{"id":75357,"date":"2018-03-29T16:02:30","date_gmt":"2018-03-29T13:02:30","guid":{"rendered":"https:\/\/www.javacodegeeks.com\/?p=75357"},"modified":"2018-03-28T11:11:14","modified_gmt":"2018-03-28T08:11:14","slug":"how-i-test-my-java-classes-for-thread-safety","status":"publish","type":"post","link":"https:\/\/www.javacodegeeks.com\/2018\/03\/how-i-test-my-java-classes-for-thread-safety.html","title":{"rendered":"How I Test My Java Classes for Thread-Safety"},"content":{"rendered":"

I touched on this problem in one of my recent webinars<\/a>, now it’s time to explain it in writing. Thread-safety is an important quality of classes in languages\/platforms like Java, where we frequently share objects between threads. The issues caused by lack of thread-safety are very difficult to debug, since they are sporadic and almost impossible to reproduce on purpose. How do you test your objects to make sure they are thread-safe? Here is how I’m doing it.
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\"\"<\/a>
Scent of a Woman (1992) by Martin Brest<\/figcaption><\/figure><\/p>\n

Let us say there is a simple in-memory bookshelf:<\/p>\n

class Books {\r\n  final Map<Integer, String> map =\r\n    new ConcurrentHashMap<>();\r\n  int add(String title) {\r\n    final Integer next = this.map.size() + 1;\r\n    this.map.put(next, title);\r\n    return next;\r\n  }\r\n  String title(int id) {\r\n    return this.map.get(id);\r\n  }\r\n}<\/pre>\n
First, we put a book there and the bookshelf returns its ID. Then we can read the title of the book by its ID:<\/p>\n
Books books = new Books();\r\nString title = \"Elegant Objects\";\r\nint id = books.add(title);\r\nassert books.title(id).equals(title);<\/pre>\n
The class seems to be thread-safe, since we are using the thread-safe ConcurrentHashMap<\/code><\/a> instead of a more primitive and non-thread-safe HashMap<\/code><\/a>, right? Let’s try to test it:<\/p>\n
class BooksTest {\r\n  @Test\r\n  public void addsAndRetrieves() {\r\n    Books books = new Books();\r\n    String title = \"Elegant Objects\";\r\n    int id = books.add(title);\r\n    assert books.title(id).equals(title);\r\n  }\r\n}<\/pre>\n
The test passes, but it’s just a one-thread test. Let’s try to do the same manipulation from a few parallel threads (I’m using Hamcrest<\/a>):<\/p>\n
class BooksTest {\r\n  @Test\r\n  public void addsAndRetrieves() {\r\n    Books books = new Books();\r\n    int threads = 10;\r\n    ExecutorService service =\r\n      Executors.newFixedThreadPool(threads);\r\n    Collection<Future<Integer>> futures =\r\n      new LinkedList<>();\r\n    for (int t = 0; t < threads; ++t) {\r\n      final String title = String.format(\"Book #%d\", t);\r\n      futures.add(service.submit(() -> books.add(title)));\r\n    }\r\n    Set<Integer> ids = new HashSet<>();\r\n    for (Future<Integer> f : futures) {\r\n      ids.add(f.get());\r\n    }\r\n    assertThat(ids.size(), equalTo(threads));\r\n  }\r\n}<\/pre>\n
First, I create a pool of threads via Executors<\/code><\/a>. Then I submit ten objects of type Callable<\/code><\/a> via submit()<\/code>. Each of them will add a new unique book to the bookshelf. All of them will be executed, in some unpredictable order, by some of those ten threads from the pool.
 
