96 static final int ATTEMPTS = 3;
97
98 /** Table of buffers. When non-null, size is a power of 2. */
99 volatile Buffer<E> @Nullable[] table;100
101 /** Spinlock (locked via CAS) used when resizing and/or creating Buffers. */
102 volatile int tableBusy;A volatile reference to an array doesn't treat the array elements as volatile.
This code declares a volatile reference to an array, which might not be what you want.
With a volatile reference to an array, reads and writes of the reference to the array are treated as volatile, but the array elements are non-volatile. To get volatile array elements, you will need to use one of the atomic array classes in java.util.concurrent (available since Java 5.0).
Bad Practice
volatile Int[] bad = new Int[10](); // Does not guarantee coherence of reads/writes to its elements.
Recommended
Using atomic classes is a good way to guarantee that all memory accesses across threads are coherent. However, because of the performance overhead, ensure that you absolutely need the concurrency guarantees before you use them.
It must be noted that an array of AtomicIntegers (AtomicInteger[]) is not the same as a single AtomicIntegerArray; the former is more prone to breakage and is a bad idea in general. AtomicIntegerArray stores ordinary integers and allows thread-safe access to them. An array of AtomicIntegers is a non-thread-safe data structure holding references to thread-safe integers.
import java.util.concurrent.atomic.AtomicIntegerArray;
AtomicInteger[] arr = new AtomicInteger[10]();
arr[2] = new AtomicInteger(); // This is not visible across threads!
volatile AtomicInteger[] volArr = new AtomicInteger[10]();
volArr[2] = new AtomicInteger(); // This is also not visible across threads!
// This would be visible across all threads,
// but it only applies when arr[2] points to an AtomicInteger and not null.
arr[2].set(3);
Changes to such an array will not be atomic or even coherent across threads unless all access to it is through a synchronized block or method.
References
AtomicIntegerArrayvsAtomicInteger[]- StackOverflow- CWE-362 - Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
- Spotbugs - VO_VOLATILE_REFERENCE_TO_ARRAY