Understanding attributes in application scope being an instance

Question

I've two JSP files, where each one is accessed by different PCs.

In the first one, I'm creating an object of type Integer:

Integer var = 0;
application.setAttribute("name_var", var);

At the second one, I'm getting the attribute created before:

var = (Integer) application.getAttribute("name_var");

At this point everything is fine. I've my value at the second JSP, in a different PC.

Since Java is pass-by-value, where that values are references (Is Java "pass-by-reference" or "pass-by-value"?), I guess I've the same instance in both the JSP files (being visited by different PCs).

So now, When I'm modifying my var value, and It doesn't appear modified in the other side, I don't understand why.

var++;
Output (in the JSP where the attribute was set): 0 (still)

Can anyone explain me what is happening? Have I to update the object with a setAttribute each time I modify its value?

Thank you in advance

gknicker · Answer 1 · 2015-03-13T18:10:59.777

1

Instead of just var++, you need to set the changed value back to the application attribute:

application.setAttribute("name_var", ++var);

The reason is autoboxing and immutability of Integer objects. When you say var++, java unboxes the Integer into a primitive int and increments that. But the result doesn't go into the attribute.

edited Mar 13 '15 at 18:10

answered Mar 13 '15 at 18:04

gknicker

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Something weird happened here. – Sotirios Delimanolis Mar 13 '15 at 18:09
no kidding - why does it show that I answered twice? – gknicker Mar 13 '15 at 18:10
2

@gknicker StackOverflow probably stored the answer count as an Integer in the application, and thought incrementing it would change the application attribute :) – JB Nizet Mar 13 '15 at 18:12

score 1 · Accepted Answer · answered Mar 13 '15 at 18:06

var++ doesn't change the internal state of the Integer instance stored in the application. An Integer is immutable. Its state never changes.

The operation is equivalent to

int tmp = var.intValue();
tmp = tmp + 1;
var = Integer.valueOf(i);

So var now references another Integer instance than the one stored in the application.

The alternative is to store the new value in the application again, or to store a thread-safe, mutable value in the application:

AtomicInteger var = new AtomicInteger(0);
application.setAttribute("name_var", var);
...
var.incrementAndGet();

Thanks for the explanation, and for the AtomicInteger class. It'll be quite usefull =) — Btc Sources, Mar 13 '15 at 18:18

Understanding attributes in application scope being an instance

2 Answers2