Java 27 and 26: runnable demos and captured output for every JEP, plus version lanes

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3

lanes/prompts/features-1-records.txt

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+Review the attached Java class and convert it to a Java 21 record if it is a pure data carrier. Preserve all validation from the old constructor inside a compact constructor. Flag fields that prevent conversion (e.g. mutable state, inheritance) and explain why.

lanes/prompts/features-2-pattern-switch.txt

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+Refactor this Java code to use Java 21 pattern matching for switch. Introduce a sealed interface if the types form a closed hierarchy. Ensure the switch is exhaustive and remove all redundant casts and intermediate variables.

lanes/prompts/features-3-virtual-threads.txt

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+Find every usage of Executors.newFixedThreadPool and Executors.newCachedThreadPool in this project. Replace with Executors.newVirtualThreadPerTaskExecutor() where the tasks are I/O-bound. List any cases where platform threads should be kept (CPU-bound work, native frames on the stack, and on Java 21 to 23 also synchronized blocks that block, which pin the carrier thread until Java 24) and justify each decision.

lanes/prompts/features-4-sequenced-collections.txt

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+Rewrite this code to use Java 21 sequenced collection APIs (getFirst, getLast, reversed) instead of list.get(0), list.get(list.size()-1), and manual reversal. Keep behaviour identical.

lanes/prompts/features-5-scoped-values.txt

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+Identify ThreadLocal usages in this file. Propose a migration to Java 25 ScopedValue, showing before/after code and explaining why ScopedValue is safer under virtual threads and structured concurrency.

lanes/prompts/features-6-structured-concurrency.txt

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+Refactor this CompletableFuture.allOf pipeline to use StructuredTaskScope. Note that StructuredTaskScope is still a preview API in Java 25, 26 and 27, so the build needs --enable-preview and the code must use the factory-method names of the JDK it targets (Joiner.anySuccessfulResultOrThrow() on 25, anySuccessfulOrThrow() on 26 and 27). Preserve error propagation semantics and show what happens on cancellation. Explain the difference from the original CompletableFuture approach.

lanes/prompts/upgrade-1-pom.txt

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+Here is my pom.xml targeting Java 21. Update it to target Java 25. Bump Spring Boot to the latest 3.5.x or 4.x release (3.4.x is documented only up to Java 24), Hibernate to 7.x (which needs Jakarta Persistence 3.2), Lombok to 1.18.40 or later, JUnit to 6.x, and Mockito to its current 5.x release. List any dependency that does not have a Java 25 compatible version and suggest alternatives.

lanes/prompts/upgrade-2-breaking-changes.txt

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+Scan this Java file for usages of SecurityManager, Thread.stop/suspend/resume, Object.finalize, sun.misc.Unsafe memory methods, System.loadLibrary and other JNI loading, and the old java.net.URL constructors. For each finding, say which Java release changed it (SecurityManager: cannot be installed since 24; Thread.suspend/resume: still compile on 21, gone by 23; Thread.stop: throws UnsupportedOperationException on 21 and 25, removed in 26; finalize: still runs, deprecated for removal; Unsafe memory access and JNI loading: run-time warnings from 24) and give a replacement with a before/after code snippet.

lanes/prompts/upgrade-3-preview-flags.txt

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+Review the attached Maven/Gradle build file and list every --enable-preview compiler and runtime argument. For each one, name the preview feature the code actually uses. Remove the flag only if that feature is final in Java 25 (scoped values, JEP 506, are final in 25; StructuredTaskScope is still a preview API in 25, 26 and 27, so a project that uses it must keep the flag). Explain each decision.

lanes/prompts/upgrade-4-modernize-idioms.txt

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+Apply these Java 25 idioms to this file: records for data carriers, pattern-matching switch for type checks, sequenced collection APIs (getFirst/getLast), and ScopedValue instead of ThreadLocal. Preserve behaviour exactly and add a comment for each change explaining why the new form is preferred.

lanes/prompts/upgrade-5-virtual-thread-audit.txt

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+Before I flip my executors to virtual threads on Java 25, audit this class for: ThreadLocal usage, JNI calls or native frames that stay on the stack while blocking (these still pin the carrier), synchronized blocks around I/O (harmless on Java 24 and later, pinning on 21 to 23), and any code that assumes a bounded thread pool. Report risks with line numbers and a severity (blocking/warning/low).

lanes/prompts/upgrade-6-checklist-runner.txt

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+Act as a Java 25 upgrade reviewer. Given the attached project files, produce a prioritized checklist of upgrade tasks, group them by risk (blocking / warning / nice-to-have), and estimate effort in developer-hours per group.

lanes/prompts/upgrade-7-25-to-27.txt

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+Act as a Java 25 to Java 27 upgrade reviewer. Scan the attached project for: reflection that writes to final fields (Field.setAccessible followed by Field.set on a final field; Java 26 warns, JEP 500), StructuredTaskScope code that uses the 25 names (Joiner.anySuccessfulResultOrThrow became anySuccessfulOrThrow in 26, and Joiner gained a third type parameter in 27), the -XX:-ZGenerational flag (ignored with a warning on 24 and 25, rejected on 26 and 27), Thread.stop and java.applet (removed in 26), JOL or Unsafe code that assumes a 12-byte object header (compact headers are the default in 27), and container start scripts that rely on the JVM choosing SerialGC in a one-CPU container (G1 is the default everywhere in 27). For each finding give the file, the line, the release that changed it, and the fix.