JDK-4513622: keeping a substring of a field prevents GC for object | Hacker News Reader

JDK-4513622: keeping a substring of a field prevents GC for object

bugs.java.com

14 pointsluu12 comments

This needs a 2001 tag, particular since it is no longer the case as others have pointed out.

It is probably the case in other languages though (I believe — though I'm open to being wrong — it is in Go, string slicing is implemented through array slicing, so a substring will share the parent string's storage and keep it alive).

A friend of mine was actually asked this in an interview (how does substring works). The initial idea of taking an advantage of the immutability of an object was nice, but it turns out to be a big GC issue if your original string is extremely large.

It is one of those cases where you can actually leak memory with a GC. But at least Java gives (gave?) you a choice: to create a new string a use the current one, in .NET substring always returns a new string.

This is why I find rust intriguing. It tracks the lifetimes using the type system, so it would be obvious to the programmer that the large strings would need to live as long as the list holding the small substrings.

I'm not a rust expert, but I believe that it's basically impossible to write something like the code in the bug report without getting a compile time error in rust. You'd either need to use a version of substring that makes a copy, or explicitly keep the large strings around.

Yeah, in Rust you'd have to go out of your way to use Rc to reference count the large strings.

This is really old, they changed the way it worked to be a copy from jdk5 to jdk6 for exactly this reason.

fixed in 6 (and backported in 5, 4) see http://bugs.java.com/view_bug.do?bug_id=6924259

The view technique was actually great. You could write performant parsing and matching code without having to keep track of positions in the string. The current situation is much worse. You get crazy amounts of allocation and GC for code that used to produce almost none.

Even better - installing java on the mac install the ask toolbar that actually adds garbage into my search results, let alone garbage collection. Howzabout then?

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