Garbage Collection in Java

Introduction

• In C/C++, programmer is responsible for both creation and destruction of objects. Usually programmer neglects destruction of useless objects. Due to this negligence, at certain point, for creation of new objects, sufficient memory may not be available and entire program will terminate abnormally causing OutOfMemoryErrors.
• But in Java, the programmer need not to care for all those objects which are no longer in use. Garbage collector destroys these objects.
• Garbage collector is best example of Daemon thread as it is always running in background.
• Main objective of Garbage Collector is to free heap memory by destroying unreachable objects.

Important terms :

1. Unreachable objects : An object is said to be unreachable iff it doesn’t contain any reference to it. Also note that objects which are part of island of isolation are also unreachable.
   
   

   Integer i = new Integer(4);
   // the new Integer object is reachable  via the reference in 'i' 
   i = null;
   // the Integer object is no longer reachable. 
   

   

2. Eligibility for garbage collection : An object is said to be eligible for GC(garbage collection) iff it is unreachable. In above image, after i = null; integer object 4 in heap area is eligible for garbage collection.

Ways to make an object eligible for GC

• Even though programmer is not responsible to destroy useless objects but it is highly recommended to make an object unreachable(thus eligible for GC) if it is no longer required.
• There are generally four different ways to make an object eligible for garbage collection.
  1. Nullifying the reference variable
  2. Re-assigning the reference variable
  3. Object created inside method
  4. Island of Isolation

  All above ways with examples are discussed in separate article : How to make object eligible for garbage collection

Ways for requesting JVM to run Garbage Collector

• Once we made object eligible for garbage collection, it may not destroy immediately by garbage collector. Whenever JVM runs Garbage Collector program, then only object will be destroyed. But when JVM runs Garbage Collector, we can not expect.
• We can also request JVM to run Garbage Collector. There are two ways to do it :
  1. Using System.gc() method : System class contain static method gc() for requesting JVM to run Garbage Collector.
  2. Using Runtime.getRuntime().gc() method : Runtime class allows the application to interface with the JVM in which the application is running. Hence by using its gc() method, we can request JVM to run Garbage Collector.
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  // Java program to demonstrate requesting  // JVM to run Garbage Collector public class Test {     public static void main(String[] args) throws InterruptedException     {         Test t1 = new Test();         Test t2 = new Test();                    // Nullifying the reference variable         t1 = null;                    // requesting JVM for running Garbage Collector         System.gc();                    // Nullifying the reference variable         t2 = null;                    // requesting JVM for running Garbage Collector         Runtime.getRuntime().gc();            }            @Override     // finalize method is called on object once      // before garbage collecting it     protected void finalize() throws Throwable     {         System.out.println("Garbage collector called");         System.out.println("Object garbage collected : " + this);     } }

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  Output:

  Garbage collector called
  Object garbage collected : Test@46d08f12
  Garbage collector called
  Object garbage collected : Test@481779b8
  

  Note :

  1. There is no guarantee that any one of above two methods will definitely run Garbage Collector.
  2. The call System.gc() is effectively equivalent to the call : Runtime.getRuntime().gc()

Finalization

• Just before destroying an object, Garbage Collector calls finalize() method on the object to perform cleanup activities. Once finalize() method completes, Garbage Collector destroys that object.
• finalize() method is present in Object class with following prototype.

  protected void finalize() throws Throwable
  

  Based on our requirement, we can override finalize() method for perform our cleanup activities like closing connection from database.

Note :

1. The finalize() method called by Garbage Collector not JVM. Although Garbage Collector is one of the module of JVM.
2. Object class finalize() method has empty implementation, thus it is recommended to override finalize() method to dispose of system resources or to perform other cleanup.
3. The finalize() method is never invoked more than once for any given object.
4. If an uncaught exception is thrown by the finalize() method, the exception is ignored and finalization of that object terminates.

For examples on finalize() method, please see Output of Java programs | Set 10 (Garbage Collection)

Related Articles :

How to make object eligible for garbage collection in Java?

Island of Isolation in Java

Output of Java programs | Set 10 (Garbage Collection)

How to find max memory, free memory and total memory in Java?

How JVM Works – JVM Architecture?

This article is contributed by Chirag Agarwal and Gaurav Miglani .Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above