Add migration-17-25 module: Java 17 class files run on 17 and 25, flags, removed APIs, jdeps and jdeprscan

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Gth99spB1CzQmBFofXmU4Z

migration-17-25/README.md

@@ -0,0 +1,20 @@
+# migration-17-25
+
+Companion module for the ankurm.com post *Java 17 to Java 25 Migration Guide: The Direct Jump*.
+
+The probes in `src/probes/` are compiled **once** with JDK 17 (`--release 17`) and run unchanged on JDK 17.0.20.1 and JDK 25.0.4.1, which is what a team skipping 21 actually does. `src/removed/` holds four tiny sources that use APIs removed or newly deprecated between 17 and 25; they are compiled with both JDKs.
+
+```bash
+export JDK17=/path/to/jdk-17 JDK25=/path/to/jdk-25
+./scripts/run-all.sh      # regenerates output/
+```
+
+| Output | Shows |
+|---|---|
+| `01-runtime-behaviour.txt` | Security Manager, `Thread.stop`, default charset, `finalize`, `Unsafe` on both runtimes |
+| `02-startup-flags.txt` | Which 17-era JVM options still start on 25 |
+| `03-compile-removed-apis.txt` | What `javac` says about four APIs on each JDK |
+| `04-jdeps-jdeprscan.txt` | The scanners run with JDK 25 tools against the JDK 17-built jar |
+| `05-gc-defaults.txt` | Default collector and heap ergonomics with no flags |
+| `06-legacy-stack-on-25.txt` | `mvn test` on JDK 25 for four generations of one Spring app (from `spring-boot-demo/openrewrite`) |
+| `07-framework-minimums.txt` | Vendor documentation quotes (not produced by running code) |

migration-17-25/output/01-runtime-behaviour.txt

@@ -0,0 +1,33 @@
+# 01-runtime-behaviour: classes compiled once with JDK 17 (--release 17), run unchanged on 17 and 25; LC_ALL=C
+--- SecurityManagerProbe
+  [17.0.20.1] WARNING: A terminally deprecated method in java.lang.System has been called
+  [17.0.20.1] WARNING: System::setSecurityManager has been called by SecurityManagerProbe (file:/tmp/migration-17-25-work/c17/)
+  [17.0.20.1] WARNING: Please consider reporting this to the maintainers of SecurityManagerProbe
+  [17.0.20.1] WARNING: System::setSecurityManager will be removed in a future release
+  [17.0.20.1] installed: true
+  [25.0.4.1] failed: java.lang.UnsupportedOperationException: Setting a Security Manager is not supported
+--- ThreadStopProbe
+  [17.0.20.1] stop() accepted; thread alive afterwards: false
+  [25.0.4.1] stop() failed: java.lang.UnsupportedOperationException
+--- CharsetProbe
+  [17.0.20.1] Charset.defaultCharset() = US-ASCII
+  [17.0.20.1] bytes written for the 4-letter word with an accent = 4 (UTF-8 would be 5)
+  [25.0.4.1] Charset.defaultCharset() = UTF-8
+  [25.0.4.1] bytes written for the 4-letter word with an accent = 5 (UTF-8 would be 5)
+--- FinalizerProbe
+  [17.0.20.1] finalize() ran: true
+  [25.0.4.1] finalize() ran: true
+--- UnsafeProbe
+  [17.0.20.1] read back: 42
+  [25.0.4.1] WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
+  [25.0.4.1] WARNING: sun.misc.Unsafe::allocateMemory has been called by UnsafeProbe (file:/tmp/migration-17-25-work/c17/)
+  [25.0.4.1] WARNING: Please consider reporting this to the maintainers of class UnsafeProbe
+  [25.0.4.1] WARNING: sun.misc.Unsafe::allocateMemory will be removed in a future release
+  [25.0.4.1] read back: 42
+--- SecurityManagerProbe with -Djava.security.manager=allow
+  [17.0.20.1] WARNING: A terminally deprecated method in java.lang.System has been called
+  [17.0.20.1] WARNING: System::setSecurityManager has been called by SecurityManagerProbe (file:/tmp/migration-17-25-work/c17/)
+  [17.0.20.1] WARNING: Please consider reporting this to the maintainers of SecurityManagerProbe
+  [25.0.4.1] Error occurred during initialization of VM
+  [25.0.4.1] java.lang.Error: A command line option has attempted to allow or enable the Security Manager. Enabling a Security Manager is not supported.
+  [25.0.4.1] 	at java.lang.System.initPhase3([email protected]/System.java:1970)

