Question

assume we have 2 threads, thread A and thread B.

thread A is the main method and contain large data structures.

is it possible to create a second thread and pass the address(a pointer) of the data structure (local to thread A) to thread B so both thread can read from the data structure?

the point of this is to avoid the need to duplicate the entire data structure on thread B or spend a lot of time pulling relevant information from the data structure for thread B to use

keep in mind that neither thread is modifying the data

Answer 1

It’s known as a reference in java, as you don’t have access directly to a pointer in a conventional sense. (For most cases it’s “safe” to think of it as every reference is a pointer that is always passed by value and the only legal operation is to dereference it. It is NOT the same as a C++ ‘reference.’)

You can certainly share references among threads. Anything that’s on the heap can be seen and used by any thread that can get a reference to it. You can either put it in a static location, or set the value of a reference on your Runnable to point to the data.

public class SharedDataTest {

  private static class SomeWork implements Runnable {
    private Map<String, String> dataTable;

    public SomeWork(Map<String, String> dataTable) {
      this.dataTable = dataTable;
    }

    @Override
    public void run() {
      //do some stuff with dataTable
    }
  }

  public static void main(String[] args) {

    Map<String, String> dataTable = new ConcurrentHashMap<String, String>();

    Runnable work1 = new SomeWork(dataTable);
    Runnable work2 = new SomeWork(dataTable);

    new Thread(work1).start();
    new Thread(work2).start();

  }

}