Java Memory Model (`pkg16advconcurrency`)

Deep concurrency beyond threads — synchronizers, visibility, structured concurrency.

Advanced synchronizers (`pkg16advconcurrency`)

Class	Use
CountDownLatch	one-shot wait until N events
CyclicBarrier	reusable barrier for N parties
Semaphore	limit concurrent access (permits)
Phaser	dynamic-party phased barrier
Exchanger	two-thread object swap

Java Memory Model (JMM)

Defines when writes by one thread are visible to another.

happens-before rules (key edges)

• Unlock → subsequent lock (same monitor)
• volatile write → volatile read (same field)
• Thread start → actions in started thread
• Actions in thread → join() returns

volatile

Forces reads/writes through main memory — no stale cached values.

Safe publication

• final fields visible after constructor completes
• volatile reference to fully constructed object
• static initializer (class holder pattern)

Classic bug: broken double-checked locking

Without volatile on the reference field, another thread may see a partially constructed object.

Structured Concurrency (Java 21+)

StructuredTaskScope owns child task lifetimes — cancel siblings on failure or first success.

Policy	Behavior
ShutdownOnSuccess	first success wins; cancel others
ShutdownOnFailure	any failure cancels all

See advconcurrency7StructuredConcurrency.java.

Files

advconcurrency1 → advconcurrency7 in `pkg16advconcurrency`.