The heap memory is garbage collected in Java.
Is the stack garbage collected as well?
How is stack memory reclaimed?
The memory on the stack contains method-parameters and local variables (to be precise: the references for objects and variables itself for primitive types). That will be automatically removed if you leave the method. If the variables are references (to objects) the objects itself are on the heap and handled by the garbage collector.
So the stack isn't garbage collected in the same way as the heap, but stack is a form of automatic memory-management in it's own (which predates garbage collection).
A more detailed answer is given by Thomas Pornin, look into that for more details.
The stack is not garbage collected in Java.
The stack allocated for a given method call is freed when the method returns. Since that's a very simple LIFO structure, there's no need for garbage collection.
One place where the stack and garbage collection interact is that references on the stack are GC roots (which means that they are the root references from which reachability is decided).
The stack could be garbage collected. However, in most JVM implementations, it is handled as, well, a "stack", which by definition precludes garbage collection.
What we call the stack is the accumulation of method activation contexts: for each invoked method, this is the conceptual structure which contains the method arguments, local variables, a hidden pointer to the context for the calling method, and a slot to save the instruction pointer. The activation context is not accessible as such from the Java language itself. A context becomes useless when the method exits (with a return or because of a thrown exception). It so happens that when a method A calls a method B, it is guaranteed that when A regains control, the context for B has become useless. This implies that the lifetime of the context for B is a subrange of the lifetime of the context for A. Therefore, activation contexts (for a given thread) can be allocated with a LIFO ("Last In, First Out") discipline. In simpler words, a stack: a new activation context is pushed on top of the stack of contexts, and the context on top will be the first to be disposed of.
In practice, the activation contexts (also called stack frames) are concatenated, in stack order, in a dedicated area. That area is obtained from the operating system when the thread is started, and the operating system gets it back when the thread terminates. The top of the stack is designated by a specific pointer, often contained in a CPU register (this depends on whether the JVM is interpreting or compiling code). The "pointer to caller's context" is virtual; the caller's context is necessarily located just below in stack order. The GC does not intervene: the area for the stack is created and reclaimed synchronously, from the thread activity itself. This is also how it works in many languages such as C, which do not have a GC at all.
Now nothing prevents a JVM implementation from doing otherwise, e.g. allocating activation contexts in the heap and having them collected by the GC. This is not usually done in Java Virtual Machines since stack allocation is faster. But some other languages need to do such things, most notably those which play with continuations while still using a GC (e.g. Scheme and its call-with-current-continuation function), because such games break the LIFO rule explained above.
The stack part of the memory works just like a "stack". I know it sounds bad, but that's exactly how it works. Data is added to the top, on top of each other (pushed onto the stack) and is automatically removed from the top (popped off the stack) as your program runs. It is not garbage collected - and it doesn't need to be since that memory is automatically reclaimed once data is popped off the stack. And when I say reclaimed I don't mean it gets de-allocated - it's just that the location in the stack memory where the next data will be stored is decreased, as data is popped off.
Of course that's not to say that you don't need to worry at all about the stack. If you run a recursive function many times it will eventually use up all the stack space. The same if you call many functions, especially if they have many parameters and/or local variables.
But the bottom line is that the memory of the stack is used and reclaimed as functions enter and leave scope - automatically. So at the end of your program's execution all the stack memory would be free and then released back to the operating system.
If you refer to the memory used on the stack, it is not garbage collected.
The java virtual machine uses explicit bytecode instructions to reserve and release memory on the stack, these instructions are generated by the compiler and manage the lifetime of primitives like int,boolean,double and object-references on the stack.
There have been plans to implement a so called tail call optimisation, which would remove some entries from the stack once it is known that they are no longer used, but I don't know any jvm which already supports this.
So no there is no garbage collection for the stack itself, only the compiler generated push and pop instructions to manage the memory use.
The stack itself is part of a thread. The stack is allocated when the thread object is created and garbage collected after the thread terminated and the thread object is no longer referenced.
All objects in Java are allocated on the heap. (At least as far as the spec goes, the actual implementation may allocate them on the stack if they transparently behave as if they were on the heap.)
Exactly what is collectible is a bit subtle. If the only reference to an object is in a single stack frame, and it can be shown that reference will not be used again, then the object may be collected. If the object is only used to read a field, then that field read may be optimised forward and the object collected earlier than you might expect.
This doesn't usually matter unless you are using finalisers (or presumably References). In that case you should be careful and use locks/volatile to enforce a happens-before relationship.
