Different ways to dynamically load a class

In this post, I am going to explain how to load a class dynamically.

a.   Using Class.forName method

b.   Using ClassLoader

Using Class.forName method

'java.lang.Class' provides forName method to dynalically load a class. 'forName' method is available in overloaded forms.

public static Class<?> forName(String className) throws ClassNotFoundException

public static Class<?> forName(String name, boolean initialize, ClassLoader loader) throws ClassNotFoundException

This method returns the Class object associated with the class or interface with the given string name.

Below table summarizes the arguments of forName method.

Argument	Description
className	Fully qualified name of the class
initialize	If true the class will be initialized.
loader	Class loader from which the class must be loaded

Example

Class<?> clazz = Class.forName("com.sample.model.Employee");

Employee emp = (Employee)clazz.newInstance();

Above statements load the class Employee at run time.

Find the below working example.

Employee.java

package com.sample.model;

public class Employee {
 private int id;
 private String firstName;
 private String lastName;

 public int getId() {
  return id;
 }

 public void setId(int id) {
  this.id = id;
 }

 public String getFirstName() {
  return firstName;
 }

 public void setFirstName(String firstName) {
  this.firstName = firstName;
 }

 public String getLastName() {
  return lastName;
 }

 public void setLastName(String lastName) {
  this.lastName = lastName;
 }

 @Override
 public String toString() {
  return "Employee [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + "]";
 }

}

Application.java

package com.sample.app;

import com.sample.model.Employee;

public class Application {

 public static void main(String args[]) throws Exception {
  Class<?> clazz = Class.forName("com.sample.model.Employee");
  
  Employee emp = (Employee)clazz.newInstance();
  
  emp.setFirstName("Krishna");
  emp.setLastName("Maj");
  emp.setId(1);
  
  System.out.println(emp);
 }
}

Output

Employee [id=1, firstName=Krishna, lastName=Maj]

Points to note

a. When you pass initialize argument to true, static block of the class is called, else not.

Employee.java

package com.sample.model;

public class Employee {
 private int id;
 private String firstName;
 private String lastName;

 static {
  System.out.println("Initializing the class");
 }

 public int getId() {
  return id;
 }

 public void setId(int id) {
  this.id = id;
 }

 public String getFirstName() {
  return firstName;
 }

 public void setFirstName(String firstName) {
  this.firstName = firstName;
 }

 public String getLastName() {
  return lastName;
 }

 public void setLastName(String lastName) {
  this.lastName = lastName;
 }

 @Override
 public String toString() {
  return "Employee [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + "]";
 }

}

Application.java

package com.sample.app;

public class Application {

 public static void main(String args[]) throws Exception {
  Class<?> clazz1 = Class.forName("com.sample.model.Employee", false, Application.class.getClassLoader()); 
 }
}

When you ran above application, you do not see anything in the console.

Let’s set the ‘initialize’ argument to true and rerun the application.

Application.java

package com.sample.app;

public class Application {

 public static void main(String args[]) throws Exception {
  Class<?> clazz1 = Class.forName("com.sample.model.Employee", true, Application.class.getClassLoader()); 
 }
}

Output

Initializing the class

Using ClassLoader load method

'java.lang.ClassLoader' class provides below methods to load the class dynamically.

public Class<?> loadClass(String name)

protected Class<?> loadClass(String name, boolean resolve)

Example

ClassLoader classLoader = Application.class.getClassLoader();

Class<?> clazz = classLoader.loadClass("com.sample.model.Employee");

Find the below working example.

Employee.java

package com.sample.model;

public class Employee {
 private int id;
 private String firstName;
 private String lastName;

 static {
  System.out.println("Initializing the class");
 }

 public int getId() {
  return id;
 }

 public void setId(int id) {
  this.id = id;
 }

 public String getFirstName() {
  return firstName;
 }

 public void setFirstName(String firstName) {
  this.firstName = firstName;
 }

 public String getLastName() {
  return lastName;
 }

 public void setLastName(String lastName) {
  this.lastName = lastName;
 }

 @Override
 public String toString() {
  return "Employee [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + "]";
 }

}

Application.java

package com.sample.app;

import com.sample.model.Employee;

public class Application {

 public static void main(String args[]) throws Exception {
  ClassLoader classLoader = Application.class.getClassLoader();
  Class<?> clazz = classLoader.loadClass("com.sample.model.Employee");

  System.out.println("Creating an object to the class Employee");

  Employee emp = (Employee) clazz.newInstance();

  emp.setId(1);
  emp.setFirstName("Krishna");
  emp.setLastName("Maj");

  System.out.println(emp);
 }
}

Output

Creating an object to the class Employee

Initializing the class

Employee [id=1, firstName=Krishna, lastName=Maj]

As you observe the ouput, loadClass method do not call the static initializer block, it is called when I requested for an object to the class Employee.

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