传递给Java线程的对象是否占用堆外空间？

Question

I have an object like:

final List<Rows> rowsToSubmit = new ArrayList<>(rows);

And I instantiate threads as in:

// submit to a thread
executorService.submit(new Callable<Boolean>() {
  @Override
  public Boolean call() throws Exception {
    Object threadObj = new Object();
    return bq.doHttpPost(rowsToSubmit); // takes about 3 seconds for IO
  }
});

Does Java utilize the thread stack memory with the rowsToSubmit object as well? In other words do I need to increase my off-heap memory (I think Java thread stack resides there?) if my rowsToSubmit is huge?

Also, is threadObj also initialized on the off-heap space?

Answer 1

All java objects are stored on the heap, so you shouldn't need to configure off-heap memory.

Java is a pass-by-reference language, meaning that objects are never duplicated between methods or threads. Only pointers are passed around, and the stack holds only primitives and pointers to objects. For example, while a pointer to the rowsToSubmit object is stored on the thread-stack, the memory for the contents of the rowsToSubmit object is still stored on the heap - the memory is shared between threads.

Likewise, I wouldn't say that threadObj is initialized on the off-heap space because all java objects are on the heap. The reference to threadObj is stored in the off-heap space.

Further reading: http://www.journaldev.com/4098/java-heap-space-vs-stack-memory

Answer 2

Java Spec

Each Java Virtual Machine thread has a private Java Virtual Machine stack, created at the same time as the thread. A Java Virtual Machine stack stores frames (§2.6). A Java Virtual Machine stack is analogous to the stack of a conventional language such as C: it holds local variables and partial results, and plays a part in method invocation and return. Because the Java Virtual Machine stack is never manipulated directly except to push and pop frames, frames may be heap allocated. The memory for a Java Virtual Machine stack does not need to be contiguous.

In The Java Virtual Machine Specification, First Edition, the Java Virtual Machine stack was known as the Java stack.

This specification permits Java Virtual Machine stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the Java Virtual Machine stacks are of a fixed size, the size of each Java Virtual Machine stack may be chosen independently when that stack is created.

Second when you pass an object through argument as in it is always pass by value.So in case it will not create any object in heap only copy the reference to the local variable and both will point to same object.

Answer 3

Java utilizes the stack memory by access the pattern which makes it trivial to allocate and deallocate memory from it (a pointer/integer is simply incremented or decremented), while the heap has much more complex bookkeeping involved in an allocation or free.

Each Thread in Java has their own stack which can be specified using -Xss JVM parameter, similarly, you can also specify heap size of Java program using JVM option -Xms and -Xmx where -Xms is starting size of the heap and -Xmx is a maximum size of java heap.