Read/write lock is slower than synchronized, even when only reading?

I have the following code ArrayList implementation

public class LongArrayListUnsafe {
public static void main(String[] args) {
LongArrayList dal1 = LongArrayList.withElements();
for (int i = 0; i < 1000; i++)
dal1.add(i);
// Runtime.getRuntime().availableProcessors()
ExecutorService executorService = Executors.newFixedThreadPool(4);
long start = System.nanoTime();
for (int i = 0; i < 100; i++) {
executorService.execute(new Runnable() {
public void run() {
for (int i = 0; i < 1000; i++)
dal1.size();
for (int i = 0; i < 1000; i++)
dal1.get(i % 100);
}
});
}
executorService.shutdown();
try {
executorService.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
} catch (InterruptedException e) {
System.out.println("mayor disaster!");
}
}
class LongArrayList {
private long[] items;
private int size;
public LongArrayList() {
reset();
}
public static LongArrayList withElements(long...initialValues) {
LongArrayList list = new LongArrayList();
for (long l: initialValues)
list.add(l);
return list;
}
// Number of items in the double list
public synchronized int size() {
return size;
}
// Return item number i
public synchronized long get(int i) {
if (0 <= i && i < size)
return items[i];
else
throw new IndexOutOfBoundsException(String.valueOf(i));
}
// Add item x to end of list
public synchronized LongArrayList add(long x) {
if (size == items.length) {
long[] newItems = new long[items.length * 2];
for (int i = 0; i < items.length; i++)
newItems[i] = items[i];
items = newItems;
}
items[size] = x;
size++;
return this;
}

Now, this concurrent drivercode simply reads of the list, which is already made.This goes pretty fast.
But I was wondering whether it would be possible
for me to do this onlyreading operation faster with a readwritelock.
In size and get, this looks like this:

synchronized public int size() {
readWriteLock.readLock().lock();
int ret = this.size.get();
readWriteLock.readLock().unlock();
return ret;
}

and

public long get(int i) {
readWriteLock.readLock().lock();
if (0 <= i && i < size.get()) {
long ret = items.get(i);
readWriteLock.readLock().unlock();
return ret;
} else {
throw new IndexOutOfBoundsException(String.valueOf(i));
}
}

However, using a readwritelock goes way slower, and even slower when I add more threads. Why is this? when my drivercode is only reading, the threads should have more or less unlimited acces to the methods?

A java.util.concurrent.locks.ReadWriteLock is an inherently more complex thing than a mutual exclusion lock like synchronized. The documentation of the class states this. The overhead of the read-write semantics are likely bigger than return this.size;, or return this.items[i];, even with a surrounding boundary check.

Let's also look at your proposal in particular. You want to replace the original

public synchronized int size() {
return size;
}

with the proposal

synchronized public int size() {           // <-- locks exclusively/mutually on "this"
readWriteLock.readLock().lock();       // <-- locks on readWriteLock.readLock()
int ret = this.size.get();             // <-- is size and AtomicInteger now?
readWriteLock.readLock().unlock();
return ret;
}

I assume the use of synchronized was a typo, or it would add another lock to the equation. Also, I assume this.size.get(); should be this.size;. (using an AttomicInteger for size makes no sense in this context and adds additional cost). If my assumptions are correct, your actual proposal would be:

public int size() {
readWriteLock.readLock().lock();
int ret = this.size; 
readWriteLock.readLock().unlock();
return ret;
}
public long get(int i) {
readWriteLock.readLock().lock();
if (0 <= i && i < this.size) {
long ret = items[i];
readWriteLock.readLock().unlock();
return ret;
} else {
throw new IndexOutOfBoundsException(String.valueOf(i));
}
}
public LongArrayList add(long x) {
readWriteLock.writeLock().lock();
if (size == items.length) {
long[] newItems = new long[items.length * 2];
for (int i = 0; i < items.length; i++)
newItems[i] = items[i];
this.items = newItems;
}
items[size] = x;
size++;
readWriteLock.writeLock().unlock();
return this;
}

The implementation of get(int) is dangerous. If an IndexOutOfBoundException is thrown, the read-lock remains locked forever. That won't slow down further reads, but it keeps all future calls to add(long) waiting. If you use a lock, it is advisable to use it in combination with finally to ensure it is unlocked:

public long get(int i) {
readWriteLock.readLock().lock();
try {
if (0 <= i && i < size) {
return items[i];
} 
throw new IndexOutOfBoundsException(String.valueOf(i));
}
finally {
readWriteLock.readLock().unlock();
}
}
public LongArrayList add(long x) {
readWriteLock.writeLock().lock();
try {
if (size == items.length) {
long[] newItems = new long[items.length * 2];
for (int i = 0; i < items.length; i++)
newItems[i] = items[i];
items = newItems;
}
items[size] = x;
size++;
}
finally {
readWriteLock.writeLock().unlock();
}
return this;
}

As mentioned, if you are reading far more than you write, using synchronized could be more performant.