21 import javax.inject.Provider;
24  * A {@link Provider} implementation that memoizes the result of another {@link Provider} using
27 public final class SingleCheck<T> implements Provider<T> {
30   private volatile Provider<T> provider;  field in SingleCheck
33   private SingleCheck(Provider<T> provider) {  in SingleCheck()  argument
34     assert provider != null;  in SingleCheck()
35     this.provider = provider;  in SingleCheck()
38   @SuppressWarnings("unchecked") // cast only happens when result comes from the delegate provider
43       // provider is volatile and might become null after the check, so retrieve the provider first  in get()
44       Provider<T> providerReference = provider;  in get()
46         // The provider was null, so the instance must already be set  in get()
52         // Null out the reference to the provider. We are never going to need it again, so we can  in get()
54         provider = null;  in get()
60   /** Returns a {@link Provider} that caches the value from the given delegate provider. */
61   // This method is declared this way instead of "<T> Provider<T> provider(Provider<T> provider)" 
63   public static <P extends Provider<T>, T> Provider<T> provider(P provider) {  in provider()  method in SingleCheck
65     if (provider instanceof SingleCheck || provider instanceof DoubleCheck) {  in provider()
66       return provider;  in provider()
68     return new SingleCheck<T>(checkNotNull(provider));  in provider()