When threads stop, then typically the entire stack will be deallocated.
No one, data is pushed and popped from stack as you have inner variables in methods, during method calls, etc. You don't need to care about this.
Everything located on stack is treated as global roots by a garbage collector. So, yes, you definitely can say that stack is "garbage collected".
No. The stack is not garbage collected in Java.
Each thread has its own stack and contains :
Method specific values (which are short lived) and
References to the objects, created on heap, and are being referred to by the method
These values are pushed as stack frames to the stack for every method call. Since the stack follows 'Last-in First-out' order, at the end of every method call, each stack frame containing all the method specific data and the references to objects, if any, is popped out.
Hence, data in stack are automatically cleaned once the method/program goes out of scope.
Related
Are there JVMs out there, that create Objects on the stack?
Or JVMs that do not interact with Java Garbage Collection via Reference Counters etc?
Assuming we have a temporary Object created in a method.
And this Object's reference never gets passed/stored/accessed outside the method.
It is just used internally.
When following the classic approach of allocating objects (on the stack, along with reference counters), the following steps would have to be take care of:
Find a spot in the Heap that is large enough to hold the Object
Allocate the space
Update reference pointer
Register Object with garbage collection
[... object gets used, eventually discarded ...]
Identify for Garbage Collection
Remove from Heap
Unregister from GC
So if now a VM created Objects on the stack, the steps 1,3,4,6,7,8 would not be necessary, and step 2 and its 7ish counterpart would be easy stack management.
So are there JVMs that optimize this?
Or any hybrid systems, like allocating the Object in Heap, but not touching the normal GC, and instead direclty remove the Object at the end of its scope?
Are there implementations with multiple Heaps (one GC-supervised and the other stack-supervised)?
Kinda, there is project called valhalla that aims to provide value types to java, and it can be already download & used, but it is NOT ready for production usage (and if it will be ready, then it will be probably just merged to one of java official releases).
You can download the early access release. You can download the EA release from https://jdk.java.net/valhalla/ and page about feature itself: https://openjdk.java.net/jeps/169
Additional notes:
Java does not use reference counting, GC works by looking for root objects that are definitely used, like object from currently executed methods, and then finds any other objects that are referenced from these roots, and remove all the rest.
Also JIT perform escape analysis and can remove the need of allocating an object at all, instead will just use the stack to store data that would normally be stored in that object. (note that this is NOT stack allocation, as object is not even created). And thanks to inlining it can also do that cross-methods, but you can't control it or have any guarantee that it will happen.
If I create a stack in a java method using Deque<Integer> stack = new LinkedList<Integer>(); Why does this stack variable store in the heap but not in the stack?
Java Heap Space
Java Heap space is used by java runtime to allocate memory to Objects and JRE classes. Whenever we create any object, it’s always created in the Heap space.
Garbage Collection runs on the heap memory to free the memory used by objects that doesn’t have any reference. Any object created in the heap space has global access and can be referenced from anywhere of the application.
Java Stack Memory
Java Stack memory is used for execution of a thread. They contain method specific values that are short-lived and references to other objects in the heap that are getting referred from the method.
Stack memory is always referenced in LIFO (Last-In-First-Out) order. Whenever a method is invoked, a new block is created in the stack memory for the method to hold local primitive values and reference to other objects in the method.
As soon as method ends, the block becomes unused and become available for next method.
Stack memory size is very less compared to Heap memory.
So when you use Deque stack = new LinkedList<>(), Deque stack is stack according to data model, but according to JVM this is just another object, like new Object(), new ArraList<>() etc. And these objects are stored in the heap.
See details in Java Heap Space vs Stack – Memory Allocation in Java
Some languages like C allow you to choose whether to allocate memory on the stack or the heap. There are a number of attractive reasons why one might want to use the stack. Memory allocated on the stack is automatically freed when the stack frame is "popped" (the function returns). You also get better performance because the stack is a contiguous chunk of memory, whereas memory can be allocated anywhere on the heap. It's easier for the compiler to optimize.
However, like many aspects of C, here there be dragons. There is no possibility of thread safety or sharing data. Whatever you allocate on the stack necessarily can't survive beyond the life of the function call. Objects on the stack are freed when the function returns, so if you need to return an object from a function it must be allocated on the heap.
Stack allocation can be dangerous and requires a low-level understanding of your program's memory usage. These go against the design principles of Java, namely that you should be able to let the computer handle memory management for you. Therefore the language designers just decided to not let you do it. As Oleg already shared, in Java all objects are allocated on the heap.
