Uwe Schindler created LUCENE-4259:
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Summary: Allow reloading of codec/postings format list when
classpath changes
Key: LUCENE-4259
URL: https://issues.apache.org/jira/browse/LUCENE-4259
Project: Lucene - Core
Issue Type: Improvement
Affects Versions: 4.0-ALPHA
Reporter: Uwe Schindler
Assignee: Uwe Schindler
Fix For: 4.0, 5.0
While implementing the SPI for analysis factories, Robert and me found out that
Solr does not see codecs/postingsformats or analysis factories outside it's
webapp/lib folder. The reason is simple: SPI uses by default the context
classloader of the thread calling the codec API for the first time (which is
Jetty's webapp classpath).
We solved the problem for analysis factories, because those are loaded with the
help of SolrResourceLoader, which exports the ClassLoader it uses, so it can
return all analysis compoenst visible to the ResourceLoader. This is cool,
because you can drop the analysis/smartchinese or analysis/icu module JAR into
your plugin folder and Solr will automatically use it.
For Codecs and PostingsFormats this is a little bit more complicated: The list
of codecs is loaded on clinit of Codecs.class or PostingsFormat.class, which
can be very early in Solr's startup. When Solr changes his classpath in
ResourceLoader, the new Codecs are never seen, so you cannot drop new codecs
into the plugin folder. Similar problems may happen with other webapps, because
Tomcat & Jetty have crazy classloader delegations (there were although reports
that Lucene did not find their codecs at all - Simon Willnauer told me!)
This patch will change NamedSPILoader to provide similar support like
java.util.ServiceLoader does in JDK: You can tell NamedSPILoader to reload
(means again list the classpath's JAR files and look for new META-INF/services
files). This API is statically exported to Codec.reloadCodecs() and
PostingsFormat.reploadPostingsFormats(), which takes a ClassLoader. This
Classloader will then be checked only for *new* codecs, old ones or those
already visible are not removed/replaced. SolrResourceLoader can then call this
static method on each change of its classloader on startup.
The implementation presented here is threadsafe, the SPI map is updated in a
copy-on-write way and assigned to a volatile field.
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