Java Singleton and Synchronization

Question

Please clarify my queries regarding Singleton and Multithreading    What is the best way to implement Singleton in Java  in a multithreaded environment  What happens when multiple threads try to access getInstance   method at the same time  Can we make singleton s getInstance   synchronized  Is synchronization really needed  when using Singleton classes

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Yes  you need to make getInstance   synchronized  If it s not there might arise a situation where multiple instances of the class can be made    Consider the case where you have two threads that call getInstance   at the same time  Now imagine T1 executes just past the instance    null check  and then T2 runs  At this point in time the instance is not created or set  so T2 will pass the check and create the instance  Now imagine that execution switches back to T1  Now the singleton is created  but T1 has already done the check  It will proceed to make the object again  Making getInstance   synchronized prevents this problem   There a few ways to make singletons thread-safe  but making getInstance   synchronized is probably the simplest

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If you are working on a multithreaded environment in Java and need to gurantee all those threads are accessing a single instance of a class you can use an Enum  This will have the added advantage of helping you handle serialization   public enum Singleton       SINGLE      public void myMethod                and then just have your threads use your instance like   Singleton SINGLE myMethod

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You can also use static code block to instantiate the instance at class load and prevent the thread synchronization issues   public class MySingleton      private static final MySingleton instance     static        instance   new MySingleton           private MySingleton            public static MySingleton getInstance         return instance

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What is the best way to implement Singleton in Java  in a multithreaded environment    Refer to this post for best way to implement Singleton   What is an efficient way to implement a singleton pattern in Java      What happens when multiple threads try to access getInstance   method at the same time    It depends on the way you have implemented the method If you use double locking without volatile variable  you may get partially constructed Singleton object   Refer to this question for more details   Why is volatile used in this example of double checked locking     Can we make singleton s getInstance   synchronized       Is synchronization really needed  when using Singleton classes    Not required if you implement the Singleton in below ways   static intitalization enum   LazyInitalaization with  Initialization-on-demand holder idiom    Refer to this question fore more details  Java Singleton Design Pattern   Questions

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Enum singleton  The simplest way to implement a Singleton that is thread-safe is using an Enum  public enum SingletonEnum     INSTANCE    public void doSomething        System out println  This is a singleton            This code works since the introduction of Enum in Java 1 5  Double checked locking  If you want to code a    classic    singleton that works in a multithreaded environment  starting from Java 1 5  you should use this one   public class Singleton      private static volatile Singleton instance   null     private Singleton            public static Singleton getInstance         if  instance    null          synchronized  Singleton class           if  instance    null              instance   new Singleton                                return instance           This is not thread-safe before 1 5 because the implementation of the volatile keyword was different   Early loading Singleton  works even before Java 1 5   This implementation instantiates the singleton when the class is loaded and provides thread safety   public class Singleton      private static final Singleton instance   new Singleton       private Singleton            public static Singleton getInstance         return instance         public void doSomething        System out println  This is a singleton

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Yes  it is necessary  There are several methods you can use to achieve thread safety with lazy initialization   Draconian synchronization   private static YourObject instance   public static synchronized YourObject getInstance         if  instance    null            instance   new YourObject              return instance      This solution requires that every thread be synchronized when in reality only the first few need to be   Double check synchronization   private static final Object lock   new Object    private static volatile YourObject instance   public static YourObject getInstance         YourObject r   instance      if  r    null            synchronized  lock          While we were waiting for the lock  another              r   instance            thread may have instantiated the object              if  r    null                      r   new YourObject                    instance   r                                    return r      This solution ensures that only the first few threads that try to acquire your singleton have to go through the process of acquiring the lock   Initialization on Demand   private static class InstanceHolder       private static final YourObject instance   new YourObject       public static YourObject getInstance         return InstanceHolder instance      This solution takes advantage of the Java memory model s guarantees about class initialization to ensure thread safety  Each class can only be loaded once  and it will only be loaded when it is needed  That means that the first time getInstance is called  InstanceHolder will be loaded and instance will be created  and since this is controlled by ClassLoaders  no additional synchronization is necessary

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This pattern does a thread-safe lazy-initialization of the instance without explicit synchronization   public class MySingleton         private static class Loader            static final MySingleton INSTANCE   new MySingleton                 private MySingleton             public static MySingleton getInstance              return Loader INSTANCE             It works because it uses the class loader to do all the synchronization for you for free  The class MySingleton Loader is first accessed inside the getInstance   method  so the Loader class loads when getInstance   is called for the first time  Further  the class loader guarantees that all static initialization is complete before you get access to the class - that s what gives you thread-safety   It s like magic   It s actually very similar to the enum pattern of Jhurtado  but I find the enum pattern an abuse of the enum concept  although it does work

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public class Elvis       public static final Elvis INSTANCE   new Elvis       private Elvis               Source   Effective Java -  Item 2   It suggests to use it  if you are sure that class will always remain singleton

[java] Java Singleton and Synchronization

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