javademos/compact-strings/README.md

compact-strings — Compact Strings (JEP 254) measured with JOL, plus compact object headers

Companion code for the ankurm.com article Compact Strings in Java: Measured with JOL on JDK 25 (plus Compact Object Headers). The explanation lives in the article, in collapsible sections; this folder holds the runnable sources and the transcripts they produced.

JDK25=/path/to/jdk-25 JDK27=/path/to/jdk-27 ./run.sh      # regenerates output/01-21 (about 40 seconds; downloads JOL 0.17 and checks its sha1)
./jmh/fetch.sh && JDK25=/path/to/jdk-25 ./jmh/run-jmh.sh   # output/30-jmh.txt (about 6 minutes; machine dependent)

• src/StringLayout.java — JOL ClassLayout of String, of a Latin-1 and a UTF-16 instance and of the byte[] behind each; a footprint table by length. Needs --add-opens java.base/java.lang=ALL-UNNAMED to read coder.
• src/MillionStrings.java — one million random lower-case strings (30 to 55 characters), measured with GraphLayout, then the same strings with one € appended.
• src/Upgrade.java — the coder of strings produced by concatenation, substring, replace, toLowerCase, new String(bytes, UTF_8), StringBuilder.toString().
• jmh/ — a JMH 1.37 benchmark of six read-only String operations with and without -XX:-CompactStrings.
• output/ — 01-06 layouts, 11-16 the million-string runs, each under six flag sets (JDK 25 default, -XX:-CompactStrings, -XX:+UseCompactObjectHeaders, both; JDK 27 default and -XX:-UseCompactObjectHeaders), 20 coder transcript, 21 PrintFlagsFinal, 30 JMH.

Thing	Version
JDK 25	Temurin 25.0.4.1+1
JDK 27	Amazon Corretto 27+35 (corretto.aws/downloads/latest/amazon-corretto-27-x64-linux-jdk.tar.gz)
JOL	0.17 (org.openjdk.jol:jol-core, latest release in Maven Central's maven-metadata.xml)
JMH	1.37

Everything except the JMH numbers is byte-stable. JOL 0.17 on JDK 27 prints one Unable to attach Serviceability Agent line (it reads a flag that no longer exists); it is kept in the transcripts because it is real.