Java Tip: ComputeIfAbsent

Often we have to loop through a list of items and convert it into a map, with the key being some sort of category and the value being a set of the associated items ie Map<Key, Set<Object>>. The old way of doing this looped through the list and before adding checked if the key was present. Using computeIfAbsent we can clean up this syntax considerably.

Instead of


// Map of item types to items
Map<String, Set<Items>> map = new HashMap<>();

for (Item item: items) {
    if (map.containsKey(item.getType())) {
        map.get(item.getType().add(item));
    } else {
        Set newSet = new HashSet<>();
        newSet.add(item);
        map.put(item.getType(), newSet));
    }
}

Do

// Map of item types to items
Map<String, Set<Items>> map = new HashMap<>();

for (Item item: items) {
    map.computeIfAbsent(item.getType(), v -> new HashSet<>()).
    add(item);
}

Jan 01, 0001
Batch Processing

Spring Batch: Query All the Steps of a Batch Job

In Spring Batch, in order to get the job_execution_id of the last batch job instance for a given batch job name use this query:

select bje.job_execution_id from batch_job_instance bji
join batch_job_execution bje
on bji.job_instance_id = bje.job_instance_id
where bji.job_name = 'jobname'
order by bje.start_time desc
limit 1;

In order to get all the steps for the latest batch job instance for a given batch job name use this query:

select *
from batch_job_execution bje
join batch_job_instance bji
on bje.job_instance_id = bji.job_instance_id
join batch_job_step_execution bse
on bse.job_execution_id = bje.job_execution_id
and bje.job_execution_id = 
(
    select bje.job_execution_id from batch_job_instance bji
    join batch_job_execution bje
    on bji.job_instance_id = bje.job_instance_id
    where bji.job_name = 'jobname'
    order by bje.start_time desc
    limit 1
)
order by bse.start_time;