Why Thread loop is not finishing without mentioning printing?

[Tushar Goel]

I was trying to read an article from here http://www.javaworld.com/article/2071214/java-concurrency/java-101--understanding-java-threads--part-3--thread-scheduling-and-wait-notify.html
I was doing the same example but loop is not ending. Thread is struck at CalculateThreadDemo run method. If i mentioned System.out.println
then it got terminated else infinite loop continues. I don't understand this. Please help me.

code:

BlockedThreadDemo

public class BlockedThreadDemo extends Thread {
private boolean isFinished = false;
BlockedThreadDemo(String name) {
super(name);
}
@Override
public void run() {
while (!isFinished) {
try {
int i = 0;
do {
i = System.in.read();
System.out.print("i: = " + i);
} while (i != '\n');
System.out.print('\n');
} catch (IOException e) {
e.printStackTrace();
}
}
System.out.println(getName() + " is finished.");
}
public void setFinished(boolean finish) {
isFinished = finish;
}
}

CalculateThread

public class CalculateThread extends Thread {
private boolean isFinished = false;
CalculateThread(String name) {
super(name);
}
@Override
public void run() {
int sum = 0;
while (!isFinished) {
sum++;
System.out.println(sum +" "); //problem is here. If i comment this then this will go infinite loop
}
System.out.println(getName() + " is finished.");
}
public void setFinished(boolean finish) {
System.out.println("isFinished: " + finish);
isFinished = finish;
}
}

Main class:

public class PriorityDemo {
public static void main(String[] args) {
BlockedThreadDemo bt1 = new BlockedThreadDemo("BlockedThreadDemo");
bt1.setPriority(Thread.NORM_PRIORITY + 1);
CalculateThread ct1 = new CalculateThread("CalculateThread");
bt1.start();
ct1.start();
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
bt1.setFinished(true);
ct1.setFinished(true);
System.out.println("main thread is dead");
}
}

Output:

With commenting the statement:

1
i: = 49i: = 13i: = 10
1
i: = 49i: = 13i: = 10
isFinished: true
main thread is dead
1
i: = 49i: = 13i: = 10
BlockedThreadDemo is finished.
1

1
1
...


Without commenting the statement:


597222
597223
597224
597225
isFinished: true
main thread is dead
597226
CalculateThread is finished.

i: = 49i: = 13i: = 10
BlockedThreadDemo is finished.

[Tim Cooke]

I would start by forgetting about an article that was written over 12 years ago. It is not current practice to extend Thread, rather you would implement Runnable instead.

For learning about how to work with concurrency (in this decade), I recommend the The Java Tutorials : Concurrency.

[Tushar Goel]

Agree with your point that this is old practice(extending Thread class) but i want to know why this is coming when it should not be? It could come using Runnable
as well as problem is in run() method. I am searching where i am getting wrong..

[S Majumder]

Hi Tushar, correct me if am wrong ..
Here you are using two different thread say BlockedThreadDemo and CalculateThread and one main thread that contains the main().The main catch is

bt1.setFinished(true);
ct1.setFinished(true);

After sleeping 5 sec the main therad wakes up and setting these two flags.In the mean while the CalculateThread thread completed its execution.The BlockedThreadDemo thread is still alive , if we make a enter (\n) there the BlockedThreadDemo thread comes up from its while and the thread ends up its execution ..The app also complete its execution.


Satya

[Tim Cooke]

If you're working on this example to learn about concurrency, then you're wasting your time by learning the wrong thing.

[Tushar Goel]

Thanks Tim, I modified the code as per current practice but still showing same thing. I am trying to learn only scheduling thing.

Updated code:

 public class BlockedThreadDemo implements Runnable {
private boolean isFinished = false;
@Override
public void run() {
while (!isFinished) {
try {
int i = 0;
do {
i = System.in.read();
System.out.print("i: = " + i);
} while (i != '\n');
System.out.print('\n');
} catch (IOException e) {
e.printStackTrace();
}
}
System.out.println("BlockedThreadDemo is finished.");
}
public void setFinished(boolean finish) {
isFinished = finish;
}
}

 public class CalculateThread implements Runnable {
private boolean isFinished = false;
@Override
public void run() {
int sum = 0;
while (!isFinished) {
sum++;
//System.out.println(sum +" ");
}
System.out.println("CalculateThread is finished.");
}
public void setFinished(boolean finish) {
System.out.println("isFinished: " + finish);
isFinished = finish;
}
}

public class PriorityDemo {
public static void main(String[] args) {
BlockedThreadDemo run1 = new BlockedThreadDemo();
Thread bt1 = new Thread(run1, "BlockedThreadDemo");
bt1.setPriority(Thread.NORM_PRIORITY + 1);
CalculateThread run2 = new CalculateThread();
Thread ct1 = new Thread(run2, "CalculateThread");
bt1.start();
ct1.start();
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
run1.setFinished(true);
run2.setFinished(true);
System.out.println("main thread is dead");
}
}

meantime i am looking for oracle docs for the same.

[Tushar Goel]

 

In the mean while the CalculateThread thread completed its execution.

It doesnot.

[Tushar Goel]

I figured it out. I have to use volatile to solve it.. Thanks Tim again.. I will stop using this practice now..