Java 中的 ConcurrentModificationException，示例

原文:https://www . geeksforgeeks . org/concurrentmodificationexception-in-Java-with-examples/

并发修改多线程环境中的异常

在 多线程环境 中，如果在资源检测期间，任何方法发现该对象存在不允许的并发修改，则可能会引发该并发修改异常。

  1. If this exception is detected, the result of iteration is undefined.
  2. Generally speaking, some iterator implementations choose to throw this exception, which is called Fail fast iterator .

例如:如果我们试图使用一个线程修改代码中的任何集合，但是另一个线程已经在使用该集合，那么这是不允许的。

并发修改单线程环境中的异常

因为，不能保证每当引发这个异常时，一个对象已经被某个不同的线程并发修改，这个异常也会在单线程环境中抛出。

如果我们在违反约定的对象上调用一系列方法，那么该对象将抛出 ConcurrentModificationException。

例如:如果在迭代集合时，我们直接尝试修改该集合，那么给定的快速失败迭代器将抛出这个 ConcurrentModificationException。

示例: 在下面的代码中，实现了一个 ArrayList。然后向其添加很少的值，并在遍历时对其进行很少的修改，

// Java program to show
// ConcurrentModificationException

import java.util.Iterator;
import java.util.ArrayList;

public class modificationError {
    public static void main(String args[])
    {

        // Creating an object of ArrayList Object
        ArrayList<String> arr
            = new ArrayList<String>();

        arr.add("One");
        arr.add("Two");
        arr.add("Three");
        arr.add("Four");

        try {
            // Printing the elements
            System.out.println(
                "ArrayList: ");
            Iterator<String> iter
                = arr.iterator();

            while (iter.hasNext()) {

                System.out.print(iter.next()
                                 + ", ");

                // ConcurrentModificationException
                // is raised here as an element
                // is added during the iteration
                System.out.println(
                    "\n\nTrying to add"
                    + " an element in "
                    + "between iteration\n");
                arr.add("Five");
            }
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

输出:

ArrayList: 
One, 

Trying to add an element in between iteration

java.util.ConcurrentModificationException

如何避免 ConcurrentModificationException？T3】

为了避免这个异常，

  • Simply put, we can modify it after the iteration is completed, or
  • Concept or method of realizing synchronization block

示例:让我们看看如何通过简单地更改修改的地方来解决这个异常。

// Java program to show
// ConcurrentModificationException

import java.util.Iterator;
import java.util.ArrayList;

public class modificationError {
    public static void main(String args[])
    {

        // Creating an object of ArrayList Object
        ArrayList<String> arr
            = new ArrayList<String>();

        arr.add("One");
        arr.add("Two");
        arr.add("Three");
        arr.add("Four");

        try {
            // Printing the elements
            System.out.println(
                "ArrayList: ");
            Iterator<String> iter
                = arr.iterator();

            while (iter.hasNext()) {
                System.out.print(iter.next()
                                 + ", ");
            }

            // No exception is raised as
            // a modification is done
            // after the iteration
            System.out.println(
                "\n\nTrying to add"
                + " an element in "
                + "between iteration: "
                + arr.add("Five"));

            // Printing the elements
            System.out.println(
                "\nUpdated ArrayList: ");
            iter = arr.iterator();

            while (iter.hasNext()) {
                System.out.print(iter.next()
                                 + ", ");
            }
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

输出:

ArrayList: 
One, Two, Three, Four, 

Trying to add an element in between iteration: true

Updated ArrayList: 
One, Two, Three, Four, Five,

从输出中可以清楚地看到，只要对代码进行最小的更改，就可以消除 ConcurrentModificationException。

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