For simple multi-threaded code, using synchronization (opens new window) is acceptable. However, using synchronization does have a liveness impact, and as a codebase becomes more complex, the likelihood goes up that you will end up with Deadlock (opens new window), Starvation, or Livelock (opens new window).

In cases of more complex concurrency, using Atomic Variables is often a better alternative, as it allows an individual variable to be accessed in a thread-safe manner without the overhead of using synchronized methods or code blocks.

Creating an AtomicInteger type:

AtomicInteger aInt = new AtomicInteger() // Create with default value 0

AtomicInteger aInt = new AtomicInteger(1) // Create with initial value 1

Similarly for other instance types.

AtomicIntegerArray aIntArray = new AtomicIntegerArray(10) // Create array of specific length
AtomicIntegerArray aIntArray = new AtomicIntegerArray(new int[] {1, 2, 3}) // Initialize array with another array

Similarly for other atomic types.

There is a notable exception that there is no float and double types. These can be simulated through the use of Float.floatToIntBits(float) and Float.intBitsToFloat(int) for float as well as Double.doubleToLongBits(double) and Double.longBitsToDouble(long) for doubles.

If you are willing to use sun.misc.Unsafe you can use any primitive variable as atomic by using the atomic operation in sun.misc.Unsafe. All primitive types should be converted or encoded in int or longs to so use it in this way. For more on this see: sun.misc.Unsafe (opens new window).

The simple way to implement multi-threaded applications is to use Java's built-in synchronization and locking primitives; e.g. the synchronized keyword. The following example shows how we might use synchronized to accumulate counts.

public class Counters {
    private final int[] counters;

    public Counters(int nosCounters) {
        counters = new int[nosCounters];
    }

    /**
     * Increments the integer at the given index
     */
    public synchronized void count(int number) {
        if (number >= 0 && number < counters.length) {
            counters[number]++;
        }
    }

    /**
     * Obtains the current count of the number at the given index,
     * or if there is no number at that index, returns 0.
     */
    public synchronized int getCount(int number) {
        return (number >= 0 && number < counters.length) ? counters[number] : 0;
    }
}

This implementation will work correctly. However, if you have a large number of threads making lots of simultaneous calls on the same Counters object, the synchronization is liable to be a bottleneck. Specifically:

1. Each synchronized method call will start with the current thread acquiring the lock for the Counters instance.
2. The thread will hold the lock while it checks number value and updates the counter.
3. Finally, the it will release the lock, allowing other threads access.

If one thread attempts to acquire the lock while another one holds it, the attempting thread will be blocked (stopped) at step 1 until the lock is released. If multiple threads are waiting, one of them will get it, and the others will continue to be blocked.

This can lead to a couple of problems:

How does one implement Atomic Types?

Let us start by rewriting the example above using AtomicInteger counters:

public class Counters {
    private final AtomicInteger[] counters;

    public Counters(int nosCounters) {
        counters = new AtomicInteger[nosCounters];
        for (int i = 0; i < nosCounters; i++) {
            counters[i] = new AtomicInteger();
        }
    }

    /**
     * Increments the integer at the given index
     */
    public void count(int number) {
        if (number >= 0 && number < counters.length) {
            counters[number].incrementAndGet();
        }
    }

    /**
     * Obtains the current count of the object at the given index,
     * or if there is no number at that index, returns 0.
     */
    public int getCount(int number) {
        return (number >= 0 && number < counters.length) ? 
                counters[number].get() : 0;
    }
}

We have replaced the int[] with an AtomicInteger[], and initialized it with an instance in each element. We have also added calls to incrementAndGet() and get() in place of operations on int values.

But the most important thing is that we can remove the synchronized keyword because locking is no longer required. This works because the incrementAndGet() and get() operations are atomic and thread-safe. In this context, it means that:

Furthermore, while two threads might actually attempt to update the same AtomicInteger instance at the same time, the implementations of the operations ensure that only one increment happens at a time on the given instance. This is done without locking, often resulting in better performance.

How do Atomic Types work?

Atomic types typically rely on specialized hardware instructions in the instruction set of the target machine. For example, Intel-based instruction sets provide a CAS (Compare and Swap (opens new window)) instruction that will perform a specific sequence of memory operations atomically.

These low-level instructions are are used to implement higher-level operations in the APIs of the respective Atomic.