<\/div> <\/div><\/p>\n
Then I fetch the results of their executors through the list of objects of type Future<\/code><\/a>. Finally, I calculate the amount of unique book IDs created. If the number is 10, there were no conflicts. I’m using the Set<\/code><\/a> collection in order to make sure the list of IDs contains only unique elements.<\/p>\n
The test passes on my laptop. However, it’s not strong enough. The problem here is that it’s not really testing the Books<\/code> from multiple parallel threads. The time that passes between our calls to submit()<\/code> is large enough to finish the execution of books.add()<\/code>. That’s why in reality only one thread will run at the same time. We can check that by modifying the code a bit:<\/p>\n
AtomicBoolean running = new AtomicBoolean();\r\nAtomicInteger overlaps = new AtomicInteger();\r\nCollection<Future<Integer>> futures = new LinkedList<>();\r\nfor (int t = 0; t < threads; ++t) {\r\n  final String title = String.format(\"Book #%d\", t);\r\n  futures.add(\r\n    service.submit(\r\n      () -> {\r\n        if (running.get()) {\r\n          overlaps.incrementAndGet();\r\n        }\r\n        running.set(true);\r\n        int id = books.add(title);\r\n        running.set(false);\r\n        return id;\r\n      }\r\n    )\r\n  );\r\n}\r\nassertThat(overlaps.get(), greaterThan(0));<\/pre>\n
With this code I’m trying to see how often threads overlap each other and do something in parallel. This never happens and overlaps<\/code> is equal to zero. Thus our test is not really testing anything yet. It just adds ten books to the bookshelf one by one. If I increase the amount of threads to 1000, they start to overlap sometimes. But we want them to overlap even when there’s a small number of them. To solve that we need to use CountDownLatch<\/code><\/a>:<\/p>\n
CountDownLatch latch = new CountDownLatch(1);\r\nAtomicBoolean running = new AtomicBoolean();\r\nAtomicInteger overlaps = new AtomicInteger();\r\nCollection<Future<Integer>> futures = new LinkedList<>();\r\nfor (int t = 0; t < threads; ++t) {\r\n  final String title = String.format(\"Book #%d\", t);\r\n  futures.add(\r\n    service.submit(\r\n      () -> {\r\n        latch.await();\r\n        if (running.get()) {\r\n          overlaps.incrementAndGet();\r\n        }\r\n        running.set(true);\r\n        int id = books.add(title);\r\n        running.set(false);\r\n        return id;\r\n      }\r\n    )\r\n  );\r\n}\r\nlatch.countDown();\r\nSet<Integer> ids = new HashSet<>();\r\nfor (Future<Integer> f : futures) {\r\n  ids.add(f.get());\r\n}\r\nassertThat(overlaps.get(), greaterThan(0));<\/pre>\n
Now each thread, before touching the books, waits for the permission given by latch<\/code>. When we submit them all via submit()<\/code> they stay on hold and wait. Then we release the latch with countDown()<\/code> and they all start to go, simultaneously. Now, on my laptop, overlaps<\/code> is equal to 3-5 even when threads<\/code> is 10.<\/p>\n
And that last assertThat()<\/code> crashes now! I’m not getting 10 book IDs, as I was before. It’s 7-9, but never 10. The class, apparently, is not thread-safe!<\/p>\n
But before we fix the class, let’s make our test simpler. Let’s use RunInThreads<\/code><\/a> from Cactoos<\/a>, which does exactly the same as we’ve done above, but under the hood:<\/p>\n
class BooksTest {\r\n  @Test\r\n  public void addsAndRetrieves() {\r\n    Books books = new Books();\r\n    MatcherAssert.assertThat(\r\n      t -> {\r\n        String title = String.format(\r\n          \"Book #%d\", t.getAndIncrement()\r\n        );\r\n        int id = books.add(title);\r\n        return books.title(id).equals(title);\r\n      },\r\n      new RunsInThreads<>(new AtomicInteger(), 10)\r\n    );\r\n  }\r\n}<\/pre>\n
\"\"<\/a><\/figure>\n
The first argument of assertThat()<\/code> is an instance of Func<\/code> (a functional interface), accepting an AtomicInteger<\/code><\/a> (the first argument of RunsInThreads<\/code>) and returning Boolean<\/code>. This function will be executed on 10 parallel thread, using the same latch-based approach as demonstrated above.<\/p>\n
This RunInThreads<\/code> seems to be compact and convenient, I’m using it in a few projects already.<\/p>\n
By the way, in order to make Books<\/code> thread-safe we just need to add synchronized<\/code> to its method add()<\/code>. Or maybe you can suggest a better solution?<\/p>\n
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Published on Java Code Geeks with permission by Yegor Bugayenko, partner at our JCG program<\/a>. See the original article here: How I Test My Java Classes for Thread-Safety<\/a><\/p>\n
Opinions expressed by Java Code Geeks contributors are their own.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
I touched on this problem in one of my recent webinars, now it’s time to explain it in writing. Thread-safety is an important quality of classes in languages\/platforms like Java, where we frequently share objects between threads. The issues caused by lack of thread-safety are very difficult to debug, since they are sporadic and almost …<\/p>\n","protected":false},"author":593,"featured_media":148,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-75357","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-core-java"],"yoast_head":"\nHow I Test My Java Classes for Thread-Safety - Java Code Geeks<\/title>\n<meta name=\"description\" content=\"I touched on this problem in one of my recent webinars, now it's time to explain it in writing. 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