A Guide to Safe Data Structure Use in Java Concurrency

A Guide to Safe Data Structure Use in Java Concurrency

Concurrency in Java allows multiple threads to execute at the same time, sharing the same resources. However, this sharing of resources can result in data inconsistencies and race conditions. Therefore, it is important to ensure that our Java code is thread-safe, meaning that it can handle multiple threads accessing it simultaneously without leading to issues such as race conditions or deadlocks.

In this article, we will explore various techniques and strategies to achieve thread-safety in Java, including immutable and stateless implementations, thread-local fields, synchronized and concurrent collections, atomic objects, and volatile fields.

Immutable Implementations

Immutable objects are objects that cannot be changed after they are created. This makes them thread-safe since they cannot be modified by multiple threads. Immutable objects can be created by:

  • Making all fields final
  • Not exposing setter methods
  • Not allowing subclassing

Stateless Implementations

Stateless objects are objects that do not maintain state between method calls. They are thread-safe since they do not change between method calls. Stateless objects can be created by:

  • Avoiding instance variables
  • Making all methods static
  • Not allowing subclassing

Thread-Local Fields

Thread-local fields are fields that are unique to each thread. This means that each thread has its own copy of the field and can modify it without affecting other threads. Thread-local fields can be created by:

  • Using the ThreadLocal class to create a thread-local variable
  • Storing thread-specific data in the Thread object itself
public class ExampleThreadLocal {
    private static final ThreadLocal<SimpleDateFormat> DATE_FORMAT 
      = ThreadLocal.withInitial(() -> new SimpleDateFormat("dd-MM-yyyy"));

    public String formatDate(Date date) {
        return DATE_FORMAT.get().format(date);

Synchronized Collections

Synchronized collections are collections that ensure thread-safety by synchronizing access to the collection. This means that only one thread can access the collection at a time. Synchronized collections can be created by:

  • Using the Collections.synchronizedXXX() methods to wrap existing collections
List<String> synchronizedList = Collections.synchronizedList(new ArrayList<>());

Concurrent Collections

Java also provides a number of concurrent collection classes, such as ConcurrentHashMap, ConcurrentSkipListMap, and ConcurrentLinkedQueue. These classes are designed to be used in multi-threaded environments and have high performance due to their non-blocking algorithms.

Concurrent collections adopt a different approach compared to their synchronized counterparts in ensuring thread-safety by partitioning their data into segments. For instance, in a ConcurrentHashMap, multiple threads can acquire locks on distinct map segments, allowing simultaneous access to the Map by several threads.

Concurrent collections demonstrate higher performance than synchronized collections, as they provide concurrent thread access advantages.

It is important to note that synchronized and concurrent collections only offer thread-safety for the collection and not its contents.

CopyOnWrite Collections

CopyOnWrite collections, such as CopyOnWriteArrayList and CopyOnWriteArraySet, create a new copy of the underlying collection whenever a modification is made. This ensures that no thread can modify the collection while another thread is iterating over it, but can be inefficient if modifications are frequent.

These collections are ideal for scenarios where reads are more frequent than writes, and where the collection size is relatively small. This is because every write operation will create a new copy of the underlying collection, which can lead to memory overhead and reduced performance.

Blocking data structures

Blocking data structures are data structures that block threads until they can perform their operation. In Java, the most commonly used blocking data structures are the blocking queue implementations, such as ArrayBlockingQueue, LinkedBlockingQueue, and PriorityBlockingQueue.

These data structures have methods that allow a thread to block if the queue is full (in the case of producers) or empty (in the case of consumers), which can be useful in scenarios where you want to limit the number of items in a queue or ensure that producers and consumers operate at the same speed.

Atomic Objects

In addition to synchronized and concurrent collections, Java provides atomic objects such as AtomicInteger, AtomicLong, AtomicBoolean, and AtomicReference. These classes are thread-safe and use a compare-and-set (CAS) operation to ensure that only one thread can modify the value at a time.

Volatile Fields

Even with synchronized methods and blocks, the values of regular class fields may be cached by the CPU, and consequent updates to a particular field may not be visible to other threads. To prevent this situation, we can use volatile class fields.

In situations where only a single task modifies the data while the others read it, the use of the volatile keyword is sufficient to avoid any data inconsistency problems without additional synchronization. However, in cases where multiple tasks modify the data, it becomes necessary to use synchronization methods.