Would the data in the following statement be stored as automatic memory allocation, or dynamic memory allocation or both
myFunction(new MyClass());
Thank You!
The terms “automatic memory allocation” and “dynamic memory allocation” make no sense in the context of Java. In Java, all memory is managed by the execution environment.
In other programming languages, the terms “automatic storage” and “dynamic storage” are used to distinguish between storage, which is automatically deallocated when going out of scope, and storage, which requires an explicit deallocation action performed by the application. In Java, there are no explicit deallocations at all. You will find that people and literature continue to distinguish between stack and heap, where only the latter contains objects, whose lifetime may exceed the execution of the method in which they were created. This, however, is only a logical separation, which might not reflect how a particular JVM implementation actually works.
The Java® Language Specification doesn’t mandate much details about the workings of this. There are only two spots at all:
15.12.4.5. Create Frame, Synchronize, Transfer Control
A method m in some class S has been identified as the one to be invoked.
Now a new activation frame is created, containing the target reference (if any) and the argument values (if any), as well as enough space for the local variables and stack for the method to be invoked and any other bookkeeping information that may be required by the implementation (stack pointer, program counter, reference to previous activation frame, and the like). If there is not sufficient memory available to create such an activation frame, a StackOverflowError is thrown.
17.4.1. Shared Variables
Memory that can be shared between threads is called shared memory or heap memory.
All instance fields, static fields, and array elements are stored in heap memory. In this chapter, we use the term variable to refer to both fields and array elements.
Local variables (§14.4), formal method parameters (§8.4.1), and exception handler parameters (§14.20) are never shared between threads and are unaffected by the memory model.
Note that this is the only place where the term “heap” is used as memory type and how it’s actually defined here…
The sections §15.9.4. Run-Time Evaluation of Class Instance Creation Expressions and §15.10.2. Run-Time Evaluation of Array Creation Expressions are much more vague, saying that “space is allocated” and an OutOfMemoryError is thrown if not enough space is available, and nothing more.
So if you go the route of distinguishing between stack and heap, you can say that your code myFunction(new MyClass()); will cause a heap allocation for an instance of MyClass, followed by a stack allocation for the activation frame of the actual implementation of the myFunction method. Not that it matters for any practical purpose.
If you want to dig more into the way, JVMs may implement it, you may refer to the Java® Virtual Machine Specification instead:
2.5.2. Java Virtual Machine Stacks
Each Java Virtual Machine thread has a private Java Virtual Machine stack, created at the same time as the thread. A Java Virtual Machine stack stores frames (§2.6). A Java Virtual Machine stack is analogous to the stack of a conventional language such as C: it holds local variables and partial results, and plays a part in method invocation and return. Because the Java Virtual Machine stack is never manipulated directly except to push and pop frames, frames may be heap allocated. The memory for a Java Virtual Machine stack does not need to be contiguous.
2.5.3. Heap
The Java Virtual Machine has a heap that is shared among all Java Virtual Machine threads. The heap is the run-time data area from which memory for all class instances and arrays is allocated.
The heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (known as a garbage collector); objects are never explicitly deallocated. The Java Virtual Machine assumes no particular type of automatic storage management system, and the storage management technique may be chosen according to the implementor's system requirements. The heap may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger heap becomes unnecessary. The memory for the heap does not need to be contiguous
Note, how these definitions only differ in their purpose while not having significant difference in their constraints, i.e. might be fixed-size or resizable and might be contiguous or not, not to speak of the explicit mentioning of the possibility to allocate stack frames on the heap.
There are three groups of memory in Java
Heap - this is where Objects are created and located when you call new MyClass();
Stack - The stack contains stack frames, and stack frames have space allocated for primitives and pointers to Objects in the Heap. Stack frames are allocated on method call and deallocated on method return
String constants: String literals defined at compile time will be added to a String constant pool. At runtime, it is possible to add strings to the pool using String.intern()
That's it
There are two things being allocated in your example;
The expression, new MyClass() allocates an instance of MyClass on the object heap.
The result of the new expression is an object reference. The object reference is saved in the activation record for a call to myFunction(). Activation records are allocated on the calling thread's call stack.
When you create a new object, it often allocated on the heap however with escape analysis, it can be unpacked onto the stack as if it was a local variable.
The only allocation is via new or a capturing lambda (or some native method in rare cases) There isn't any explicit distinction between different ways of allocating an object.
Related
If a variable is declared within a function in Java, will that variable be deallocated automatically upon that function's completion regardless of its type? Would the memory occupied by a primitive type, a non-primitive Object, and/or an array of either primitives or Objects be freed up once that variable's scope has been exited?
Primitive types in Java are allocated on the stack, so their memory is automatically deallocated when they go out of scope. Object references are primitives which are similarly managed, but the objects themselves are garbage collected. They will be removed automatically by the garbage collector, but it is not guaranteed how long that will take.
