Heap Memory¶

1. Heap Memory¶

"Heap" memory, also known as "dynamic" memory, is an alternative to local stack memory. Local memory is quite automatic. Local variables are allocated automatically when a function is called, and they are deallocated automatically when the function exits. Heap memory is different in every way. The programmer explicitly requests that space be allocated, using the new operator in Java or C++. This memory "block" will be of a particular size, ususally determined automatically from the size of the object being created. That memory block (your object) continues to be allocated until something happens that makes it go away. In some languages (noteably, C and C++), an object in heap memory only goes away when the programmer explicitly requests that it be deallocated. So the programmer has much greater control of memory, but with greater responsibility since the memory must now be actively managed. Dropping all references to a memory location without deallocating it is a signficant source of errors in C/C++, and this is so common that it has a name: memory leak. (In fact, many commercial programs implemented in C++ have memory leaks, that will eventually make them crash after being used for a long time.) Java removes this source of errors by handling memory deallocation automatically, using garbage collection. The downside is that garbage collection is a slow process that happens at unpredictable times.

The advantages of heap memory are:

Lifetime. Because the programmer now controls exactly when memory is allocated, it is possible to build a data structure in memory, and return that data structure to the caller. This was never possible with local memory, which was automatically deallocated when the function exited.
Size. The size of allocated memory can be controlled with more detail. For example, a string buffer can be allocated at run-time that is exactly the right size to hold a particular string. With local memory, the code is more likely to declare a buffer of size 1000 and hope for the best. (See the StringCopy() example below.)

The disadvantages of heap memory are:

More Work. Heap allocation needs to arranged for explicitly in the code, which is just more work.
More Bugs. Because it's now done explicitly in the code, on occasion the allocation will be done incorrectly leading to memory bugs. Local memory is constrained, but at least it's never wrong.

Nonetheless, there are many problems that can only be solved with heap memory, so that's the way it has to be.

The following are some important points about Garbage Collection.

Garbage collection is a mechanism that is invoked by the Java Virtual Machine to get ride of the unused heap memory objects. It removes every object that is not being used anymore by the running Java program. The result of this process is freeing up the unused memory so other new objects can use that piece of memory.
In other programming languages like C and C++, it is the responsibility of the programmer to take care of freeing up the memory from unused objects and arrays. Doing so consumes programmer's time and increases code complexity. On the other hand, garbage collection makes programming easier by taking care of the memory management for the programmer.
Before removing an object from memory, garbage collection invokes the finalize() method of that object and gives an opportunity to perform any sort of cleanup required. If the programmer does not override this method, the default finializer method will be invoked (the method defined in the Object class).
The Java Virtual Machine invokes garbage collection based on the size of the dynamically allocated memory from the heap. Garbage collection is slow, and hard to predict. This can be a problem for programs that have real-time performance constraints.

The following are some cases that make an object subject to be removed from heap memory by garbage collection:

When the programmer changes all references to an object to something else.

If the object is defined inside a block of code, and all references to that object are local. When excecution of that block is complete, the local variables are destroyed automatically, leaving the object in heap memory without any references. (So this is really a special case of rule (1).) Here is an example

void test(boolean found)
{
  if (found)
  {
    // employee1 is a variable local to this if statement
    // but the Employee object is in heap memory.
    Employee employee1 = new Employee("John", 1000);
    // We can do things here with employee1, and its associated object
    Printout(employee1.name());
  }

  // At this point, the only reference to the created object is out of scope
  // since employee1 was local to the if statement block.
  // So the Employee object will be eligible for garbage collection.
}

Assume an object A contains a reference to another object B, and A contains the only such reference to B. Object B will be eligible for garbage collection if A is also eligible for garbage collection. Here is an example.

// Date class to represent dates
class Date {
  int day;
  int month;
  int year;

  // Date constructor
  public Date(int d, int m, int y)
  {
    day = d; month = m; year = y;
  }
}

// An Employee class that includes a Date
class Employee {
  String name;
  Date dateOfBirth;

  // Employee constructor to initialize both name and date
  public Employee(String name , int d, int m, int y)
  {
    this.name = name;
    dateOfBirth = new Date(d, m, y);
  }
}

// Now let's test this code
class Test {

  public static void main() {
    Employee empPtr = new Employee("Sam", 3, 9, 1983);
    // Now empPtr references an object that contains a reference to a Date object

    empPtr = null; // Now the Employee object is eligible for garbage collection.
    // So its Date object will also be eligible for garbage collection
    // because nothing else points to it.
  }
}

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Before seeing the exact details, let's look at a rough example of allocation and deallocation in the heap.

1.1. Allocation¶

The heap is a large area of memory available for use by the program. The program can request areas, or "blocks", of memory for its use within the heap. In order to allocate a block of some size, the program makes an explicit request by calling the heap allocation operation. In Java or C++, this is the new operator. The allocation function reserves a block of memory of the requested size in the heap (usually, the size of the object that you want) and returns a refernece to it. Suppose a program makes three allocation requests to allocate memory to hold three separate GIF images in the heap, each of which takes 1024 bytes of memory. After the three allocation requests, memory might look like.

