How does GC determine objects to collect?

Understanding Garbage Collection Object Identification in Java

This tutorial explains how the Java Virtual Machine's (JVM) Garbage Collector (GC) identifies which objects are eligible for collection, thereby reclaiming memory. We'll explore the concepts of reachability, garbage collection roots, and different garbage collection algorithms used to determine object eligibility.

Reachability: The Core Concept

What is Reachability?

The central concept behind garbage collection is reachability. An object is considered reachable if it can be accessed directly or indirectly from a garbage collection root. If an object is not reachable, it means there are no active references pointing to it, making it eligible for garbage collection.

Garbage Collection Roots

Understanding GC Roots

Garbage collection roots are the starting points the GC uses to traverse the object graph and identify reachable objects. Common GC roots include:

• Local variables: Variables within the current execution stack frame.
• Static variables: Static variables of loaded classes.
• Active threads: Threads that are currently running.
• JNI references: References created through Java Native Interface (JNI).

The GC starts from these roots and follows all references to other objects. Any object found during this traversal is marked as reachable and therefore kept alive.

Mark and Sweep Algorithm

The Mark and Sweep Algorithm

One of the earliest and simplest garbage collection algorithms is the Mark and Sweep algorithm. It operates in two phases:

1. Mark Phase: The GC traverses the object graph starting from the GC roots and marks each reachable object.
2. Sweep Phase: The GC iterates through the entire heap and identifies unmarked objects (those not reachable). These unmarked objects are then reclaimed, freeing up the memory they occupied.

While conceptually simple, the Mark and Sweep algorithm can lead to memory fragmentation over time because it doesn't compact the memory.

Mark and Compact Algorithm

The Mark and Compact Algorithm

The Mark and Compact algorithm builds upon Mark and Sweep by adding a compaction phase to address memory fragmentation. It involves three phases:

1. Mark Phase: Same as Mark and Sweep, marking all reachable objects.
2. Compact Phase: After marking, live objects are moved to one end of the heap, leaving all free memory in a contiguous block at the other end.
3. Update References: All references to the moved objects are updated to reflect their new memory locations.

Mark and Compact reduces fragmentation, improving memory utilization, but it's generally slower than Mark and Sweep due to the overhead of moving objects and updating references.

Copying Garbage Collection

Copying Garbage Collection

In Copying Garbage Collection, the heap is divided into two regions. At any given time, only one region is actively used. When this region becomes full, the GC copies all reachable objects from the active region to the other region. This process automatically compacts the objects. The roles of the two regions are then swapped.

Copying GC is very efficient for short-lived objects, but its major drawback is that it effectively halves the available heap space.

Generational Garbage Collection

Generational Garbage Collection

Modern JVMs, like HotSpot, employ Generational Garbage Collection, which leverages the observation that most objects are short-lived. The heap is divided into generations:

• Young Generation: This is where new objects are allocated. It's further divided into Eden space and Survivor spaces (S0 and S1). Minor GC occurs frequently in the Young Generation.
• Old Generation (Tenured Generation): Objects that survive multiple minor GC cycles are promoted to the Old Generation. Major GC (or Full GC) occurs less frequently in the Old Generation.
• Permanent Generation (PermGen) / Metaspace: (Deprecated in Java 8 and replaced with Metaspace) Holds class metadata, interned strings, and other permanent data.

By focusing GC efforts on the Young Generation, the JVM can reclaim memory more efficiently.

Example Code Illustrating Object Reachability

Code Explanation

In this example:

• obj1 is initially reachable but becomes eligible for garbage collection when it's set to null.
• obj2 remains reachable throughout the execution.
• obj3 initially refers to a new object. When obj3 = null, the object is still referenced by obj4, so it's not immediately garbage collected. It becomes eligible for collection only if obj4 is also set to null or goes out of scope.
• System.gc() is a suggestion to the JVM to run the garbage collector, but it's not guaranteed to run immediately or at all.

public class GarbageCollectionExample {

    public static void main(String[] args) {
        Object obj1 = new Object(); // obj1 is reachable
        Object obj2 = new Object(); // obj2 is reachable

        obj1 = null; // obj1 is now eligible for garbage collection

        Object obj3 = new Object();
        Object obj4 = obj3;

        obj3 = null; // obj3 is not reachable directly but obj4 still refers to it, so it's not garbage collected yet

        System.gc(); // Suggest garbage collection (not guaranteed)
        
        System.out.println("Garbage collection suggested.");

    }
}

Concepts Behind the Snippet

Key Concepts Illustrated

• Object Lifespan: Objects live in memory as long as they are reachable.
• Explicit Nullification: Setting references to null can make objects eligible for garbage collection.
• Indirect Reachability: An object remains reachable if any other reachable object holds a reference to it.

Real-Life Use Case Section

Database Connection Management

In applications that interact with databases, connections are often created and used. Failing to properly close these connections after use can lead to resource exhaustion. By ensuring that database connection objects are set to null or properly closed when they are no longer needed, you allow the garbage collector to reclaim those resources, preventing connection leaks and improving the application's overall stability and performance.

Best Practices

Garbage Collection Best Practices

• Minimize Object Creation: Creating excessive objects puts unnecessary pressure on the GC. Reuse objects where possible, especially in performance-critical sections of code.
• Avoid Premature Optimization: Don't introduce unnecessary complexity to optimize GC behavior unless you've identified it as a performance bottleneck.
• Use Try-With-Resources: For resources like streams or database connections, use the try-with-resources statement to ensure they are closed properly, even if exceptions occur. This helps prevent resource leaks.

Interview Tip

Interview Tip: GC and Reachability

When discussing garbage collection in interviews, emphasize the importance of reachability as the core criterion for determining object eligibility. Explain the concept of GC roots and how different GC algorithms use reachability to reclaim memory.

When to Use Them

When to Consider GC Impact

Pay close attention to garbage collection impact in scenarios such as:

• High-Throughput Systems: Applications that require consistently low latency and high throughput.
• Memory-Constrained Environments: Applications running on devices with limited memory (e.g., mobile devices, embedded systems).

Memory Footprint

Memory Overhead

The garbage collector itself consumes memory and CPU resources. Choose the appropriate GC algorithm based on your application's requirements and the available hardware. Different GC algorithms have different trade-offs in terms of memory footprint, pause times, and throughput.

Alternatives

Alternatives to JVM GC

While the JVM's garbage collector is highly optimized, there are alternatives to managing memory. Manual memory management using technologies like C++ can provide more control over memory allocation and deallocation, but it also introduces the risk of memory leaks and other errors. Off-heap memory management libraries offer a middle ground, allowing you to manage memory outside the JVM heap.

Pros

Advantages of Automatic Garbage Collection

• Reduced Risk of Memory Leaks: The GC automatically reclaims unused memory, reducing the risk of memory leaks caused by unreleased resources.
• Simplified Development: Developers don't need to manually manage memory, simplifying development and reducing the likelihood of memory-related errors.

Cons

Disadvantages of Automatic Garbage Collection

• Performance Overhead: GC activity can introduce pauses in application execution, impacting performance.
• Unpredictable Behavior: The exact timing of garbage collection cycles is not always predictable, which can be problematic in real-time systems.