Functions and variables are stored on stack while string and object references are stored on heap. Why is there a difference on how they are stored?
Some of the comments above offer links to the difference. The stack is temporary. Chunks of the stack are used as your code nests. Think of a big chunk of memory in which you keep track only of the very top of the memory. When you call a method, the system knows how much memory is required to remember where to return to when the method exits and enough space for the variables required for the method. The stack pointer then points higher up in memory, giving you all the space it just skipped over. When your method returns, the stack pointer is returned to where it was before your method was called. Any variables that were there are now gone.
It's not entirely that simple in a complex world like Java, but I still think of the stack in assembly language, which is where I first encountered it. (I'm old.) Close enough for this discussion.
The heap is different. The heap is managed memory with complex structures that keep track of the memory you've used. If you say new Foo(), Java knows how big a Foo is and it asks the heap for enough space to hold one. Much more complex things happen around managing that. But when your method returns, that object still exists. If it were allocated on the stack, there'd be real problems, because the stack unwinds when your method returns. But your memory in the heap is still allocated, and your object can continue to exist.
Again, it's not that simple, but maybe it makes sense.
Space on the stack exists only as long as your method is running. (I presume if you nest inside {}, it might allocate more space. I don't know.) Space on the heap persists until objects are freed, but that can be far longer than the duration of a method call.
I know that a retain cycle (at least in Objective-C and Swift) is when two objects claim ownership of one another (they have references to each other). And in Objective-C we can solve the issue by declaring one of them weak.
From what I have read and understood, the Java GC is not affected by retain cycles, and we do not have to worry about weak references. How does it solve it?
The Java (JVM) garbage collector works by looking for "reachable" objects - from the root(s) of the object tree. If they can't be reached (if they have no outside object references) then entire object graphs can be discarded.
Essentially it just just traverses the tree from root(s) to leaf nodes and marks all objects it encounters. Any memory not taken up by marked objects in the heap is swept (marked as free). This is called mark and sweep. img src
This can't be done easily in objective-c because it uses reference counting, not mark and sweep which has it's flaws
The reason there can be no retain cycles is because if they aren't linked to the "tree" anywhere, they aren't marked and can be discarded.
The garbage collector looks for reachable objects, starting from the roots (typically: variables on the call stack or global variables). So if two objects reference each other but are not otherwise reachable they won't be flagged as "live" and will be collected.
As the name suggests, Garbage Collection refers to removing of objects
which are no longer in use. It is a well known fact that irrespective
of their scope objects, Java stores objects in heap. Thus, if we keep
on creating objects without clearing the heap, our computers might run
out of heap space and we get ‘Out of Memory’ error. Garbage Collection
in Java is a mechanism which is controlled and executed by the Java
Virtual Machine (JVM) to release the heap space occupied by the
objects which are no more in use. In contrast to C++, garbage
collection in java relives the developer from the Memory Management
related activities. The JVM executes this process with the help of a
demon thread called the ‘Garbage Collector’. The garbage collector
thread first invokes the finalize method of the object. This performs
the cleanup activity on the said object. As a developer we cannot
force the JVM to run the garbage collector thread. Though there are
methods e.g Runtime.gc () or System.gc(), but none of these assures
the execution of garbage collector thread. These methods are used to
send garbage collection requests to the JVM. It is up to the Java
Virtual machine when it will initiate the garbage collection process.
Take a look at this stuff
How Garbage Collection works in Java
In basic terms, Garbage Collection works by walking the object graphs from a number of predefined roots. Anything not accessible from those roots is garbage, therefore one object referencing another is irrelevant unless either can be accessed from one or more roots.
It's all explained in more detail in How Garbage Collection Really Works.
The behavior of a tracing garbage collector may be viewed as analogous to that of a bowling alley pinsetter, which automatically sweeps up all pins that have been knocked over without disrupting pins that are still standing. Rather than trying to identify knocked-over pins, the pinsetter grabs all of the pins that are still standing, lifts them off the alley, and then runs a sweeper bar over the alley surface, removing wholesale any pins that might happen to be there without knowing or caring where they are.
A tracing GC works by visiting a certain set of "rooted" object references (which are regarded as always "reachable") and objects that are reachable via references held in reachable objects. The GC will mark such objects and protect their contents somehow. Once all such objects have been visited, the system will then visit some "special" objects (e.g. lists of weak or phantom references, or references to objects with finalizers) and others which are reachable from them but weren't reachable from ordinary rooted references, and then regard any storage which hasn't been guarded as eligible for reuse.
