how to use notify and wait

Question

Can wait/notify be used within one thread? I'm mean I have a listener and in the moment when that listener gets called I wanna enable a thread to do his work.How could I do that?

UPDATE:My data is written in a database...and is written each time the listener is called.Now the thread that I've created reads that data and sends it somewhere.... Next...I get some other data and do the same thing....The other thread needs to know what was the last data he read it so he can start reading from where he left....

Take a look in here: using wait and notify within one thread This is how my problem looks like.Thx

I have the following:

synchronized (syncToken)
{
    try {
        syncToken.wait();

    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}
System.out.println("MyThread: " + s);

in MyThread....so when I do

MyThread t = new MyThread(syncToken);
t.start();

I put my thread on waiting...yes?

And when I do this:

syncToken.notify();

I get my thread back on track....but the execution of the next line is the one after wait()?

I mean this: System.out.println("MyThread: " + s); ????

When u notify a thred does he continues his execution with the line after wait()???Thx

Answer 1

The following is a simple example of concurrency between two different threads. In the example the main thread start a MyThread thread and every 3 seconds it sets a data to the MyThread instance and then MyThread prints it. The idea is to have a synchronized object that you wait on it and notify in the end of the usage to other threads that they can use it:

Test.java:

package stack;

public class Test {
    public static void main (String args[])
    {
        Object syncToken = new Object();
        MyThread t = new MyThread(syncToken);
        t.start();
        for (int i = 0; i < 10; i++)
        {
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized(syncToken)
            {
                t.setText("Iteration " + i);
                syncToken.notify();
            }
        }
    }
}

MyThread.java:

package stack;

public class MyThread extends Thread{

    String s;
    Object syncToken;
    public MyThread(Object syncToken)
    {
        this.s = "";
        this.syncToken = syncToken;
    }

    public void run()
    {
        while(true) // you will need to set some condition if you want to stop the thread in a certain time...
        {
            synchronized (syncToken)
            {
                try {
                    syncToken.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println("MyThread: " + s);
        }
    }

    public void setText(String s)
    {
        this.s = s;
    }
}

In this example, the main thread sets a string (every 3 seconds) and the MyThread thread prints it.

Adapt it to your needs, it shouldn't be too hard.

Answer 2

I had similar problem. I created an arbiter used by two threads (in your case it can be listeners thread and your task thread): listener:

arbiter.waitConsumer();
// prepare data
arbiter.dataLoaded();

task thread:

while(true){
  arbiter.waitProducer();
  // consume data
  arbiter.dataConsumed();
}

arbiter:

public class Arbiter {
private boolean dataLoaded = false;
public synchronized void waitProducer(){
    while(!dataLoaded){
        try {
            wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
public synchronized void waitConsumer(){
    while(dataLoaded){
        try {
            wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
public synchronized void dataLoaded(){
    dataLoaded = true;
    notify();
}public synchronized void dataConsumed(){
    dataLoaded = false;
    notify();
}}

Listener and task will synchronize themselfes against arbiters monitor. Probably you can call your arbiter queue or pipe and store date for consuming in it?