27 June 2012

The ++ operator is not thread safe

Findbugs is great tools for analyzing java code. It can find potential bugs. One of the warnings is the VO: An increment to a volatile field isn't atomic (VO_VOLATILE_INCREMENT). It means that the ++ operator is not an atomic operation and thus not thread safe. To demonstrate this looks at this code:

public class PlusPlusOperatorThreadSaftey {
    public void testThreadSaftey() throws InterruptedException {
        class IntegerHolder {
            private volatile int value = 0;
            private void increase() {
            private int getValue() {
                return value;
        final IntegerHolder integerHolder = new IntegerHolder();
        final int numberOfIncreasePerThread = 50;
        final int numberOfThreads = 100;
        ExecutorService threadPool = Executors.newFixedThreadPool(numberOfThreads);
        for (int i = 0; i < numberOfThreads; i++) {
            threadPool.submit(new Runnable() {
                public void run() {
                    for (int i = 0; i < numberOfIncreasePerThread; i++) {
        threadPool.awaitTermination(10, TimeUnit.SECONDS);
        assertEquals(integerHolder.getValue(), numberOfIncreasePerThread *
When running this code on my dual core CPU sometimes the test case passes, and sometimes I get:
java.lang.AssertionError: expected:<4998> but was:<5000>. Thus, the ++ operator is not atomic and updates can be lost.

To fix this you could add a synchronized block, but a better approach is to use an AtomicInteger like this:
class IntegerHolder {
        private AtomicInteger value = new AtomicInteger(0);
        private void increase() {
        private int getValue() {
            return value.get();

11 June 2012

Create named "screen"s to continue work later

The Unix command screen is a great command that lets you manage your login sessions. If you for some reasons shares an account with several people or have many different contexts it is useful to crate a named screen session:

lennart@pingvinen:~$ screen -S myScreen

To list all the screens use:
lennart@pingvinen:~$ screen -list
There are screens on:
        14825.myScreen  (2012-06-11 08.15.28)   (Attached)
        14692.pts-0.pingvinen   (2012-06-11 08.13.25)   (Detached)
2 Sockets in /var/run/screen/S-lennart.
Note that  one of the screens are named (the myScreen) and one is unnamed and gets a default name.

To attach to a named screen:
lennart@pingvinen:~$ screen -r myScreen