migration-17-25/output/02-startup-flags.txt

@@ -0,0 +1,11 @@
+# flag | JDK 17 | JDK 25   (exit code: first warning or error line)
+-Djava.security.manager=allow | exit 0: (no message) | exit 1: Error occurred during initialization of VM
+-XX:+UseBiasedLocking | exit 0: Option UseBiasedLocking was deprecated in version 15.0 and will likely be removed in a future release. | exit 1: Unrecognized VM option 'UseBiasedLocking'
+-XX:+UseConcMarkSweepGC | exit 1: Unrecognized VM option 'UseConcMarkSweepGC' | exit 1: Unrecognized VM option 'UseConcMarkSweepGC'
+-XX:+AggressiveOpts | exit 1: Unrecognized VM option 'AggressiveOpts' | exit 1: Unrecognized VM option 'AggressiveOpts'
+-XX:MaxPermSize=128m | exit 1: Unrecognized VM option 'MaxPermSize=128m' | exit 1: Unrecognized VM option 'MaxPermSize=128m'
+-XX:+UseParallelOldGC | exit 1: Unrecognized VM option 'UseParallelOldGC' | exit 1: Unrecognized VM option 'UseParallelOldGC'
+-Xverify:none | exit 0: Options -Xverify:none and -noverify were deprecated in JDK 13 and will likely be removed in a future release. | exit 0: Options -Xverify:none and -noverify were deprecated in JDK 13 and will likely be removed in a future release.
+-XX:-UseCompressedClassPointers | exit 0: (no message) | exit 0: Option UseCompressedClassPointers was deprecated in version 25.0 and will likely be removed in a future release.
+-XX:+ParallelRefProcEnabled | exit 0: (no message) | exit 0: (no message)
+-XX:+UseStringDeduplication | exit 0: (no message) | exit 0: (no message)

migration-17-25/output/03-compile-removed-apis.txt

@@ -0,0 +1,9 @@
+# 03-compile-removed-apis: javac -Xlint:removal on JDK 17 and JDK 25
+UsesCompiler.java            [17.0.20.1] [removal] Compiler in java.lang has been deprecated and marked for removal
+UsesCompiler.java            [25.0.4.1] cannot find symbol
+UsesRunFinalization.java     [17.0.20.1] compiles cleanly
+UsesRunFinalization.java     [25.0.4.1] [removal] runFinalization() in Runtime has been deprecated and marked for removal
+UsesThreadGroupDestroy.java  [17.0.20.1] [removal] destroy() in ThreadGroup has been deprecated and marked for removal
+UsesThreadGroupDestroy.java  [25.0.4.1] [removal] destroy() in ThreadGroup has been deprecated and marked for removal
+UsesThreadSuspend.java       [17.0.20.1] [removal] suspend() in Thread has been deprecated and marked for removal
+UsesThreadSuspend.java       [25.0.4.1] cannot find symbol

migration-17-25/output/04-jdeps-jdeprscan.txt

@@ -0,0 +1,16 @@
+# 04-jdeps-jdeprscan (scanning probes.jar, built by JDK 17, with the JDK 25 tools)
+--- jdeps --jdk-internals
+probes.jar -> jdk.unsupported
+   UnsafeProbe                                        -> sun.misc.Unsafe                                    JDK internal API (jdk.unsupported)
+--- jdeprscan --release 17 --for-removal
+Jar file probes.jar:
+class SecurityManagerProbe uses deprecated class java/lang/SecurityManager (forRemoval=true)
+class SecurityManagerProbe uses deprecated method java/lang/System::setSecurityManager(Ljava/lang/SecurityManager;)V (forRemoval=true)
+class SecurityManagerProbe uses deprecated method java/lang/System::getSecurityManager()Ljava/lang/SecurityManager; (forRemoval=true)
+--- jdeprscan --release 25 --for-removal
+Jar file probes.jar:
+class FinalizerProbe$Res overrides deprecated method java/lang/Object::finalize()V (forRemoval=true)
+class SecurityManagerProbe uses deprecated class java/lang/SecurityManager (forRemoval=true)
+class SecurityManagerProbe uses deprecated method java/lang/System::setSecurityManager(Ljava/lang/SecurityManager;)V (forRemoval=true)
+class SecurityManagerProbe uses deprecated method java/lang/System::getSecurityManager()Ljava/lang/SecurityManager; (forRemoval=true)
+class ThreadStopProbe uses deprecated method java/lang/Thread::stop()V (forRemoval=true)

