Requirements

Let's design a simplified garbage collector (GC) for a Java-like environment. We're not aiming for the full complexity of HotSpot's GC, but rather a demonstration of the underlying principles and the challenges of memory management in a managed runtime. This exercise will focus on implementing a basic Mark and Sweep garbage collector. Imagine a simplified Java Virtual Machine (JVM) where objects are allocated on a heap. Our GC will periodically scan this heap, identify objects that are no longer reachable from the "roots" (e.g., local variables on the stack, static fields), and reclaim the memory occupied by these unreachable objects. We'll simulate object creation and deletion. The key is to accurately track object references, handle potential cycles, and ensure thread-safety during the GC process. Our focus is on the correctness and efficiency of the garbage collection algorithm, not necessarily on performance metrics like pause times. This is a challenging problem, requiring a good understanding of memory management, data structures, and concurrency. The real-world garbage collectors are much more complex and consider generational collection, different GC algorithms (CMS, G1), and advanced optimizations. # Requirements ## Functional Requirements 1. **Heap Management:** * The system must provide a way to allocate memory for objects on a simulated heap. A simple `allocate(size)` method should suffice. The heap should have a fixed maximum size. * Implement a method `free(object)` to explicitly deallocate memory (for testing and simulation purposes *before* the GC is triggered). This method should only be used for testing and will be removed for real GC operation. * The `allocate` method should return a unique object identifier (e.g., an integer representing a memory address). * Handle OutOfMemoryError conditions gracefully if allocation fails due to insufficient memory. Throw an exception or return a null object. * Represent objects as simple blocks of memory. No complex object headers are needed beyond what's necessary for the GC. 2. **Object Representation:** * Objects can contain references to other objects. The system needs a way to define these references. Assume each object contains an array of object references. * Implement a method `setReference(object, index, target)` to update an object's reference at a specific index. * Implement a method `getReference(object, index)` to retrieve an object's reference at a specific index. 3. **Garbage Collection (Mark and Sweep):** * Implement a Mark and Sweep garbage collection algorithm. * The GC should identify reachable objects starting from a set of root objects. Assume a method `getRoots()` provides a list of root object identifiers. * **Mark Phase:** Traverse the object graph, starting from the roots, and mark all reachable objects. * **Sweep Phase:** Scan the entire heap and reclaim the memory occupied by objects that are not marked. * Clear the mark bits after the sweep phase. * The GC should run automatically when memory is low (e.g., when allocation fails). The trigger mechanism should be configurable to simulate different GC frequencies. * Implement a method `runGC()` that explicitly triggers the garbage collector. 4. **Root Identification:** * Implement a method to define and retrieve root objects. This simulates the JVM's knowledge of objects currently in use on the stack and in static fields. 5. **Object Creation and Interlinking (Simulation):** * Provide methods to create objects and set references between them to simulate complex object graphs with cycles. This is crucial for testing the GC's correctness. ## Non-Functional Requirements 1. **Thread Safety:** The garbage collector must be thread-safe. Multiple threads might be allocating memory and modifying object references concurrently. Use appropriate synchronization mechanisms (locks, atomic variables) to prevent race conditions. Pay special attention to the mark and sweep phases. 2. **Modularity:** The code should be well-modularized, with clear separation of concerns. The heap management, object representation, and garbage collection logic should be in separate classes/modules. This enables easier testing and maintenance. 3. **Extensibility:** Design the code to be extensible. Consider how you might add different garbage collection algorithms (e.g., generational GC) in the future. Use interfaces and abstract classes to provide extension points. For example, have an interface for `GarbageCollector` with different implementations (MarkAndSweepGC, etc.). 4. **Testability:** The code must be easily testable. Write comprehensive unit tests to verify the correctness of the garbage collection algorithm, including edge cases like cycles, empty heaps, and large object graphs. Use dependency injection or other techniques to isolate components for testing. 5. **Efficiency:** While not the primary focus, the garbage collector should be reasonably efficient. Avoid unnecessary iterations or memory copies. Choose appropriate data structures for tracking object reachability (e.g., sets, bit vectors). Consider using lock-free data structures if appropriate for performance. 6. **Error Handling:** Handle potential errors gracefully, such as OutOfMemoryError and invalid object references. Provide informative error messages. 7. **Memory Leaks:** Ensure that the garbage collector itself does not introduce memory leaks. Pay attention to resource management and avoid holding onto objects longer than necessary. 8. **Clean Code:** Follow SOLID principles to ensure code is maintainable, readable, and reusable. Pay attention to single responsibility principle and dependency inversion. ## Core Entities 1. **Heap:** Represents the memory area where objects are allocated. Manages the allocation and deallocation of memory blocks. This is a critical class for managing memory and should be thread-safe. 2. **Object:** Represents a Java-like object on the heap. Contains data and references to other objects. We'll represent it as a simple data structure containing an array of references. 3. **Reference:** A pointer from one object to another. This can be represented as an integer (object ID). 4. **GarbageCollector:** An interface or abstract class that defines the contract for garbage collection algorithms. Concrete implementations (e.g., MarkAndSweepGC) implement specific algorithms. 5. **MarkAndSweepGC:** Implements the Mark and Sweep garbage collection algorithm. Responsible for identifying reachable objects and reclaiming unreachable memory. 6. **RootSet:** Represents the set of root objects from which the GC starts its traversal. This could be a simple list or set of object references. It needs to be updated as the simulated program runs.

Design and Implement a Simple Java GC (Java 7+)

Requirements

Think like an Architect

Premium Content