Each allocation request reserves a contiguous area of the requested size in the heap and returns a reference to that new block to the program. Since each block is always referred to by a reference, the block always plays the role of a "pointee" (Section 1) and the program always manipulates its heap blocks through references. The heap block references are sometimes known as "base address" pointers since by convention they point to the base (lowest address byte) of the block. In this example, the three blocks have been allocated contiguously starting at the bottom of the heap, and each block is 1024 bytes in size as requested. In reality, the heap manager can allocate the blocks wherever it wants in the heap so long as the blocks do not overlap and they are at least the requested size. At any particular moment, some areas in the heap have been allocated to the program, and so are "in use". Other areas have yet to be committed and so are "free" and are available to satisfy allocation requests. The heap manager has its own, private data structures to record what areas of the heap are committed to what purpose at any moment. The heap manager satisfies each allocation request from the pool of free memory and updates its private data structures to record which areas of the heap are in use.

1.2. Deallocation¶

When the program is finished using a block of memory, in some languages the block must be explicitly deallocated. In such cases, the block will be marked unused. In Java, typically space is "made available" by not having any references to it. This allows Java garbage collection to know that this area must be cleaned. Garbage collection will implicitly free up the unused memory blocks in the heap. The heap manager updates its private data structures to show that the area of memory occupied by the block is free again and so may be re-used to satisfy future allocation requests. Here's what the heap would look like if the garbage collection deallocates the second of the three blocks.

After deallocation, the reference continues to point to the now deallocated block. The program can no longer reach the deallocated pointee. In a language (like C++) with explicit memory deallocation and no garbage collection, the programmer must make sure that he or she does not try to follow the old reference to the deallocated block. This is why the pointer is drawn in gray—the pointer is there, but it must not be used. Of course, in Java the code will have set the pointer to null or to point to somewhere else, so as to tell the garbage collection that this object is now unused. This is a big part of why Java references are safer to use than C++ pointers.

1.3. Programming the Heap¶

Programming the heap looks pretty much the same in most languages. The basic features are:

The heap is an area of memory available to allocate areas ("blocks") of memory for the program.
There is some "heap manager" library code which manages the heap for the program. The programmer makes requests to the heap manager, which in turn manages the internals of the heap.
The heap manager uses its own private data structures to keep track of which blocks in the heap are "free" (available for use) and which blocks are currently in use by the program and how large those blocks are. Initially, all of the heap is free.
The heap may be of a fixed size (the usual conceptualization), or it may appear to be of a fixed but extremely large size backed by virtual memory. In either case, it is possible for the heap to get "full" if all of its memory has been allocated and so it cannot satisfy an allocation request. The allocation function will communicate this run-time condition in some way to the program—usually by raising an OutOfMemoryError run-time exception.
The allocation function requests a block in the heap of a particular size. The heap manager selects an area of memory to use to satisfy the request, marks that area as "in use" in its private data structures, and returns a reference to the heap block. The caller is now free to use that memory by following the reference. The block is guaranteed to be reserved for the sole use of the caller—the heap will not hand out that same area of memory to some other caller. The block does not move around inside the heap—its location and size are fixed once it is allocated.
The Java virtual machine invokes the garbage collection to remove any unused block of memory, free its space and return this space of memory to the heap free area for later re-use.

1.4. A Heap Example¶

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1.5. Arrays¶

In Java, array memory is allocated in the heap. The size of the array memory block is the size of each element multiplied by the number of elements. So the following code heap allocates an array of 100 Fraction objects in the heap, sets them all to 22/7, and deallocates the heap array.

// Fraction class
class Fraction 
  int numerator, denominator;

  // Fraction constructor
  public Fraction(int num, int den) {
    numerator = num;
    denominator = den;
  }
}

// Test class
class TestHeapArray {
  static void main() {
    Fraction[] fracts;
    int i;
    int length = 100;
    
    // allocate the array
    fracts = new Fraction[length];
    // use it like an array -- in this case set them all to 22/7
    for (i = 0; i < length; i++) {
      fracts[i] = new Fraction(22, 7);
    }
    // Remove access to the array, making it garbage.
    // If the Fraction objects that were created are also garbage now.
    fracts = null;
  }
}

In this example the array is dynamically allocated memory in two steps:

The first step when the array is created using fracts = new Fraction[100];. This line is used to allocate dynamic array of 100 references to Fractions. All references are initialized to null.
The second step was inside the loop. Every loop iteration uses new to dynamically allocate an object of type Fraction. The initial value of each object determined by the values sent to the Fraction constructor.

Heap memory provides greater control for the programmer—the blocks of memory can be requested in any size, and they remain allocated until they are no longer pointed to and recovered by the garbage collector. An object in heap memory can be passed back to the caller of a function, since it is not deallocated when that function exits. And it can be used to build linked structures such as linked lists and binary trees. The disadvantage of heap memory is that the program must make explicit allocation calls to manage the heap memory, and the program has to wait when the garbage collector runs. The heap memory does not operate automatically and conveniently the way local memory does.

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