Make reflective call to Lookup lazy on first default method invocation

[Dragas]

After moving my project to fully support JPMS (JDK11), and retrofit has become unusable due to the following error:

Caused by: java.lang.reflect.InaccessibleObjectException: Unable to make private java.lang.invoke.MethodHandles$Lookup(java.lang.Class,int) accessible: module java.base does not "opens java.lang.invoke" to module retrofit2
	at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:340)
	at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:280)
	at java.base/java.lang.reflect.Constructor.checkCanSetAccessible(Constructor.java:189)
	at java.base/java.lang.reflect.Constructor.setAccessible(Constructor.java:182)
	at retrofit2@2.9.0/retrofit2.Platform.<init>(Platform.java:59)
	at retrofit2@2.9.0/retrofit2.Platform.findPlatform(Platform.java:44)
	at retrofit2@2.9.0/retrofit2.Platform.<clinit>(Platform.java:35)
	... 11 more

It seems that retrofit deeply depends on Java's Lookup class and making its constructor visible to itself. The issue becomes apparent, when using the JPMS. A workaround for this is to add --add-opens java.base/java.lang.invoke=retrofit2 but it would be nice if there wasn't such need to expose the base module's packages myself. Perhaps add a call to Module#addOpen(String, Module) when initializing retrofit's class?

[JakeWharton]

If you can move to JDK 14 or newer this reflective call is no longer required. I suspect we'll probably make it lazy rather than eager, but I don't see us adding a call to Module (unless that also was done with reflection).

[JakeWharton]

#3444 was the motivation for making it lazy. This is yet another reason!

The reflection is only needed when your service interfaces have default methods (on pre-14).

[Dragas]

I doubt reflectively calling Module class will solve the issue, considering open declaration in module definition is what permits reflective operations against module exposing a package. See the following for JDK11 https://hg.openjdk.java.net/jdk/jdk11/file/1ddf9a99e4ad/src/java.base/share/classes/module-info.java, which does not contain an open declaration on java.lang package. Nor does JDK14 https://hg.openjdk.java.net/jdk/jdk14/file/6c954123ee8d/src/java.base/share/classes/module-info.java

[JakeWharton]

I can't reflectively invoke public API? That's pretty nonsensical on the JDKs part. But since this isn't a problem with JDK 14 any more I'm inclined to make the reflective lookup on Lookup lazy on first use and if you are using default methods on pre-14 in the module system then you'll just have to live with opening up java.base until you can upgrade to 14 or newer.

[Dragas]

Fair enough.

[robertvazan]

@JakeWharton You can definitely access all public APIs via reflection even with strong module encapsulation.

[nienienienie]

true :)

If you can move to JDK 14 or newer this reflective call is no longer required.

I had JDK 11 and warning was there.

Then I moved to JDK 17 and it's gone.

[JakeWharton]

This happened, at some point. The lookup is now lazy.

retrofit/retrofit/src/main/java/retrofit2/DefaultMethodSupport.java

Lines 36 to 37 in e34aee6

	Constructor<Lookup> constructor = lookupConstructor;
	if (constructor == null) {