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The main thread calls a method which asks for user input and sets the appropriate ActionListener (using Swing). The main thread needs to know the user's input before it can continue meaningfully. Under normal circumstances, the main thread will continue and complete itself before the ActionListener is activated. I have been able to work around this by pushing the main thread into a timer, which checks for input from the user and reacts only after that input has arrived. This is inconvenient, because the continuation of the main thread's logic has to occur within the timer block. It seems clumsy, and does not help the readability of the program.
My question : Surely there is a more elegant way of achieving the same result?
Ideally, I suppose, I would want a type of conditional suspension of the main thread.
I received 4 very useful answers to my question - many thanks to the people who suggested answers. After receiving those 4 answers, my question was closed, for the reason given : "Provide more detail and clarity".
So now I will provide code, detail and clarity, at least for part of the problem.
I made a thorough investigation into the suggestion made by Pavel : "so you need to wait for the other thread to finish its job? have you explored thread.join() functionality? It will be helpful if you could share some of your code."
I found the following code on javatpoint.com, which illustrates the functionality of the thread.join method.
package packThreads;
public class JoinExample1 extends Thread
{
public void run()
{
for(int i=1; i<=4; i++)
{
try
{
Thread.sleep(500);
}catch(Exception e){System.out.println(e);}
System.out.println(i);
}
}
public static void main(String args[])
{
// creating three threads
JoinExample1 t1 = new JoinExample1();
JoinExample1 t2 = new JoinExample1();
JoinExample1 t3 = new JoinExample1();
// thread t1 starts
t1.start();
// starts second thread when first thread t1 is dead.
try
{
t1.join();
}catch(Exception e){System.out.println(e);}
// start t2 and t3 thread
t2.start();
t3.start();
}
}
The following output is produced, showing how the join method achieved the desired result :
1
2
3
4
1
1
2
2
3
3
4
4
Next, I modified the code to apply it to my particular problem. Here is the modified "Main" class :
package packThreads;
public class MainJoin {
public static void main(String[] args) {
// creating three threads
SpecialThread t1 = new SpecialThread();
JoinExample1 t2 = new JoinExample1();
JoinExample1 t3 = new JoinExample1();
// thread t1 starts
t1.start();
// starts second thread when first thread t1 is dead.
try
{
t1.join();
}catch(Exception e){System.out.println(e);}
// start t2 and t3 thread
t2.start();
t3.start();
}
}
And the new "SpecialThread" class :
package packThreads;
public class SpecialThread extends Thread {
public void run()
{
for(int i=1; i<=4; i++)
{
try
{
Thread.sleep(500);
}catch(Exception e){System.out.println(e);}
System.out.println(i);
}
System.out.println("Now the special thread will");
System.out.println(" start doing special things");
UserInputFrame inputFrame1 = new UserInputFrame();
}
}
And finally, the "UserInputFrame" class :
package packThreads;
import javax.swing.*;
import java.awt.event.*;
public class UserInputFrame extends JFrame implements ActionListener {
JButton b;
UserInputFrame()
{
System.out.println("Setting up the click button");
b=new JButton("Click");
b.setBounds(100,150,80,30);
b.addActionListener(this);
this.add(b);
setSize(300,300);
setLocation(800,300);
setLayout(null);
setVisible(true);
}
public void actionPerformed(ActionEvent e){
b.setText("Clicked");
System.out.println("Button has been clicked");
this.dispose();
}
}
This modified code produces the output :
1
2
3
4
Now the special thread will
start doing special things
Setting up the click button
1
1
2
2
3
3
4
4
Button has been clicked
The output shows that the thread.join method has not solved my problem. Thread 1 (which asks for the input) terminates itself (despite the "join" coding) before the user has clicked the button. I'm guessing that when calling the UserInputFrame method, a new thread is created, which runs independently of its parent thread (Thread1).
I will now show the clumsy way I have worked around the problem, using a timer.
