Java: Aggregate Data Off-Heap

by Per Minborg

on December 18, 2018

Java: Aggregate Data Off-Heap

Explore how to create off-heap aggregations with a minimum of garbage collect impact and
maximum memory utilization.

Creating large aggregations using Java Map, List and Object normally creates a lot of heap memory overhead. This also means that the garbage collector will have to clean up these objects once the aggregation goes out of scope.

Read this short article and discover how we can use Speedment Stream ORM to create off-heap aggregations that can utilize memory more efficiently and with little or no GC impact.


Let’s say we have a large number of Person objects that take the following shape:

public class Person {
private final int age;
private final short height;
private final short weight;
private final String gender;
private final double salary;

// Getters and setters hidden for brievity

For the sake of argument, we also have access to a method called persons() that will create a new Stream with all these Person objects.

Salary per Age

We want to create the average salary for each age bucket. To represent the results of aggregations we will be using a data class called AgeSalary which associates a certain age with an average salary.

public class AgeSalary {
private int age;
private double avgSalary;

// Getters and setters hidden for brievity

Age grouping for salaries normally entails less than 100 buckets being used and so this example is just to show the principle. The more buckets, the more sense it makes to aggregate off-heap.


Using Speedment Stream ORM, we can derive an off-heap aggregation solution with these three steps:

Create an Aggregator

var aggregator = Aggregator.builderOfType(Person.class, AgeSalary::new)

The aggregator can be reused over and over again.

Compute an Aggregation

var aggregation = persons().collect(aggregator.createCollector());

Using the aggregator, we create a standard Java stream Collector that has its internal state completely off-heap.

Use the Aggregation Result


Since the Aggregation holds data that is stored off-heap, it may benefit from explicit closing rather than just being cleaned up potentially much later. Closing the Aggregation can be done by calling the close() method, possibly by taking advantage of the AutoCloseable trait, or as in the example above by using streamAndClose() which returns a stream that will close the Aggregation after stream termination.

Everything in a One-Liner

The code above can be condensed to what is effective a one-liner:

persons().collect(Aggregator.builderOfType(Person.class, AgeSalary::new)

There is also support for parallel aggregations. Just add the stream operation Stream::parallel and aggregation is done using the ForkJoin pool.


Download Speedment here

Read more about off-heap aggregations here


Per Minborg

Per Minborg is a Palo Alto based developer and architect, currently serving as CTO at Speedment, Inc. He is a regular speaker at various conferences e.g. JavaOne, DevNexus, Jdays, JUGs and Meetups. Per has 15+ US patent applications and invention disclosures. He is a JavaOne alumni and co-author of the publication “Modern Java”.