migration-17-25/output/05-gc-defaults.txt

@@ -0,0 +1,17 @@
+# 05-gc-defaults (no flags, same 2-CPU machine)
+--- JDK 17.0.20.1
+  InitialHeapSize = 98566144
+  MaxHeapSize = 1570766848
+  UseG1GC = true
+  collector: G1 Young Generation
+  collector: G1 Old Generation
+  processors: 2
+--- JDK 25.0.4.1
+  InitialHeapSize = 98566144
+  MaxHeapSize = 1570766848
+  UseCompactObjectHeaders = false
+  UseG1GC = true
+  collector: G1 Young Generation
+  collector: G1 Concurrent GC
+  collector: G1 Old Generation
+  processors: 2

migration-17-25/output/06-legacy-stack-on-25.txt

@@ -0,0 +1,5 @@
+# 06-legacy-stack-on-25: mvn test on JDK 25 for four generations of the same app (bytecode target is release 17 in every row)
+Spring Boot 2.7.18 (Spring 5.3, Hibernate 5.6):      Tests run: 1, Failures: 0, Errors: 0, Skipped: 0 BUILD SUCCESS 
+Spring Boot 3.5.16:                                  Tests run: 1, Failures: 0, Errors: 0, Skipped: 0 BUILD SUCCESS 
+Spring Boot 4.0.8:                                   Tests run: 1, Failures: 0, Errors: 0, Skipped: 0 BUILD SUCCESS 
+Spring Boot 4.1.1:                                   Tests run: 1, Failures: 0, Errors: 0, Skipped: 0 BUILD SUCCESS 

migration-17-25/output/07-framework-minimums.txt

@@ -0,0 +1,6 @@
+# 07-framework-minimums (quoted from vendor documentation on 2026-09-28; NOT produced by running code)
+Spring Boot 4.1.1 system requirements (https://docs.spring.io/spring-boot/system-requirements.html):
+  "Spring Boot 4.1.1 requires at least Java 17 and is compatible with versions up to and including Java 26."
+  Spring Framework 7.0.9 or above; Maven 3.6.3 or later; Gradle 8.14 or later, and 9.x
+Gradle Java compatibility (https://docs.gradle.org/current/userguide/compatibility.html), "support for running Gradle":
+  Java 21: 8.5 and after | Java 24: 8.14 and after | Java 25: 9.1.0 and after | Java 26: 9.4.0 and after

migration-17-25/scripts/flags.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# Which JVM options from Java 17-era startup scripts still work? Runs `java <flag> -version` on both JDKs; records exit code and first warning/error line.
+J17=${JDK17:-/tmp/tools/j17}; J25=${JDK25:-/tmp/tools/jdk-25.0.4.1+1}
+echo "# flag | JDK 17 | JDK 25   (exit code: first warning or error line)"
+for f in -Djava.security.manager=allow -XX:+UseBiasedLocking -XX:+UseConcMarkSweepGC -XX:+AggressiveOpts -XX:MaxPermSize=128m -XX:+UseParallelOldGC -Xverify:none -XX:-UseCompressedClassPointers -XX:+ParallelRefProcEnabled -XX:+UseStringDeduplication; do
+  line="$f |"
+  for j in "$J17" "$J25"; do
+    all=$("$j/bin/java" $f -version 2>&1); rc=$?
+    msg=$(echo "$all" | grep -E 'Unrecognized|warning|Error' | head -1 | sed 's/^OpenJDK 64-Bit Server VM warning: //' | cut -c1-120)
+    [ -z "$msg" ] && msg="(no message)"
+    line="$line exit $rc: $msg |"
+  done
+  echo "${line% |}"
+done