Changes have been made to the "SpecialThread" class :
package packThreads;
import java.util.Timer;
import java.util.TimerTask;
public class SpecialThread extends Thread {
public void run()
{
for(int i=1; i<=4; i++)
{
try
{
Thread.sleep(500);
}catch(Exception e){System.out.println(e);}
System.out.println(i);
}
System.out.println("Now the special thread will");
System.out.println(" start doing special things");
UserInputFrame inputFrame1 = new UserInputFrame();
new Timer().schedule ( new TimerTask()
{
public void run()
{
if (inputFrame1.b.getText().equals("Clicked"))
{
System.out.println("I have the user input");
System.out.println("and can continue with the logic,");
System.out.println("but I'm stuck within the timer block.");
this.cancel();
}
else { } // do nothing
}
}, 100,100);// start time -- run every x time in milliseconds
System.out.println("Thread1 is now completing.");
}
}
Here is the output produced :
1
2
3
4
Now the special thread will
start doing special things
Setting up the click button
Thread1 is now completing.
1
1
2
2
3
3
4
4
Button has been clicked
I have the user input
and can continue with the logic,
but I'm stuck within the timer block.
So, I'm back to my original question : Surely there's a more elegant way to do this?
Related
I was working on Threads and decided to add some extra text before and after my focused lines of code are run, for reference. I expected to get one 'extra-text' towards the start and the other at the end. However... that's not happening and the second 'extra-text' just comes at the fourth position when I run it. I am a beginner and need to know why this is happening...
---CODE---
class Hi extends Thread{
public void run(){
for(int i=1; i<=5; i++){
System.out.println("HI!");
try{
Thread.sleep(500);
} catch(InterruptedException e){}
}
}
}
class Hey extends Thread{
public void run(){
for(int i=1; i<=5; i++){
System.out.println("HEY!");
try{
Thread.sleep(500);
} catch(InterruptedException e){}
}
}
}
public class MyClass {
public static void main(String[] args){
Hi hiObj = new Hi();
Hey heyObj = new Hey();
System.out.println("extra-text");
hiObj.start();
heyObj.start();
try {
Thread.currentThread().sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("extra-text");
}
}
---OUTPUT---
extra-text
HI!
HEY!
extra-text
HEY!
HI!
HEY!
HI!
HEY!
HI!
HEY!
HI!
This is a common concurrency error.
The main method of your program runs on the main thread. Thus, before you've started the hiObj and heyObj threads, you already have one thread. After you start both of the new threads, you have three. Each executes concurrently. This means that each thread can execute code without waiting for the others. Order is not guaranteed between threads.
This causes the behavior you observe. Before hiObj or heyObj are started, the main method running on the main thread prints "extra-text". Next, hiObj and heyObj are started. The main thread reaches the line Thread.currentThead().sleep(10) which causes it to suspend execution for 10 milliseconds. On most machines (including yours), this is enough time for the other two threads to begin execution. Each thread begins the for loop in its run method and prints either "HI" or"HEY". Thus, the first three lines of output are (the order of "HI" and "HEY" are not guaranteed):
"extra-text"
"HI"
"HEY"
Next, the hiObj and heyObj threads reach the line Thread.sleep(500) which causes them to suspend execution for 500 milliseconds. After 10 milliseconds have passed, the main thread will be finished sleeping a will resume. Note that neither the hiObj or heyObj threads could have resumed by now. Thus, the next line printed will be the from the next line executed in main. This is "extra-text". Thus, the expected output is:
"extra-text"
"HI"
"HEY"
"extra-text"
Over the next few seconds, the remaining prints from the hiObj and heyObj threads will occur. In Java, the main thread exits only after all other threads have exited (unless System.exit is called or there is an uncaught exception). In this case this means the program will only exit when main reaches the end of execution and when both hiObj's and heyObj's run methods return.
To change your program so that the last "extra-text" always prints at the end, you have to cause the main thread to wait for the hiObj and heyObj threads to finish. In Java, there is a method on Thead called join which causes the calling thread to wait until the joined thread dies. In your program, you can modify MyClass to look like this:
public class MyClass {
public static void main(String[] args){
Hi hiObj = new Hi();
Hey heyObj = new Hey();
System.out.println("extra-text");
hiObj.start();
heyObj.start();
try {
Thread.currentThread().sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
hiObj.join();
heyObj.join();
System.out.println("extra-text");
}
}
With this change, main will first wait for hiObj to finish and then wait for heyObj to finish before it prints "extra-text".
If you get rid of the
Thread.currentThread().sleep(10);
in the main Method you will see that your two extra texts are printed to the console immediately after execution. By using the sleep(10) you just delay the second extra text and in the meantime your 2 threads print their first output.
This question already has answers here:
How Thread.sleep() works internally
(5 answers)
Closed 3 years ago.
