序列化数组列表时 SynchronizedCollection 失败

Question

这是错误堆栈，

java.util.ConcurrentModificationException
    at java.util.ArrayList.writeObject(ArrayList.java:766)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:977)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1545)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1481)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1227)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1597)
    at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:456)
    at java.util.Collections$SynchronizedCollection.writeObject(Collections.java:2125) // 1
    at java.lang.reflect.Method.invoke(Native Method)
    at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:977)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1545)

我的 ArrayList 由 SynchronizedCollection 包装。 并且从错误栈判断ArrayList的writeObject方法是在scope的SynchronizedCollection.writeObject()方法内部调用的，后面是同步的。 似乎其他线程更改了 ArrayList，即使所有操作都由synchronized包装。 所以，我想知道为什么其他线程可以更改 ArrayList？

该问题源于一个Android APP。 所以我写了一个 java 程序来重现这个问题，

public class MultiThreadSerialTest2 {

    private static final int TOTAL_TEST_LOOP = 100;
    private static final int TOTAL_THREAD_COUNT = 20;

    private static volatile int writeTaskNo = 0;

    private static final List<String> list = Collections.synchronizedList(new ArrayList<>());

    private static final Executor executor = Executors.newFixedThreadPool(TOTAL_THREAD_COUNT);

    public static void main(String...args) throws IOException {
        for (int i = 0; i < TOTAL_TEST_LOOP; i++) {
            executor.execute(new WriteListTask());
            for (int j=0; j<TOTAL_THREAD_COUNT-1; j++) {
                executor.execute(new ChangeListTask());
            }
        }
    }

    private static final class ChangeListTask implements Runnable {

        @Override
        public void run() {
            list.add("hello");
            System.out.println("change list job done");
        }
    }

    private static final class WriteListTask implements Runnable {

        @Override
        public void run() {
            File file = new File("temp");
            OutputStream os = null;
            ObjectOutputStream oos = null;
            try {
                os = new FileOutputStream(file);
                oos = new ObjectOutputStream(os);
                oos.writeObject(list);
                oos.flush();
                os.flush();
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                try {
                    oos.close();
                    os.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(String.format("write [%d] list job done", ++writeTaskNo));
        }
    }
}

这是示例代码的错误堆栈跟踪，

Exception in thread "pool-1-thread-88" java.util.ConcurrentModificationException
    at java.base/java.util.ArrayList.writeObject(ArrayList.java:901)
    at java.base/jdk.internal.reflect.GeneratedMethodAccessor4.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at java.base/java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1145)
    at java.base/java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1497)
    at java.base/java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1433)
    at java.base/java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1179)
    at java.base/java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1553)
    at java.base/java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:442)
    at java.base/java.util.Collections$SynchronizedCollection.writeObject(Collections.java:2086)
    at java.base/jdk.internal.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at java.base/java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1145)
    at java.base/java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1497)
    at java.base/java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1433)
    at java.base/java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1179)
    at java.base/java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:349)
    at me.shouheng.thread.MultiThreadSerialTest2$WriteListTask.run(MultiThreadSerialTest2.java:49)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:829)

Answer 1

这是令人惊讶的行为，并且似乎是一个 JDK 错误，从 OpenJDK 19.0.2 开始仍未修复。

https://bugs.openjdk.org/browse/JDK-8208779

出现此问题是因为Collections.synchronizedList(new ArrayList<>()返回的 object 是SynchronizedRandomAccessList的实例。出于向后兼容的原因，此 class 实现了一个writeReplace方法，该方法在写入之前替换SynchronizedList的实例。然后，它是writeObject替换列表中调用以生成序列化数据的方法。不幸的是，此替换列表中使用的互斥锁与原始列表中使用的互斥锁不同，因此在writeObject期间持有的锁与在add期间持有的锁不同。这这就是为什么并发修改是可能的。

作为解决方法，您可以显式同步列表上的writeObject调用：

synchronized (list) {
    oos.writeObject(list);
}

这样就避免了错误。