Garbage Collection in Java

Garbage Collection in Java

Introduction

Garbage collection is a crucial aspect of Java's memory management system. It automates the process of reclaiming memory occupied by objects that are no longer in use. This helps prevent memory leaks and ensures efficient memory utilization.

How Garbage Collection Works

The Java Virtual Machine (JVM) automatically performs garbage collection to identify and clean up objects that are no longer reachable. It follows these basic steps:

1. Marking Phase: The JVM starts by traversing all the objects from the root set (global variables, references from the current stack, etc.) and marks them as reachable.

2. Sweeping Phase: In this step, the JVM identifies and removes objects that were not marked as reachable. These unreached objects become eligible for garbage collection.

3. Compacting Phase (optional): In some garbage collection algorithms, a compacting phase may be included to optimize memory usage by rearranging the remaining objects to create larger continuous blocks of free memory.

Example

Consider the following Java code example to understand garbage collection:

java
public class GarbageCollectionExample {
    public static void main(String[] args) {
        GarbageCollectionExample obj1 = new GarbageCollectionExample();
        GarbageCollectionExample obj2 = new GarbageCollectionExample();
        obj1 = null; // obj1 is eligible for garbage collection
        System.gc(); // Requesting garbage collection explicitly (not guaranteed to run immediately)
    }
    
    @Override
    protected void finalize() throws Throwable {
        System.out.println("Garbage Collection is performed by the JVM.");
        super.finalize();
    }
}

Explanation

In the example above, we create two instances of the GarbageCollectionExample class, namely obj1 and obj2. After that, we set obj1 to null, making it eligible for garbage collection. By calling System.gc(), we request garbage collection to run, but it is not guaranteed to execute immediately.

When the garbage collection takes place, the finalize() method (provided by the Object class) is called before reclaiming the memory occupied by objects eligible for garbage collection. In this example, the finalize() method will be called for obj1. However, it is essential to note that relying on finalize() for cleanup is generally discouraged, and explicit resource cleanup using try-with-resources or other mechanisms is preferred.

In conclusion, garbage collection in Java is a vital process that helps manage memory efficiently by automatically reclaiming memory from unreachable objects, thus preventing memory leaks and improving application performance.