The JVM garbage collector automatically runs when the memory pressure becomes tight, so as long as there are no references to an object anymore, you can effectively make the assumption that its memory will be freed.
If there are no reference to the object, Garbage collector will remove it automatically. But we cant say when will that happen.
This article have a pretty good explanation about how garbage collector works.
in addition to Allexis King's Answer
there is one more technique in newer Java/ JVM which decides whether local object should be allocated in Stack or Heap. local objects allocated in stack by this technique will be de-allocated when it goes out of scope
Escape Analysis
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.
Escape analysis is supported and enabled by default in Java SE 6u23 and later.
If I create a static variable in Java, does it automatically go into the perm gen space on the heap? it seems obvious that the answer is yes, but i cannot find the confirmation anywhere.
I know the static variable (also strings and enums) are alive for the life of the JVM so it must go on the permanent heap. IS this correct?
The idea of the "PermGen" is completely implementation-dependent, and JVMs are free to handle the "physical" memory management however makes sense to them--they're not even actually required to provide garbage collection!
The PermGen is just a feature of some JVM implementations (including the Sun/Oracle HotSpot JVM for many years), and it's actually being eliminated with a new approach in the Oracle Java 8 JVM. It's quite likely that JVMs that include the concept of a PermGen will put static variables there for performance, but it's entirely up to the programmer.
JLS #17.4.1 Shared Variables
Memory that can be shared between threads is called shared memory or heap memory.
All instance fields, static fields and array elements are stored in heap memory. In this chapter, we use the term variable to refer to both fields and array elements. Local variables (§14.4), formal method parameters (§8.4.1) or exception handler parameters are never shared between threads and are unaffected by the memory model.
Nice description here By #Stephen:static allocation in java - heap, stack and permanent generation
Java heap only stores objects, and stack only stores primitive data and object reference.
Consider A.a = B.b , where A.a and B.b are int.
In my understanding, a JVM will first GET the value of A.a from the heap to the stack, and then PUT the value to B.b which is on the heap. It seems that the only way to change data on heap is to PUT the value from the stack.
My question is: is there some way to operate data on Java heap without stack? E.g., copy the value of A.a direct to B.b without stack operation.
If you say "it depends on the implementation of JVM", then my question is about Dalvik.
As far as the abstract machine called JVM (not to be confused with the various pieces of software which go by the same name and implement that abstract machine by mapping it onto real hardware) is concerned, A.a = B.b does indeed load the value of B.b on the stack, then stores it to A.a.1
However, as the name abstract machine may tell you, this is only a way of thinking about semantics. An implementation may do whatever it pleases, as long as it preserves the effect of the program. As you should know, most implementations don't actually interpret JVM instructions most of the time, but instead compile it to machine code for the CPU they run on. If you're concerned about performance or memory traffic, you need to go deeper.
When compiling, the JVM's stack is mostly discarded in favor of the registers most physical CPUs use. If there aren't enough registers available, the hardware stack (which is distinct from the JVM stack!) may also be used. But I digress. On most architectures, there's no instruction for moving from one memory location to the other (see Assembly: MOVing between two memory addresses). However, the hardware stack is also memory, so it's actually impossible to go heap -> stack -> heap. Instead, you'll find that the code loads the value from memory into a register, then stores it to memory from a register.
Finally, if the objects A and B are short-lived and aren't aliased, they may even be elided with their fields ending up on the stack or in registers. Then, this operation becomes even simpler (or may even be removed entirely if it has no effect).
1 These two steps actually take several JVM instructions each, but that's not important here.
When considering JIT, things get complicated. I think my question is actually about Java compiler, not JVM
If you are thinking of the javac you should assume that it does almost no optimisation and gives in byte code almost a literal translation of the code, which is very stack based.
In fact the byte code will be using the stack more than your example suggests. It does operations like
push B
getAndPushField b
push A
popAndSetField a
i.e. instead of one stack operation, there is notionally 3.
The JIT on the other hand might optimise this away so it is not even using a register for the value. Depending on the processor
// R3 contains A, R7 contains B,
// a starts at the 14th byte
// b start at the 16th byte
MOVI [R3+14], [R7+16]
It doesn't depend on the implementation of the JVM. It depends on the Java Virtual Machine Specification, which doesn't provide any other way than via the stack.
I am trying to fully understand how the Java works with its memory arrangement. The discussions on Internet are very confused, and sometimes contradicts each other, so I found no one I can trust. This thing can be very complicate if it mixed with static, static method, local variable, thread, volatile and so on. So I am thinking if there is a way I can study that myself my doing some Java coding experiment. A class MemoryTrack does something like this,
public myMethod(){
int i = 0;
MemoryTrack.show(new myClass()); //print out "Heap at address 111"
MemoryTrack.show(new myClass()); //print out "Heap at address 222"
MemoryTrack.show(i); //print out "Stack at address 333"
MemoryTrack.show("a static method"); //print out "stack at address 444"
}
The use of memory is described in Section 2.5 of the Java Virtual Machine Specification. The stack stores stack frames (containing local variables and partial results). The heap is where all class instances and arrays come from. Stack frames can also be allocated from the heap (and then pushed onto the stack). There are also method areas and runtime constant pool memory. The details are spelled out in the spec.
