Add jdk28-preview module: JDK 28 early-access build, value classes, strict init flag, PEM, JSON incubator

Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01Gth99spB1CzQmBFofXmU4Z
2026-09-28 08:48:04 +00:00
parent 57a04dc393
commit c8ff3acf09
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+# jdk28-preview
+
+Companion module for the ankurm.com post *What's Coming in JDK 28: Value Classes Preview and Strict Field Initialization*.
+
+Runs small programs on an **early-access** OpenJDK 28 build (`28-ea+17`, downloaded from jdk.java.net/28) and on JDK 25.0.4.1 for comparison. Early-access builds change weekly, and JEP status changes with them, so treat every transcript as a snapshot and rerun `scripts/run-all.sh` against a newer build.
+
+```bash
+export JDK28=/path/to/jdk-28-ea JDK25=/path/to/jdk-25
+./scripts/run-all.sh
+```
+
+| Output | Shows |
+|---|---|
+| `00-environment.txt` | The exact builds used |
+| `01-value-classes.txt` | JEP 401: `==` on value records, `Integer` identity, synchronizing on a value object |
+| `02-forgot-the-preview-flag.txt` | The two errors you get without `--enable-preview` |
+| `03-strict-init-flags.txt` | JEP 539: the `ACC_STRICT_INIT` field flag, value class vs identity class |
+| `04-pem-encodings.txt` | JEP 542: `PEMEncoder`/`PEMDecoder` on 28 (no flag) and 25 (preview) |
+| `05-json-incubator.txt` | JEP 540: `jdk.incubator.json` |
+| `06-shenandoah.txt` | JEP 535 could not be tested on this build (see file) |
+| `07-jep-status.txt` | Published JEP status, quoted (not run) |
@@ -0,0 +1,7 @@
+# 00-environment
+--- JDK 28 early-access:
+openjdk version "28-ea" 2027-03-23
+OpenJDK 64-Bit Server VM (build 28-ea+17-1282, mixed mode, sharing)
+--- JDK 25:
+openjdk version "25.0.4.1" 2026-08-18 LTS
+OpenJDK 64-Bit Server VM Temurin-25.0.4.1+1 (build 25.0.4.1+1-LTS, mixed mode, sharing)
@@ -0,0 +1,8 @@
+# 01-value-classes: ValueClasses.java on JDK 28 with --enable-preview
+p == q: true, equals: true
+isValue: true
+synchronized on a value object: java.lang.IdentityException: Cannot synchronize on an instance of value class ValueClasses$Point
+--- IntegerIdentity.java: same source, three runs
+JDK 25:                 Integer 1000 == Integer 1000: false
+JDK 28, no preview:     Integer 1000 == Integer 1000: false
+JDK 28, --enable-preview: Integer 1000 == Integer 1000: true
@@ -0,0 +1,9 @@
+# 02-forgot-the-preview-flag
+--- javac ValueClasses.java (no --enable-preview):
+src/ValueClasses.java:3: error: value classes are a preview feature and are disabled by default.
+	value record Point(int x, int y) {}
+	^
+  (use --enable-preview to enable value classes)
+--- java ValueClasses (compiled with the flag, run without):
+Error: LinkageError occurred while loading main class ValueClasses
+	java.lang.UnsupportedClassVersionError: Preview features are not enabled for ValueClasses (class file version 72.65535). Try running with '--enable-preview'
@@ -0,0 +1,7 @@
+# 03-strict-init-flags (javap -v -p, field flags only)
+--- StrictFieldProbe$Money
+  final long cents;
+    flags: (0x0810) ACC_FINAL, ACC_STRICT_INIT
+--- StrictFieldProbe$Legacy
+  final long cents;
+    flags: (0x0010) ACC_FINAL
@@ -0,0 +1,11 @@
+# 04-pem-encodings (JEP 542): PemRoundTrip.java
+--- JDK 28, no flags:
+-----BEGIN PUBLIC KEY-----
+round trip equal: true
+--- JDK 25, no flags:
+src/PemRoundTrip.java:10: error: PEMEncoder is a preview API and is disabled by default.
