Guava Cache of maximum size 1

Question

Is it ok to use a Guava Cache of maximum size 1?

I'm reading that sometimes there can be evictions even before that maximum size is reached.

However, I only need one cache entry. So I'm wondering what value to set for maximum size that would be safe, but not excessive.

Answer 1

You should be able to use Cache.cleanUp() as explained in When Does Cleanup Happen? and test whether a maximum size of 1 suites your needs or not.

e.g. The following shows that using a LoadingCache with maximum size of 1 will not evict an existing entry until a different entry is loaded to take its place:

final LoadingCache<Character, Integer> loadingCache = CacheBuilder.newBuilder()
        .maximumSize(1)
        .build(new CacheLoader<Object, Integer>() {
            private final AtomicInteger loadInvocationCount = new AtomicInteger();

            @Override
            public Integer load(Object key) throws Exception {
                return loadInvocationCount.getAndIncrement();
            }
        });
assert loadingCache.size() == 0;

assert loadingCache.getUnchecked('a') == 0;
assert loadingCache.size() == 1;

loadingCache.cleanUp();
assert loadingCache.size() == 1;

assert loadingCache.getUnchecked('a') == 0;
assert loadingCache.size() == 1;

assert loadingCache.getUnchecked('b') == 1;
assert loadingCache.size() == 1;

loadingCache.cleanUp();
assert loadingCache.size() == 1;

assert loadingCache.getUnchecked('a') == 2;
assert loadingCache.size() == 1;

loadingCache.cleanUp();
assert loadingCache.size() == 1;

Note that this may be specific to the type of LoadingCache being built so you will need to test whatever configuration you plan to use.