Learn to use pattern matching in switch to simplify type checks and reduce boilerplate. Why it matters: Cleaner and safer polymorphic logic without casting. Challenge: Refactor a switch with instanceof checks into a pattern-matching switch. JEP 441 JEP 441: Pattern Matching for switch
Combine record patterns with pattern-matching to destructure immutable data directly. Why it matters: Enables concise, declarative data extraction. Challenge: Write a record Point(int x,int y) and destructure it in a switch. JEP 440 JEP 440: Record Patterns
Understand new interfaces SequencedCollection, SequencedSet, SequencedMap. Why it matters: Unified first/last element access and reverse iteration. Challenge: Implement an LRU cache using SequencedMap. JEP 431 JEP 431 Sequenced Collections
Work with embedded expressions inside string literals. Why it matters: Eliminates messy concatenation and improves readability. Challenge: Write a log message builder using template expressions. JEP 430 JEP 430 String Templates
Adopt records for immutable DTOs in APIs. Why it matters: Reduces boilerplate, supports equality and serialization easily. Challenge: Replace POJOs with records in a REST DTO layer.
Use scoped values for thread-local-like behavior without inheritance issues. Why it matters: Safer contextual data sharing in virtual threads. Challenge: Pass a security context using scoped values in a request handler. JEP 429 JEP Explore Scoped Values
Limit subclassing explicitly to improve domain modeling. Why it matters: Makes hierarchies explicit and maintainable. Challenge: Redesign a payment hierarchy with sealed interfaces. JEP 409
Simplify control flow with expression switches returning values. Why it matters: Removes boilerplate break statements. Challenge: Replace if-else chains with switch expressions. JEP 361
Adopt triple-quoted text for SQL, JSON, and HTML strings. Why it matters: Improves readability and reduces escape noise. Challenge: Store SQL queries as text blocks in a DAO. JEP 378
Use pattern matching with null labels safely in switches. Why it matters: Prevents NPEs and documents null cases explicitly. Challenge: Handle null branches in pattern switches gracefully.
Experiment with lightweight threads to simplify concurrency. Why it matters: Enables millions of concurrent tasks with minimal overhead. Challenge: Rewrite a blocking I/O server using virtual threads. JEP 444
Measure scheduling and memory overhead differences. Why it matters: Quantifies benefits before migration. Challenge: Benchmark 10 000 concurrent tasks under both models.
Manage child tasks as a unit for predictable cancellation. Why it matters: Simplifies error handling and resource cleanup. Challenge: Run concurrent HTTP calls with structured concurrency. JEP 453
Adopt new factory methods for simplicity. Why it matters: Reduces thread-pool configuration overhead. Challenge: Replace fixed pool executor with virtual thread executor.
They’re costly and unnecessary with scoped values. Why it matters: Ensures scalability and correctness. Challenge: Refactor ThreadLocal usage to scoped values.
Blend reactive streams with virtual threads where useful. Why it matters: Learn migration paths from reactive to structured concurrency. Challenge: Convert one reactive flow into Loom-based structured concurrency.
Understand scheduler implications. Why it matters: Ensures optimal CPU utilization. Challenge: Experiment with custom schedulers and measure throughput.
Learn to propagate cancellation via structured concurrency. Why it matters: Prevents leaked subtasks. Challenge: Implement a timeout-aware multi-API aggregator.
Low-level creation for custom strategies. Why it matters: Gives fine-grained control of virtual thread creation. Challenge: Write a custom virtual thread naming factory.
Apply Loom inside web frameworks. Why it matters: Modernizes enterprise workloads easily. Challenge: Enable Loom executor in Spring Boot 3.2 app.
Simplify edge-element access. Why it matters: Cleaner alternative to index math. Challenge: Refactor algorithms using list.get(0) and list.get(list.size()-1).
Why it matters: Built-in reverse views simplify code. Challenge: Print lists backward using reversed() without copying.
Why it matters: Efficient boundary lookups. Challenge: Build a bounded cache that removes lastEntry() when full.
Immutable patterns prevent concurrent bugs. Challenge: Create read-only historical logs using unmodifiable sequenced lists.
Why it matters: Returns unmodifiable list; understand when copying is needed. Challenge: Test mutation attempts on Stream.toList() output.
Why it matters: Offers fine control for flattening. Challenge: Replace flatMap() with mapMulti() for conditional expansion.
Why it matters: Avoids null checks elegantly. Challenge: Chain fallbacks using Optional.or().
