Is it safe to share a stream instance among multiple threads?

Question

I have a set of keys.

class X { private static String[] keys = {"k1", "k2", ... };

I have to extract values for the keys from a request. I think I can use map to extract values and create a list necessary objects in some request processing method like this:

 public void processReq(Request req) { ... Stream.of(keys).map(k-> new Pack(k, req.getHeader(k)));

But creating Stream per every request looks unnecessary task. If sharing Stream instance among multiple threads is safe, I think I can modify the code like this:

class X { private static Stream<String> keys = Stream.of("k1", "k2", ...); ... public void processReq(Request req) { ... keys..map(k-> new Pack(k, req.getHeader(k)));

So, is sharing Stream instance among multiple threads like this safe?

And creating a Stream over an array doesn't take any time. What takes time is to traverse it. — JB Nizet
– JB Nizet, Commented Apr 6, 2016 at 16:29

Peter Lawrey · Accepted Answer · 2016-04-06 16:39:39Z

Streams are not intended to be used more than once, even in the same thread. If you want to have a collection, use a List (or an array)

 private static final List<String> keys = Arrays.asList("k1", "k2", ...);

This can be used multiple times.

List<Pack> packs = keys.stream() .map(k-> new Pack(k, req.getHeader(k))) .collect(Collectors.toList());

In your code, the new Pack or req.getHeader is where most of the time is spent.

Jan Nielsen · Accepted Answer · 2016-04-06 16:34:23Z

No; it's not generally safe. "Unless the source was explicitly designed for concurrent modification, unpredictable or erroneous behavior may result from modifying the stream source while it is being queried... A stream should be operated on only once."

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