inter thread communication in java

Question

How do threads that rely on one another communicate in Java?

For example, I am building a web crawler with threads that need data that comes from other threads.

Answer 1

Below is example for Inter thread communication:

public class Main {     public static void main(String[] args) {         Chat m = new Chat();         new T1(m);         new T2(m);     } }  class Chat {     boolean flag = false;      public synchronized void FromSam(String msg) {         if (flag) {             try {                 wait();             } catch (InterruptedException e) {                 e.printStackTrace();             }         }         System.out.println(msg);         flag = true;         notify();     }      public synchronized void FromJam(String msg) {         if (!flag) {             try {                 wait();             } catch (InterruptedException e) {                 e.printStackTrace();             }         }          System.out.println(msg);         flag = false;         notify();     } }  class T1 implements Runnable {     Chat m;     String[] s1 = { "Hello Jam", "How are you ?", "I am also doing fine!" };      public T1(Chat m1) {         this.m = m1;         new Thread(this, "Sam").start();     }      public void run() {         for (int i = 0; i < s1.length; i++) {             m.FromSam(s1[i]);         }     } }  class T2 implements Runnable {     Chat m;     String[] s2 = { "HI Sam", "I am good,And U ?", "ha haa" };      public T2(Chat m2) {         this.m = m2;         new Thread(this, "Jam").start();     }      public void run() {         for (int i = 0; i < s2.length; i++) {             m.FromJam(s2[i]);         }     } } 

Answer 2

That depends on the nature of the communication.

• Is it duplex (ie A talks to B and B talks to A)?
• Is it communication of data or communication of completion?

and so on.

The simplest and most advisable form of inter-thread communication is simply to wait for the completion of other threads. That's most easily done by using Future:

ExecutorService exec = Executors.newFixedThreadPool(50); final Future f = exec.submit(task1); exec.submit(new Runnable() {     @Override     public void run() {         f.get();         // do stuff     } }); 

The second task won't execute until the first completes.

Java 5+ has many concurrent utilities for dealing with this kind of thing. This could mean using LinkedBlockingQueues, CountDownLatch or many, many others.

For an in-depth examination of concurrency Java Concurrency in Practice is a must-read.