Java Video Tutorial 18
In this Java Video Tutorial, I continue to teach you about Java Threads. If you missed the last part, watch it first here last Java Tutorial.
Today I cover how to lock down methods, synchronized, thread pools, scheduleAtFixedRate and numerous other Java Thread methods.
Use the code that follows the video to make it easier to follow this tutorial. It is heavily commented and will help you.
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Code From the Video
LESSONEIGHTEEN.JAVA
// In the last tutorial I coordinated threads using
// a timing method. Here I show you how to execute code based
// on a predefined time schedule and much more
// Used to schedule when certain events should be triggered
import java.util.concurrent.ScheduledThreadPoolExecutor;
// Used to tell Java what unit of time I want to use
import static java.util.concurrent.TimeUnit.*;
public class LessonEighteen{
public static void main(String[] args){
addThreadsToPool();
}
public static void addThreadsToPool(){
// Allows you to schedule code execution at some time in the future
// You can also have code execute repetitively based on a time period
// It must be big enough to hold all potential Threads
ScheduledThreadPoolExecutor eventPool = new ScheduledThreadPoolExecutor(5);
// Adds a Thread to the pool. Tells that thread to start executing
// after 0 seconds (immediately) and then execute every 2 seconds
eventPool.scheduleAtFixedRate(new CheckSystemTime(), 0, 2, SECONDS);
eventPool.scheduleAtFixedRate(new PerformSystemCheck("Mail"), 5, 5, SECONDS);
eventPool.scheduleAtFixedRate(new PerformSystemCheck("Calendar"), 10,10, SECONDS);
// Thread.activeCount returns the number of threads running
System.out.println("Number of Threads: " +Thread.activeCount());
// Quiz: Why does it say there are 4 threads when we expect 3?
// Create an array of threads with enough spaces for all active threads
Thread[] listOfThreads = new Thread[Thread.activeCount()];
// enumerate fills the Thread array with all active threads
Thread.enumerate(listOfThreads);
// Cycle through all the active threads and print out their names
for(Thread i : listOfThreads){
System.out.println(i.getName());
}
// Get priority of all the threads (Priority is equal to the thread
// that created the new thread. Top priority is 10, lowest priority is 1
for(Thread i : listOfThreads){
System.out.println(i.getPriority());
}
// threadName.setPriority can be used to set the priority
// This allows the above threads to run for approximately 20 seconds
try{
Thread.sleep(20000);
}
catch(InterruptedException e)
{}
// Shuts down all threads in the pool
// eventPool.shutdown();
}
}
CHECKSYSTEMTIME.JAVA
import java.text.DateFormat;
import java.util.*;
public class CheckSystemTime implements Runnable{
public void run(){
Date rightNow;
Locale currentLocale;
DateFormat timeFormatter;
String timeOutput;
rightNow = new Date();
currentLocale = new Locale("en");
timeFormatter = DateFormat.getTimeInstance(DateFormat.DEFAULT, currentLocale);
timeOutput = timeFormatter.format(rightNow);
System.out.println("Time: " + timeOutput);
}
}
PERFORMSYSTEMCHECK.JAVA
// You could also lock down a method and then unlock it
// when you are finished with it. This library does that
import java.util.concurrent.locks.ReentrantLock;
public class PerformSystemCheck implements Runnable{
private String checkWhat;
// Creates a lock for your method
ReentrantLock lock = new ReentrantLock();
public PerformSystemCheck(String checkWhat){
this.checkWhat = checkWhat;
}
// By putting synchronized before a method, you make
// sure only one thread at a time can execute it.
// Synchronizing slows down Java, so it should only
// be used when necessary.
/* synchronized */ public void run(){
// this locks down the method just like synchronized
// You can't use synchronized and lock, that's why
// synchronized is commented out above
lock.lock();
System.out.println("Checking " + this.checkWhat);
// this unlocks the method just like synchronized
lock.unlock();
}
}
Please put the codes of Java Video Tutorial 18 on this page.
Fixed it. Thanks for pointing out that error
Hi, great videos! Just wondering about why the TimeUnit library has to be static? Line 3 in JavaLessonEighteen.
It doesn’t have to be. I’m not sure why I did that? I improv my way through these tutorials and on occasion I do something silly
Ok, thank you for quick answer! And the tutorials are just great, have been through 1-24 now and my understanding of Java Programming is so much better than after all the other java tutorials I’ve tried out!
Thank you. I’m very happy to hear that. I always wonder who my competition is. I don’t watch any other tutorials so I don’t know what I’m up against.
There are alot of other talented guys making tutorials on Java, but your pace suits me very well. The others are often a bit slow. Also the fact that you edit your tutorials is as far as I can tell very rare but very nice for us viewers!
Well, thank you very much. It is time consuming to edit them, but I think the extra time sets me apart. I’m glad you took the time to say Hi 🙂
good work sir, excellent
Thank you very much 🙂
hello derek, i am getting error on line no 30,32,34 . the eclipse throws error “SECONDS cannot be resolved to a variable”.I completely copied this lessons code, i its throwing this error.plz do explain
Just put static to java.util.concurrent.TimeUnit.*; I used to have the same error.
hello derek, i am getting error on line no 30,32,34 . the eclipse throws error “SECONDS cannot be resolved to a variable”.I completely copied this lessons code, i its throwing this error.plz do explain
Derek instead of library here i used TIMEUNIT.SECONDS directly as argument so i got perfect result.
I’m confused on why java.util.concurrent.TimeUnit.* didn’t work? That is odd. Thank you for pointing that out
So this pretty much explains the concept of multi threading? Or is that something else? I would think multi-threading involves running the threads at the same time not scheduled threads.
More threads than cores / cpus will slow things down due to the overhead of switches with multi-threading
Exception in thread “main” java.lang.Error: Unresolved compilation problems:
SECONDS cannot be resolved to a variable
SECONDS cannot be resolved to a variable
SECONDS cannot be resolved to a variable
at LessonEighteen.addThreadsToPool(LessonEighteen.java:15)
at LessonEighteen.main(LessonEighteen.java:10)
I replace SECONDS with timeUnit.SECONDS
I’m glad you got it working. Sorry I couldn’t get to you quicker
Exception in thread “main” java.lang.Error: Unresolved compilation problems:
You should write TimeUnit.SECONDS, that`s enum in java except using only SECONDS
Hi, your video tutorials are awesome!! Thank you for this great vidoes.
I have a question. Why we used try block in addThreadToPool() method?
try{
Thread.sleep(20000);
}
catch(InterruptedException e){}
Best Regard,
Burak
That is there to solve issues that an interrupt may cause.
Is there a limit to how many threads can be started? Is there any type of architecture dependency for threads, like does it have to be multi-CPU etc for the threads to run simultaneously?
The JVM has an upper limit to the number of threads, but it is in the 10s of thousands. For each new thread you create you will take a performance hit.