Fast-fail mechanism of java container

The fast-fast mechanism of Java containers is an important feature in the Java collection framework. It is mainly used to detect and process concurrent modifications of collections during iteration. Here is a detailed explanation of the mechanism:

1. Definition and Principles

The core idea of ​​the fast failure mechanism is that during the iteration process, once the structure of the collection is detected to be modified (such as adding or deleting elements), it will be thrown immediately.ConcurrentModificationExceptionException, thus preventing potential errors or inconsistent states. This mechanism is maintained by maintaining a modified counter (modCount) to achieve it.

• Modify the counter (modCount）: This is an important property in the collection class, which is used to record the number of times the collection is modified. Whenever a set changes structurally (such as adding or deleting elements),modCountThe value of .
• The expected number of modifications in the iterator (expectedModCount）: When the iterator is created, it will put it inside itexpectedModCountSet the property to the current collectionmodCountvalue. During the iteration, each callnext()Before the method, the iterator will check the collection'smodCountIs the value internal to itexpectedModCountThe values ​​are equal. If it is not equal, it means that the set has been modified during the iteration process, so it is thrownConcurrentModificationExceptionabnormal.

2. Application scenarios and examples

The fast failure mechanism is mainly used in the Java collection frameworkArrayList、HashMapetc. Here is a typical example:

ArrayList<Integer> list = new ArrayList<>();
(1);
(2);
(3);
Iterator<Integer> iterator = ();
while (()) {
    Integer value = ();
    if (value == 2) {
        (value); // Modify the collection structure and trigger the fast failure mechanism    }
}

In the above example, when the iterator traverses an element with a value of 2, it tries to pass through the collection'sremoveMethod to delete the element. This will result in the collection'smodCountThe value increases, while the iterator'sexpectedModCountThe value remains the same. Therefore, in the next callnext()When the method is used, the iterator will detectmodCountandexpectedModCountNot equal, thus thrownConcurrentModificationExceptionabnormal.

3. Precautions and solutions

• Things to note：

  • The fast failure mechanism does not guarantee that exceptions can be thrown in all cases. Since the modification check is not performed under synchronization, there is a visibility problem. If the container is modified, it will causemodCountChanges, iterators may see invalidmodCountvalue, thus not realizing that a modification has occurred.
  • Even in a single-threaded environment, the fast failure mechanism can trigger an exception. For example, in the iteration process, theaddorremoveMethods modify the collection structure.

• Solution：

  • During iteration, avoid direct access to the collectionaddorremoveMethods modify the collection structure. If you need to modify the collection, you can collect the elements to be modified and modify them after the iteration is over.
  • Use the iterator providedremove()Method to delete the current element. This keeps the internal state of the iterator consistent and avoids throwing exceptions. But please note that the iterator itselfremove()The method will not modify the collectionmodCountother states other than values.
  • In a multithreaded environment, use thread-safe collection classes (e.g.CopyOnWriteArrayList、ConcurrentHashMapWait) to avoidConcurrentModificationExceptionabnormal. These collection classes adopt different mechanisms to ensure thread safety and allow concurrent modifications during the iteration.

4. Summary

The fast failure mechanism of Java containers is an important error detection mechanism, which helps to promptly detect and handle unexpected modifications of collection structures during the iteration process. However, developers need to pay attention to their limitations when using them and take corresponding solutions to avoid potential problems. The robustness and stability of Java applications can be ensured by rationally using fast failure mechanisms and other thread-safe collection classes.

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