As far as monitoring memory usage, several profilers have tools for that. For monitoring from within the program, take a look at the MemoryMXBean class (and related classes in the java.lang.management package). It's very easy to use. While it probably doesn't give you everything that it sounds like you want, but it's probably the best thing available.
The rule is pretty simple: heap contains objects, stack contains local variables and method parameters. Object fields are inside the objects, in the heap. Don't know about static fields. Methods and constructors are not stored in the stack, nor in the heap. Threads and volatile don't matter.
Method calls are on the stack. Each has a space reserved to it, the stack frame, that contains the local variables and the parameters.
Two things you need to know
its not as complicated as you think.
it doesn't matter 99% of the time.
Variables are always on the stack.
Objects are always on the heap. (There are exceptions but I wouldn't worry about them)
Methods and classes are always in the perm gen.
In the HotSpot/OpenJDK JVM, static fields are collected in a singleton object for the class. You can see the instance if you do a heap dump. Other JVMs may do this differently.
A class MemoryTrack does something like this,
Such a method wouldn't do anything useful as the argument will always be on the stack and object it refers to will always be on the heap. You can't get the memory location of an Object in a standard way and it unlikely to be useful if it did as it can change at any time.
You may think all variables in Java are on heap. Actually it is implementation dependent.
Why do we use Heap Memory, Can we use Stack memory for the same?
EDITED
One more question came in my mind after reading answers
1) is there any other kind of memory which we can think of alternative to Heap and Stack?
Edited
I came across the string pool, is that memory associated with the heap or Stack?
Well, you have to use the heap if you want to use objects. Objects are inherently pointers on the stack (or inside other objects) to memory chunks in the heap.
Unlike C++ where you can put objects on the stack or heap, Java does things differently.
Even in C++, it's a good idea to use the heap for objects that must outlive the function they were created in. You probably can avoid it but you may find yourself with a performance problem with all the copy constructors.
As to your edit:
Is there any other kind of memory which we can think of alternative to Heap and Stack?
Sure there is: Static data members, the ones where there's only one per class rather than one per instantiated object must go somewhere. These can't be on the stack since they may disappear when you exit from a function. And they can't belong to an particular object.
These (at least in the Sun/Oracle JVM) go into the Method area.
In addition, you should not think of there being one stack. Every thread gets its own stack on creation.
There's also the runtime constant pool and stacks for native (non-Java) calls.
There's lots of information on the JVM internals here regarding that aspect but keep in mind there may be a distinction between the logical machine and the implementation.
Heap memory is central to Java. All objects are allocated on the heap, and local variables hold only references (essentially pointers) to them. C# allows you to have objects (value types) that live on the stack. Java chose not to borrow this from C++, partly in order to simplify the language.
Across languages, heap memory is the way to provide long-lived objects of arbitrary size.
Can we use Stack memory for the same?
Basically no.
In Java, stack memory is used solely for parameters and local variables of methods (and some hidden book-keeping info which is not relevant to this discussion). And for those variables, only primitive types use just stack memory. Any object or array is represented as a reference to something on the heap, and Java provides no way to allocate an object or array on the stack.
[Aside: it is theoretically possible to perform any calculation in Java using just primitive types and recursive static methods. However, it would be horribly messy. Even writing a simple message to the console will entail your application causing objects to be allocated on the heap. So we can dismiss this as being totally impractical.]
In the JVM, instead of a thread local stack for objects it uses a Thread Local Allocation Buffer or (TLAB) This gives much of the performance benifits of a Stack, without the developer needing to worry about wether an object is on the stack or not.
Heap memory is used to store objects in Java. No matter, where object is created in code.
e.g. as member variable, local variable or class variable, they are always created inside heap space in Java.
If there is no memory left in stack for storing function call or local variable, JVM will throw java.lang.StackOverFlowError,
While if there is no more heap space for creating object, JVM will throw
java.lang.OutOfMemoryError Java Heap Space.
Another important factor to consider is scoping. If all objects were to be allocated on stack then they go out of context as stack frame gets rolled out. In layman terms, think of stack as a dump that stores all the variable values and object references that are local to a subroutine/method which is currently in scope. As soon as it finishes executing (or the method goes out of scope), then all in it would be lost.
It makes it easier for compiler to manage large &/or dynamic sized variables too- they still take small constant storage on call stack- that of 4 bytes ( a heap pointer).