Java threads calling parent functions

Im making some software written in java, it involves a single 'Parent' class that invokes dozens of classes(nodes) that run in their own thread (extends Thread).

The classes store the parent class as a variable. If the node class invokes a method from the parent class, does each node store a copy of the parent object or do all the nodes act upon the same object? if all nodes act on the same object, do statements in the called function run in the thread of the node or in the thread of the parent class?

EX code:

class parent { ArrayList<node> nodes = new ArrayList<node>(); void createStartNodes() { for(int i =0; i < 36;++i) { nodes.put(i,new node(this)); nodes.get(i).start(); } } void callSomeFunc() { /* things */ } } class node extends Thread { parent par; public node(parent p) { par=p; } @Override public void run() { par.callSomeFunc(); } }

-------------Problems Reply------------

does each node store a copy of the parent object or do all the nodes act upon the same object?

They each store a pointer to the memory location of the parent object. They indeed act upon the same object.

if all nodes act on the same object, do statements in the called function run in the thread of the node or in the thread of the parent class?

They run in the thread of the node that tries to modify the parent object. You may want to make sure you use synchronization properly when doing that, for example by using a synchronized block. Otherwise, when there is concurrent access to the parent object, you may get incorrect / inconsistent behaviour. Example:

class Node extends Thread
{
Parent par;

public Node(Parent p)
{
par=p;
}

@Override
public void run()
{
synchronized(par) {
par.callSomeFunc();
}
}
}

The synchronized block will give this thread exclusive access to the parent object.

Also, thread specific logic should be in the run method, not in the start method.

I think you're confusing things. I just want to clarify these before answering your questions:

  • A class does not "run in a thread", because it does not "run". You can run methods, not classes. A method of a class could be executed in different threads depending on where you called it.
  • When extending the Thread class (note the capital T), you should override the run() method, not the start() method, otherwise the multithreading stuff won't happen. However, you indeed have to call start(), defined in the Thread class, which in turn will call your run() method in the created thread, as you would expect. [EDIT: this point was apparently solved in the OP's edit]

That being said, back to your actual questions:

does each node store a copy of the parent object or do all the nodes act upon the same object?

Every java object (read non-primitive type) is just a reference, you don't pass object values. Therefore, all your nodes store a reference to the same object, because you gave them the reference (this) to the same object when calling their constructor.

do statements in the called function run in the thread of the node or in the thread of the parent class?

In the current state of your code, you're not overriding run() but start(), so you don't have multiple threads anyway. (solved by OP)

The code executed in the run() method will be executed in the thread of the node, even when calling other methods from run().

Sidenote: this is because you called node.start() properly. If you directly called node.run(), it would be executed in the thread from which you call run().

First of all you are not supposed to override the start() method of Thread class, you are supposed to call it. And Similarly you are not supposed to call run() method of Thread class, you supposed to override it.

So when you call the method which is written in the your "parent" class from the run() method, it acts in the new instance and its not shared by all the other thread that you have created.

Edit: As you have passed "this" to constructor, they will share the same object.

Category:java Views:6 Time:2018-03-13
Tags: java

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