使用Guava适配不同的callback

Cache<Key,Value> cache =CacheBuilder.newBuilder()
    .maximumSize(1000)
    .build();// look Ma, no CacheLoader
...
try{
    // If the key wasn't in the "easy to compute" group, we need to
    // do things the hard way.
    cache.get(key,newCallable<Value>(){
        @Override
        publicValue call()throwsAnyException{
            return doThingsTheHardWay(key);
        }
    });
}catch(ExecutionException e){
    thrownewOtherException(e.getCause());
}