Use a registry
Use a singleton registry to:
Specify singleton classes at runtime
Prevent singleton subclasses from allowing multiple instances
Example 8 lists a singleton class that maintains a registry of singletons, registered by class name:
Example 8. A singleton with a registry
import java.util.HashMap;
import org.apache.log4j.Logger;
public class Singleton {
private static HashMap map = new HashMap();
private static Logger logger = Logger.getRootLogger();
protected Singleton() {
// Exists only to thwart instantiation
}
public static synchronized Singleton getInstance(String classname) {
if(classname == null) throw new IllegalArgumentException("Illegal classname");
Singleton singleton = (Singleton)map.get(classname);
if(singleton != null) {
logger.info("got singleton from map: " + singleton);
return singleton;
}
if(classname.equals("SingeltonSubclass_One"))
singleton = new SingletonSubclass_One();
else if(classname.equals("SingeltonSubclass_Two"))
singleton = new SingletonSubclass_Two();
map.put(classname, singleton);
logger.info("created singleton: " + singleton);
return singleton;
}
// Assume functionality follows that's attractive to inherit
}
The preceding base class creates subclass instances and stores them in a map. But that base class is high maintenance because you must update its getInstance() method for every subclass. Luckily, we can use reflection to skirt that issue.
Use reflection
Example 9 lists a singleton with a registry that uses reflection to instantiate a particular class's objects. With this implementation, as opposed to Example 8, the Singleton.getInstance() method does not need to update when new subclasses are implemented.
Example 9. Use reflection to instantiate singletons
import java.util.HashMap;
import org.apache.log4j.Logger;
public class Singleton {
private static HashMap map = new HashMap();
private static Logger logger = Logger.getRootLogger();
protected Singleton() {
// Exists only to thwart instantiation
}
public static synchronized Singleton getInstance(String classname) {
Singleton singleton = (Singleton)map.get(classname);
if(singleton != null) {
logger.info("got singleton from map: " + singleton);
return singleton;
}
try {
singleton = (Singleton)Class.forName(classname).newInstance();
}
catch(ClassNotFoundException cnf) {
logger.fatal("Couldn't find class " + classname);
}
catch(InstantiationException ie) {
logger.fatal("Couldn't instantiate an object of type " + classname);
}
catch(IllegalAccessException ia) {
logger.fatal("Couldn't access class " + classname);
}
map.put(classname, singleton);
logger.info("created singleton: " + singleton);
return singleton;
}
}
One more thing concerning singleton registries: they should be encapsulated in their own class for maximum reuse.
Encapsulate the registry
Example 10 lists a singleton registry class:
Example 10. A SingletonRegistry class
import java.util.HashMap;
import org.apache.log4j.Logger;
public class SingletonRegistry {
public static SingletonRegistry REGISTRY = new SingletonRegistry();
private static HashMap map = new HashMap();
private static Logger logger = Logger.getRootLogger();
protected SingletonRegistry() {
// Exists to defeat instantiation
}
public static synchronized Object getInstance(String classname) {
Object singleton = map.get(classname);
if(singleton != null) {
return singleton;
}
try {
singleton = Class.forName(classname).newInstance();
logger.info("created singleton: " + singleton);
}
catch(ClassNotFoundException cnf) {
logger.fatal("Couldn't find class " + classname);
}
catch(InstantiationException ie) {
logger.fatal("Couldn't instantiate an object of type " +
classname);
}
catch(IllegalAccessException ia) {
logger.fatal("Couldn't access class " + classname);
}
map.put(classname, singleton);
return singleton;
}
}
Notice I implemented the SingletonRegistry class as a singleton. I also generalized the registry so it can store and retrieve any type of object. Example 11 shows a Singleton class that uses the registry:
Example 11. A Singleton class that uses the registry
import java.util.HashMap;
import org.apache.log4j.Logger;
public class Singleton {
protected Singleton() {
// Exists only to thwart instantiation.
}
public static Singleton getInstance() {
return (Singleton)SingletonRegistry.REGISTRY.getInstance(classname);
}
}
The preceding Singleton class uses the registry's singleton instance to retrieve singleton objects by class name.
