RFR: 8258588: MD5 MessageDigest in java.util.UUID should be cached

[Alan Bateman]

On Fri, 18 Dec 2020 20:11:26 GMT, Stuart Marks <smarks at openjdk.org> wrote:

> I have to say that introducing a ThreadLocal here seems like a step in the wrong direction. With a ThreadLocal, if I read this correctly, a MessageDigest will be cached with each thread that ever calls this API, and it won't be garbage collected until the thread dies. Some threads live indefinitely, e.g., the ones in the fork-join common pool. Is this what we want? Is UUID creation so frequent that each thread needs its own, or could thread safety be handled using a simple locking protocol?

This is a good point. Significant effort has gone into recent releases to reduce the use of TLs in the (JDK-8245309, JDK-8245308, JDK-8245304, JDK-8245302) so adding new usages is a disappointing. So I think this PR does need good justifications, and probably a lot more exploration of options.

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PR: https://git.openjdk.java.net/jdk/pull/1821