Communicating between two threads

Question

I have a thread, A which has a list. List holds some objects. Now I want to design a mechanisim by which I can send some message to thread A.

Thread A runs in a loop (it does not wait or sleep). Some other thread, B, sends some message to thread A and thread A empties all its queues.

How can I send messages between threads?

class A extends Thread {     List<Object> objs = something; //Init it     void run() {         while(true) {             //Body which works on objects.             //After receiving an external message, "A" should perform some action, for example, empty objects.         }     } } 

EDIT: Can I do it like this?

class A extends Thread {     List<Object> objs = something; //Init it     Boolean flag = false;      public void setFlag(boolean value) {         synchronized(flag) {             this.flag = value;         }     }      public void getFlag() {         synchronized(flag) {             return this.flag;         }     }      void run() {         while(true) {             //Body which works on objects.             //After receiving an external message, A should perform some action, for example, empty objects.             if (getFlag == true)                 //Empty list         }     } } 

Answer 1

You could have a BlockingQueue of message objects. Other threads would place messages onto the queue. As part of the while(true) loop, thread A would poll the queue and process any messages that have arrived.

In code:

class A extends Thread{  List<Object>  objs = something ;//init it  BlockingQueue<Message> queue = new LinkedBlockingQueue<Message>();  void run(){      while(true){        Message msg;        while ((msg = queue.poll()) != null) {          // process msg        }        // do other stuff      }    } } 

Other threads can now call queue.put() to send messages to thread A.

Answer 2

In a simplistic case you can add some instance variable to thread A class and have thread B set its value to indicate that thread A must clear its' queues. In more advanced case you can use some queue of messages that both threads A and B can access. B would place a message there and A would read and act on it.

In all cases, access to variable or queue must be properly guarded for multiple threads access.