Does a synchronized block trigger a full memory fence for arrays?

Question

I am confused about sharing arrays safely between threads in Java, specifically memory fences and the keyword synchronized.

This Q&A is helpful, but does not answer all of my questions: Java arrays: synchronized + Atomic*, or synchronized suffices?

What follows is sample code to demonstrate the issue. Assume there is a pool of worker threads that populates the SharedTable via method add(...). After all worker threads are done, a final thread reads and saves the data.

Sample code to demonstrate the issue:

public final class SharedTable {

    // Column-oriented data entries
    private final String[] data1Arr;
    private final int[] data2Arr;
    private final long[] data3Arr;
    private final AtomicInteger nextIndex;

    public SharedTable(int size) {
        this.data1Arr = new String[size];
        this.data2Arr = new int[size];
        this.data3Arr = new long[size];
        this.nextIndex = new AtomicInteger(0);
    }

    // Thread-safe: Called by worker threads
    public void addEntry(String data1, int data2, long data3) {
        final int index = nextIndex.getAndIncrement();
        data1Arr[index] = data1;
        data2Arr[index] = data2;
        data3Arr[index] = data3;
    }

    // Not thread-safe: Called by clean-up/joiner/collator thread...
    // after worker threads are complete
    public void save() {
        // Does this induce a full memory fence to ensure thread-safe reading of 
        synchronized (this) {
            final int usedSide = nextIndex.get();
            for (int i = 0; i < usedSide; ++i) {
                final String data1 = data1Arr[i];
                final int    data2 = data2Arr[i];
                final long   data3 = data3Arr[i];
                // TODO: Save data here
            }
        }
    }
}

The sample code above could also be implemented using Atomic*Array, which acts as an "array of volatile values/references".

public final class SharedTable2 {

    // Column-oriented data entries
    private final AtomicReferenceArray<String> data1Arr;
    private final AtomicIntegerArray  data2Arr;
    private final AtomicLongArray data3Arr;
    private final AtomicInteger nextIndex;

    public SharedTable2(int size) { ... }

    // Thread-safe: Called by worker threads
    public void addEntry(String data1, int data2, long data3) {
        final int index = nextIndex.getAndIncrement();
        data1Arr.set(index, data1);
        ...
    }

    // Not thread-safe: Called by clean-up/joiner/collator thread...
    // after worker threads are complete
    public void save() {
        final int usedSide = nextIndex.get();
        for (int i = 0; i < usedSide; ++i) {
            final String data1 = data1Arr.get(i);
            final int    data2 = data2Arr.get(i);
            final long   data3 = data3Arr.get(i);
            // TODO: Save data here
        }
    }
}

1. Is SharedTable thread-safe (and cache coherent)?
2. Is SharedTable (much?) more efficient as only a single memory fence is required, whereas SharedTable2 invokes a memory fence for each call to Atomic*Array.set(...)?

If it helps, I am using Java 8 on 64-bit x86 hardware (Windows and Linux).

Answer 1

No, SharedTable is not thread-safe. A happens-before is only guaranteed if you read, from a synchronized block, something that has been written from a synchronized block using the same lock.

Since the writes are made out of a synchronized block, the JMM doesn't guarantee that the writes will be visible by the reader thread.