loadClass() Method - Loading Classes Explicitly

JVM Tutorials - Herong's Tutorial Examples

∟loadClass() Method - Loading Classes Explicitly

This section provides a tutorial example on how to load classes into JVM in 3 ways: loadClass() method, Class.forName() method, and 'new' operator.

There are several ways to load a Java class into the JVM:

1. Using the "new" operator: When you use the "new" operator and a class constructor to create a new instance of a Java class, JVM will use the default ClassLoader to load the class. This the most common way of loading a Java class. For example:

Object o = new Hello();
Class c = o.getClass();

2. Using the Class.forName() method: When you use the Class.forName() method, JVM will use the default ClassLoader or the specified ClassLoader to the load the specified class. For example:

Class c1 = Class.forName("ClassLoaderTest");
Class c2 = Class.forName("Hello", true, currentLoader);

3. Using the ClassLoader.loadClass() method: When you use the Class.forName() method, JVM will use the given ClassLoader to the load the specified class. For example:

Class c3 = currentLoader.loadClass("SystemInOut");

Here is tutorial example program showing you how to load a Java class in different ways:

/* ClassLoaderLoadClass.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
class ClassLoaderLoadClass {
   public static void main(String[] a) {
      java.io.PrintStream out = System.out;
      Object o = null;
      Class c = null;
      ClassLoader l = null;

      out.println("");
      out.println("Loading with 'new' operator...");
      o = new Hello();
      c = o.getClass();

      out.println("");
      out.println("Loading with Class.forName() method...");
      try {
         c = Class.forName("ClassLoaderTest");
      } catch (Exception e) {
         e.printStackTrace();
      }

      out.println("");
      out.println("Loading with ClassLoader.loadClass() method...");
      l = ClassLoader.getSystemClassLoader();
      try {
         c = l.loadClass("LongSleep");
      } catch (Exception e) {
         e.printStackTrace();
      }

      out.println("");
      out.println("Loading the same class again...");
      l = ClassLoader.getSystemClassLoader();
      try {
         c = l.loadClass("LongSleep");
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

Prepare some other classes to support the above program:

herong> javac Hello.java
herong> javac SystemInOut.java
herong> javac LongSleep.java

Run the above program with HotSpot JVM:

herong> java -verbose:class ClassLoaderLoadClass

Loading with 'new' operator...
[0.359s][info][class,load] Hello source: file:/C:/herong/ClassLoader/

Loading with Class.forName() method...
[0.361s][info][class,load] ClassLoaderTest source:
   file:/C:/herong/ClassLoader/

Loading with ClassLoader.loadClass() method...
[0.362s][info][class,load] LongSleep source: file:/C:/herong/ClassLoader/

Loading the same class again...

The '-verbose:class' output confirms that all 3 ways of loading classes are working. If you trying to load the same class again, the JVM will ignore your request.