It may be a dumb question but i have researched it alot and i am not getting any satisfactory knowledge.
Can anyone help me to understand?
my code
public class sleepclass extends Thread {
public static void main(String[] args) {
sleepclass t1 = new sleepclass();
sleepclass t2 = new sleepclass();
t1.start();
t2.start();
}
public void run() {
for (int i = 1; i < 5; i++) {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
System.out.println(e);
}
System.out.println(i);
}
}
}
Output :
1
1
2
2
3
3
4
4
Now as per the output i can see both thread gone to a sleep state. and both comes after at the same time
My question is :
1 . Since sleep is a static method how both works and sleep at same time? (explain in layman please).
Can i sleep only t1 and left t2 to run?
Thread.sleep() sleeps the current thread. It has no effect elsewhere.
If you want one thread to sleep and one not in your current code, you would have to have some kind of indicator to control it. Such as a boolean flag which you would then check:
if(shouldSleep) {
Thread.sleep(5000);
}
Otherwise all threads will execute the same code, and all threads will sleep for 5 seconds.
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I"M TRYING TO PUT A 5 SECONDS GAP BETWEEN 2 THREADS,THAT RUNS ONE AFTER ANOTHER I.E. SUPPOSE MY 1ST THREAD PRINTS "X" ,THERE WILL BE 5 SECONDS DELAY & THEN ANOTHER THREAD IS PRINTED "Y", AGAIN 5 SECONDS DELAY & THEN "X" & THIS IS GOES ON , SAY 30 TIMES.
import java.lang.*;
import java.util.concurrent.TimeUnit;
class PingPong implements Runnable
{ String word;
PingPong(String s){
word = s;
}
public void run()
{
try
{
for(int i = 0; i<30; i++)
{
System.out.println(word);
Thread.sleep(100) ;
}
} catch (InterruptedException e)
{ e.printStackTrace(); }
}
public static void main(String[] args){
Runnable p1 = new PingPong("ping");
Thread t1 = new Thread(p1);
t1.start();
Runnable p2 = new PingPong("pong");
Thread t2 = new Thread(p2);
t2.start();
}
}
Threads are independent of each other unless you introduce some kind of synchronisation mechanism. So the first thing you need to do is change your PingPong class to take something to synchronize on, on which each thread is going to wait.
Let's call this object ball. You can pass it in the constructor of PingPong. It can be any object you want (even just Object) or you can create your own small class for it.
Then in your loop, you can do:
synchronized(ball) {
System.out.println(word);
Thread.sleep(5000);
}
Thread.sleep(1000);
This way each thread will block for 5seconds until it allows another thread to 'take' the ball's monitor and output it's word.
The second sleep is arbitrary but important so that the same thread doesn't get the monitor again.
A slightly more complex but more correct way to do it is to use a second ReentrantLock. Again you have to pass it through the constructor together with the previous ball object. Let's call this lock.
lock.lock();
synchronized(ball) {
try {
System.out.println(word);
} finally {
lock.unlock();
}
Thread.sleep(5000);
}
The unlock() is in a finally block to ensure that if any exception is thrown the lock doesn't remain locked forever.
The System.out didn't actually need to be inside the try block, but this makes the code a bit more elegant, rather than having an empty try. The sleep() has to be outside, to make sure the other thread goes in through the first lock while this thread is sleeping.
This ensures that if thread Ping is sleeping, thread Pong takes the lock, so it will be next to go inside the synchronized block. When Ping wakes up and goes out of the synchronized block, even if coincidentally gets scheduled before Pong, it won't be able to proceed because it can't take the lock, and has to wait for Pong to go inside the synchronized block and output its word.
I'm trying to write a program that asks simple questions with a time limit on them.
So far I have the following:
public static void main(String[]args) throws IOException, InterruptedException{
Thread thread = new Thread();
Scanner scan = new Scanner(System.in);
System.out.println("1. What is 1+1?");
System.out.println("a. 2\tb. Cannot Be.\tc. 3\td. All of the above.");
String question1 = scan.next();
for(int i = 3; i>=0; i--){
System.out.print("\b"+i);
Thread.sleep(1000);
}
}
This properly asks the question and takes the answer, but it doesn't put a time limit on input and counts down from 3 to 0 after the input is given. What am I doing wrong?
This can be done using a little bit of black multithreading magic.
