IllegalMonitorStateException notify () and wait ()

Question

I have a problem. When I use notify() in a synchronized block, I have an IllegalMonitorStateException. Can someone help me solve this problem?

I have to do this, one thread will send a char to the second thread, then this thread should wait, and the second thread will print this char. After this second thread, wait and send the next char again first

Main.java:

 import java.util.logging.Level; import java.util.logging.Logger; import javax.swing.JFrame; import javax.swing.JOptionPane; /* * To change this template, choose Tools | Templates * and open the template in the editor. */ public class Main extends JFrame { Thread t1, t2; Consumer con; public Main() { con = new Consumer(); startThreads(); } private synchronized void startThreads() { t1 = new Thread(new Producent("grudzien", con)); t1.start(); t2 = new Thread(con); t2.start(); } public class Producent implements Runnable { String m_atom; char[] atoms; Consumer m_c; public Producent(String atom, Consumer c) { m_atom = atom; m_c = c; } @Override public void run() { synchronized (this) { atoms = m_atom.toCharArray(); System.out.print("Tablica znaków: "); for (int i = 0; i < atoms.length; i++) { System.out.print(atoms[i] + ", "); } } for (int i = 0; i < atoms.length; i++) { synchronized (this) { con.setChar(atoms[i]); t2.notify(); try { wait(); } catch (InterruptedException ex) { JOptionPane.showMessageDialog(null, "Blad w wait()", "Blad!", JOptionPane.ERROR_MESSAGE); } } } } } public class Consumer implements Runnable { char atom; public void setChar(char c) { atom = c; } @Override public void run() { while (true) { synchronized (this) { try { wait(); } catch (InterruptedException ex) { JOptionPane.showMessageDialog(null, "Blad w wait()", "Blad!", JOptionPane.ERROR_MESSAGE); } System.out.println(atom); t1.notify(); } } } } public static void main(String[] args) { new Main(); } } 

Answer 1

you need to be the "owner of the object monitor" in order to be able to call a notification about this. for now your methods are all synchronized(this) , but they call notify () on other objects (which they are not synchronized). in other words:

 synchronized(t2) { t2.notify(); } 

and

 synchronized(t1) { t1.notify(); } 

for a full explanation of monitors and synchronization in java, see here or look for similar questions here on SO, like this one - Java Wait and Notify: IllegalMonitorStateException