FangYongs commented on PR #22718:
URL: https://github.com/apache/flink/pull/22718#issuecomment-1578869662
@zentol I got some help from our JVM team and the detailed structure is in
JVM native codes. For Java 10 and later, each classloader has a
`ClassLoaderData` and there is a `Dictionary` in `ClassLoaderData` to cache the
loaded class:
class ClassLoaderData : public CHeapObj<mtClass> {
...
**Dictionary* _dictionary; // The loaded InstanceKlasses,
including initiated by this class loader**
// Loaded class dictionary
Dictionary* dictionary() const { return _dictionary; }
Dictionary* ClassLoaderData::create_dictionary() {
...
// const int _boot_loader_dictionary_size = 1009;
// const int _default_loader_dictionary_size = 107;
return new Dictionary(this, size, resizable);
}
}
Each classloader tries to find class from dictionary in its
`ClassLoaderData` first.
Klass* SystemDictionary::resolve_instance_class_or_null(Symbol* name,
Handle class_loader,
Handle
protection_domain,
TRAPS) {
class_loader = Handle(THREAD,
java_lang_ClassLoader::non_reflection_class_loader(class_loader()));
ClassLoaderData* loader_data = register_loader(class_loader);
**Dictionary* dictionary = loader_data->dictionary();**
unsigned int d_hash = dictionary->compute_hash(name);
...
{
**Klass* probe = dictionary->find(d_hash, name, protection_domain);**
if (probe != NULL) return probe;
}
...
}
After a classloader load class from its parent classloader, it will put the
class to the dictionary as a local cache
void SystemDictionary::update_dictionary(unsigned int d_hash,
int p_index, unsigned int p_hash,
InstanceKlass* k,
Handle class_loader,
TRAPS) {
...
ClassLoaderData *loader_data = class_loader_data(class_loader);
...
**// Make a new dictionary entry.**
Dictionary* dictionary = loader_data->dictionary();
InstanceKlass* sd_check = find_class(d_hash, name, dictionary);
if (sd_check == NULL) {
dictionary->add_klass(d_hash, name, k);
}
...
The issue of JDK is https://bugs.openjdk.org/browse/JDK-7133093
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