Using java.awt.Toolkit.getDefaultToolkit().getScreenSize() reflectively causes InaccessibleObjectException

Peter Levart peter.levart at
Sat Jan 7 18:16:48 UTC 2017

Hi Rony,

On 01/07/2017 03:53 PM, Rony G. Flatscher wrote:
> Hi Peter,
> thank you very much for your efforts!
> However, in this context there is a problem at hand, that there is no 
> information available what Java method returned what object and what 
> cast was carried out, if any. To understand this, maybe I should give 
> a little bit more information about the Rexx-Java bridge: Rexx/ooRexx 
> (originally developed by IBM, now in opensource) is an interpreter for 
> a dynamically typed, caseless programming language with a rather easy 
> to learn syntax, yet powerful implemented concepts. ooRexx is 
> implemented in C++.
> The Rexx-Java-bridge uses JNI and a Java package (for ooRexx 
> programmers it is an external function package called BSF4ooRexx, 
> which allows to camouflage all of Java as the dynamically typed, 
> caseless ooRexx). It is possible with this package to create Rexx 
> proxy objects for Java objects (and the other way around as well). 
> This is realized by storing proxied Java objects on the Java side in a 
> Map ("registry") and using a common (unique) string value as the key.
> So when the Rexx side invokes a Java method, briefly the following 
> steps take place (there is much more to this, but not important in 
> this context):
>   * the Rexx side uses JNI and supplies the string identifying the
>     Java object in the Map, the method name in uppercase (caselessness
>     is realized in Rexx by uppercasing all Rexx tokens outside of
>     quotes) and the arguments, if any,
>   * the Java side fetches the Java object from the Java registry and
>     inspects it for its available methods, picks those that have
>     caselessly the same name as the supplied method name, then checks
>     whether the arguments are type-compatible and invokes the method;
>     any returned Java object will be placed in the Java "registry" and
>     its key (a unique string) is returned to Rexx.

Couldn't you save also the method's return type besides the result under 
the same key into the registry, so next time you have to invoke a method 
on such object, you retrive the object and the type you use to find 
methods on?

> So after returning control to Rexx, there is no information available 
> about the Java object in the Java registry other than the string 
> serving as the key to fetch that Java object on the Java side.

Not on the Rexx side, but on the Java side in the registry. Right where 
you need it, right?

> Take this Rexx code as an example (the tilde is the message operator 
> in ooRexx and can have white space around it):
>     clzToolkit = bsf.import("java.awt.Toolkit")
>     dim = clzToolkit ~getDefaultToolkit ~getScreenSize
> will be transformed internally by Rexx into:
>     CLZTOOLKIT=BSF.IMPORT("java.awt.Toolkit")
> and the Java bridge gets used (via JNI) as follows:
>   * step 1: BSF.IMPORT() is an external Rexx function that will use
>     JNI and cause a Java class object to be loaded (and stored in the
>     Java registry) and boxed as an ooRexx proxy class object upon
>     return and assigned to the Rexx variable CLZTOOLKIT,

This time the 'type' to search methods on is the same as the class 
object you just "imported".

>  *
>   * step 2: the CLZTOOLKIT~GETDEFAULTTOOLKIT statement contains a Rexx
>     message that will cause JNI to be used and the Java method
>     GETDEFAULTTOOLKIT to be executed for the Java object referenced by
>     CLZTOOLKIT (which incorporates the unique string for that proxied
>     Java class object); the returned Java object will be stored in the
>     Java registry, its unique key (a string) returned, boxed as an
>     ooRexx proxy object value which will be the receiver of the next
>     Rexx message,

Right and if you also save the return type of the method you just called 
into the registry besides the returned object on the Java side, you can 
use it later...

>  *
>   * step 3: the returned value gets the GETSCREENSIZE Rexx message
>     sent to it causing JNI to be used and the Java method
>     GETSCREENSIZE to be located and executed for the Java object
>     returned from the previous step;

The method should then use the saved method return type from previous 
step for looking up the GETSCREENSIZE method...

