If you have that:
while(true) byte[] fillbuffer = new byte[400]; What will happen? Will create thousand of objects and each time just link the pointer of the fillbuffer with the new object's pointer? or something else?
Add a comment |
3 Answers
Yes, then the old objects will be garbage collected as needed since they are no longer referenced. You can roughly see this yourself if you hook up VisualVM or similar and watch the memory usage (consider adding a sleep).
As pointed out, to be technical the array is the only object. You are allocating 400 bytes, and one object, whose main job is to know where the 400 bytes are, each loop.
I'm not aware of any optimizations that are done to avoid the allocations, but in general compilers/virtual machines in any language have a lot of license to take shortcuts. "Logically" my answer explains what happens here, but YMMV (specifically, YMMV depending on how much of the JVM spec you have read.)
3 Comments
JamesB
byte is a primitive, not an Object. The only Object is the array.
Louis Wasserman
It may not even be necessary to eliminate the allocation: very short-lived objects are very fast to allocate and collect.
Has QUIT--Anony-Mousse
What about Escape Analysis in the Hotspot JVM?
If you are lucky, the Hotspot JVM will succeed at escape analysis and notice that the array never escapes the loop. It may then be optimized away altogether.
Escape analysis is a technique by which the Java Hotspot Server Compiler can analyze the scope of a new object's uses and decide whether to allocate it on the Java heap.
But at least for the first 1000 or so iterations - before optimization kicks in - it will likely allocate these, and eventually garbage collect them.
Congratulations, you have written an infinite loop.
answered Jan 14, 2015 at 0:40
Has QUIT--Anony-Mousse
77.8k14 gold badges146 silver badges198 bronze badges
Comments
What will happen?
Upon each iteration of the while loop, a new array of 400 bytes is allocated on the heap.
Will create thousand of objects and each time just link the pointer of the fillbuffer with the new object's pointer?
Yes, a new array object is created each time. Since the variable fillbuffer is in scope only within the body of the while loop, the referenced byte array becomes immediately available for garbage collection upon completion of each loop iteration.
Edit: Note
If you were to define fillbuffer outside the loop, then its value would not be immediately available for garbage collection upon completion of each loop iteration, but the old value would become available for garbage collection as soon as the variable was assigned a new value. I.e.
byte[] fillbuffer; while(true) fillbuffer = new byte[400]; 
1 Comment
user207421
That's actually not the reason. The reason is that the
fillbuffer variable takes on a new value each time around the loop, so there are no references to the old value. Java doesn't know when you hit an inner }.