Understanding Java Memory Management

Question

Java programmers know that JVM runs a Garbage Collector, and System.gc() would just be a suggestion to JVM to run a Garbage Collector. It is not necessarily that if we use System.gc(), it would immediately run the GC.

Please correct me if I misunderstand Java's Garbage Collector.

Is/are there any other way/s doing memory management other than relying on Java's Garbage Collector?
If you intend to answer the question by some sort of programming practice that would help managing the memory, please do so.

Answer 1

You cannot avoid garbage collection if you use Java. Maybe there are some obscure JVM implementations that do, but I don't know of any.

A properly tuned JVM shouldn't require any System.gc() hints to operate smoothly. The exact tuning you would need depends heavily on what your application does, but in my experience, I always turn on the concurrent-mark-and-sweep option with the following flag: -XX:+UseConcMarkSweepGC. This flag allows the JVM to take advantage of the extra cores in your CPU to clean up dead memory on a background thread. It helps to drastically reduce the amount of time your program is forcefully paused when doing garbage collections.

Answer 2

I would suggest to take a look at the following tutorials and its contents

This is a four part tutorial series to know about the basics of garbage collection in Java:

1. Java Garbage Collection Introduction

2. How Java Garbage Collection Works?

3. Types of Java Garbage Collectors

4. Monitoring and Analyzing Java Garbage Collection

I found This tutorial very helpful.

Answer 3

From my experience, in java you should rely on the memory management that is provided by JVM itself.

The point I'd focus on in this topic is to configure it in a way acceptable for your use case. Maybe checking/understanding JVM tuning options would be useful: http://docs.oracle.com/cd/E15523_01/web.1111/e13814/jvm_tuning.htm

Answer 4

You are correct in saying that System.gc() is a request to the compiler and not a command. But using below program you can make sure it happens.

import java.lang.ref.WeakReference;

public class GCRun {

    public static void main(String[] args) {

        String str = new String("TEMP");
        WeakReference<String> wr = new WeakReference<String>(str);
        str = null;
        String temp = wr.get();
        System.out.println("temp -- " + temp);
        while(wr.get() != null) {
            System.gc();
        }
    }
}