Example
public class myThread extends Thread{
@override
public void run(){
while(true){
threadCondWait();// Circle waiting...
//bla bla bla bla
}
}
public synchronized void threadCondWait(){
while(myCondition){
wait();//Comminucate with notify()
}
}
}
public class myAnotherThread extends Thread{
@override
public void run(){
//Bla Bla bla
notify();//Trigger wait() Next Step
}
}
Even though you asked for wait()
and notify()
specifically, I feel that this quote is still important enough:
Josh Bloch, Effective Java 2nd Edition, Item 69: Prefer concurrency utilities to wait
and notify
(emphasis his):
Given the difficulty of using
wait
andnotify
correctly, you should use the higher-level concurrency utilities instead [...] usingwait
andnotify
directly is like programming in "concurrency assembly language", as compared to the higher-level language provided byjava.util.concurrent
. There is seldom, if ever, reason to usewait
andnotify
in new code.
Example for wait() and notifyall() in Threading.
A synchronized static array list is used as resource and wait() method is called if the array list is empty. notify() method is invoked once a element is added for the array list.
public class PrinterResource extends Thread{
//resource
public static List<String> arrayList = new ArrayList<String>();
public void addElement(String a){
//System.out.println("Add element method "+this.getName());
synchronized (arrayList) {
arrayList.add(a);
arrayList.notifyAll();
}
}
public void removeElement(){
//System.out.println("Remove element method "+this.getName());
synchronized (arrayList) {
if(arrayList.size() == 0){
try {
arrayList.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}else{
arrayList.remove(0);
}
}
}
public void run(){
System.out.println("Thread name -- "+this.getName());
if(!this.getName().equalsIgnoreCase("p4")){
this.removeElement();
}
this.addElement("threads");
}
public static void main(String[] args) {
PrinterResource p1 = new PrinterResource();
p1.setName("p1");
p1.start();
PrinterResource p2 = new PrinterResource();
p2.setName("p2");
p2.start();
PrinterResource p3 = new PrinterResource();
p3.setName("p3");
p3.start();
PrinterResource p4 = new PrinterResource();
p4.setName("p4");
p4.start();
try{
p1.join();
p2.join();
p3.join();
p4.join();
}catch(InterruptedException e){
e.printStackTrace();
}
System.out.println("Final size of arraylist "+arrayList.size());
}
}
Not a queue example, but extremely simple :)
class MyHouse {
private boolean pizzaArrived = false;
public void eatPizza(){
synchronized(this){
while(!pizzaArrived){
wait();
}
}
System.out.println("yumyum..");
}
public void pizzaGuy(){
synchronized(this){
this.pizzaArrived = true;
notifyAll();
}
}
}
Some important points:
1) NEVER do
if(!pizzaArrived){
wait();
}
Always use while(condition), because
while(!pizzaExists){ wait(); }
.2) You must hold the lock (synchronized) before invoking wait/nofity. Threads also have to acquire lock before waking.
3) Try to avoid acquiring any lock within your synchronized block and strive to not invoke alien methods (methods you don't know for sure what they are doing). If you have to, make sure to take measures to avoid deadlocks.
4) Be careful with notify(). Stick with notifyAll() until you know what you are doing.
5)Last, but not least, read Java Concurrency in Practice!
Have you taken a look at this Java Tutorial?
Further, I'd advise you to stay the heck away from playing with this kind of stuff in real software. It's good to play with it so you know what it is, but concurrency has pitfalls all over the place. It's better to use higher level abstractions and synchronized collections or JMS queues if you are building software for other people.
That is at least what I do. I'm not a concurrency expert so I stay away from handling threads by hand wherever possible.
Source: Stackoverflow.com