Cocurrency Problem

Concurrency problems happen when multiple threads access shared resources simultaneously, leading to unpredictable or incorrect results.

Common Concurrency Problems:

Race Conditions

• Multiple threads modify shared data at the same time.
• Result depends on timing, leading to bugs.
• Example: Two threads incrementing the same counter can miss updates.

Deadlocks

• Two or more threads wait for each other to release locks.
• They end up stuck forever.

Livelocks

• Threads keep changing state in response to each other but don’t make progress.

Starvation

• A thread is blocked from getting CPU or resources for a long time.

Memory Consistency Errors

• Changes made by one thread are not visible to others due to caching or lack of synchronization.

Ways to Solve Concurrency Problems

Locks (synchronized, Lock interface)

• Ensure only one thread accesses a critical section at a time.
• Types: ReentrantLock, ReentrantReadWriteLock, StampedLock

Atomic Variables

• From java.util.concurrent.atomic (e.g., AtomicInteger, AtomicReference)
• Lock-free operations for simple data types.
• Internally use CPU-level atomic instructions.

Volatile Keyword

• Ensures visibility of changes to variables across threads.
• Does not ensure atomicity.

Immutable Objects

• Use objects that cannot be modified (e.g., String, custom immutable classes).
• Eliminates shared mutable state.

Thread-safe Collections

• Use from java.util.concurrent, like:
  • ConcurrentHashMap
  • CopyOnWriteArrayList
  • BlockingQueue

Executor Framework

• Avoid manual thread creation; use thread pools.
• Executors.newFixedThreadPool(), ScheduledExecutorService, etc.
• Reduces errors from low-level thread management.

Fork/Join Framework

• Breaks tasks into subtasks using RecursiveTask/RecursiveAction.
• Efficient for parallel computation (Java 7+).

Thread-local Storage

• Each thread has its own isolated copy of a variable using ThreadLocal.

Summary

Method	Handles Race?	Handles Visibility?	Lock-Free?	Notes
synchronized / Lock	✅	✅	❌	Basic and widely used
Atomic variables	✅ (simple)	✅	✅	Best for counters, flags
volatile	❌	✅	✅	Only for visibility
Immutable objects	✅	✅	✅	Avoids shared state
Thread-local	✅	✅	✅	Per-thread data
Thread-safe collections	✅	✅	✅	For shared structures
ExecutorService	❌	❌	✅	Manages thread lifecycle
Fork/Join	❌	❌	✅	For compute-intensive tasks