Garbage Collection in Java

1. Garbage Collection

1.1 Introduction

The process of releasing the heap memory occupied by objects which do not have any live references in the Java program is known as garbage collection.

1.2 Features

• Garbage Collection is done automatically in java by JVM (Java Virtual Machine)
• There is no delete keyword in Java as in C++ to free up the memory
• Those memory blocks which are not referenced by any pointer are known as garbage blocks
• The code which is responsible for garbage collection is known as garbage collector code
• The main advantage of garbage collection is protection from memory leaks
  

2. Process Of Garbage Collection

Whenever a Java program is compiled and executed, JVM creates three threads

• Main thread - this thread is responsible for running the main method() in the Java program

• Thread Scheduler - this thread controls the scheduling of the threads in the program like which thread should execute first or which thread should execute after which thread etc.

• Garbage Collector Thread - this thread is responsible to release the garbage blocks present in the memory and is the main focus of this article

Garbage Collector thread works in the background, performs its tasks in background and comes in the category of daemon threads or low priority threads

Garbage Collector Thread

• When the garbage collector starts running it identifies all the live objects present in the memory which have no reference in the program
• Before releasing their memory it calls a method named finalize(), to make sure that the objects have completed their processes or not
• Here finalize() method can be considered to have almost the same functionality as destructors() in C++
• After the finalize() method is executed, the garbage collector releases the memory of the objects and hence protects the program from unwanted memory leaks
  

3. Object Dereferncing Methods

Assing Null to Object

School obj = new School();
obj = null;

Here the obj variable, first refers to an object in the heap memory but after nulling the obj there is no other reference to that object, hence garbage collector identifies it and releases that object’s memory.

Assign the Reference to Another Object

School obj1 = new School();
School obj2 = new School();
obj1 = obj2;

Now the object previously referenced by obj1 in the heap memory is available for garbage collection as no live reference is available for that object now.

Anonymous Object

new School();

Anonymous objects don’t have any reference so garbage collector releases their memory once they serve their purpose

4. gc() Method

• In Java, JVM implicitly calls the garbage collector but this method can be used to call the garbage collector explicitly
• This method doesn’t guarantee that the garbage collector will be executed at that point
• This method is not a command, it’s just a request, the final decision is taken by JVM whether to execute the garbage collector or not

Example

File : Test.java

import java.io.*;
public class Test{  

  public void finalize()
  {
    System.out.println("Memory is freed");
  }  

  public static void main(String args[]){
 
  Test obj1=new Test();  
  Test obj2=new Test();  
  obj1=null;  
  obj2=null;  
  System.gc();  
  }  
   
}

Output

$ javac Test.java
$ java Test
Memory is freed
Memory is freed

• In above example obj1 and obj2 are dereferenced by assinging null, after which the objects don’t have any live reference in the program and are now available for garbage collection
• The gc() method requests to invoke the garbage collector
• The garbage collector thread before releasing the memory calls the finalize() method for both objects
• After the execution of finalize() method it frees up the memory