JVM.GC.How do you trigger garbage collection manually, and why is it discouraged?

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 the gc() method of the JVM’s Runtime 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.

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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.

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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! 🚀