Java's concurrency model is thread-based: the JVM runs multiple threads within a single process, and Java provides a layered set of tools — from the low-level Thread class up through the java.util.concurrent package — to create, coordinate, and synchronize those threads safely.
Key Points: • At the lowest level, Thread and Runnable let you create and start units of concurrent execution directly. • Intrinsic locking (synchronized, wait/notify) provides the original built-in mechanism for mutual exclusion and inter-thread coordination. • The java.util.concurrent package (added in Java 5) layered on higher-level abstractions: ExecutorService for managed thread pools, locks like ReentrantLock, concurrent collections like ConcurrentHashMap, and coordination utilities like CountDownLatch and Semaphore. • The Java Memory Model formally defines how and when writes by one thread become visible to reads by another, underpinning constructs like volatile and synchronized. • More recent additions like CompletableFuture and, in newer JDKs, virtual threads (Project Loom) continue evolving the model toward higher-level, more scalable concurrency.
Example: A typical modern application rarely creates raw Thread objects directly; instead it submits tasks to an ExecutorService-managed pool, coordinates using concurrent collections and locks from java.util.concurrent, and composes asynchronous results with CompletableFuture — all built on top of the same underlying thread and memory model.
Interview Tip: A concise interview answer is:
"Java's concurrency model is built around threads managed by the JVM, with the Thread and Runnable classes at the foundation, synchronized/wait/notify as the original coordination primitives, and the java.util.concurrent package providing higher-level tools like thread pools, locks, concurrent collections, and CompletableFuture for building safe, scalable concurrent applications with much less boilerplate."