In C and C++ I know that there could be a huge difference in performance between instantiating objects on the stack vs. using 'new' to create them on the heap.
Is this the same in Java?
The 'new' operator in Java is very convenient (especially when I don't have to remember freeing/deleting the objects created with 'new'), but does this mean that I can go wild with 'new'?
Erm, there is no other way in java to instantiate an object.
All objects are created with new, and all objects are created on the heap.
in Java, when you say
MyObject foo;
You're simply declaring a variable (reference). It isn't instantiated until you say
foo = new MyObject();
When all references to that object are out of scope, the object becomes elegible for garbage collection. You'll note there's no such thing as delete in java :)
There is no allocation of objects on the stack in Java.
Only local variables (and parameters) can live on the stack and those can only contain references or primitive values, but never objects.
You can't create objects on the stack, you can only have primitives and references on the stack, so the question doesn't apply to Java.
There have been attempts to use escape analysis to optimise objects which are short lived (and possibly put them on the stack instead) however I haven't seen any evidence this improved performance.
Part of the reason there isn't the same performance hit/benifit as there would be in C/C++ is that Java has thread local allocation on the heap and objects are not recycled as agressively. C/C++ has thread local stacks, but you need additional libraires to support multi-thread object allocation. Objects are recycled more aggresively which increases the cost of object allocation.
One of the biggest changes coming from C/C++ world is to find that Java has far less features, but tries to do make the most of them (There is alot of complex optimisation going on in the JVM) On the other hand Java has a rich/baffling array of open sources libraries.
Repeat after me: there is no allocation of objects on the stack in Java
In Java, unlike C++, all objects are allocated on the heap, and the only way out is when they are garbage collected.
In Java, unlike C++, the variable falling out of scope does not mean that the destructor of the object runs; in fact, there is no destructor. So the variable might fall out of scope, but the object remains alive on the heap.
Can I go wild with 'new'?
Yes. First, because it's the only way to instantiate an object. Second, because the JVM is so good it can create up to 2^32 ightweight objects in less than a second.
In Java, there is no way to manually allocate objects on the Stack, though the compiler may decide to allocate objects created with 'new' on the stack, see Java theory and practice: Urban performance legends, revisited.
There's really nothing to compare here: you can't create objects on the stack in Java.
If it's any comfort, however, heap-based allocation in Java is (at least usually) quite fast. Java's garbage collector periodically "cleans up" the heap, so it basically looks a lot like a stack, and allocating from it is a lot like allocating from a stack as well -- in a typical case, you have a pointer to the beginning (or end) of the free memory area, and allocating a chunk of memory simply means adding (or subtracting) the amount from that pointer, and returning the address of the beginning (then, of course, constructing an object (or objects) in that area, etc.)
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.
Quoting Wikipedia 'A heap is a useful data structure when you need to remove the object with the highest (or lowest) priority'.
I am familiar with what a heap is and the kind of problems I can solve with them, but I was wondering why this data structure is the one used for the allocation of Objects in Java? Also, what determines the priority of an Object?
The quoted text is referring to a kind of data structure called a heap.
The word heap is also used for a form of dynamic memory management.
This is a case where one IT English word has taken on two different and independent meanings. (This is a fairly common phenomenon in normal English ...)
I was wondering why this data structure is the one used for the allocation of Objects in Java?
Simply, it isn't. A dynamic memory heap (such as the Java heap) is not organized using a heap data structure.
In fact, the Java heap isn't really a data structure at all. Rather it is an area of memory in which objects are allocated. Space is reclaimed by tracing the reachable objects, and then deleting the remaining objects and consolidating the remaining space.
By contrast, a C or C++ heap cannot be traced and consolidated (because there is insufficient reliable type information to allow pointers to be identified unambiguously). Therefore a C / C++ heap will include a data structure to organize the free space. However, this isn't a heap data structure in the sense of the quoted text. Typically it is an array of lists of "nodes" of the same size.
I will explain that with a reference to C++.
You got local variables that get created on the stack when initializing the variable and destroyed when leaving the block. Basically that means that every local variable lives inside the stack frame of the block. Hence, dies the block, dies the variable.
If you don't know in advance how big your object is going to be, you have to allocate memory on the heap. An example would be a dynamically resizable array. In C++ this is done with the "new" operator (or malloc, calloc, realloc etc.). In Java you are doing this with the "new" operator too. That means you are responsible for creating and releasing the memory.
