Mastering Java’s ReentrantReadWriteLock: When to Use Read/Write Locks

Understanding ReentrantReadWriteLock in Java

ReentrantLock provides exclusive mutual exclusion, allowing only one thread to execute the code after lock(). In contrast, ReentrantReadWriteLock introduces two separate locks: a shared read lock and an exclusive write lock.

The read lock permits multiple threads to acquire it simultaneously when no thread holds the write lock, enabling concurrent read operations. The write lock is exclusive; only one thread can hold it at a time, blocking both other writers and readers.

Key characteristics of ReentrantReadWriteLock:

Read‑read sharing

Write‑write exclusion

Read‑write mutual exclusion

Write‑read exclusion

Below are example demonstrations (images) of each behavior:

Read‑read sharing example

The output shows two threads acquiring the lock at almost the same time, confirming that lock.readLock().lock(); allows concurrent execution.

Write‑write exclusion example

The result shows a roughly 5‑second difference, indicating that write threads are mutually exclusive.

Read‑write (or write‑read) exclusion example

Again, the execution time difference demonstrates that read and write threads cannot run concurrently.

These examples illustrate how ReentrantReadWriteLock can improve performance over a plain ReentrantLock by allowing concurrent reads while still protecting writes.