Why it matters: Performs dual reductions. Challenge: Compute min and max in one pass.
Why it matters: Encourages functional design. Challenge: Replace mutable constants with immutable factory methods.
Why it matters: Enables efficient LIFO/FIFO behaviors. Challenge: Implement deque-backed caching system.
Why it matters: JDK 21 refines latency and footprint. Challenge: Profile GC pause times under G1 vs ZGC.
Adds generational support to ZGC for better throughput. Challenge: Enable -XX:+UseZGC and compare heap metrics. JEP 439
Why it matters: Reduces memory duplication of identical strings. Challenge: Enable -XX:+UseStringDeduplication and inspect memory savings.
Why it matters: Built-in low-overhead profiling. Challenge: Record a 60-second app session and analyze hotspots.
Why it matters: Unlocks SIMD performance. Challenge: Implement vectorized array math benchmark. JEP 448
Why it matters: Replaces JNI with safer, faster interop. Challenge: Call a native C function via MemorySegment. JEP 442
Why it matters: Simplifies diagnosis with -Xlog:gc*. Challenge: Collect GC logs and visualize with GCViewer.
Why it matters: Faster startup for heavy apps. Challenge: Enable CDS and measure JVM cold-start times.
Why it matters: Shares pre-loaded classes across JVMs. Challenge: Generate and apply a custom CDS archive.
Why it matters: Understand how Latin-1 vs UTF-16 affects memory. Challenge: Measure heap for ASCII vs emoji-rich datasets.
Why it matters: Improves REPL exploration. Challenge: Prototype pattern-matching snippets in JShell.
Why it matters: Build minimal runtime images. Challenge: Create a 50 MB self-contained CLI tool.
Why it matters: Helps modularize monoliths. Challenge: Run jdeps --summary on a legacy jar.
Why it matters: Enables distribution as modular runtimes. Challenge: Package and inspect a module file.
Why it matters: Verify concurrency behavior correctly. Challenge: Write concurrent tests using Loom executors.
Why it matters: Ensure Gradle/Maven compatibility. Challenge: Upgrade toolchains to Java 21 target.
Why it matters: Lightweight continuous monitoring. Challenge: Capture async profiler traces during peak load.
Why it matters: Visualize dependencies between modules. Challenge: Generate and interpret a module graph via IDE.
Why it matters: Needed for String Templates or Scoped Values. Challenge: Configure IntelliJ or VS Code for preview flags.
Why it matters: Maintain clarity across language evolution. Challenge: Annotate which JEP introduced each API.
Leverage StructuredTaskScope to supervise concurrent subtasks. Why it matters: Simplifies cancellation, exception aggregation, and lifecycle. Challenge: Implement a service calling 3 APIs concurrently, failing fast on error. JEP 453
Combine structured concurrency with async pipelines. Why it matters: Integrates legacy async codebases with Loom. Challenge: Wrap blocking calls in CompletableFuture executed by virtual threads.
Why it matters: Fair locks prevent starvation but reduce throughput. Challenge: Benchmark fair vs non-fair ReentrantLock under contention.
Why it matters: Offers optimistic reads improving performance on read-heavy workloads. Challenge: Replace ReentrantReadWriteLock with StampedLock in a cache.
Why it matters: Reduces synchronization overhead. Challenge: Use Holder idiom or AtomicReference for on-demand singleton creation.
Why it matters: Centralized timeout handling reduces complexity. Challenge: Add per-request timeout with StructuredTaskScope::shutdown.
Why it matters: Some legacy JDBC drivers block OS threads internally. Challenge: Identify such cases and switch to Loom-friendly drivers.
Why it matters: Debugging thousands of fibers requires tooling awareness. Challenge: Use jcmd Thread.print and visualize Loom thread hierarchies.
Why it matters: Transition period will require hybrid designs. Challenge: Implement a reactive adapter invoking blocking virtual thread APIs.
Why it matters: Replaces ThreadLocal for correlation IDs in distributed logs. Challenge: Pass trace IDs through nested virtual threads using scoped values. JEP 429
Why it matters: Combines low pause times with generational heuristics. Challenge: Compare throughput vs standard ZGC on long-lived workloads. JEP 439
Why it matters: Identifies leaks in new thread-rich apps. Challenge: Capture heap histograms before and after traffic spikes.
Why it matters: Allocation pressure causes hidden GC churn. Challenge: Record 30 s JFR profile, detect high-frequency object types.
