Java GC 如何处理从大 Stream 加载的已处理 object 超过可用堆 ZCD69BZ4957F06CD8181？

Question

Let's say I have a stream of objects loaded from a database (using Spring Data JPA as follow)

public interface MyJpaRepository extends JpaRepository<Foo, String> {

  Stream<Foo> findAll();
}

And let's say there's millions of Foo objects stored in my database using way more GB than my max heap memory size.

I'm expecting consuming the stream as follow would let the JVM handle properly its heap memory by garbage collecting processed objects as more are loaded from the database:

try (Stream<Foo> fooStream =
    myJpaRepository.findAll()) {
  fooStream.forEach(entity -> logger.info("Hello !"));
}

But in facts, this exact code throws an out of memory exception.

• How does the garbage collector acts in this case?
• How consuming this stream using a forEach requires the JVM to entirely load the data from the stream in memory (as per my understanding)?

Thank you

Answer 1

Java Stream won't fetch all the data from the underlying database. Streams do not store data; rather, they provide data from a source such as a collection, array, or IO channel. Generally, these are lazily evaluated. So, when the looger.info gets called on each entity, stream will fetch the data from the underlying data store and apply the command. Since the stream just provides an iterator, it only needs to fetch the next data in the iteration not the whole set. And the GC will remove the fetched data once the lambda function has been applied to it.

Answer 2

In your scenario, garbage collector will not get time spot to act and clean up your memory. Let me try to explain in more details. When you start your java process, you configured heap memory as well as garbage collection algorithm. In case if you didn't fine tune either of them, JVM take for granted the default settings and proceed. Once your process starts allocating heap, JVM internally collects statistics and schedule garbage collection process. But if your process doesn't provide the breathing space to take a decision on when and how to collect garbage, JVM will throw Out of Memory(OOM) error and crash as you observed.

Answer 3

@ernest_k was 100% in his comment, this issue has nothing to do with Streams. As @avishek-bhattacharya explained:

Streams do not store data; rather, they provide data from a source such as a collection, array, or IO channel. Generally, these are lazily evaluated.

In fact, Postgres (the underlying DB in my case) always returns the entire ResultSet unless configured otherwise (samething for MySQL). And to configure it to use a database Cursor, you need to do as follow:

public interface MyJpaRepository extends JpaRepository<Foo, String> {

  @QueryHints(
    value = {
      @QueryHint(name = HINT_FETCH_SIZE, value = "1000"),
      @QueryHint(name = HINT_CACHEABLE, value = "false"),
      @QueryHint(name = HINT_READONLY, value = "true")
  })
  Stream<Foo> findAll();
}