migration-17-25/scripts/run-all.sh

@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+# Regenerates output/. Needs JDK 17 and JDK 25 (set JDK17 / JDK25), and, for 06, the openrewrite module's legacy app from spring-boot-demo (LEGACY_APP).
+# The probes are compiled ONCE with JDK 17 (--release 17) and run unchanged on both runtimes: that is the "direct jump" scenario.
+set -uo pipefail
+cd "$(dirname "$0")/.."
+J17=${JDK17:-/tmp/tools/j17}; J25=${JDK25:-/tmp/tools/jdk-25.0.4.1+1}; OUT=$PWD/output; C=/tmp/migration-17-25-work; rm -rf "$C"; mkdir -p "$C" "$OUT"
+export JDK17=$J17 JDK25=$J25
+"$J17/bin/javac" -encoding UTF-8 --release 17 -Xlint:-options -nowarn -d "$C/c17" src/probes/*.java 2>/dev/null
+q() { grep -v -E '^Picked up'; }
+
+{ echo "# 01-runtime-behaviour: classes compiled once with JDK 17 (--release 17), run unchanged on 17 and 25; LC_ALL=C"
+  for p in SecurityManagerProbe ThreadStopProbe CharsetProbe FinalizerProbe UnsafeProbe; do
+    echo "--- $p"
+    for j in "$J17" "$J25"; do
+      v=$("$j/bin/java" -version 2>&1 | q | head -1 | sed -E 's/^openjdk version "([^"]+)".*/\1/')
+      LC_ALL=C "$j/bin/java" -cp "$C/c17" $p 2>&1 | q | sed "s/^/  [$v] /"
+    done
+  done
+  echo "--- SecurityManagerProbe with -Djava.security.manager=allow"
+  for j in "$J17" "$J25"; do
+    v=$("$j/bin/java" -version 2>&1 | q | head -1 | sed -E 's/^openjdk version "([^"]+)".*/\1/')
+    LC_ALL=C "$j/bin/java" -Djava.security.manager=allow -cp "$C/c17" SecurityManagerProbe 2>&1 | q | head -3 | sed "s/^/  [$v] /"
+  done
+} > "$OUT/01-runtime-behaviour.txt"
+
+./scripts/flags.sh > "$OUT/02-startup-flags.txt" 2>&1
+
+{ echo "# 03-compile-removed-apis: javac -Xlint:removal on JDK 17 and JDK 25"
+  for f in src/removed/*.java; do
+    for j in "$J17" "$J25"; do
+      v=$("$j/bin/java" -version 2>&1 | q | head -1 | sed -E 's/^openjdk version "([^"]+)".*/\1/')
+      r=$("$j/bin/javac" -encoding UTF-8 -Xlint:removal -d "$C/x" "$f" 2>&1 | q | grep -E 'error:|warning:' | head -1 | sed -E 's/^[^ ]+ (error|warning): //')
+      printf '%-28s [%s] %s\n' "$(basename "$f")" "$v" "${r:-compiles cleanly}"
+    done
+  done
+} > "$OUT/03-compile-removed-apis.txt"
+
+"$J17/bin/jar" cf "$C/probes.jar" -C "$C/c17" .
+{ echo "# 04-jdeps-jdeprscan (scanning probes.jar, built by JDK 17, with the JDK 25 tools)"
+  echo "--- jdeps --jdk-internals"; "$J25/bin/jdeps" --jdk-internals "$C/probes.jar" 2>&1 | q | sed -n '1,2p'
