Lazy loading and caching are extremely useful tools in the developer toolbox, like most tools they can be often overused / abused so use them sparingly. Lazy loading can be useful when creating a singleton with any variety of double checked locking, static holder class, enum singleton pattern, etc. Lazy loading is also great for expensive operations that you only need to handle some of the time. Hibernate heavily utilizes lazy loading. Lazy loading can either be request scoped or in a more global scope. Google's Guava provides an extremely convenient way to create lazy loaded values as well as caching individual values with or without an expiration.

Supplier

We will be using a very simple "hello world" Supplier that will log every time it is called.

public static String helloWorldSupplier() {
    log.info("supplying");
    return "hello world";
}

Suppliers.memoize

Suppliers.memoize is a simple method that takes a Supplier and returns a new Supplier that caches the value returned from the supplied Supplier.get() method. This convenient helper allows us to turn any Supplier into a lazily loaded cached value. The returned Supplier is thread-safe backed with double checked locking and volatile fields. It even allows the original Supplier to be garbage collected once it has been called. If you are utilizing this pattern for code that is not multi-threaded it might be useful to make a non thread-safe version.

log.info("Memoized");
Supplier<String> memoized = Suppliers.memoize(SuppliersExamples::helloWorldSupplier);
log.info(memoized.get());
log.info(memoized.get());

2017-09-06 08:50:58.069 [main] INFO  c.s.e.common.SuppliersExamples - Memoized
2017-09-06 08:50:58.156 [main] INFO  c.s.e.common.SuppliersExamples - supplying
2017-09-06 08:50:58.157 [main] INFO  c.s.e.common.SuppliersExamples - hello world
2017-09-06 08:50:58.157 [main] INFO  c.s.e.common.SuppliersExamples - hello world

Suppliers.memoizeWithExpiration

Suppliers.memoizeWithExpiration is also straightforward. It allows us to memoize a value from a given Supplier but have it update anytime we exceed the expiration time. This is a great caching mechanism for any data you know changes infrequently. A minor drawback is if the operation is expensive you may see a hiccup every time the object needs to be reloaded. Often times this is not a major concern. If it is an issue you can investigate refreshing the object asynchronously with a background thread. Suppliers.memoizeWithExpiration is used to cache the scraped results for our HTML / CSS Themes page and can be seen in the Web scraping in Java with jsoup and OkHttp.

log.info("Memoized with Expiration");
Supplier<String> memoizedExpiring = Suppliers.memoizeWithExpiration(
    SuppliersExamples::helloWorldSupplier, 50, TimeUnit.MILLISECONDS);
log.info(memoizedExpiring.get());
log.info(memoizedExpiring.get());
log.info("sleeping");
TimeUnit.MILLISECONDS.sleep(100);
log.info(memoizedExpiring.get());
log.info(memoizedExpiring.get());
log.info("sleeping");
TimeUnit.MILLISECONDS.sleep(100);
log.info(memoizedExpiring.get());
log.info(memoizedExpiring.get());

2017-09-06 08:50:58.157 [main] INFO  c.s.e.common.SuppliersExamples - Memoized with Expiration
2017-09-06 08:50:58.171 [main] INFO  c.s.e.common.SuppliersExamples - supplying
2017-09-06 08:50:58.171 [main] INFO  c.s.e.common.SuppliersExamples - hello world
2017-09-06 08:50:58.171 [main] INFO  c.s.e.common.SuppliersExamples - hello world
2017-09-06 08:50:58.171 [main] INFO  c.s.e.common.SuppliersExamples - sleeping
2017-09-06 08:50:58.272 [main] INFO  c.s.e.common.SuppliersExamples - supplying
2017-09-06 08:50:58.272 [main] INFO  c.s.e.common.SuppliersExamples - hello world
2017-09-06 08:50:58.272 [main] INFO  c.s.e.common.SuppliersExamples - hello world
2017-09-06 08:50:58.272 [main] INFO  c.s.e.common.SuppliersExamples - sleeping
2017-09-06 08:50:58.377 [main] INFO  c.s.e.common.SuppliersExamples - supplying
2017-09-06 08:50:58.377 [main] INFO  c.s.e.common.SuppliersExamples - hello world
2017-09-06 08:50:58.377 [main] INFO  c.s.e.common.SuppliersExamples - hello world