+		String pem = PEMEncoder.of().encodeToString(kp.getPublic());
+		             ^
+--- JDK 25, --enable-preview:
+-----BEGIN PUBLIC KEY-----
+round trip equal: true
@@ -0,0 +1,24 @@
+# 05-json-incubator (JEP 540): JsonDemo.java on JDK 28
+--- module and API surface:
+jdk.incubator.json@28-ea
+exports jdk.incubator.json
+requires java.base mandated
+  public static jdk.incubator.json.JsonValue parse(java.lang.String);
+  public static jdk.incubator.json.JsonValue parse(char[]);
+  public static java.lang.String toDisplayString(jdk.incubator.json.JsonValue, java.lang.String);
+--- without --add-modules:
+src/JsonDemo.java:1: error: package jdk.incubator.json is not visible
+import jdk.incubator.json.*;
+--- with --add-modules jdk.incubator.json:
+name = Ada
+second language = fr
+age + 1 = 37
+missing key -> Optional.empty
+{
+  "name": "Ada",
+  "langs": [
+    "en",
+    "fr"
+  ],
+  "age": 36
+}
@@ -0,0 +1,8 @@
+# 06-shenandoah (JEP 535: generational mode by default)
+--- JDK 28 EA build, -XX:+UseShenandoahGC:
+Error occurred during initialization of VM
+Option -XX:+UseShenandoahGC not supported
+--- JDK 25 Temurin, -XX:+UseShenandoahGC -Xlog:gc*:
+[0.007s][info][gc     ] Using Shenandoah
+[0.009s][info][gc,init] Mode: Snapshot-At-The-Beginning (SATB)
+[0.016s][info][gc,metaspace] CDS archive(s) mapped at: [0x0000000020000000-0x0000000020d96000-0x0000000020d96000), size 14245888, SharedBaseAddress: 0x0000000020000000, ArchiveRelocationMode: 1.
@@ -0,0 +1,7 @@
+# 07-jep-status (quoted from published articles on 2026-09-28; NOT produced by running code; status changes weekly)
+InfoQ, "JDK 27 and JDK 28: What We Know So Far" (https://www.infoq.com/news/2026/08/java-27-so-far/), August 2026:
+  Targeted to JDK 28: JEP 541 Deprecate the macOS/x64 Port for Removal; JEP 540 Simple JSON API (Incubator);
+  JEP 539 Strict Field Initialization in the JVM (Preview); JEP 535 Shenandoah GC: Generational Mode by Default; JEP 401 Value Objects (Preview)
+  Proposed to target: JEP 542 PEM Encodings of Cryptographic Objects
+  "Scheduled for a GA release in March 2027"
+Inside.java, 2026-09-20 (https://inside.java/2026/09/20/jep401-target-jdk28/): "JEP targeted to JDK 28: 401: Value Objects (Preview)"
@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+# Regenerates output/. Needs JDK 25 and a JDK 28 early-access build (default location below; see output/00-environment.txt).
+# Early-access builds change weekly: expect these transcripts to need regenerating, and re-read the post's status callout.
+set -uo pipefail
+cd "$(dirname "$0")/.."
