ConcurrentHashMap's copy constructor calls `putAll()` -> `tryPresize()` -> `transfer()`. When coming from the copy constructor, the Map is empty, so there is nothing to transfer. But `transfer()` will still copy all the empty nodes from the old table to the new table.
This patch avoids this work by only calling `tryPresize()` if the table is already initialized. If `table` is null, the initialization is deferred to `putVal()`, which calls `initTable()`. --- ### JMH results for testCopyConstructor before patch: Result "org.openjdk.bench.java.util.concurrent.Maps.testCopyConstructor": 937395.686 ±(99.9%) 99074.324 ns/op [Average] (min, avg, max) = (825732.550, 937395.686, 1072024.041), stdev = 92674.184 CI (99.9%): [838321.362, 1036470.010] (assumes normal distribution) after patch: Result "org.openjdk.bench.java.util.concurrent.Maps.testCopyConstructor": 620871.469 ±(99.9%) 59195.406 ns/op [Average] (min, avg, max) = (545304.633, 620871.469, 689013.573), stdev = 55371.419 CI (99.9%): [561676.063, 680066.875] (assumes normal distribution) Average time is decreased by about 33%. ------------- Commit messages: - 8322149: ConcurrentHashMap copy constructor should not transfer from old table on presizing Changes: https://git.openjdk.org/jdk/pull/17116/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17116&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8322149 Stats: 22 lines in 2 files changed: 19 ins; 1 del; 2 mod Patch: https://git.openjdk.org/jdk/pull/17116.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/17116/head:pull/17116 PR: https://git.openjdk.org/jdk/pull/17116
