Thread-Safe Collection Classes
Thread-Safe Collection Classes
In Java, various thread-safe collection classes are available to handle concurrent access to data structures. These classes ensure that multiple threads can safely manipulate data without causing race conditions or inconsistencies. Below are some categories of thread-safe collections:
Blocking Collections
Blocking collections are based on locks and use blocking mechanisms to manage concurrent access. They provide synchronization through thread blocking and unblocking.
CopyOnWrite Collections
CopyOnWrite collections have high modification overhead but are designed for read-heavy use cases. They create a new copy of the collection when modified, ensuring that reads are always consistent with the state when the iterator was created.
Concurrent Collections
Concurrent collections are optimized for throughput and utilize CAS (Compare-And-Swap) operations for efficient concurrent access. They provide weak consistency guarantees, allowing iterators to continue traversing the collection even if modifications occur during traversal. This means that size operations may not always be 100% accurate.
Non-Thread-Safe Collections
Non-thread-safe collections throw ConcurrentModificationException when modifications occur during iteration, enforcing a “fail-fast” behavior.
ConcurrentHashMap
ConcurrentHashMap is a thread-safe alternative to HashMap. While individual operations are atomic, combining operations may not be inherently thread-safe. Special methods like computeIfAbsent() ensure atomicity for specific combinations of get and put operations.
ConcurrentHashMap Internals
- Lazy initialization: Actual table creation occurs on first use.
- Java 8 uses an array + linked list + red-black tree structure.
- Array length starts at 16 and expands when the load factor threshold is reached.
- Linked lists transform into red-black trees when their length exceeds 8.
- JDK 7 used segment-based locks, while JDK 8 employs CAS (Compare-And-Swap) and synchronized blocks to ensure thread safety.
Thread-safe collection classes are crucial for concurrent programming in Java, allowing multiple threads to safely manipulate data structures without compromising data integrity.