Java Memory Management and Garbage Collection (Simple Version)

Here's a summarized explanation of Java memory management and garbage collection:

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✅ Type of Memory: Stack vs Heap

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📦 What Kind of Data Is Stored?

🧠 Stack

• Primitive values (int, boolean, etc.)
• References to heap objects Example:

int x = 5;          // x stored in stack
String s = "abc";   // reference stored in stack, actual String in heap

🧠 Heap

• All objects and arrays Example:

Person p = new Person();  // Person object stored in heap

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🔗 Types of References

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💪 Strong Reference

String s = new String("hello");

As long as s is in scope, the object won’t be garbage collected.

🪶 Weak Reference

WeakReference<String> weakStr = new WeakReference<>(new String("weak"));

Collected when GC runs, even if memory is sufficient.

🌬️ Soft Reference

SoftReference<String> softStr = new SoftReference<>(new String("soft"));

Only collected if JVM runs low on memory.

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🧱 Heap Memory Structure (Managed by GC)

1. Young Generation (Young Gen)

• Where new objects are allocated.
• Collected frequently (minor GC).

• Subdivided into:

  • Eden: Where objects are first created.
  • Survivor Spaces (S0, S1): Objects that survive Eden collection.

2. Old Generation (Tenured Gen)

• Stores long-living objects.
• Collected less frequently (major GC).

3. Metaspace (Java 8+)

• Stores class metadata (formerly PermGen).
• Resides in native memory (outside heap).

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🧹 How Garbage Collector Works (Simplified)

Example:

Person p1 = new Person();  // Allocated in Eden
p1 = null;                 // No strong reference
System.gc();               // Hint to GC to run

GC Workflow:

1. Minor GC: Cleans Eden → survivors go to S0/S1.
2. If object survives multiple GCs → moved to Old Gen.
3. Major GC: Cleans Old Gen.
4. GC frees objects with no strong references.

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🚮 Types of Garbage Collectors

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