Why it matters: Each virtual thread allocates stack segments dynamically. Challenge: Spawn 1 M virtual threads and observe RSS growth.
Why it matters: Reduces memory overhead per object. Challenge: Enable -XX:+UseCompactObjectHeaders and inspect with jol-cli. JEP 450
Why it matters: Streams add abstraction overhead; measure trade-offs. Challenge: Use JMH to benchmark 10^6 element processing.
Why it matters: The JVM may allocate objects on stack if non-escaping. Challenge: Verify with -XX:+PrintEscapeAnalysis flags.
Why it matters: Controls pause predictability for moderate heaps. Challenge: Tune -XX:MaxGCPauseMillis and observe GC logs.
Why it matters: Frees memory when many duplicate strings exist. Challenge: Activate -XX:+UseStringDeduplication and track reclaimed memory.
Why it matters: Monitors off-heap allocations from FFM or buffers. Challenge: Run with -XX:NativeMemoryTracking=detail and parse output.
Why it matters: Records generate stable hashCode and equals. Challenge: Replace composite key classes with records in caching layer.
Why it matters: Perfect for algebraic data modeling. Challenge: Model Shape as a sealed interface with record variants.
Why it matters: Enables concise and type-safe conditions. Challenge: Replace instanceof casting with pattern binding. JEP 394
Why it matters: Destructure complex record graphs inline. Challenge: Decompose nested record structures with one pattern.
Why it matters: Compile-time exhaustiveness checking improves safety. Challenge: Create exhaustive switch over enum variants with default elimination.
Why it matters: Enables annotations but can obscure types. Challenge: Annotate lambda parameters with @Nonnull using var.
Why it matters: Prevents NPEs in switch branches. Challenge: Use when clauses to guard against null inputs.
Why it matters: You can build custom templating logic. Challenge: Implement a safe HTML escaping template processor. JEP 430
Why it matters: Simplifies multi-line formatting. Challenge: Format structured JSON logs using text blocks.
Why it matters: Encourages immutability even in helpers. Challenge: Refactor configuration value objects to records.
Why it matters: Greatly increases concurrency for blocking IO. Challenge: Create a virtual-thread-based HTTP downloader.
Why it matters: Built-in HttpClient supports multiplexing. Challenge: Measure difference vs Apache HttpClient under load.
Why it matters: Security and performance balance is critical. Challenge: Benchmark handshake times using JDK 21 default cipher suites.
Why it matters: Efficient binary comparison built in. Challenge: Write a deduplication tool for backups.
Why it matters: Combines NIO scalability with Loom simplicity. Challenge: Build simple chat server using blocking APIs in virtual threads.
Why it matters: Prevents thread blocking on large reads/writes. Challenge: Benchmark parallel file processing pipelines.
Why it matters: Cryptographically strong RNGs prevent vulnerabilities. Challenge: Generate secure tokens for authentication system.
Why it matters: Enforces least privilege model. Challenge: Scan and fix incorrect file permission bits.
Why it matters: Monitor and control OS processes safely. Challenge: Spawn subprocesses and capture CPU usage.
Why it matters: Evaluate JDBC → R2DBC or Loom hybrids. Challenge: Test latency between virtual thread JDBC vs R2DBC queries.
Why it matters: Tests scale without thread-pool exhaustion. Challenge: Enable virtual-thread test executor in JUnit 5.
Why it matters: Ensure tooling compatibility post-migration. Challenge: Configure Jacoco for preview-feature modules.
Why it matters: Prepares for JDK 25 upgrades. Challenge: Run jdeprscan and document replacements.
Why it matters: Uses javaToolchains for consistent CI builds. Challenge: Define multiple Java versions in Gradle build.
Why it matters: Detects common Loom misuses early. Challenge: Enable ErrorProne in build and fix reported issues.
Why it matters: Empirically verify performance claims. Challenge: Benchmark thread-per-task vs virtual thread per task.
Why it matters: Expose domain-specific metrics in Flight Recorder. Challenge: Add JFR annotations to capture transaction latency.
Why it matters: Ensures JVM resource awareness in containers. Challenge: Run Java 21 in Docker and observe cgroup metrics.
Why it matters: Smooth upgrades minimize risk. Challenge: Write migration checklist including module and API updates.
Why it matters: Knowledge transfer solidifies expertise. Challenge: Conduct an internal workshop showcasing Java 21 concurrency improvements.