>   * the returned Java object will be stored in the Java registry, its
>     unique key (a string) returned, boxed as an ooRexx proxy object
>     value that gets assigned to the Rexx variable DIM.

You should then store the getScreenSize() method's return type besides 
the returned object under the key... You see the pattern?

>  *
> Each step gets carried out contextless, i.e. there is no Java context 
> available, that we (or the Java compiler) can see/infer when looking 
> at a Java program.

When you invoke them method you not only store the returned object but 
also the method's return type.

If you need casting, then this would need to be explicit (like in Java).

There's one problem with this scheme. What is the key you use to 
register returned object? Is it based on object identity? When methods 
return the same instance, is it saved under the same key? If yes, which 
is understandable, then there might be a problem when two methods with 
different return types return the same instance. Which return type 
should you use to find methods for following invocations then? Maybe the 
most specific type (if they are related) or both (all) of them if they 
are not and then use them all to search for methods.

Regards, Peter

> ---rony
> P.S.: Also it might be interesting to know, that with that same 
> Rexx-Java bridge it is possible to implement Java methods from 
> interface or abstract classes in Rexx! In that case there is a Java 
> proxy class available for proxying Rexx objects and on the Rexx side 
> there is a Rexx Directory to maintain the proxied Rexx objects for 
> their Java proxies. Fun stuff! :)
> On 06.01.2017 23:22, Peter Levart wrote:
>> Hi Rony,
>> On 01/06/2017 02:28 PM, Rony G. Flatscher wrote:
>>>> >The j.l.r.Method object on which you call invoke() should not be obtained by inspecting the
>>>> >methods of the implementation class given by getDefaultToolkit().getClass(). Implementation
>>>> >classes (i.e. classes in non-exported packages) cannot be instantiated, nor their members
>>>> >manipulated, by code outside their module.
>>>> >
>>>> >The j.l.r.Method object on which you call invoke() should be obtained by inspecting the methods of
>>>> >the "public Java class" java.awt.Toolkit. The first argument that you pass to invoke(), indicating
>>>> >the receiver, can still be instanceof the implementation class.
>>> As was noted earlier, the information that some Java object xyz was created by some public method
>>> "getDefaultToolkit()" and hence knowing that its return type would be that of the java.desktop
>>> public class java.awt.Toolkit is not available at runtime.
>> But it is. The method Toolkit.getDefaultToolkit() has a return type. 
>> You can use reflection to find out that return type of that method:
>> Method getDefKitMeth = Toolkit.class.getMethod("getDefaultToolkit");
>> Class<?> tkClass = getDefKitMeth.getReturnType();
>> // now you can obtain the toolkit instance:
>> Object tkInst = getDefKitMeth.invoke(null);
>> // and obtain a method to be called upon it
>> Method getScrSizMeth = tkClass.getMethod("getScreenSize");
>> // and invoke it:
>> Object screenSize = getScrSizMeth.invoke(tkInst);
>> ... and so on...
>> You see, I never had to mention java.awt.Toolkit type explicitly to 
>> invoke getScreenSize on an object of that type (or subtype). If you 
>> think what a programmer does when he codes this in straight Java 
>> without using reflection, it is the following:
>> 1. He finds out a factory method on Toolkit class: 
>> Toolkit.getDefaultToolkit()
>> 2. He looks up the return type of that method (in javadocs).
>> 3. He uses that type to declare a local variable to which it assigns 
>> the result of the method invocation:
>> java.awt.Toolkit tkInst = java.awt.Toolkit.getDefaultToolkit();
>> 4. He looks up and finds an instance method to call in type 
>> java.awt.Toolkit: java.awt.Toolkit.getScreenSize() and writes it down:
>> tkInst.getScreenSize();
>> Above invocation is using static type java.awt.Toolkit - the return 
>> type of Toolkit.getDefaultToolkit().
>> You can do similar things with reflection. Instead of using 
>> anInstance.getClass() to get the runtime class of the instance, you 
>> can use Method.getReturnType() of the method that was used to obtain 
>> the instance. If API is designed so that no casts are needed when you 
>> chain calls, then this should work.
>> Regards, Peter

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