Sébastien Dubois Sébastien Dubois - 10 months ago 52
Java Question

Why doesn't Java close() stream after a terminal operation is issued?

After reading https://www.airpair.com/java/posts/spring-streams-memory-efficiency, I am tempted to stream results out of a database, but as I discussed with a colleague (cfr. comment he added to that article), one needs to remember to use the try-with-resources construct to avoid any memory leaks.


  1. Why doesn't the Java 8 library take care of closing streams itself after each terminal operation (without having to wrap the stream instantiation in a try-with-resources)?

  2. If applicable, are there any plans for this functionality to be added to Java, or would it make sense to request it?


Answer Source

Because streams that require explicit resource release is actually a pretty unusual case. So we chose not to burden all stream execution with something that is only valuable for .01% of usages.

We made Stream Autocloseable so that you can release resources from the source if you want to, but this is where we stopped, and for a good reason.

Not only would doing this automagically burden the majority of users with extra work that they don't need, but this would also violate a general principle: he who allocates the resource is responsible for closing the resource. When you call

BufferedReader.lines().op().op()...

you are the one opening the resource, and you should close it. In fact, since closing a stream resulting from calling an accessor method on some resource-holding object will sometimes close the underlying object, you probably don't want the stream closing the BufferedReader for you -- you might want it to stay open after the call.

You're probably using streams in a particular way, so it probably seems "obvious" what streams should do -- but there are more use cases out there than yours. So rather than catering to specific use cases, we approached it from the general principle: if you opened the stream, and you want it closed, close it yourself, but if you didn't open it, it's not for you to close.