+  echo "--- jdeprscan --release 17 --for-removal"; "$J25/bin/jdeprscan" --release 17 --for-removal "$C/probes.jar" 2>&1 | q | sed 's/^Jar file .*/Jar file probes.jar:/'
+  echo "--- jdeprscan --release 25 --for-removal"; "$J25/bin/jdeprscan" --release 25 --for-removal "$C/probes.jar" 2>&1 | q | sed 's/^Jar file .*/Jar file probes.jar:/'
+} > "$OUT/04-jdeps-jdeprscan.txt"
+
+{ echo "# 05-gc-defaults (no flags, same 2-CPU machine)"
+  for j in "$J17" "$J25"; do
+    v=$("$j/bin/java" -version 2>&1 | q | head -1 | sed -E 's/^openjdk version "([^"]+)".*/\1/')
+    echo "--- JDK $v"
+    "$j/bin/java" -XX:+PrintFlagsFinal -version 2>/dev/null | grep -E ' (UseG1GC|MaxHeapSize|InitialHeapSize|UseCompactObjectHeaders) ' | awk '{print "  " $2, $3, $4}'
+    "$j/bin/java" -cp "$C/c17" DefaultsProbe 2>&1 | q | sed 's/^/  /'
+  done
+} > "$OUT/05-gc-defaults.txt"
+
+# The four apps come from spring-boot-demo/openrewrite: legacy-app is checked in, step1..step3 are produced by its scripts/run-all.sh under $OR_WORK.
+LEG=${LEGACY_APP:-/tmp/spring-boot-demo/openrewrite/legacy-app}; ORW=${OR_WORK:-/tmp/or-work}
+{ echo "# 06-legacy-stack-on-25: mvn test on JDK 25 for four generations of the same app (bytecode target is release 17 in every row)"
+  for row in "Spring Boot 2.7.18 (Spring 5.3, Hibernate 5.6)|$LEG" "Spring Boot 3.5.16|$ORW/step1" "Spring Boot 4.0.8|$ORW/step2" "Spring Boot 4.1.1|$ORW/step3"; do
+    name=${row%%|*}; dir=${row#*|}
+    if [ -d "$dir" ]; then
+      rm -rf "$C/app"; cp -r "$dir" "$C/app"; rm -rf "$C/app/target"
+      r=$(cd "$C/app" && JAVA_HOME=$J25 PATH=$J25/bin:$PATH mvn -B test 2>&1 | grep -E 'Tests run:.*Fail|BUILD' | grep -v 'Time elapsed' | sed 's/\[INFO\] //' | tr '\n' ' ')
+      printf '%-52s %s\n' "$name:" "$r"
+    else printf '%-52s %s\n' "$name:" "(not found: $dir)"; fi
+  done
+} > "$OUT/06-legacy-stack-on-25.txt"
+cat > "$OUT/07-framework-minimums.txt" <<'EOT'
+# 07-framework-minimums (quoted from vendor documentation on 2026-09-28; NOT produced by running code)
+Spring Boot 4.1.1 system requirements (https://docs.spring.io/spring-boot/system-requirements.html):
+  "Spring Boot 4.1.1 requires at least Java 17 and is compatible with versions up to and including Java 26."
+  Spring Framework 7.0.9 or above; Maven 3.6.3 or later; Gradle 8.14 or later, and 9.x
+Gradle Java compatibility (https://docs.gradle.org/current/userguide/compatibility.html), "support for running Gradle":
+  Java 21: 8.5 and after | Java 24: 8.14 and after | Java 25: 9.1.0 and after | Java 26: 9.4.0 and after
+EOT
+ls -1 "$OUT"; rm -rf "$C"

