To test all thread interleavings, we put the complete test in a while loop iterating over all thread interleavings using the class AllInterleavings from vmlens , line We can see this in the report from vmlens:.
In the case of the error, both threads first read the variable counter in parallel. And then, both create the same id. To solve this problem, we make the method atomic by using a synchronized block:. Now, the method is atomic. The synchronized block makes sure that other threads cannot see the intermediate state of the method. Methods that do not access shared state are automatically atomic.
The same is true for classes with the read-only state. Therefore, stateless and immutable classes are an easy way to implement thread-safe classes. All their methods are automatically atomic. Not all usages of atomic methods are automatically thread-safe.
Combining multiple atomic methods for the same values typically leads to race conditions. Let us look at the atomic method get and put from ConcurrentHashMap to see why. Let us use those methods to insert a value in the map when no previous mapping exists:. The test is similar to the previous test. Again, we use two threads to test if our method is thread-safe, lines 18 and An again, we test after both threads finished if the result is correct, line Running the test, we see the following error:.
What we demand from a threadsafe data type is that when clients call its atomic operations concurrently, the results are consistent with some sequential ordering of the calls. In this case, clearing and inserting, that means either clear -followed-by- insert , or insert -followed-by- clear. This property is called serializability : for any set of operations executed concurrently, the result the values and state observable by clients must be a result given by some sequential ordering of those operations.
For each pair of concurrent calls and their result, does that outcome violate serializability and therefore demonstrate that MyStringBuffer is not threadsafe? This reading talked about three major ways to achieve safety from race conditions on shared mutable data:. Safe from bugs. Easy to understand. Applying these general, simple design patterns is far more understandable than a complex argument about which thread interleavings are possible and which are not.
Ready for change. Software in 6. Communicating clearly with future programmers, including future you. Designed to accommodate change without rewriting. There are basically four ways to make variable access safe in shared-memory concurrency: Confinement. Make the shared data immutable. Threadsafe data type. Encapsulate the shared data in an existing threadsafe data type that does the coordination for you. Use synchronization to keep the threads from accessing the variable at the same time.
Synchronization is what you need to build your own threadsafe data type. What Threadsafe Means A data type or static method is threadsafe if it behaves correctly when used from multiple threads, regardless of how those threads are executed, and without demanding additional coordination from the calling code.
Hover or tap on each step to update the diagram: When we start the program, we start with one thread running main. Unlike local variables, static variables are not automatically thread confined. The call to computeFact starts before the call to computeFact 99 starts missing answer. The call to computeFact 99 starts before the call to computeFact starts missing answer. The call to computeFact finishes before the call to computeFact 99 starts missing answer.
The call to computeFact 99 finishes before the call to computeFact starts missing answer. About to execute lines 1 and 3 Yes, it could be violated missing answer.
Yes, it could be violated missing answer. In the following code, which variables are confined to a single thread? Stronger definition of immutability So in order to be confident that an immutable data type is threadsafe without locks, we need a stronger definition of immutability: no mutator methods all fields are private and final no representation exposure no mutation whatsoever of mutable objects in the rep — not even beneficent mutation If you follow these rules, then you can be confident that your immutable type will also be threadsafe.
Which of the following elements would you have to look at? Threadsafe Collections The collection interfaces in Java — List , Set , Map — have basic implementations that are not threadsafe. Consider this code, which checks whether a list has at least one element and then gets that element: if!
Even the isPrime method still has potential races: if cache. The race between containsKey and get is not harmful because we never remove items from the cache — once it contains a result for x, it will continue to do so. Java Concurrency — Thread Safety? Last Updated: August 27, Java Concurrency. A piece of code is thread-safe if it only manipulates shared data structures in a manner that guarantees safe execution by multiple threads at the same time.
And there are more similar definitions. What is Correctness in thread safety? Correctness means that a class conforms to its specification. Example: A Stateless Servlet A good example of thread safe class is java servlets which have no fields and references, no fields from other classes etc. Happy Learning!! Was this post helpful? Let us know if you liked the post. Java Concurrency — Difference between yield and join. Thanks a lot. This documentation page provides good programming constructs to achieve thread safety.
Lock Objects support locking idioms that simplify many concurrent applications. Executors define a high-level API for launching and managing threads. Executor implementations provided by java. Concurrent Collections make it easier to manage large collections of data, and can greatly reduce the need for synchronization.
Atomic Variables have features that minimize synchronization and help avoid memory consistency errors. ThreadLocalRandom in JDK 7 provides efficient generation of pseudorandom numbers from multiple threads.
Refer to java. You are clearly working in a WinForms environment. WinForms controls exhibit thread affinity, which means that the thread in which they are created is the only thread that can be used to access and update them. That is why you will find examples on MSDN and elsewhere demonstrating how to marshall the call back onto the main thread. For example I have seen code that formatted floating point numbers to string, if two of these are run in different threads the global value of decimalSeparator can be permanently changed to '.
Producing Thread-safe code is all about managing access to shared mutable states. When mutable states are published or shared between threads, they need to be synchronized to avoid bugs like race conditions and memory consistency errors. I recently wrote a blog about thread safety. You can read it for more information.
To understand thread safety, read below sections :. As a more substantial example of delegation, let's construct a version of the vehicle tracker that delegates to a thread-safe class. We store the locations in a Map, so we start with a thread-safe Map implementation, ConcurrentHashMap. We also store the location using an immutable Point class instead of MutablePoint , shown in Listing 4. Point is thread-safe because it is immutable.
Immutable values can be freely shared and published, so we no longer need to copy the locations when returning them. DelegatingVehicleTracker in Listing 4.
If we had used the original MutablePoint class instead of Point, we would be breaking encapsulation by letting getLocations publish a reference to mutable state that is not thread-safe. This means that if thread A calls getLocations and thread B later modifies the location of some of the points, those changes are reflected in the Map returned to thread A.
We can also delegate thread safety to more than one underlying state variable as long as those underlying state variables are independent, meaning that the composite class does not impose any invariants involving the multiple state variables.
VisualComponent in Listing 4. It maintains a list of registered listeners of each type, so that when an event occurs the appropriate listeners can be invoked. But there is no relationship between the set of mouse listeners and key listeners; the two are independent, and therefore VisualComponent can delegate its thread safety obligations to two underlying thread-safe lists. VisualComponent uses a CopyOnWriteArrayList to store each listener list; this is a thread-safe List implementation particularly suited for managing listener lists see Section 5.
Each List is thread-safe, and because there are no constraints coupling the state of one to the state of the other, VisualComponent can delegate its thread safety responsibilities to the underlying mouseListeners and keyListeners objects. Most composite classes are not as simple as VisualComponent : they have invariants that relate their component state variables.
NumberRange in Listing 4. Listing 4. Don't do this. NumberRange is not thread-safe ; it does not preserve the invariant that constrains lower and upper. The setLower and setUpper methods attempt to respect this invariant, but do so poorly. Both setLower and setUpper are check-then-act sequences, but they do not use sufficient locking to make them atomic.
If the number range holds 0, 10 , and one thread calls setLower 5 while another thread calls setUpper 4 , with some unlucky timing both will pass the checks in the setters and both modifications will be applied.
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