy.newProxyInstance(
            Thread.currentThread().getContextClassLoader(),
            new Class[] { Duck.class },
            (proxy, method, args) -> {
                MethodHandles.lookup()
                    .findSpecial( 
                         Duck.class, 
                         "quack",  
                         MethodType.methodType(void.class, new Class[0]),  
                         Duck.class)
                    .bindTo(proxy)
                    .invokeWithArguments(args);
                return null;
            }
        );

        duck.quack();
    }
}

Solution

Simply implement both of the above solutions and check if your code is running on JDK 8 or on a later JDK and you'll be fine. Until you're not :)

Answer 2

In Java 16 (from the documentation, which also has more complex examples):

Object proxy = Proxy.newProxyInstance(loader, new Class[] { A.class },
        (o, m, params) -> {
            if (m.isDefault()) {
                // if it's a default method, invoke it
                return InvocationHandler.invokeDefault(o, m, params);
            }
        });
}

Answer 3

If you use a concrete impl class as lookupClass and caller for the invokeSpecial it should correctly invoke the default implementation of the interface (no hack for private access needed):

Example target = new Example();
...

Class targetClass = target.getClass();
return MethodHandles.lookup()
                    .in(targetClass)
                    .unreflectSpecial(method, targetClass)
                    .bindTo(target)
                    .invokeWithArguments();

This of course only works if you have a reference to a concrete object implementing the interface.

Edit: this solution will only work if the class in question (Example in the code above), is private accessible from the caller code, e.g. an anonymous inner class.

The current implementation of the MethodHandles/Lookup class will not allow to call invokeSpecial on any class that is not private accessible from the current caller class. There are various work-arounds available, but all of them require the use of reflection to make constructors/methods accessible, which will probably fail in case a SecurityManager is installed.

Answer 4

If all you have is an interface, and all you have access to is a class object is an interface that extends your base interface, and you want to call the default method without a real instance of a class that implements the interface, you can:

Object target = Proxy.newProxyInstance(classLoader,
      new Class[]{exampleInterface}, (Object p, Method m, Object[] a) -> null);

Create an instance of the interface, and then construct the MethodHandles.Lookup using reflection:

Constructor<MethodHandles.Lookup> lookupConstructor = 
    MethodHandles.Lookup.class.getDeclaredConstructor(Class.class, Integer.TYPE);
if (!lookupConstructor.isAccessible()) {
    lookupConstructor.setAccessible(true);
}

And then use that lookupConstructor to create a new instance of your interface that will allow private access to invokespecial. Then invoke the method on the fake proxy target you made earlier.

lookupConstructor.newInstance(exampleInterface,
        MethodHandles.Lookup.PRIVATE)
        .unreflectSpecial(method, declaringClass)
        .bindTo(target)
        .invokeWithArguments(args);

Answer 5

T. Neidhart answer almost worked but I got the java.lang.IllegalAccessException: no private access for invokespecial

Changing to use MethodHandles.privateLookupIn(...) solved it

return MethodHandles.privateLookupIn(clazz,MethodHandles.lookup())
                        .in(clazz)
                        .unreflectSpecial(method, clazz)
                        .bindTo(proxy)
                        .invokeWithArguments(args);

Here's a full example, the idea is that a user that extends a provided IMap can access nested nested map's with he's custom interface

interface IMap {
    Object get(String key);

    default <T> T getAsAny(String key){
        return (T)get(key);
    }


    default <T extends IMap> T getNestedAs(String key, Class<T> clazz) {
        Map<String,Object> nested = getAsAny(key);
        return (T)Proxy.newProxyInstance(this.getClass().getClassLoader(), new Class[]{clazz},  (proxy, method, args) -> {
                    if (method.getName().equals("get")){
                        return nested.get(args[0]);
                    }
                    return MethodHandles.privateLookupIn(clazz, MethodHandles.lookup())
                            .in(clazz)
                            .unreflectSpecial(method, clazz)
                            .bindTo(proxy)
                            .invokeWithArguments(args);
                }
        );
    }
}

interface IMyMap extends IMap{

    default Integer getAsInt(String key){
        return getAsAny(key);
    }
    default IMyMap getNested(String key){
        return getNestedAs(key,IMyMap.class);
    }
}

@Test
public void test(){
    var data =Map.of("strKey","strValue", "nstKey", Map.of("intKey",42));
    IMyMap base = data::get;

    IMyMap myMap = base.getNested("nstKey");
    System.out.println( myMap.getAsInt("intKey"));
}

Answer 6

Use:

Object result = MethodHandles.lookup()
    .in(method.getDeclaringClass())
    .unreflectSpecial(method, method.getDeclaringClass())
    .bindTo(target)
    .invokeWithArguments();

Answer 7

We can see how spring process default method.

1. try invoke public method MethodHandles.privateLookupIn(Class,Lookup) first. This should success on jdk9+.
2. try create a Lookup with package private constructor MethodHandles.Lookup(Class).
3. fallback to MethodHandles.lookup().findSpecial(...)

https://github.com/spring-projects/spring-data-commons/blob/2.1.8.RELEASE/src/main/java/org/springframework/data/projection/DefaultMethodInvokingMethodInterceptor.java

Answer 8

Lukas' answer works on Android 8+ (earlier releases do not have default methods) but relies on a private API that was blocked in later Android releases. Fortunately, the alternative constructor also works and is in grey list (unsupported) for now. The example (written in Kotlin) can be seen here.

@get:RequiresApi(26)
private val newLookup by lazy @TargetApi(26) {
    MethodHandles.Lookup::class.java.getDeclaredConstructor(Class::class.java, Int::class.java).apply {
        isAccessible = true
    }
}

@RequiresApi(26)
fun InvocationHandler.invokeDefault(proxy: Any, method: Method, vararg args: Any?) =
    newLookup.newInstance(method.declaringClass, 0xf)   // ALL_MODES
        .unreflectSpecial(method, method.declaringClass)
        .bindTo(proxy)
        .invokeWithArguments(*args)