migration-17-25/src/probes/CharsetProbe.java

@@ -0,0 +1,15 @@
+import java.io.*;
+import java.nio.charset.Charset;
+import java.nio.file.*;
+
+/** Writes the word cafe with an accented e with the platform default charset and reads the bytes back. Default is UTF-8 from Java 18 (JEP 400). */
+public class CharsetProbe {
+	public static void main(String[] args) throws Exception {
+		Path f = Files.createTempFile("charset", ".txt");
+		try (Writer w = new FileWriter(f.toFile())) { w.write("caf\u00e9"); }
+		byte[] bytes = Files.readAllBytes(f);
+		System.out.println("Charset.defaultCharset() = " + Charset.defaultCharset());
+		System.out.println("bytes written for the 4-letter word with an accent = " + bytes.length + " (UTF-8 would be 5)");
+		Files.delete(f);
+	}
+}

migration-17-25/src/probes/DefaultsProbe.java

@@ -0,0 +1,9 @@
+import java.lang.management.ManagementFactory;
+
+/** Prints which garbage collector the JVM chose with no flags at all. */
+public class DefaultsProbe {
+	public static void main(String[] args) {
+		ManagementFactory.getGarbageCollectorMXBeans().forEach(b -> System.out.println("collector: " + b.getName()));
+		System.out.println("processors: " + Runtime.getRuntime().availableProcessors());
+	}
+}

migration-17-25/src/probes/FinalizerProbe.java

@@ -0,0 +1,12 @@
+/** finalize() is deprecated for removal since 18. Does it still run on 25? */
+public class FinalizerProbe {
+	static volatile boolean finalized;
+	@SuppressWarnings({"removal", "deprecation"})
+	static class Res { @Override protected void finalize() { finalized = true; } }
+
+	public static void main(String[] args) throws Exception {
+		new Res();
+		for (int i = 0; i < 20 && !finalized; i++) { System.gc(); Thread.sleep(50); }
+		System.out.println("finalize() ran: " + finalized);
+	}
+}

migration-17-25/src/probes/SecurityManagerProbe.java

@@ -0,0 +1,12 @@
+/** Java 17: installing a SecurityManager works (deprecated for removal). Java 24+: it cannot be installed at all. */
+public class SecurityManagerProbe {
+	@SuppressWarnings({"removal", "deprecation"})
+	public static void main(String[] args) {
+		try {
+			System.setSecurityManager(new SecurityManager());
+			System.out.println("installed: " + (System.getSecurityManager() != null));
+		} catch (Throwable t) {
+			System.out.println("failed: " + t.getClass().getName() + ": " + t.getMessage());
+		}
+	}
+}

migration-17-25/src/probes/ThreadStopProbe.java

@@ -0,0 +1,16 @@
+/** Thread.stop() worked (deprecated) on 17. From Java 20 it throws UnsupportedOperationException. */
+public class ThreadStopProbe {
+	@SuppressWarnings({"removal", "deprecation"})
+	public static void main(String[] args) throws Exception {
+		Thread t = new Thread(() -> { try { Thread.sleep(5000); } catch (InterruptedException e) { } });
+		t.start();
+		try {
+			t.stop();
+			t.join(1000);
+			System.out.println("stop() accepted; thread alive afterwards: " + t.isAlive());
+		} catch (Throwable e) {
+			System.out.println("stop() failed: " + e.getClass().getName());
+			t.interrupt();
+		}
+	}
+}

migration-17-25/src/probes/UnsafeProbe.java

@@ -0,0 +1,14 @@
+import java.lang.reflect.Field;
+
+/** sun.misc.Unsafe memory access: JEP 471 deprecates it for removal (23), JEP 498 warns at first use (24). */
+public class UnsafeProbe {
+	public static void main(String[] args) throws Exception {
+		Field f = Class.forName("sun.misc.Unsafe").getDeclaredField("theUnsafe");
+		f.setAccessible(true);
+		sun.misc.Unsafe u = (sun.misc.Unsafe) f.get(null);
+		long addr = u.allocateMemory(8);
+		u.putLong(addr, 42L);
+		System.out.println("read back: " + u.getLong(addr));
+		u.freeMemory(addr);
+	}
+}

migration-17-25/src/removed/UsesCompiler.java

@@ -0,0 +1,5 @@
+public class UsesCompiler {
+	public static void main(String[] args) {
+		java.lang.Compiler.disable();
+	}
+}

migration-17-25/src/removed/UsesRunFinalization.java

@@ -0,0 +1,5 @@
+public class UsesRunFinalization {
+	public static void main(String[] args) throws Exception {
+		Runtime.getRuntime().runFinalization();
+	}
+}

migration-17-25/src/removed/UsesThreadGroupDestroy.java

@@ -0,0 +1,5 @@
+public class UsesThreadGroupDestroy {
+	public static void main(String[] args) {
+		new ThreadGroup("g").destroy();
+	}
+}

migration-17-25/src/removed/UsesThreadSuspend.java

@@ -0,0 +1,6 @@
+public class UsesThreadSuspend {
+	@SuppressWarnings("deprecation")
+	public static void main(String[] args) {
+		Thread.currentThread().suspend();
+	}
+}