RFR: 8277072: ObjectStreamClass caches keep ClassLoaders alive

[Peter Levart]

On Mon, 15 Nov 2021 19:10:17 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> > If the intent of this change is to alter the lifetimes of the objects in question in a meaningful way, I recommend a CSR for the behavioral compatibility impact.
> 
> It would be hard for application code to observe this change: before the change, a ClassLoader and its classes could be lingering in the cache longer than necessary, even if otherwise not reachable. With the change, they would be reclaimed as soon as they become unreachable. This could only be observed, if application code holds onto ClassLoader or Class instances via Weak or PhantomReference, and even then I am not sure if that qualifies as 'meaningful'.

Hi Roman,

What your patch changes is the following:

ConcurrentHashMap<WeakReference<Class<?>>, SoftReference<ObjectStreamClass>>

into:

ConcurrentHashMap<WeakReference<Class<?>>, WeakReference<ObjectStreamClass>>


While it is true that when the Class<?> object used in a weak key is not reachable any more by the app, it is not sensible to hold on to the value any longer so in that respect SoftReference<Value> is to "storng" of a weakness. But while the Class<?> object is still reachable by the app, the app expects to obtain the ObjectStreamClass (the value) from the cache at least most of the time. If you change the SoftReference<ObjectStreamClass> into WeakReference<ObjectStreamClass>, the ObjectStreamClass might get GC-ed even while in the middle of stream deserialization.

ObjectStream class pre-dates java.lang.invoke (MethodHandles), so it uses its own implementation of weak caching. But since MethodHandles, there is a class called ClassValue<T> that would solve these problem with more elegance, because it ties the lifetime of the value (ObjectStreamClass) to the lifetime of the Class<?> key (Class object has a strong reference to the associated value) while the Class<?> key is only Weakly referenced.

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PR: https://git.openjdk.java.net/jdk/pull/6375