Throwable types handled specially in the VM?

[Joe Darcy]

Hi David.

David Holmes wrote:
> Hi Joe,
>
> Joe Darcy said the following on 08/20/10 12:03:
>> As part of Project Coin, try-with-resources/ARM blocks have recently 
>> been integrated into JDK 7.  Part of this feature is changes to 
>> java.lang.Throwable to support recording of suppressed exception 
>> information (6911258 "Project Coin: Add essential API support for 
>> Automatic Resource Management (ARM) blocks").  Subsequently, some 
>> more changes in Throwable were necessary to fix a regression in the 
>> ability to print the stack traces of specially handled 
>> OutOfMemoryErrors (6973831 "NPE when printing stack trace of OOME").
>>
>> Are there any other Throwable types which receive special handling or 
>> creation by the VM?  If there are any, I'd like to examine them to 
>> find and correct and additional issues.
>
> All the preallocation of exceptions occurs in universe_post_init in 
> universe.cpp. These are as follows
>
>     // Setup preallocated OutOfMemoryError errors
>     Universe::_out_of_memory_error_java_heap = 
> k_h->allocate_permanent_instance(CHECK_false);
>     Universe::_out_of_memory_error_perm_gen = 
> k_h->allocate_permanent_instance(CHECK_false);
>     Universe::_out_of_memory_error_array_size = 
> k_h->allocate_permanent_instance(CHECK_false);
>     Universe::_out_of_memory_error_gc_overhead_limit =
>       k_h->allocate_permanent_instance(CHECK_false);
>
>     // Setup preallocated NullPointerException
>     // (this is currently used for a cheap & dirty solution in 
> compiler exception handling)
>     Universe::_null_ptr_exception_instance = 
> instanceKlass::cast(k)->allocate_permanent_instance(CHECK_false);
>     // Setup preallocated ArithmeticException
>     // (this is currently used for a cheap & dirty solution in 
> compiler exception handling)
>     Universe::_arithmetic_exception_instance = 
> instanceKlass::cast(k)->allocate_permanent_instance(CHECK_false);
>     // Virtual Machine Error for when we get into a situation we can't 
> resolve
>     Universe::_virtual_machine_error_instance =
>       instanceKlass::cast(k)->allocate_permanent_instance(CHECK_false);
>
> As you're aware from 6973831 these are raw allocations that invoke no 
> constructor, so all fields are implicitly set to their default (null, 
> 0) values.

Right.

>
> I can't think of any other specialized exception handling.
>
 From the name "allocate_permanent_instance," I assume that these 
exception objects are saved and reused for the lifetime of the JVM, 
correct?  For better or worse, Throwable and its subclasses are mutable; 
the suppressed warning information is a new possible mutation, setting 
the stack trace is an existing mutability avenue.

If so, as Remi has recently pointed out, perhaps these fields should be 
cleared when the exceptions are resued?

 From an API perspective, perhaps the suppression feature should be 
modified so that self-suppression is interpreted as not allowing a 
particular exception instance to record addition suppressed exceptions...

-Joe