[jira] [Comment Edited] (SOLR-14579) Comment SolrJ 'Utils' generic map functions

["Uwe Schindler (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SOLR-14579?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17188582#comment-17188582 ] 

Uwe Schindler edited comment on SOLR-14579 at 9/1/20, 3:50 PM:
---------------------------------------------------------------

Hi,

sorry the explanation by [~noble.paul] in the code is completely wrong and the warnings can be resolved without some static methods! Please read the specification about how lambdas are working. Lambdas are NOT converted to plain new instances. It depends on the type of lambda and waht parameters it takes.

Before telling other such completely wrong information, please read the bytecode and understand it!

For the {{k -> new HashMap()}} (the "key" is String, for demonstatrion purposes, see example below) there is exactly one instance created. It works like this:
- The lambda does not use any variable parameter from outside. This is called a non-capturing lambda (it captures no parameters and therefor needs to bind them to the functional interface).
- Javac creates a static method in the class where the lambda is localed: {{private static HashMap lambda$somesuffix(String key)}}
- At the place of the lambda, javac inserts an invokedynamic to - indeed - create a methodHandle that generates an anonymous class. This anonymous class is only generated on the first call of the invokedynamic (that's the whole trick of invokedynamic, its called bootstrapping).
- The bootsrapper gets back a methodHandle to receive an instance of the functional interface. And here it get's interesting: As you see, the above method is private static and the parameters passed to it are all coming from the lambda call, there's only exactly one instance produced.

So please fix the code and apply the patch and remove Utils.


was (Author: thetaphi):
Hi,

sorry the explanation by [~noble.paul] in the code is completely wrong and the qarnings can be resolved with some static methods! Please read the specification about how lambdas are working. Lambdas are NOT converted to plain new instances. It depends on the type of lambda and waht parameters it takes.

Before telling other such completely wrong information, please read the bytecode and understand it!

For the {{k -> new HashMap()}} (the "key" is String, for demonstatrion purposes, see example below) there is exactly one instance created. It works like this:
- The lambda does not use any variable parameter from outside. This is called a non-capturing lambda (it captures no parameters and therefor needs to bind them to the functional interface).
- Javac creates a static method in the class where the lambda is localed: {{private static HashMap lambda$somesuffix(String key)}}
- At the place of the lambda, javac inserts an invokedynamic to - indeed - create a methodHandle that generates an anonymous class. This anonymous class is only generated on the first call of the invokedynamic (that's the whole trick of invokedynamic, its called bootstrapping).
- The bootsrapper gets back a methodHandle to receive an instance of the functional interface. And here it get's interesting: As you see, the above method is private static and the parameters passed to it are all coming from the lambda call, there's only exactly one instance produced.

So please fix the code and apply the patch and remove Utils.

> 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: 2.5h
>  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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