Singleton, DCL, Holder Idiom, and When Singleton is not a Singleton

For eager-initialization singleton, the simplest way is to use an enum type with only 1 INSTANCE.

public enum EnumSingleton implements Calc {
    INSTANCE;

    public int add(int a, int b) {
        return a + b;
    }
}

A traditional, non-enum implementation of eager-initialization singleton is:

public class EagerSingleton {
    private static EagerSingleton instance = new EagerSingleton();

    private EagerSingleton() {}

    public static EagerSingleton getInstance() { return instance; }
} 

A lazy-initialization singleton implemented with double-checked locking (DCL), which works in Java 5 onward:

package test.concurrent;
public final class MySingleton1 {
 private static volatile MySingleton1 instance;

 private MySingleton1() {}

 public static MySingleton1 getInstance() {
   MySingleton1 result = instance;
   if(result == null) {
     synchronized (MySingleton1.class) {
       result = instance;
       if(result == null) {
         instance = result = new MySingleton1();
       }
     }
   }
   return result;
 }

A lazy-initialization singleton implemented with holder idoim (applicable to lazy static field):

package test.concurrent;
public final class MySingleton2 {
 private MySingleton2() {}

 private static class Holder {
   private static final MySingleton2 instance = new MySingleton2();
 }

 public static MySingleton2 getInstance() {
   return Holder.instance;
 }

A main method that spawns multiple threads calling getInstance():

public static void main(String[] args) {
   int numOfThreads = Integer.parseInt(args[0]);
   for(int i = 0; i < numOfThreads; i++) {
     Thread t = new Thread(new Runnable() {
       public void run() {
         System.out.println(MySingleton1.getInstance());
       }
     });
     t.start();
   }
 }

When the same singleton class is loaded by different class loaders, each loader has its own class data. MySingleton1.class loaded by classloader1 is a distinct class from MySingleton1.class loaded by classloader2. Hence multiple instances will be created when calling MySingleton1.getInstance().

To test this scenario, write a servlet that calls MySingleton1.getInstance, and print it. Package the servlet class and MySingleton1 class in test.war, deploy it to appserver. Redeploy the same war in a different context root test2. Invoke the 2 webapp to see if the same instance is printed.

The servlet class:

package test;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

@javax.servlet.annotation.WebServlet(urlPatterns = "/*")
public class TestServlet extends HttpServlet {
 protected void processRequest(HttpServletRequest request, HttpServletResponse response)
 throws ServletException, IOException {
   PrintWriter out = response.getWriter();
   out.println("singleton: " + MySingleton1.getInstance());
 }
 @Override
 protected void doGet(HttpServletRequest request, HttpServletResponse response)
 throws ServletException, IOException {
   processRequest(request, response);
 }
 @Override
 protected void doPost(HttpServletRequest request, HttpServletResponse response)
 throws ServletException, IOException {
   processRequest(request, response);
 }
}

The content of test.war:

WEB-INF/classes/test/MySingleton1$1.class
WEB-INF/classes/test/MySingleton1.class
WEB-INF/classes/test/TestServlet.class

To deploy the webapp to GlassFish:

$ asadmin deploy test.war
$ asadmin deploy --name test2 test.war
$ asadmin list-applications

To run the 2 webapp and notice 2 different instances of MySingleton class are instantiated within the same JVM, one for each webapp:

$ curl http://localhost:8080/test/
singleton: test.MySingleton1@114536a5

$ curl http://localhost:8080/test/
singleton: test.MySingleton1@114536a5

$ curl http://localhost:8080/test2/
singleton: test.MySingleton1@3997ebf6

$ curl http://localhost:8080/test2/
singleton: test.MySingleton1@3997ebf6