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https://issues.apache.org/jira/browse/SOLR-14579?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17189086#comment-17189086
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Uwe Schindler edited comment on SOLR-14579 at 9/2/20, 8:52 AM:
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If you are interested, here is the implementation of a Bootstrap for lambdas in
Elasticsearch's Painless (written by me), especially see the comment:
[LambdaBootstrap lines
126-133|https://github.com/elastic/elasticsearch/blob/0a135a358e4736ee54d76bad0932cab1b5cc1a74/modules/lang-painless/src/main/java/org/elasticsearch/painless/LambdaBootstrap.java#L126-L133]
If no captures are there: it's a singleton and the bootstrap code returns a
call site just returning the singleton: [LambdaBootstrap lines
481-491|https://github.com/elastic/elasticsearch/blob/0a135a358e4736ee54d76bad0932cab1b5cc1a74/modules/lang-painless/src/main/java/org/elasticsearch/painless/LambdaBootstrap.java#L481-L491];
you see it returns a {{ConstantCallSite}} always returning the same singleton
using {{MethodHandles.constant()}} created there with {{newInstance}}. At a
later stage, hotspot can even inline this, so code is fast as if the lambda
won't exist at all!
You can check similar code in LambdaBootstraps code in the JDK, but those are
doing a bit more magic, as Java is type-safe, while Elasticsearch Painless has
more freedom of types (e.g. def type), so this is easier to read.
was (Author: thetaphi):
If you are interested, here is the implementation of a Bootstrap for lambdas in
Elasticsearch's Painless (written by me), especially see the comment:
[LambdaBootstrap lines
126-133|https://github.com/elastic/elasticsearch/blob/0a135a358e4736ee54d76bad0932cab1b5cc1a74/modules/lang-painless/src/main/java/org/elasticsearch/painless/LambdaBootstrap.java#L126-L133]
If no captures are there: it's a singleton and the bootstrap code returns a
method handle that: [LambdaBootstrap lines
481-491|https://github.com/elastic/elasticsearch/blob/0a135a358e4736ee54d76bad0932cab1b5cc1a74/modules/lang-painless/src/main/java/org/elasticsearch/painless/LambdaBootstrap.java#L481-L491];
you see it returns a {{ConstantCallSite}} always returning the same singleton
created there with {{newInstance}}.
You can check similar code in LambdaBootstraps code in the JDK, but those are
doing a bit more magic, as Java is type-safe, while Elasticsearch Painless has
more freedom of types (e.g. def type), so this is easier to read.
> Comment SolrJ 'Utils' generic map functions
> -------------------------------------------
>
> Key: SOLR-14579
> URL: https://issues.apache.org/jira/browse/SOLR-14579
> Project: Solr
> Issue Type: Improvement
> Affects Versions: master (9.0)
> Reporter: Megan Carey
> Assignee: Erick Erickson
> Priority: Minor
> Fix For: 8.7
>
> Attachments: SOLR-14579.patch
>
> Time Spent: 2h 50m
> Remaining Estimate: 0h
>
> Remove the map functions like `NEW_HASHMAP_FUN` from the Utils class in solrj
> module to reduce warnings and improve code quality.
> [https://github.com/apache/lucene-solr/blob/master/solr/solrj/src/java/org/apache/solr/common/util/Utils.java#L92]
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