RFR: 8375580: Avoid using ArrayDeque in jdk.internal.loader.URLClassPath [v2]

[Jaikiran Pai]

On Wed, 21 Jan 2026 10:22:07 GMT, Eirik Bjørsnøs <eirbjo at openjdk.org> wrote:

>> Please review this PR which proposes we replace `j.u.ArrayDeque` with `j.u.ArrayList` in `jdk.internal.loader.URLClassPath`.
>> 
>> The motivation for using a double-ended queue may have been that "original" search path URLs are added to the tail of the queue while any URLs discovered during JAR `Class-Path` expansion are inserted at the head such that they are processed first.
>> 
>> By splitting these two concerns and processing  loader discovered class path URLs separately from the original search path, we no longer need a double-ended queue. We can replace `ArrayDeque` with `ArrayList`.
>> 
>> Advantages:
>> 
>> * A "hello world" Java program no longer  loads `ArrayDeque`, `Deque` and `Queue` (`URLClassPath` is the only consumer of these classes during startup using a directory class path)
>> * One data structure is simpler than two
>> * We no longer need to manage search path URLs across two different collections 
>> * Code and comments to dance around `ArrayDeque` calling into lambda too early is no longer a concern and can be removed
>> * I think this PR leaves the code overall simpler and easier to follow.
>> 
>> Testing:
>> 
>> This PR introduces a new test to verify that URLs disovered via a multi-level tree paths discovered via `Class-Path` JAR attributes are found in the expected DFS order. The ordering aspect seems to lack existing coverage.
>
> Eirik Bjørsnøs has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use "list" instead of "queue" when referring to expandedPath which is processed FIFO and as such is not a queue

src/java.base/share/classes/jdk/internal/loader/URLClassPath.java line 488:

> 486:      * Adds the specified URLs to the list of 'Class-Path' expanded URLs
> 487:      */
> 488:     private void addExpandedPaths(URL[] urls) {

Once we rename the field to `manifestClassPath`, we should rename this method to `addManifestClassPaths` too.

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PR Review Comment: https://git.openjdk.org/jdk/pull/29288#discussion_r2727651738