+J28=${JDK28:-/tmp/tools/jdk28}; J25=${JDK25:-/tmp/tools/jdk-25.0.4.1+1}; OUT=$PWD/output; W=/tmp/jdk28-preview-work; rm -rf "$W"; mkdir -p "$W" "$OUT"
+q() { grep -v -E '^Picked up'; }
+ver() { "$1/bin/java" -version 2>&1 | q | sed -n '1p;3p'; }
+
+{ echo "# 00-environment"; echo "--- JDK 28 early-access:"; ver "$J28"; echo "--- JDK 25:"; ver "$J25"; } > "$OUT/00-environment.txt"
+
+{ echo "# 01-value-classes: ValueClasses.java on JDK 28 with --enable-preview"
+  "$J28/bin/javac" --enable-preview --release 28 -d "$W" src/ValueClasses.java 2>&1 | q | grep -v '^Note:'
+  "$J28/bin/java" --enable-preview -cp "$W" ValueClasses 2>&1 | q
+  echo "--- IntegerIdentity.java: same source, three runs"
+  "$J25/bin/javac" -d "$W/i25" src/IntegerIdentity.java 2>&1 | q; "$J28/bin/javac" --enable-preview --release 28 -d "$W/i28" src/IntegerIdentity.java 2>&1 | q | grep -v '^Note:'
+  echo "JDK 25:                 $("$J25/bin/java" -cp "$W/i25" IntegerIdentity 2>&1 | q)"
+  echo "JDK 28, no preview:     $("$J28/bin/java" -cp "$W/i25" IntegerIdentity 2>&1 | q)"
+  echo "JDK 28, --enable-preview: $("$J28/bin/java" --enable-preview -cp "$W/i28" IntegerIdentity 2>&1 | q)"
+} > "$OUT/01-value-classes.txt"
+
+{ echo "# 02-forgot-the-preview-flag"
+  echo "--- javac ValueClasses.java (no --enable-preview):"; "$J28/bin/javac" -d "$W/n" src/ValueClasses.java 2>&1 | q | head -4
+  echo "--- java ValueClasses (compiled with the flag, run without):"; "$J28/bin/java" -cp "$W" ValueClasses 2>&1 | q | head -2
+} > "$OUT/02-forgot-the-preview-flag.txt"
+
+"$J28/bin/javac" --enable-preview --release 28 -d "$W/s" src/StrictFieldProbe.java 2>&1 | q >/dev/null
+{ echo "# 03-strict-init-flags (javap -v -p, field flags only)"
+  for c in 'StrictFieldProbe$Money' 'StrictFieldProbe$Legacy'; do echo "--- $c"; "$J28/bin/javap" -v -p -cp "$W/s" "$c" 2>&1 | q | grep -A2 -E '^  (final )?long cents;' | grep -E 'cents|flags' ; done
+} > "$OUT/03-strict-init-flags.txt"
+
+{ echo "# 04-pem-encodings (JEP 542): PemRoundTrip.java"
+  echo "--- JDK 28, no flags:"; "$J28/bin/javac" -d "$W/p28" src/PemRoundTrip.java 2>&1 | q; "$J28/bin/java" -cp "$W/p28" PemRoundTrip 2>&1 | q
+  echo "--- JDK 25, no flags:"; "$J25/bin/javac" -d "$W/p25" src/PemRoundTrip.java 2>&1 | q | head -3
+  echo "--- JDK 25, --enable-preview:"; "$J25/bin/javac" --enable-preview --release 25 -d "$W/p25" src/PemRoundTrip.java 2>&1 | q | grep -v '^Note:'; "$J25/bin/java" --enable-preview -cp "$W/p25" PemRoundTrip 2>&1 | q
+} > "$OUT/04-pem-encodings.txt"
+
+{ echo "# 05-json-incubator (JEP 540): JsonDemo.java on JDK 28"
+  echo "--- module and API surface:"; "$J28/bin/java" --describe-module jdk.incubator.json 2>&1 | q | head -3
+  "$J28/bin/javap" --module jdk.incubator.json -public jdk.incubator.json.Json 2>&1 | q | grep -E 'parse|toDisplay'
+  echo "--- without --add-modules:"; "$J28/bin/javac" -d "$W/j" src/JsonDemo.java 2>&1 | q | head -2
+  echo "--- with --add-modules jdk.incubator.json:"
+  "$J28/bin/javac" --add-modules jdk.incubator.json -d "$W/j" src/JsonDemo.java 2>&1 | q | grep -v -E '^(Note:|warning:|1 warning)'
+  "$J28/bin/java" --add-modules jdk.incubator.json -cp "$W/j" JsonDemo 2>&1 | q | grep -v '^WARNING'
+} > "$OUT/05-json-incubator.txt"
+
+{ echo "# 06-shenandoah (JEP 535: generational mode by default)"
+  echo "--- JDK 28 EA build, -XX:+UseShenandoahGC:"; "$J28/bin/java" -XX:+UseShenandoahGC -Xlog:gc=info -version 2>&1 | q | head -2
+  echo "--- JDK 25 Temurin, -XX:+UseShenandoahGC -Xlog:gc*:"; "$J25/bin/java" -XX:+UseShenandoahGC -Xlog:gc*=info -version 2>&1 | q | grep -E 'Using Shenandoah|Mode:'