First, you'll need two threads like this:
Thread thread1 = Thread.currentThread();
Thread thread2 = new Thread(() -> {
try {
for (int seconds = 3; seconds > 0; seconds--) {
System.out.println(seconds+" second"+(seconds == 1 ? "s" : "")+" left");
Thread.sleep(1000);
}
System.out.println("Time's up!");
thread1.stop();
}catch(InterruptedException weCanIgnoreThisException){}
});
where thread1 is the thread that asks the question and thread2 is the countdown.
Then what is left is to ask the question. Don't forget to start() thread2 before asking for input and to stop() it after receiving the input!
System.out.println("1. What is 1+1?");
System.out.println("a. 2\tb. Cannot Be.\tc. 3\td. All of the above.");
thread2.start();
String answer = scan.next();
thread2.stop();
Alright, so here's why I used the deprecated method Thread#stop().
The official documentation of java.lang.Thread explains why is stop() deprecated and what circumstances make it screw up programs:
Stopping a thread with Thread.stop causes it to unlock all of the monitors that it has locked (as a natural consequence of the unchecked ThreadDeath exception propagating up the stack). If any of the objects previously protected by these monitors were in an inconsistent state, the damaged objects become visible to other threads, potentially resulting in arbitrary behavior.
In short, if a thread is stop()ped while it's locked on an object using a synchronized block or method, the lock on the object is released in a dangerously abrupt manner. Since asking multiple choice questions and placing a time limit on input doesn't require a thread to be synchronized on something, we can ignore this.
I would create separate functions to call during the loop, so you don't have a long winded declaration of global variables and such. If you need to control what is called randomly, then you can put a rand in a function and use one global that way, or you can simply put them in a order you want it to be called and completed as.
As you rightly guessed, you need two separate threads running like in the below explanation & code.
The below explanation will provide you more details on what and how you need to do with the two threads.
(1) Thread 1: Timer thread (Inner class implements Runnable) runs in a separate thread and counts the seconds while waiting for the user's input. Once user enters the input this thread needs to be stopped using a signal (stopTimer variable acts as a signal), ensure that stopTimer variable is volatile (to receive the data written by Thread2), otherwise this thread will wait infinitely.
(2) Thread 2: This is the main thread which waits for the user's input. Once the user inputs the data, this main thread signals to stop the Timer thread using a separate method call - signalStopTimer()
public class TimerTest {
public static class Timer implements Runnable {
private volatile boolean stopTimer = false;
private long timerMilliSeconds =0;
#Override
public void run() {
try {
while(!stopTimer) {
Thread.sleep(1000);
timerMilliSeconds = timerMilliSeconds+1000;
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public void signalStopTimer() {
stopTimer = true;
}
//this method will be helpful to find the elapsed time in seconds
public long getTotalTimeInSeconds() {
return timerMilliSeconds/1000;
}
}
public static void main(String[] args) {
TimerTest.Timer timer = new TimerTest.Timer();
//Start the Timer Thread now
Thread thread = new Thread(timer);
thread.start();
Scanner scan = new Scanner(System.in);
System.out.println("1. What is 1+1?");
System.out.println("a. 2\tb. Cannot Be.\tc. 3\td. All of the above.");
String input = scan.next();
//received the input, signal to stop timer
timer.signalStopTimer();
System.out.println(" input is:"+input+" seconds");
System.out.println(" total time :"+timer.getTotalTimeInSeconds());
}
}
In case you only need to count the time it took until user put his input, the better way and the easiest way is to use System.currentTimeMillis().
before the scan code you can save the current time in a variable (Long), then in while loop (when the loop condition will be stopped when the user put his input) in the end of the loop just save the same way mentioned above the current time in millisecond and then all left is subtraction.
if this is your direction let me know i can supply a code for that ;)
I have the following code that runs whenever you click the Start button on my program. I have denoted via comments where I want the timer to go, problem is, when I do thread.sleep(time) it freezes my program! So, I was wondering if someoen could just simply add atimer to my code so it runs the first bit, waits, then runs it again based on bumpNum.
