Although Garbage Collection (GC) in Java is automatic, you can suggest it manually using the following methods:
1️⃣ Using System.gc()
System.gc();
- This requests the JVM to run garbage collection.
- The JVM may or may not execute GC immediately.
2️⃣ Using Runtime.getRuntime().gc()
Runtime.getRuntime().gc();
- Similar to
System.gc()
, but explicitly calls thegc()
method of the JVM’sRuntime
instance. - Again, GC execution is not guaranteed.
3️⃣ Calling finalize()
(Discouraged)
- Before Java 9, objects could override
finalize()
, which was called before an object was garbage collected. - However, it does not guarantee immediate GC execution and is deprecated since Java 9.
Example:
@Override
protected void finalize() throws Throwable {
System.out.println("Finalizing object...");
}
🔴 Deprecated in Java 9 and removed in Java 18.
Why Is Manually Triggering GC Discouraged?
1️⃣ GC Is Optimized by the JVM
- The JVM automatically manages memory efficiently.
- Manual GC calls disrupt optimization and can cause unnecessary overhead.
2️⃣ System.gc()
Causes Stop-the-World Pauses
- Calling
System.gc()
forces a full garbage collection (if the JVM allows it). - All application threads stop (Stop-the-World event) while GC runs.
- This reduces performance and can cause noticeable lag in real-time applications.
3️⃣ It Does Not Guarantee Immediate Collection
System.gc()
is only a suggestion.- The JVM may ignore it if it determines GC is not necessary.
4️⃣ Full GC Can Lead to Performance Issues
- A Full GC (triggered by
System.gc()
) can be very expensive in large applications. - It can cause latency spikes, making applications unresponsive.
5️⃣ Encourages Bad Coding Practices
- If your application relies on manual GC, it may indicate poor memory management.
- Proper memory handling (object reuse, weak references, GC tuning) is a better alternative.
When Should You Trigger GC Manually?
Although discouraged, manual GC triggering can be useful in some cases:
✅ 1. Before a Large Memory-Intensive Task
- If an application is about to load a large dataset, forcing GC beforehand may free up space.
- Example:
System.gc(); // Free memory before loading a large file
loadLargeFile();
✅ 2. In Testing & Benchmarking
- In performance testing, you might use
System.gc()
to measure memory usage before and after an operation. - Example:
System.gc();
long beforeMemory = Runtime.getRuntime().freeMemory();
performTask(); // Run task
long afterMemory = Runtime.getRuntime().freeMemory();
System.out.println("Memory used: " + (beforeMemory - afterMemory));
✅ 3. In Embedded or Low-Memory Systems
- In memory-constrained environments, a manual GC call might help prevent OutOfMemoryErrors.
How to Optimize GC Without Manual Calls
🚀 Instead of calling System.gc()
, use these optimizations: ✔ Use Proper Data Structures – Avoid unnecessary object creation.
✔ Use Weak References – Prevent memory leaks with WeakReference
or SoftReference
.
✔ Tweak JVM GC Settings – Use -XX:+UseG1GC
or -XX:+UseZGC
for optimized GC.
✔ Monitor Memory Usage – Use VisualVM, JConsole, or JProfiler to analyze GC behavior.
Conclusion
- Manual GC triggering (
System.gc()
) is discouraged because it disrupts JVM optimizations and can slow down applications. - The JVM automatically manages memory, making manual GC unnecessary in most cases.
- Use GC tuning and better coding practices instead of relying on explicit garbage collection.
📌 In short: Let the JVM handle GC—manual calls do more harm than good! 🚀