A Platform Thread Against a Virtual Thread

A server holding ten thousand open connections. One thread per connection was the obvious design and was impossible for twenty years, because each thread reserved about a megabyte of stack and asked the operating system for a scheduling slot.

Java 21 finalised virtual threads: threads the virtual machine schedules itself, on a small pool of ordinary threads underneath.

The two kinds of thread, on cost, scheduling and how many you can have
Compared on	Platform thread	Virtual thread
Backed by	One operating-system thread	A heap object, run on a carrier thread
Stack	Reserved up front, around a megabyte	Grows and shrinks on the heap
Scheduled by	The operating system	The virtual machine
Practical number	Thousands	Millions
While blocked on input or output	Its operating-system thread is idle	It releases its carrier for someone else

java

Thread.ofVirtual().start(() -> handle(connection));

try (var pool = Executors.newVirtualThreadPerTaskExecutor()) {
    connections.forEach(c -> pool.submit(() -> handle(c)));
}

The decision rule. Use virtual threads for work that spends its time waiting — network calls, database queries, file reads. Use platform threads for work that spends its time computing, where there is nothing to release the carrier for and a pool sized to the processor count is the right shape.