Now that we've seen how to implement thread-safe singletons and how to use a registry to specify singleton class names at runtime, let's examine how to deal with classloaders and serialization.
Classloaders
Because multiple classloaders are commonly used in many situations—including servlet containers—you can wind up with multiple singleton instances no matter how carefully you've implemented your singleton classes. If you want to make sure the same classloader loads your singletons, you must specify the classloader yourself; for example:
private static Class getClass(String classname)
throws ClassNotFoundException {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
if(classLoader == null)
classLoader = Singleton.class.getClassLoader();
return (classLoader.loadClass(classname));
}
}
The preceding method tries to associate the classloader with the current thread; if that classloader is null, the method uses the same classloader that loaded a singleton base class. The preceding method can be used instead of Class.forName().
Serialization
If you serialize a singleton and then deserialize it twice, you will have two instances of your singleton, unless you implement the readResolve() method, like this:
Example 12. A serializable singleton
import org.apache.log4j.Logger;
public class Singleton implements java.io.Serializable {
public static Singleton INSTANCE = new Singleton();
protected Singleton() {
// Exists only to thwart instantiation.
}
private Object readResolve() {
return INSTANCE;
}
}
The previous singleton implementation returns the lone singleton instance from the readResolve() method; therefore, whenever the Singleton class is deserialized, it will return the same singleton instance.
Example 13 tests Example 12's singleton:
Example 13. Test a serializable singleton
import java.io.*;
import org.apache.log4j.Logger;
import junit.framework.Assert;
import junit.framework.TestCase;
public class SingletonTest extends TestCase {
private Singleton sone = null, stwo = null;
private static Logger logger = Logger.getRootLogger();
public SingletonTest(String name) {
super(name);
}
public void setUp() {
sone = Singleton.INSTANCE;
stwo = Singleton.INSTANCE;
}
public void testSerialize() {
logger.info("testing singleton serialization...");
writeSingleton();
Singleton s1 = readSingleton();
Singleton s2 = readSingleton();
Assert.assertEquals(true, s1 == s2);
}
private void writeSingleton() {
try {
FileOutputStream fos = new FileOutputStream("serializedSingleton");
ObjectOutputStream oos = new ObjectOutputStream(fos);
Singleton s = Singleton.INSTANCE;
oos.writeObject(Singleton.INSTANCE);
oos.flush();
}
catch(NotSerializableException se) {
logger.fatal("Not Serializable Exception: " + se.getMessage());
}
catch(IOException iox) {
logger.fatal("IO Exception: " + iox.getMessage());
}
}
private Singleton readSingleton() {
Singleton s = null;
try {
FileInputStream fis = new FileInputStream("serializedSingleton");
ObjectInputStream ois = new ObjectInputStream(fis);
s = (Singleton)ois.readObject();
}
catch(ClassNotFoundException cnf) {
logger.fatal("Class Not Found Exception: " + cnf.getMessage());
}
catch(NotSerializableException se) {
logger.fatal("Not Serializable Exception: " + se.getMessage());
}
catch(IOException iox) {
logger.fatal("IO Exception: " + iox.getMessage());
}
return s;
}
public void testUnique() {
logger.info("testing singleton uniqueness...");
Singleton another = new Singleton();
logger.info("checking singletons for equality");
Assert.assertEquals(true, sone == stwo);
}
}
The preceeding test case serializes Example 12's singleton and deserializes it twice. Then the test case checks to see if the deserialized singletons are the same object. Here's the test case output:
Buildfile: build.xml
init:
[echo] Build 20030422 (22-04-2003 11:32)
compile:
run-test-text:
[java] .INFO main: testing singleton serialization...
[java] .INFO main: testing singleton uniqueness...
[java] INFO main: checking singletons for equality
[java] Time: 0.1
[java] OK (2 tests)
Singleton sign-off
The Singleton pattern is deceivingly simple, especially for Java developers. In this article, I've demonstrated how Java developers implement singletons, considering multithreading, classloaders, and serialization. I've also shown how you can implement singleton registries that let you specify singleton classes at runtime.
