Allow configurable class loader sharing between cores
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Key: SOLR-409
URL: https://issues.apache.org/jira/browse/SOLR-409
Project: Solr
Issue Type: Sub-task
Affects Versions: 1.3
Reporter: Henri Biestro
Priority: Minor
Fix For: 1.3
WHAT:
This patch allows to configure in the solrconfig.xml the library directory and
associated class loader used to dynamically create instances.
The solr config XML can now specify a libDir element (the same way the dataDir
can be specified).
That element can also specify through the 'shared' attribute whether the
library is shared.
By default, the shared attribute is true; if you specify a libDir, its class
loader is made shareable.
Syntax:
<libDir shared='true*|false'>/path/to/shareable/dir</libDir>
WHY:
Current behavior allocates one class loader per config & thus per core.
However, there are cases where one would like different cores to share some
objects that are dynamically instantiated (ie, where the class name is used to
find the class through the class loader and instantiate). In the current form;
since each core possesses its own class loader, static members are indeed
different objects. For instance, there is no way of implementing a singleton
shared between 2 request handlers.
Originally from
http://www.nabble.com/Post-SOLR215-SOLR350-singleton-issue-tf4776980.html
HOW:
The libDir element is extracted from the XML configuration file and parsed in
the Config constructor.
A static map of weak reference to classloaders keyed by library path is used to
keep track of already built shareable class loaders.
The weak reference is here to allow class eviction by the GC, avoiding leakage
that would result by keeping hard reference to class loaders.
STATUS:
initial code drop
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