Evolution of Java Multithreading: From Manual Gear to Virtual Threads

Introduction: The article outlines the evolution of Java multithreading, starting from JDK 1.4 and earlier (the manual‑gear era) through JDK 5.0 (the automatic‑gear era) to JDK 19 (the automatic‑driving era with virtual threads).

Manual‑gear era: JDK 1.0 introduced Thread and Runnable with primitive stop/resume/suspend methods, which caused dead‑lock problems; JDK 1.2 replaced them with wait/notify/sleep and added ThreadLocal and Collections, laying the groundwork for later concurrency utilities.

Automatic‑gear era: JDK 5.0 brought the java.util.concurrent (JUC) package, offering atomic classes, locks, thread pools, and concurrent collections; JDK 6 refined synchronized with biased, lightweight, and heavyweight locks; JDK 7 introduced the Fork/Join framework; JDK 8 added CompletableFuture and the Stream API, expanding functional‑style parallelism.

Later improvements: JDK 9 improved lock‑contention handling and introduced Reactive Streams; JDK 10 added thread‑local handshakes to avoid global safepoints; JDK 15 removed biased locking, reflecting shifting performance priorities.

Automatic‑driving era: JDK 19 introduced virtual threads, providing lightweight, JVM‑scheduled threads that dramatically reduce memory usage and improve IO‑bound throughput while preserving the familiar thread‑per‑request programming model without new syntax.

Conclusion: By presenting a milestone‑by‑milestone overview, the article helps readers grasp both the historical context and the modern tools available for Java concurrency.