Garbage Collection in Java

19 May

Garbage Collection in Java

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 :

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.
garbage collection

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 the 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.
Nullifying the reference variable
Re-assigning the reference variable
Object created inside method
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 the garbage collector.
Whenever JVM runs the Garbage Collector program, then only the 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 :

Using System.gc() method : System class contain static method gc() for requesting JVM to run Garbage Collector.
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.

// Java program to demonstrate requesting
// JVM to run Garbage Collector
public class Test
{
public static void main(String[] args) throws InterruptedException
{
Test tmp1 = new Test();
Test tmp2 = new Test();

// Nullifying the reference variable
tmp1 = null;

// requesting JVM for running Garbage Collector
System.gc();

// Nullifying the reference variable
tmp2 = 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);
}
}
Output:

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

There is no guarantee that any one of above two methods will definitely run Garbage Collector.
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 :

The finalize() method called by Garbage Collector not JVM. Although Garbage Collector is one of the module of JVM.
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.
The finalize() method is never invoked more than once for any given object.
If an uncaught exception is thrown by the finalize() method, the exception is ignored and finalization of that object terminates.

Let’s take a real-life example, where we use the concept of garbage collector.

Suppose you go for the internship at PrutordotAi and their you were told to write a program, to count the number of Employees working in the company(excluding interns).To make this program, you have to use the concept of a garbage collector.
This is the actual task you were given at the company:-

Q.Write a program to create a class called Employee having the following data members.
1.An ID for storing unique id allocated to every employee.
2.Name of employee.
3.age of an employee.

Also, provide the following methods-

A parameterized constructor to initialize name and age. The ID should be initialized in this constructor.
A method show() to display ID, name, and age.
A method showNextId() to display the ID of the next employee.

Now any beginner, who doesn’t have knowledge on garbage collector will code like this:

//Program to count number
//of employees working
//in a company

class Employee
{
private int ID;
private String name;
private int age;
private static int nextId=1;
//it is made static because it
// is keep common among all and
// shared by all objects
public Employee(String name,int age)
{
this.name = name;
this.age = age;
this.ID = nextId++;
}
public void show()
{
System.out.println
("Id="+ID+"nName="+name+"nAge="+age);
}
public void showNextId()
{
System.out.println
("Next employee id will be="+nextId);
}
}
class UseEmployee
{
public static void main(String []args)
{
Employee E=new Employee("Prutor1",56);
Employee F=new Employee("Prutor2",45);
Employee G=new Employee("Prutor3",25);
E.show();
F.show();
G.show();
E.showNextId();
F.showNextId();
G.showNextId();

{ //It is sub block to keep
// all those interns.
Employee X=new Employee("Prutor4",23);
Employee Y=new Employee("Prutor5",21);
X.show();
Y.show();
X.showNextId();
Y.showNextId();
}
//After countering this brace, X and Y
//will be removed.Therefore,
//now it should show nextId as 4.
E.showNextId();//Output of this line
//should be 4 but it will give 6 as output.
}
}
Output:

Id=1
Name=Prutor1
Age=56
Id=2
Name=Prutor2
Age=45
Id=3
Name=Prutor3
Age=25
Next employee id will be=4
Next employee id will be=4
Next employee id will be=4
Id=4
Name=Prutor4
Age=23
Id=5
Name=Prutor5
Age=21
Next employee id will be=6
Next employee id will be=6
Next employee id will be=6

Now to get the correct output:
Now garbage collector(gc) will see 2 objects free. Now to decrement nextId,gc(garbage collector) will call method finalize() only when we programmers have override it in our class. And as mentioned previously, we have to request gc(garbage collector) and for this, we have to write the following 3 steps before closing brace of sub-block.

Set references to null(i.e X = Y = null;)
Call, System.gc();
Call, System.runFinalization();
Now the correct code for counting the number of employees(excluding interns)

// Correct code to count number
// of employees excluding interns.
class Employee
{
private int ID;
private String name;
private int age;
private static int nextId=1;
//it is made static because it
// is keep common among all and
// shared by all objects
public Employee(String name,int age)
{
this.name = name;
this.age = age;
this.ID = nextId++;
}
public void show()
{
System.out.println
("Id="+ID+"nName="+name+"nAge="+age);
}
public void showNextId()
{
System.out.println
("Next employee id will be="+nextId);
}
protected void finalize()
{
--nextId;
//In this case,
//gc will call finalize()
//for 2 times for 2 objects.
}
}

// it is closing brace of Employee class
class UseEmployee
{
public static void main(String []args)
{
Employee E=new Employee("Prutor1",56);
Employee F=new Employee("Prutor2",45);
Employee G=new Employee("Prutor3",25);
E.show();
F.show();
G.show();
E.showNextId();
F.showNextId();
G.showNextId();

Garbage Collection in Java

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Garbage Collection in Java