java - Thread-safe updating of a cached resource -

the logic pretty straightforward:

foo foo = cache.get(); if (isuptodate(foo)) {     return foo; } else {     foo = getupdatedfoo(); // slow or expensive     cache.put(foo);     return foo; } 

however, want make sure that

1. only 1 thread calls getupdatedfoo() @ time
2. if thread calling getupdatedfoo(), thread b doesn't call it, instead waiting thread a's results

i cobble based on memoizer pattern jcip, suspect there's simpler way -- possibly using guava cachebuilder? not obvious how, though.

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update: implemented double-checked locking pattern per frankpl's answer below:

foo foo = cache.get(); if (!isuptodate(foo)) {     lock.lock(); // block if other thread refreshing     try {         // see if other thread refreshed         foo = cache.get();         if (!isuptodate(foo)) {             // guess not, we'll refresh ourselves             foo = getupdatedfoo();             cache.put(foo);         }     } {         lock.unlock();     } } return foo; 

see http://en.wikipedia.org/wiki/double-checked_locking double checked locking pattern use. instead of new helper() used in code samples, use getupdatedfoo().