Objects on the heap don't just get destroyed when you leave a block. Except you define it in your main function and the program exits after that.
In C++ you either call delete or free() to free the created memory of your heap object. In Java on the other hand, the garbage collector does this for you. It is doing that by basically keeping a reference count to the instance (of course its a bit more complicated than that).
In C++, dynamically allocated array has to be deleted unless it is lost in the memory. In java, do we have to do the same, and if so. How do you do that
In the Java programming language, dynamic allocation of objects is achieved using the new operator. An object once created uses some memory and the memory remains allocated till there are references for the use of the object. When there are no references for an object, it is assumed to be no longer needed and the memory occupied by the object can be reclaimed.There is no explicit need to destroy an object as java handles the de-allocation automatically. The technique that accomplishes this is known as Garbage Collection.Programs that do not de-allocate memory can eventually crash when there is no memory left in the system to allocate. These programs are said to have memory leaks.
In Java,Garbage collection happens automatically during the lifetime of a java program, eliminating the need to de-allocate memory and avoiding memory leaks.
In C language, it is the programmer’s responsibility to de-allocate memory allocated dynamically using free() function.
Read more at http://www.javatutorialhub.com/java-garbage-collection.html#wpewoJfMWffgXd8O.99
I don't understand your definition of dynamic array. Do you mean a collection of items where its length is unknown initialiazing it? If yes, you're talking about collections such as an ArrayList.
You don't have to worry about the deletion of your objects. The JVM will take care of your objects when they're not used anymore or when they're out of scope.
The JVM analyses the execution of your code and it will invoke a special component, named Garbage Collector, that it will clean the execution code from useless/out of scope objects.
For example, when you have this code:
if(condition)
{
string myStr = "test";
//other code here
}
The variable myStr, because it's not used anymore (it's out of scope of the if statement), will be marked by the JVM to be garbage collected.
The developer need not worry about deallocating the memory in java. The Java Garbage Collector do that for you. You can read how it works here
You create a variable to store a value that you can refer to that variable in the future. I've heard that you must set a variable to 'null' once you're done using it so the garbage collector can get to it (if it's a field var).
If I were to have a variable that I won't be referring to agaon, would removing the reference/value vars I'm using (and just using the numbers when needed) save memory? For example:
int number = 5;
public void method() {
System.out.println(number);
}
Would that take more space than just plugging '5' into the println method?
I have a few integers that I don't refer to in my code ever again (game loop), but I've seen others use reference vars on things that really didn't need them. Been looking into memory management, so please let me know, along with any other advice you have to offer about managing memory
I've heard that you must set a variable to 'null' once you're done using it so the garbage collector can get to it (if it's a field var).
This is very rarely a good idea. You only need to do this if the variable is a reference to an object which is going to live much longer than the object it refers to.
Say you have an instance of Class A and it has a reference to an instance of Class B. Class B is very large and you don't need it for very long (a pretty rare situation) You might null out the reference to class B to allow it to be collected.
A better way to handle objects which don't live very long is to hold them in local variables. These are naturally cleaned up when they drop out of scope.
If I were to have a variable that I won't be referring to agaon, would removing the reference vars I'm using (and just using the numbers when needed) save memory?
You don't free the memory for a primitive until the object which contains it is cleaned up by the GC.
Would that take more space than just plugging '5' into the println method?
The JIT is smart enough to turn fields which don't change into constants.
Been looking into memory management, so please let me know, along with any other advice you have to offer about managing memory
Use a memory profiler instead of chasing down 4 bytes of memory. Something like 4 million bytes might be worth chasing if you have a smart phone. If you have a PC, I wouldn't both with 4 million bytes.
In your example number is a primitive, so will be stored as a value.
If you want to use a reference then you should use one of the wrapper types (e.g. Integer)
So notice variables are on the stack, the values they refer to are on the heap. So having variables is not too bad but yes they do create references to other entities. However in the simple case you describe it's not really any consequence. If it is never read again and within a contained scope, the compiler will probably strip it out before runtime. Even if it didn't the garbage collector will be able to safely remove it after the stack squashes. If you are running into issues where you have too many stack variables, it's usually because you have really deep stacks. The amount of stack space needed per thread is a better place to adjust than to make your code unreadable. The setting to null is also no longer needed
It's really a matter of opinion. In your example, System.out.println(5) would be slightly more efficient, as you only refer to the number once and never change it. As was said in a comment, int is a primitive type and not a reference - thus it doesn't take up much space. However, you might want to set actual reference variables to null only if they are used in a very complicated method. All local reference variables are garbage collected when the method they are declared in returns.