Code:
public class startReplyButtonListener implements ActionListener{
public void actionPerformed(ActionEvent ev){
int length = textAreaReplyMessage.getText().length();
int remLen = 400 - length;
String strHTML = neo.get("http://www.neopets.com/neoboards/topic.phtml?topic=" + txtTopicID.getText());
/*strHTML = neo.post("/neoboards/process_topic.phtml?", new String[][] {{"boardType", "topic_id", "board_id", "message", "next", "remLen"}, {"reply", txtTopicID.getText(), "4", textAreaReplyMessage.getText() , "1", ((Integer)remLen).toString()}});
if(strHTML.contains("No topic with ID")){
txtLog.append("Invalid Topic ID! \n");
}
else{
txtLog.append("Bumped Topic ID " + txtTopicID.getText() + "\n");
}
*/
System.out.println(strHTML);
bumpNum = 5;
wait = Integer.parseInt(textWait1.getText()) * 1000; //converting to miliseconds
int i=1;
do{
strHTML = neo.post("/neoboards/process_topic.phtml?", new String[][] {{"boardType", "topic_id", "board_id", "message", "next", "remLen"}, {"reply", txtTopicID.getText(), "4", textAreaReplyMessage.getText() , "1", ((Integer)remLen).toString()}});
txtLog.append("Board Bumped. Waiting "+ ((Integer)(wait/1000)).toString() +" Seconds..." + "\n");
//ADD TIMER HERE
i++;
}while(i <= bumpNum);
}
}
What I wish to accomplish:
User indicates how many times they want to "post"(indicated by bumpNum), the loop will first, post once:
strHTML = neo.post("/neoboards/process_topic.phtml?", new String[][] {{"boardType", "topic_id", "board_id", "message", "next", "remLen"}, {"reply", txtTopicID.getText(), "4", textAreaReplyMessage.getText() , "1", ((Integer)remLen).toString()}});
Then:
Based on users input, it will wait for however many seconds(txtWait1) and THEN repeat the posting code above until it has reached bumpNum.
And it will update txtLog with the following EACH TIME it bumps(so the program cannot be frozen):
txtLog.append("Board Bumped. Waiting "+ ((Integer)(wait/1000)).toString() +" Seconds..." + "\n");
Edit:
Sigh. Ok, now I understand. I don't know the answer. You are talking about drawing a GUI element. I suspect you want to fork a thread to do a job and then show the GUI display that you are waiting for it. You need to wait for the thread to finish (see my join code below) all of the time having the GUI element refresh UNTIL it finishes when you display some result.
This depends more on the GUI code than sleep/timer. I would start a new question now and explain !!!NOT WITH CODE!!! but with pseudo code from 1000 foot view what you want. Something like:
I am trying to fork a thread that runs in the background in [Swing/Android/etc]. I want to display to the user that the thread has been forked, I want the user interface to wait for the thread without freezing, and then I want the user interface to join with the thread and display the results.
Think about the problem like we have to think of it. Anticipate questions that we will ask. Figure out what we don't and can't know about your environment.
Best of luck.
Edit:
If you are just trying to call sleep then you don't need to fork a thread for that. All you need to do in your code is:
try {
Thread.sleep(waitingTime);
System.out.println(waitingTime);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
e.printStackTrace();
}
This will pause the current thread (which could be the main thread) for waitingTime milliseconds.
So you are forking 3 threads very quickly which I guess you don't want to do. If you are trying to wait for each thread to finish then you will have to do something like:
Thread thread = new Thread(new Counter(wait));
thread.start();
thread.join();
Couple of other comments:
It is considered bad form to start a thread in the constructor of a class: new Thread(this).start();
You are creating 2 thread objects inside of your Runnable. You should just create one outside of your Runnable. See above.
Thread myCounter = new Thread(this); << #1
public Counter(int waitingTime) {
new Thread(this).start(); << #2
}
I would not initialize waitingTime = 0; when defined and initialize it in the constructor. This is confusing. Remove the = 0.
int waitingTime; << remove the =0 here
public Counter(int waitingTime) {
this.waitingTime = waitingTime;
When you catch InterruptedException, be sure to handle it right. A good pattern is to reset the interrupt flag and/or quit the thread:
} catch (InterruptedException e) {
// resets the interrupt flag cleared by catching the exception
Thread.currentThread.interrupt();
// or stops the thread immediately
return;
}
You're starting a new thread each time through the loop. Rather than creating a new thread in the constructor, move the do/while loop into a normal method rather than the run method of a new thread. What you're doing is spawning a new thread that does in fact sleep, but it's not the thread that's executing the loop so that thread just continues as normal.