Java 21+ Features

A virtual thread is a lightweight, JVM-managed thread that unmounts from its carrier OS thread when it blocks on I/O, enabling millions of concurrent threads at roughly 1 KB each.

Pinning is when a virtual thread cannot unmount from its carrier during a blocking call, typically because it is inside a synchronized block or a JNI call.

Pooling virtual threads is an anti-pattern because they cost roughly 1 KB each and are meant to be created per task; use a Semaphore to limit access to finite resources instead of limiting the number of threads.

Virtual threads remove the thread bottleneck but every other constraint remains: database pool size, CPU cores, downstream rate limits, and memory are unchanged.

Java 21 is an LTS release that finalizes virtual threads, pattern matching for switch, record patterns, sequenced collections, and generational ZGC.

StructuredTaskScope forks concurrent subtasks, joins them as one unit, and automatically cancels siblings when any subtask fails or the first succeeds.

ScopedValue is an immutable, auto-cleaning, efficiently inheritable alternative to ThreadLocal, designed for virtual threads and structured concurrency.

Text blocks are multi-line string literals delimited by triple quotes that preserve newlines, strip incidental whitespace, and eliminate the need to escape double quotes.

Switch expressions return a value using arrow syntax with no fall-through, enforce exhaustiveness at compile time, and use yield for multi-statement blocks.

Helpful NullPointerExceptions enhance the error message to tell you exactly which reference was null, but they do not prevent the crash.