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Tip of the Day
Language: Java
Expertise: Beginner
Aug 26, 2012

Create an Instance of a Class Whose Name Will Be Supplied at Runtime

You need to create an instance of a class whose class name you do not know at design/implementation time. You’ll be supplied with the class name during runtime, in other words, your program will NOT know which class to instantiate until it is executing.

This is a common scenario for many a frameworks and products as they need to create instances of user defined classes of which the framework authors have no knowledge of. Hence, the framework can’t use the new operator to create such instances. As an example, a Servlet Container needs to create instances of Servlet classes defined by servlet developers. How does the servlet container do that? Can we do the same in our programs?

Yes and here is how:

Say, DynamicInstantiation.java needs to create an instance of a class supplied to it as a command line argument as follows:

java DynamicInstantiation com.sun.cust.CustomerImpl

Here is how DynamicInstantiation can instantiate CustomerImpl class or any other class that is supplied to it by using Java's Reflection API:

     Class c = Class.forName(className);
     c.newInstance();

Here is the complete implementation of DynamicInstantiation class:

public class DynamicInstantiation {
  public Object createInstance(String className) throws Exception {
    Class c = Class.forName(className);
    return c.newInstance();
  }
  public static void main(String args) throws Exception {
    if(args.length != 1) {
      System.err.println("Usage: java DynamicInstantiation &ltclass_name&gt");
      return;
    }
    DynamicInstantiation di = new DynamicInstantiation();
    Object o = di.createInstance(args);  
    System.out.println("Instance of " + args + " created.");
  }
}

Things to Note:

1. Similar to any Java class, JVM should be able to locate the user defined class using CLASSPATH.

2. The user defined class must define a public default constructor.

3. You must supply fully qualified class name to Class.forName().

Amar Gunda
 
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