Java crumbs

Threads are expensive. They need lot of system resources, it's time-consuming to create them and JVM is able to manage only few of them. Therefore we need asynchronous processing, new libraries, new languages etc. At least, that's what I am hearing from everywhere. But is it really true? Are threads really so expensive? There is only one way how to find out. Just ask the machine.

The only thing we need to do is write a simple test. We just have to create as many sleeping or waiting threads as possible and we will see when it breaks. The reason why we are interested only in waiting threads is obvious. It has no sense to create thousands of active threads if we have only few CPUs. We want to simulate a situation, when threads are waiting for database data, incoming message or other CPU unrelated tasks.

public class CreateThreads {
	/**
	 * Thread class
	 * @author Lukas Krecan
	 *
	 */
	private static final class MyThread extends Thread
	{
		private final int number;

		public MyThread(int number) {
			this.number = number;
		}
		@Override
		public void run() {
			if (shouldPrintMessage(number))
			{
				System.out.println("Thread no. "+number+" started.");
			}
			try {
				//sleep forever
				Thread.sleep(Long.MAX_VALUE);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		long startTime = System.currentTimeMillis();
		final int noOfThreads = 1000000;
		int i=0;
		try {
			for (i=0; i < noOfThreads; i++)
			{
				if (shouldPrintMessage(i))
				{
					System.out.println("Creating thread "+i+" ("+(System.currentTimeMillis()-startTime)+"ms)");
				}
				new MyThread(i).start();
			}
		} catch (Throwable e) {
			System.out.println("Error thrown when creating thread "+i);
			e.printStackTrace();
		}
	}

	private static boolean shouldPrintMessage(int i)
	{
		return i % 100 == 0;
	}
}

As you can see, there is nothing special in the test. Every hundredth threads prints a message to console and all of them sleep forever. In the main method we are creating threads and when an exception is thrown, number of created threads is printed out.

Even though there is nothing special on the test, the results are really interesting and for me quite surprising. I will let you guess the results. The test is executed on my two years old laptop with Intel Core 2 Duo T8100 2.1GHz. There is 64bit Linux 2.6.31 and OpenJDK (IcedTea6 1.6.1) running on top of it.

Try to guess how many threads are created if I do not tinker with JVM arguments at all. Is it 1K? Or 10K? Or even more? Well, it's slightly more than 32K. Usually you see result like this

Creating thread 31900 (37656ms)
Thread no. 31900 started.
Creating thread 32000 (37736ms)
Thread no. 32000 started.
Creating thread 32100 (37818ms)
Thread no. 32100 started.
Error thrown when creating thread 32172
java.lang.OutOfMemoryError: unable to create new native thread
	at java.lang.Thread.start0(Native Method)
	at java.lang.Thread.start(Thread.java:614)
	at CreateThreads.main(CreateThreads.java:43)

The question is, why we can not create more threads. Are we constrained by the heap size? Or maybe stack size? No in fact, 32K is apparently Linux kernel limit. Therefore there is nothing we can do in Java, Linux is just not able to handle more threads. But I think that 32K threads is not bad at all. Please keep in mind, that you will be able to reproduce the results only on modern operating systems and JVMs. I am afraid that Windows XP results will be much worse.

Please also note, that once the threads are created and all of them sleep, the application does not consume any CPU and it consumes around 800MB of memory. I am not sure why so much memory is used, it has to be investigated.

As we have seen, in this artificial scenario threads are relatively cheap. I do not want to say, that we should not use new libraries or tools. Most of them are quite useful. The only thing I want to say is that we should think twice before prematurely optimizing our applications. Sometimes the most straightforward and easiest approach is the best.

Next time, I will try to do some more real life example with servlet container.

What a nice and controversial title. But I really mean it. Level of test coverage is really interesting only when it's low. Of course, we should aim for highest coverage possible, but the higher the coverage is, the less useful information it brings. Let take the following example and let's assume there are no tests for this method.

	/**
	 Adds positive numbers. If one of the parameters is negative,
	 IllegalArgumentException is thrown.
	 * @param a
	 * @param b
	 * @return
	 */
	public int addPositive(int a, int b)
	{
		if ((a<0) || (b<0))
		{
			throw new IllegalArgumentException("Argument is negative");
		}
		return a+a;
	}

No coverage - wow, that's interesting

The fact that some piece of code is not covered at all is really interesting. It either means, that we have to start writing tests at once or that the piece of code has to be deleted. Either way we know that we should do something. Moreover, we know exactly what to do. For example, we can start with the following test.

	@Test
	public void testAddition()
	{
		assertEquals(2, math.addPositive(1, 1));
	}

It will get us partial coverage like this

Partial coverage - cool, I know what to test next

If I have partial coverage, I can see what other tests I am supposed to write. In our example I see that I have to test what happens if I pass in a negative numbers.

	@Test(expected=IllegalArgumentException.class)
	public void testNegative1()
	{
		math.addPositive(-1, 1);
	}
	@Test(expected=IllegalArgumentException.class)
	public void testNegative2()
	{
		math.addPositive(1, -1);
	}

Total coverage - I have no clue

Now we have 100% coverage. Well done! Unfortunately, I have no clue what to do next. I have no idea if I need to write more tests, I do not know if I have complete functional coverage. I have to stop looking at the coverage and I have to start thinking. You see, when the coverage was low it gave me lot of useful information, when it's high I am left on my own. For example I can think hard and come up with following complex test scenario.

	@Test
	public void testComplicatedAddition()
	{
		assertEquals(3, math.addPositive(1, 2));
	}

Please note, that the coverage was already 100% before I have written the test. So one more test could not make it higher. Nevertheless the test actually detected a bug that was there from the beginning. It means that 100% code coverage is not enough, we have to aim higher. We should care about functional coverage and not about code coverage!

That's also one of many reasons why we should do test driven development. Or at least test first development. If we write the tests before the code, we are forced to think from the beginning. In TDD we do not care about the coverage. We just think about the functionality, write corresponding tests and then add the code. Total coverage is automatically achieved.

In the opposite situation, when we write the code first and then add the tests as an afterthought, we are not forced to think. We can just write some tests, achieve the target coverage and be happy. Test coverage can be really deceitful. It can give us false sense of security. So please remember, test coverage is a good servant but a bad master.

Spring WS project provides nice and versatile exception handling tools. But in some scenarios predefined Exception Resolvers are not sufficient. For example if you want to provide additional error info in the soap:fault detail like in this example:

   
   
      
         SOAP-ENV:Client
         Something wrong happened
         
            Error
            BigTrouble
         
      
   

Fortunately it is quite easy to add similar behavior using Spring WS. You can easily extend existing SoapFaultMappingExceptionResolver and customize the fault (please note that EndpointExeption is project specific exception that provides necessary data):

 public class EndpointExceptionResolver extends SoapFaultMappingExceptionResolver {
	private static final QName CODE = new QName("code");
	private static final QName SUB_CODE = new QName("sub-code");

	@Override
	protected void customizeFault(Object endpoint, Exception ex, SoapFault fault) {
		logger.warn("Exception processed ",ex);
		if (ex instanceof EndpointException) {
			EndpointException ee = (EndpointException) ex;
			SoapFaultDetail detail = fault.addFaultDetail();
			detail.addFaultDetailElement(CODE).addText(ee.getCode());
			detail.addFaultDetailElement(SUB_CODE).addText(ee.getSubCode());
		}
	}
}