The system will need to specially treat objects that were reachable from special objects but weren't reachable from ordinary ones, but otherwise won't need to care about "ordinary" objects that become eligible for collection. If an object doesn't have a finalizer and isn't targeted by a weak or phantom reference, the GC may reuse its associated storage without ever bothering to look at any of it. There's no need for the GC to worry about the possibility that a group of objects that aren't reachable via any rooted references might hold references to each other because the GC wouldn't bother examining of those references even if they existed.
I see only disadvantage of this: you can get StackOverflow :) Why not use only Heap?
In Java, C, C++ the parameters to functions are passed on stack. The plain variables inside functions bodies are created in stack.
As I know the stack is limited per thread, has some default values, but relative low: 1-8 Mb.
Why not use the Heap instead of Stack. Both are in memory, just the OS make a separation from Address A to B is Heap and from C to D is Stack.
There are variable arguments. It says there are 10 variable of 4 byte each. If you read 11 than you maybe read some data a "memory" trash, and maybe exactly that you want for hacking or maybe you get a Segmentation fault ... if the OS detects you as bad boy. :) - So security can't be a reason for use Stack.
Performance is one of many reasons: memory in the stack is trivial to book-keep; it has no holes; it can be mapped directly into the cache; it is attached on a per-thread basis.
In contrast, memory in the heap is, well, a heap of stuff; it is more difficult to book-keep; it can have holes.
Check out this answer (excellent, in my opinion) explaining some other differences.
Others have already mentioned that the stack can be faster due to simplicity of incrementing/decrementing the stack pointer. This is, however, quite a ways from the whole story.
First of all, if you're using a garbage collector that compacts the heap (i.e., most modern collectors) allocation on the heap isn't much different from allocation on the stack. You simply keep a pointer to boundary between allocated and free memory, and to allocate some space, you just move that pointer, just like you would on the stack. Objects that will have extremely short lives (like the locals in most functions) cost next to nothing in a GC cycle too. Keeping a live object accessible takes (a little) work, but an object that's no longer accessible normally involves next to no work.
There is, however, often still a substantial advantage to using the stack for most variables. Many typical programs tend to run for fairly extended periods of time using nearly constant amounts of stack space. They enter one function, create some variables, use them for a while, pop them off the stack, then repeat the same cycle in another function.
This means most of the memory toward the top of the stack is almost always in the cache. Most function calls are re-using memory that was just vacated by the previous function call. By reusing the same memory continuously, you end up with considerably better cache usage.
By contrast, when you allocate items in the heap, you typically end up allocating separate space for nearly every item. You cache is in a constant state of "churn", throwing away the memory for objects you're no longer user to make space for newly allocated ones. Unless you use a minuscule heap, the chances of re-using an address while it's still in the cache are nearly nonexistent.
I'm sure this is answered a million times online, but...
Because you don't want every method call to be a memory allocation (slow). So, you pre-allocate your stack.
Some more reasons listed here (including security).
The answer is that you get holes when you allocate and de-allocate on the heap. This means that it gets more and more difficult to allocate memory since the places that are available are different sizes. The stack only reserves what is needed and gives it all back when you get out of scope. No hassle.
If everything was on the stack, each time you passed those values on, they would have to be copied. However, unlike the heap, it doesn't need to be cleverly managed - items on the heap require garbage collection.
So they work in two different ways that suit two different uses. The stack is a quick and lightweight home for values to be held for a short time whereas the heap allows you to pass objects around without copying them.
Neither stack nor heap is perfect for every scenario - that is why they both exist.
Using the heap requires "requesting" a bit of memory from the heap, using new or some similar function. Then, when it's finished, you delete the it again. This is very useful for variables that are long-lived and/or that take up quite a bit of space (or take up an "unknown at compile-time" space - for example if you read a string into a variable from a file, you don't necessarily know how much space it needs, and it's REALLY annoying to get a message from the program saying "String too large on line X in file Y").
On the other hand, the stack is "free" both when it comes to allocating and de-allocating (technically, any function that uses stack-space will need one extra instruction for the allocation of the stackspace, but compared to the several hundred or thousands that a call to new will involve, it's not noticeable). Of course, class objects will still have to have their respective constructors called, which may take almost any amount of time to complete, but that is true regardless of how/where the storage is allocated from.