Well, the JVM memory model works something like this: values are stored on one pile of memory stack and objects are stored on another pile of memory called the heap. The garbage collector looks for garbage by looking at a list of objects you've made and seeing which ones aren't pointed at by anything. This is where setting an object to null comes in; all nonprimitive (think of classes) variables are really references that point to the object on the stack, so by setting the reference you have to null the garbage collector can see that there's nothing else pointing at the object and it can decide to garbage collect it. All Java objects are stored on the heap so they can be seen and collected by the garbage collector.
Nonprimitive (ints, chars, doubles, those sort of things) values, however, aren't stored on the heap. They're created and stored temporarily as they're needed and there's not much you can do there, but thankfully the compilers nowadays are really efficient and will avoid needed to store them on the JVM stack unless they absolutely need to.
On a bytecode level, that's basically how it works. The JVM is based on a stack-based machine, with a couple instructions to create allocate objects on the heap as well, and a ton of instructions to manipulate, push and pop values, off the stack. Local variables are stored on the stack, allocated variables on the heap.* These are the heap and the stack I'm referring to above. Here's a pretty good starting point if you want to get into the nitty gritty details.
In the resulting compiled code, there's a bit of leeway in terms of implementing the heap and stack. Allocation's implemented as allocation, there's really not a way around doing so. Thus the virtual machine heap becomes an actual heap, and allocations in the bytecode are allocations in actual memory. But you can get around using a stack to some extent, since instead of storing the values on a stack (and accessing a ton of memory), you can stored them on registers on the CPU which can be up to a hundred times (maybe even a thousand) faster than storing it on memory. But there's cases where this isn't possible (look up register spilling for one example of when this may happen), and using a stack to implement a stack kind of makes a lot of sense.
And quite frankly in your case a few integers probably won't matter. The compiler will probably optimize them out by itself in this case anyways. Optimization should always happen after you get it running and notice it's a tad slower than you'd prefer it to be. Worry about making simple, elegant, working code first then later make it fast (and hopefully) simple, elegant, working code.
Java's actually very nicely made so that you shouldn't have to worry about nulling variables very often. Whenever you stop needing to use something, it will usually incidentally be disappearing from the scope of your program (and thus becoming eligible for garbage collection). So I guess the real lesson here is to use local variables as often as you can.
*There's also a constant pool, a local variable pool, and a couple other things in memory but you have close to no control over the size of those things and I want to keep this fairly simple.
In java Can objects be created with both static memory allocation and dynamic memory allocation?
If by static memory, you mean on the stack, no, all objects are allocated on the heap. Only primitives are allocated on the stack.
Edit: I'm still not sure if by dynamic and static you mean heap and stack, respectively, but that is usually where the question comes from for people with a C/C++ background, because those languages give the developer control over that.
In Java, when you do a typical:
Object o = new Object();
That will allocate memory on the heap. If inside a method you do:
int i = 1;
Then that int is allocated on the stack (if it is a field in a class, then it will be allocated on the heap).
All instance memory (by calling new) is allocated on the heap, all parameters are allocated on the stack. But java (non primitive) paramters are all passed by reference (excepty primitives).
'Static' doesn't mean 'on the stack'.
Objects allocated in the initialisation of class-static variables, or in static code blocks, are statically allocated, in the sense that allocation is done at class-load time (which can be made to happen statically immediately after program startup).
You could, in theory, write a java program using only such allocations, and it would be statically allocated, the same as a C program that never called malloc, just had fixed buffers for the stuff it wanted to do.
If such a program successfully starts up, that proves there is enough memory available for everything it can do, and so it will never get an out of memory error, fragmentation problem, or GC pause.
It will just, if correctly written, return a lot of error messages saying 'I can't do that'.
The answers claiming that non-primitives are always allocated on the heap are dead wrong.
JVMs can do escape analysis to determine whether objects will always be confined to a single thread and that the object's lifetime is bounded by the lifetime of a given stack frame. If it can determine that an object can be allocated on the stack, a JVM may allocate it there.
See this article for details.