为什么这段代码不触发 ConcurrentModificationException？

Question

我正在从多个线程修改同一个列表，它不应该触发吗迭代列表时出现 ConcurrentModificationException？

如何触发此异常？

public class ConcurrentTest {

    static List<String> list = setupList();

    public static List<String> setupList() {
        System.out.println("setup predefined list");

        List<String> l = new ArrayList();
        for(int i = 0; i < 50;i++) {
            l.add("test" + i);
        }

        return l;
    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(50);

        for(int i = 0; i < 50; i++) {
            executorService.submit( () -> {     
                list.add("key1");       

                Thread currentThread = Thread.currentThread();
                System.out.println( Thread.currentThread().getName() + ", " + list.size() );

                for(String val: list) {
                    try {
                        Thread.sleep(25);
                    }
                    catch(Exception e) {

                    }
                }

            });
        }

        executorService.shutdown();
    }
}

Answer 1

ConcurrentModificationException 当您在迭代列表时修改列表时触发。在你的代码中 qhen 你在列表上迭代时，你只休眠线程而不修改 it.This 会触发异常：

for(String s: list) {
    list.add("something");
}

Answer 2

你有没有检查下面的块是否抛出异常

try {
    list.add("key1");       

    Thread currentThread = Thread.currentThread();
    System.out.println( Thread.currentThread().getName() + ", " + list.size() );

    for(String val: list) {
        try {
            Thread.sleep(25);
        }
        catch(Exception e) {

        }
    }
}catch (Exception e) {
    e.printStackTrace();
}

请环绕 try...catch 块和 print() 错误消息以检查其是否抛出错误。

Answer 3

您的代码确实（并且可以）生成 ConcurrentModificationException。你不是抓住它来打印它。

从下面可以看出确实抛出了很多ConcurrentModificationException.

try {
    for (String val : list) {
        try {
          Thread.sleep(25);
        } catch (Exception e) {
        }
    }
} catch (Exception e) {
    e.printStackTrace();
}

注意： 来自 javadoc，

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw {@code ConcurrentModificationException} on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs

另见：java.util.ConcurrentModificationException not thrown when expected

或者，您可以获得返回的 Future 并将它们收集在列表中。通过这样做，您将在（至少）其中一个期货上调用 get 时得到异常。

为什么这段代码不触发 ConcurrentModificationException？

Why this code does not trigger ConcurrentModificationException?

java

java.util.concurrent

threadpoolexecutor