Java - java.lang.IllegalStateException : source already consumed or closed

Question

我有一些代码:

    Stream<String> previewImagesURLsList = uploadedVideoItemObj.getPreviewImagesURLsList().parallel();

    Stream<HashMap<String, Object>> imagesStream = previewImagesURLsList
            .map(new Function<String, HashMap<String, Object>>() {
                @Override
                public HashMap<String, Object> apply(String fileName) {
                    HashMap<String, Object> m = new HashMap<>();
                    m.put("preview_file", fileName);
                    m.put("parent_id", gotId);
                    return m;
                }
            });

    HashMap<String, Object>[] filesArr = imagesStream.toArray(HashMap[]::new);

我上线了

 HashMap<String, Object>[] filesArr = imagesStream.toArray(HashMap[]::new);

一个异常(exception)

java.lang.IllegalStateException: source already consumed or closed
    at java.util.stream.AbstractPipeline.sourceSpliterator(AbstractPipeline.java:455)
    at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:255)
    at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:438)
    at my.site.objects.videoitems.uploadedVideoItems.dao.impl.JdbcUploadedVideoItemDAO.insert(JdbcUploadedVideoItemDAO.java:79)
    at my.site.processors.files.threads.ServerUploadedFileProcessor.processFileServer(ServerUploadedFileProcessor.java:53)
    at my.site.processors.files.threads.ServerUploadedFileProcessor.call(ServerUploadedFileProcessor.java:28)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

我看不懂，哪里有bug。这个异常是什么意思，在什么情况下可以抛出？

Answer 1

似乎您的 getPreviewImagesURLsList() 正在返回一个 Stream，尽管它的名称。 Stream 只能使用一次，因此您必须确保每次调用 getPreviewImagesURLsList() 都会返回一个新的 Stream。

或者更好的是，让它返回 List，正如它的名字所暗示的那样，让调用者对其调用 stream()。

顺便说一下，使用 Stream 的通常方法是链接方法调用而不是将中间结果存储在变量中以避免错误地多次使用这些变量:

HashMap<String, Object>[] filesArr =
  uploadedVideoItemObj.getPreviewImagesURLsList().parallel()
  // or better if getPreviewImagesURLsList() was returning a List:
  // uploadedVideoItemObj.getPreviewImagesURLsList().parallelStream()
  .map(fileName -> {
    HashMap<String, Object> m = new HashMap<>();
    m.put("preview_file", fileName);
    m.put("parent_id", gotId);
    return m;
  }).toArray(HashMap[]::new);