Java 8 introduced default methods to interfaces. Existing InvocationHandler implementations will not invoke default interface methods. This short article documents the necessary changes. Note: It first describes the implementation based on a reading of the documents and then provides a working implementation for Java 8.
Given the InvocationHandler implementation:
@Override
public Object invoke(Object proxy,
Method method, Object[] argv) throws Throwable {
Object result = null;
/*
* Logic to calculate result.
*/
return result;
}
the Java 8 solution appears to be to extend the implementation to invoke any interface default methods through a MethodHandle:
@Override
public Object invoke(Object proxy,
Method method, Object[] argv) throws Throwable {
Object result = null;
Class<?> declaringClass = method.getDeclaringClass();
if (method.isDefault()) {
result =
MethodHandles.lookup()
.in(declaringClass)
.unreflectSpecial(method, declaringClass)
.bindTo(proxy)
.invokeWithArguments(argv);
} else {
/*
* Logic to calculate result.
*/
}
return result;
}
If the Method is "default" then the target interface method is invoked. Otherwise, the InvocationHandler implementation processes as before. Any interface default method should be invoked by:
- Finding the
MethodHandles.LookupthroughMethodHandles.lookup().in(declaringClass), - Get a
MethodHandlebypassing any overriding methods through.unreflectSpecial(method, declaringClass), and, - Invoke the method on the
proxywith.bindTo(proxy).invokeWithArguments(argv)
Unfortunately, this does not work if the declaringClass is not "private-accessible" to the caller (which is most of the time) resulting in:
Caused by: java.lang.IllegalAccessException: no private access for invokespecial: interface package1.SomeInterface, from package1.SomeInterface/public
at java.lang.invoke.MemberName.makeAccessException(MemberName.java:850)
at java.lang.invoke.MethodHandles$Lookup.checkSpecialCaller(MethodHandles.java:1572)
at java.lang.invoke.MethodHandles$Lookup.unreflectSpecial(MethodHandles.java:1231)
at package2.InvocationHandlerImpl.invoke(InvocationHandlerImpl.java:59)
The actual Java 8 solution is:
@Override
public Object invoke(Object proxy,
Method method, Object[] argv) throws Throwable {
Object result = null;
Class<?> declaringClass = method.getDeclaringClass();
if (method.isDefault()) {
Constructor<MethodHandles.Lookup> constructor =
MethodHandles.Lookup.class.getDeclaredConstructor(Class.class);
constructor.setAccessible(true);
result =
constructor.newInstance(declaringClass)
.in(declaringClass)
.unreflectSpecial(method, declaringClass)
.bindTo(proxy)
.invokeWithArguments(argv);
} else {
/*
* Logic to calculate result.
*/
}
return result;
}
This will not work in Java 9+. In Java 9 and subsequent releases, the solution should be based on MethodHandles.Lookup.findSpecial() and/or MethodHandles.privateLookupIn().
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