Difference between Class.forName("ClassName") and ClassName.class

[Muthukrishnan Manoharan]

Hi all,

May I know how does Class.forName("ClassName") different from ClassName.class. For example, I tried to execute the following

public final class FinalClassExample { 
  
public static void main(String[] args) throws Exception 
{
		
    	Class cls = Class.forName("sample1");   
    	
    	sample1 object = (sample1)cls.newInstance();
    	
    	System.out.println(" end of main");
}
  
}

class sample1
{
	static
	{
		System.out.println(" loading sample class");
	}
}

My output :

loading sample class
end of main

Also, I tried the following,

public final class FinalClassExample { 
  
public static void main(String[] args) throws Exception 
{		
         Class cls = sample1.class;    	
    	sample1 object = (sample1)cls.newInstance();    	
    	System.out.println(" end of main");
}
  
}

class sample1
{
	static
	{
		System.out.println(" loading sample class");
	}
}

My output:

end of main

So doesn't it load the class when I say ClassName.class, but loads only when I use the Class Object to get constructors(cls.getConstructors()) or methods?

[Christophe Verré]

You'll find some explanations in the Java Reflection Tutorial.

[Muthukrishnan Manoharan]

Thanks Chris,

I actually followed the tutorial first and tried to execute an example from my understanding.


But the output I get from using Class.forName("ClassName") and ClassName.class are different as I said above. Is it normal or am I missing something.

[Sunny Bhandari]

Muthukrishnan Manoharan wrote:Thanks Chris,

I actually followed the tutorial first and tried to execute an example from my understanding.


But the output I get from using Class.forName("ClassName") and ClassName.class are different as I said above. Is it normal or am I missing something.

Please recheck the output again carefully.

I am getting the same out put for both.

loading sample class  
end of main 

[Seetharaman Venkatasamy]

@Muthukrishnan : what is your jdk version?
@Sunny :what is your jdk version?

[Sunny Bhandari]

Version is not making any change for the output.

I have checked with 1.4, 1.5 and 1.6

[Rob Spoor]

The main difference is in the required availability of the class. With ClassName.class the class needs to be available at compile time. If the class is missing while you are compiling you will get a compiler error. With Class.forName("ClassName") the class doesn't need to be there at compile time, only at runtime.

Another important difference is the generic type. ClassName.class returns a Class<ClassName>. Class.forName("ClassName") returns a Class<?> - the actual type is not known.

[Seetharaman Venkatasamy]

Sunny Bhandari wrote:
I am getting the same out put for both.

As muthu mentioned the output is different for me.

[Muthukrishnan Manoharan]

Hi all,

Please ignore the previous code. Since I am creating newInstance from the class obtained, it is obvious that it executes the static content.

But this is my actual code and corresponding output. My version of JDK is 1.5.

Example1:

public final class FinalClassExample { 
  
public static void main(String[] args) throws Exception 
{		
         Class cls = sample1.class;    	
         System.out.println(" end of main");
}
  
}

class sample1
{
	static
	{
		System.out.println(" loading sample class");
	}
}

output:

 end of main

Example2:

public final class FinalClassExample { 
  
public static void main(String[] args) throws Exception 
{		        
	Class cls = Class.forName("sample1");
    	System.out.println(" end of main");
}
  
}

class sample1
{
	static
	{
		System.out.println(" loading sample class");
	}
}

output:

 loading sample class
 end of main

[Seetharaman Venkatasamy]

Muthukrishnan Manoharan wrote:Since I am creating newInstance from the class obtained, it is obvious that it executes the static content.

dont worry, I already ignored that

[Rob Spoor]

Class.forName actually loads the class. ClassName.class doesn't load the classbecause it's not necessary to do so. If you instantiate the class or access a static member (method, field) then the class is loaded, but not just by calling ClassName.class.

[Seetharaman Venkatasamy]

well rob, one query : if ClassName.class doest not load a class then how come below code is working. please can you explain?

Class cls =  Sample1.class;   
Constructor[] cons = cls.getDeclaredConstructors();
			 
for(Constructor c : cons)
{
	System.out.println("entry :::: ");
	System.out.println(c.getName());
}

[Rob Spoor]

Guess I was wrong. It's loaded all right (as can be seen when running the JVM with -verbose), just not initialized. That only happens when instantiating or accessing a static member.

[Seetharaman Venkatasamy]

Thank you for your kind reply Rob

[Sunny Bhandari]

But how come I get the output as;

 loading sample class
 end of main

for the code:

public final class FinalClassExample { 
  
public static void main(String[] args) throws Exception 
{		
         Class cls = sample1.class;    	
         System.out.println(" end of main");
}
  
}

class sample1
{
	static
	{
		System.out.println(" loading sample class");
	}
}

[Rob Spoor]

Which JVM are you using? Not just the version but also the vendor. Because my Sun / Oracle 1.6.0_23 JVM doesn't initialize the class with that code.

[Sunny Bhandari]

I have tried with SUN/Oracle 1.4,1.5 and 1.6 but same output

 loading sample class
 end of main