Does Google Guava Cache load on same thread?

Question

Does Google Guava Cache load the cache on the same thread by default?

Code:

cache = CacheBuilder
    .newBuilder()
    .refreshAfterWrite(2, TimeUnit.SECONDS)
    .build(new CacheLoader<String,String>() {
        @Override
        public String load(String s) throws Exception {
            return addCache(s);
        }
});

Will the call to addCache be made on a different thread? As far as I know, it is a synchronous call but I am not sure.

score 8 · Accepted Answer · answered Oct 25 '15 at 12:34

Here's a simple test allowing to know:

    System.out.println("Thread.currentThread() = " + Thread.currentThread());
    LoadingCache<String, String> cache = CacheBuilder
        .newBuilder()
        .refreshAfterWrite(2, TimeUnit.SECONDS)
        .build(new CacheLoader<String, String>() {
            @Override
            public String load(String s) throws Exception {
                System.out.println("Thread.currentThread() = " + Thread.currentThread());
                return "world";
            }
        });
    cache.get("hello");

Output:

Thread.currentThread() = Thread[main,5,main]
Thread.currentThread() = Thread[main,5,main]

Of course, as the documentation indicates, if another thread has already started loading the value for the key, the current thread won't reload it: it will wait for the value to be loaded by the other one:

If another call to get(K) or getUnchecked(K) is currently loading the value for key, simply waits for that thread to finish and returns its loaded value.

Thanks! Actually I had written this test and it did return the same thread ids. But the documentation confused me a bit. I thought that I was not using the cache properly. So, just to be clear, we need to overload `CacheLoader.reload` for the call to be asynchronous, right? — tourniquet_grab, Oct 25 '15 at 20:05
Its quite late to answer , but yes you have to overload it for your CacheLoader. — Prakhar Asthana, Sep 19 '17 at 13:26

score 2 · Answer 2 · answered Jul 06 '21 at 04:42

To add to JB Nizet's answer, you can find out why Guava avoids making the cache multi-threaded by default here:

The reason for this is as follows: if we wanted to perform Cache maintenance continuously, we would need to create a thread, and its operations would be competing with user operations for shared locks. Additionally, some environments restrict the creation of threads, which would make CacheBuilder unusable in that environment.

Does Google Guava Cache load on same thread?

2 Answers2