Java List的SubList使用问题

一、Sublist导致OOM

代码

@Slf4j
public class SubListDemo {

    public static void subListOOM() {
        List<List<Integer>> data = new ArrayList<>();
        for (int i = 0; i < 1000; i++) {
            List<Integer> rawList = IntStream.rangeClosed(1, 100000).boxed().collect(Collectors.toList());//构建一个100000个元素的list
            data.add(rawList.subList(0, 1));
        }
        log.info("data.size(): " +data.size());
    }
}

OOM

Exception in thread "File Watcher" java.lang.OutOfMemoryError: GC overhead limit exceeded
    at java.io.File.listFiles(File.java:1212)
    at org.springframework.boot.devtools.filewatch.FolderSnapshot.collectFiles(FolderSnapshot.java:63)
    at org.springframework.boot.devtools.filewatch.FolderSnapshot.collectFiles(FolderSnapshot.java:67)
    at org.springframework.boot.devtools.filewatch.FolderSnapshot.collectFiles(FolderSnapshot.java:67)
    at org.springframework.boot.devtools.filewatch.FolderSnapshot.collectFiles(FolderSnapshot.java:67)
    at org.springframework.boot.devtools.filewatch.FolderSnapshot.<init>(FolderSnapshot.java:58)
    at org.springframework.boot.devtools.filewatch.FileSystemWatcher$Watcher.getCurrentSnapshots(FileSystemWatcher.java:277)
    at org.springframework.boot.devtools.filewatch.FileSystemWatcher$Watcher.scan(FileSystemWatcher.java:251)
    at org.springframework.boot.devtools.filewatch.FileSystemWatcher$Watcher.run(FileSystemWatcher.java:236)
    at java.lang.Thread.run(Thread.java:748)
Exception in thread "restartedMain" java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49)
Caused by: java.lang.OutOfMemoryError: GC overhead limit exceeded
    at java.lang.Integer.valueOf(Integer.java:832)
    at java.util.stream.IntPipeline$$Lambda$492/1604894031.apply(Unknown Source)
    at java.util.stream.IntPipeline$4$1.accept(IntPipeline.java:250)
    at java.util.stream.Streams$RangeIntSpliterator.forEachRemaining(Streams.java:110)
    at java.util.Spliterator$OfInt.forEachRemaining(Spliterator.java:693)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
    at com.example.newdemo.SubListDemo.subListOOM(SubListDemo.java:17)
    at com.example.newdemo.NewdemoApplication.main(NewdemoApplication.java:13)

分析

出现 OOM 的原因是，循环中的 1000 个具有 10 万个元素的 List 始终得不到回收，因为它始终被 subList 方法返回的 List 强引用。

    public List<E> subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }

parent 字段就是原始的 List。SubList没有copy一份自己的数据，而是完整的保留了原始的list。 SubList 是原始 List 的视图，并不是独立的 List， SubList 强引用了原始的 List，所以大量保存这样的 SubList 会导致 OOM。

解决

不直接使用 subList 方法返回的 SubList，而是重新使用 new ArrayList，在构造方法传入 SubList，来构建一个独立的 ArrayList。sublist直接释放-》原始的list也被释放。

   public static void subListWithoutOOM() {
        List<List<Integer>> data = new ArrayList<>();
        for (int i = 0; i < 1000; i++) {
            List<Integer> rawList = IntStream.rangeClosed(1, 100000).boxed().collect(Collectors.toList());//构建一个100000个元素的list
            data.add(new ArrayList<>(rawList.subList(0, 1)));
        }
        log.info("data.size(): " +data.size());
    }

另外一个例子

   public static void removeSubList() {
        List<Integer> rawList = IntStream.rangeClosed(1, 10).boxed().collect(Collectors.toList());
        List<Integer> subList = rawList.subList(0, 3);
        subList.remove(0);
        rawList.forEach(System.out::print);
    }

2345678910

可以看到，移除sublist的元素后，直接影响到了原始list。

二、修改原始列表后SubList循环报错

测试代码

 public static void addItemToOriginalList() {
        List<Integer> rawList = IntStream.rangeClosed(1, 10).boxed().collect(Collectors.toList());
        List<Integer> subList = rawList.subList(0, 3);

        rawList.add(11);
        try {
            subList.forEach(System.out::print);
        }
        catch (Exception ex)
        {
            ex.printStackTrace();
        }
    }

java.util.ConcurrentModificationException
    at java.util.ArrayList$SubList.checkForComodification(ArrayList.java:1239)
    at java.util.ArrayList$SubList.listIterator(ArrayList.java:1099)
    at java.util.AbstractList.listIterator(AbstractList.java:299)
    at java.util.ArrayList$SubList.iterator(ArrayList.java:1095)
    at java.lang.Iterable.forEach(Iterable.java:74)
    at com.example.newdemo.SubListDemo.addItemToOriginalList(SubListDemo.java:46)
    at com.example.newdemo.NewdemoApplication.main(NewdemoApplication.java:13)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49)

分析

modCount 的字段，表示结构性修改的次数--影响list size修改测次数，add肯定影响size，导致modCount加1.但是sublist的modCount没有变，所有抛出了异常。

代码

 private void checkForComodification() {
            if (ArrayList.this.modCount != this.modCount)
                throw new ConcurrentModificationException();
        }