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7.3 Memory Leak

← Back to Chapter 7: Garbage Collection & Serialization

What is memory Leak in Java?​

Even after GC memory leak can still happen:

Objects are no longer logically needed but are still reachable, so GC cannot remove them.

Example:

static List<User> users = new ArrayList<>();

public void addUser(User user) {
    users.add(user);
}

Suppose millions of users are added.

Even if your business logic doesn't need old users anymore:

GC Root
   ↓
static users
   ↓
User 1
User 2
User 3
...
User 1,000,000

They are still reachable.

Therefore GC says: "These objects are reachable. I cannot remove them."

Heap keeps growing. Eventually:

OutOfMemoryError

Common causes of Memory Leaks​

1. Static collections​

static List<Object> list;

2. Objects stored unnecessarily in caches​

Especially caches without proper eviction.

3. Listeners/callbacks not removed​

Objects remain referenced.

4. ThreadLocal misuse​

Very important in server applications.

5. Long-lived objects holding references to short-lived objects​

Example:

Singleton
   ↓
Huge Object
   ↓
many objects

finalize() and Garbage Collection​

Older Java versions had:

protected void finalize()

It was intended to be called before an object was reclaimed.

But: Finalization is deprecated and should not be used in modern Java.

Why? Because it is:

unpredictable
slow
difficult to reason about
can delay memory reclamation

For resource management, use mechanisms such as:

Can an object become eligible for GC without assigning null? ✅​

Yes. When the method finishes:

user reference disappears
       ↓
User object may become unreachable
       ↓
eligible for GC

Can two objects reference each other and still be garbage? ✅​

Yes.

Object A ─────> Object B
   ↑              |
   └──────────────┘

Suppose no GC Root points to A or B.

Then both are unreachable.

Therefore:

A → garbage
B → garbage

What is OutOfMemoryError?​

It happens when the JVM cannot satisfy a memory allocation.

Heap is full. Result:

java.lang.OutOfMemoryError

Does OutOfMemoryError always mean a memory leak? ❌​

No. Possible reasons include:

1. Actual memory leak
2. Heap too small
3. Application legitimately needs more memory
4. Huge objects
5. Too many objects
6. Poor cache configuration
7. Excessive concurrency

What is StackOverflowError?​

Usually caused by excessive method recursion/call depth. Example:

void test() {
    test();
}

-Xms and -Xmx​

-Xms​

Initial/minimum heap size.

-Xmx​

Maximum heap size.