+} > "$OUT/06-shenandoah.txt"
+
+
+cat > "$OUT/07-jep-status.txt" <<'EOT'
+# 07-jep-status (quoted from published articles on 2026-09-28; NOT produced by running code; status changes weekly)
+InfoQ, "JDK 27 and JDK 28: What We Know So Far" (https://www.infoq.com/news/2026/08/java-27-so-far/), August 2026:
+  Targeted to JDK 28: JEP 541 Deprecate the macOS/x64 Port for Removal; JEP 540 Simple JSON API (Incubator);
+  JEP 539 Strict Field Initialization in the JVM (Preview); JEP 535 Shenandoah GC: Generational Mode by Default; JEP 401 Value Objects (Preview)
+  Proposed to target: JEP 542 PEM Encodings of Cryptographic Objects
+  "Scheduled for a GA release in March 2027"
+Inside.java, 2026-09-20 (https://inside.java/2026/09/20/jep401-target-jdk28/): "JEP targeted to JDK 28: 401: Value Objects (Preview)"
+EOT
+ls -1 "$OUT"
@@ -0,0 +1,7 @@
+/** Compiles on every JDK. Does == on two boxed 1000s say true? */
+public class IntegerIdentity {
+	public static void main(String[] args) {
+		Integer a = 1000, b = 1000;
+		System.out.println("Integer 1000 == Integer 1000: " + (a == b));
+	}
+}
@@ -0,0 +1,13 @@
+import jdk.incubator.json.*;
+
+/** JEP 540: the incubating JSON API in module jdk.incubator.json. Needs --add-modules jdk.incubator.json. */
+public class JsonDemo {
+	public static void main(String[] args) {
+		JsonValue doc = Json.parse("{\"name\":\"Ada\",\"langs\":[\"en\",\"fr\"],\"age\":36}");
+		System.out.println("name = " + doc.get("name").asString());
+		System.out.println("second language = " + doc.get("langs").get(1).asString());
+		System.out.println("age + 1 = " + (doc.get("age").asInt() + 1));
+		System.out.println("missing key -> " + doc.tryGet("email"));
+		System.out.println(Json.toDisplayString(doc, "  "));
+	}
+}
@@ -0,0 +1,15 @@
+import java.security.*;
+import java.security.spec.ECGenParameterSpec;
+
+/** JEP 542: PEMEncoder / PEMDecoder. Preview in 25; does it still need --enable-preview on 28? */
+public class PemRoundTrip {
+	public static void main(String[] args) throws Exception {
+		KeyPairGenerator g = KeyPairGenerator.getInstance("EC");
+		g.initialize(new ECGenParameterSpec("secp256r1"));
+		KeyPair kp = g.generateKeyPair();
+		String pem = PEMEncoder.of().encodeToString(kp.getPublic());
+		System.out.println(pem.lines().findFirst().get());
+		PublicKey back = PEMDecoder.of().decode(pem, PublicKey.class);
+		System.out.println("round trip equal: " + back.equals(kp.getPublic()));
+	}
+}
@@ -0,0 +1,6 @@
+/** JEP 539 (preview in JDK 28): a JVM field flag. javac sets it on value-class fields; an identity class is the control. */
+public class StrictFieldProbe {
+	static value class Money { final long cents; Money(long c) { cents = c; } }
+	static class Legacy { final long cents; Legacy(long c) { cents = c; } }
+	public static void main(String[] args) { new Money(1); new Legacy(1); }
+}
@@ -0,0 +1,16 @@
+/** JEP 401 (preview in JDK 28): value classes and value records give up identity, so == compares state. */
+public class ValueClasses {
+	value record Point(int x, int y) {}
+
+	public static void main(String[] args) {
+		Point p = new Point(1, 2), q = new Point(1, 2);
+		System.out.println("p == q: " + (p == q) + ", equals: " + p.equals(q));
+		System.out.println("isValue: " + p.getClass().isValue());
+		Object o = p;
+		try {
+			synchronized (o) { System.out.println("locked"); }
+		} catch (Throwable t) {
+			System.out.println("synchronized on a value object: " + t);
+		}
+	}
+}