Initialization on demand holder idiom

In software engineering, the Initialization on Demand Holder idiom (design pattern) is a lazy-loaded singleton. The idiom can be implemented in both single-threaded/serial and concurrent environments, but care must be taken to correctly implement the idiom under concurrent conditions.

Example Java Implementation

This implementation from Bill Pugh (see links below) is a well-performing and concurrent implementation valid in all versions of Java. The original implementation from Bill Pugh has been modified to reduce the scope of LazyHolder.something to package and to make the field final.

public class Something {
      private Something() {
      }

      private static class LazyHolder {
              private static final Something something = new Something();
      }

      public static Something getInstance() {
              return LazyHolder.something;
      }
} 

How it works

The implementation relies on the well-specified initialization phase of execution within the Java Virtual Machine (JVM); see section 12.4 of Java Language Specification (JLS) for details.

When the class Something is loaded by the JVM, the class goes through initialization. Since the class does not have any static variables to initialize, the initialization completes trivially. The static class definition LazyHolder within it is not initialized until the JVM determines that LazyHolder must be executed. The static class LazyHolder is only executed when the static method getInstance is invoked on the class Something, and the first time this happens the JVM will load and initialize the LazyHolder class. The initialization of the LazyHolder class results in static variable something being initialized by executing the (private) constructor for the outer class Something. Since the class initialization phase is guaranteed by the JLS to be serial, i.e., non-concurrent, no further synchronization is required in the static getInstance method during loading and initialization. And since the initialization phase writes the static variable something in a serial operation, all subsequent concurrent invocations of the getInstance will return the same correctly initialized something without incurring any additional synchronization overhead.

When to use it

Use this pattern if the initialization of the class is expensive and it cannot be done safely at class-loading time and the initialization is highly concurrent. The crux of the pattern is the safe removal of the synchronization overhead associated with accessing a singleton instance.