I have a thread created by implementing the Runnable interface. My task is this: the thread should count (starting from 0) and save the value it has reached to a variable which is part of the main class. I overrode the run method and did the counting but how I can save this value to the main's variable? Only with a getter is the solution?
My thread class:
public class myThread implements Runnable {
private static int count = 0;
private final int length;
public myThread (int length) {
this.length = length;
}
#Override
public void run() {
while (count < this.length) {
increaseValue();
}
}
private void increaseValue() {
synchronized (ThreadFirstExercise.class) {
if (count < this.length) {
this.count++;
}
}
}
}
main:
public class Main {
public static void main(String[] args) {
int result; // this is the variable where I want to save the counting result of my thread
(new Thread(new ThreadFirstExercise(10000))).start();
}
}
You can either use a Callable that returns a Future.
Or you could use a wrapper Object in the main method that contains the integer.
Related
I am facing a problem with a Runnable class. The value that is passed to the runnable class is overwritten by the last value it received when the same value is passed to another class.
The function of the runnable class is to pass the values to another function in another class to print them. But only the last value received by the Runnable class is printed.
Here is my code,
This is the main class from where the values are passed.
public class MainClass {
private int intVal = -1;
public void MainMethod() {
ExecutorUtil theExecutor = ExecutorUtil.GetInstance();
for(int i = 0; i < 3; i++) {
intVal = i;
synchronized (this) {
theExecutor.SubmitTask(new ActionExecutor(intVal));
}
}
}
}
This is the executorUtil that I use to call the thread.
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.BlockingQueue;
public class ExecutorUtil {
private static ExecutorUtil theInstance;
private ExecutorService theExecutor;
private BlockingQueue<Runnable> theQueue;
protected ExecutorUtil() {
theExecutor = CreateThreadPoolExecutor();
}
private ExecutorService CreateThreadPoolExecutor() {
theQueue = new LinkedBlockingQueue<Runnable>();
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(10, 10, 900, java.util.concurrent.TimeUnit.SECONDS, theQueue);
threadPoolExecutor.allowCoreThreadTimeOut(true);
return threadPoolExecutor;
}
public static ExecutorUtil GetInstance() {
if (theInstance == null) {
synchronized(ExecutorUtil.class) {
if (theInstance == null) {
theInstance = new ExecutorUtil();
}
}
}
return theInstance;
}
public void SubmitTask(Runnable runnable) {
theExecutor.submit(runnable);
}
}
This is the thread that passes the received value to the function that prints these values.
public class ActionExecutor implements Runnable {
int iVal = -1;
public ActionExecutor(int iVal) {
this.iVal = iVal;
}
public void run() {
SecondClass sc = new SecondClass();
sc.printIntVal(iVal);
}
}
And this is the class that prints the values.
public class SecondClass {
public void printIntVal(int i) {
System.out.println(i);
}
}
Expected Output:
0
1
2
Obtained Output:
2
2
2
No idea why this is behaving in this way!
UPDATE:
The issue occurs only when a non-primitive data type is used. In my example here, I've used an integer value (intVal). Since java passes the value for primitive data types, the output was obtained as expected. But in my original code, I've used a JSONObject. And since java passes the reference of the object for non-primitive data types, the value was over-written.
I have solved this by creating new JSONObject for each iteration.
It seems that somewhere you have static field, which store your number.
So, I suppose, that you have 3 instances of ActionExecutor, but it looks like field has static modifier, so each of this instance will have latest value.
Check this case...
So after I read your question and studied your code I might as well try it out. And lo and behold it's working as expected. The only difference here is that there is a race condition so the output my vary in order, but it will not print the same numbers.
Output might
1st: 0 1 2
2nd: 0 1 2
3rd: 2 1 0
Here is the full code to try it out:
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
public class MainClass {
private int intVal = -1;
public void MainMethod() {
ExecutorUtil theExecutor = ExecutorUtil.GetInstance();
for(int i = 0; i < 3; i++) {
intVal = i;
synchronized (this) {
theExecutor.SubmitTask(new ActionExecutor(intVal));
}
}
}
public class ActionExecutor implements Runnable {
int iVal = -1;
public ActionExecutor(int iVal) {
this.iVal = iVal;
}
public void run() {
SecondClass sc = new SecondClass();
sc.printIntVal(iVal);
}
}
public class SecondClass {
public void printIntVal(int i) {
System.out.println(i);
}
}
public static class ExecutorUtil {
private static ExecutorUtil theInstance;
private ExecutorService theExecutor;
private BlockingQueue<Runnable> theQueue;
protected ExecutorUtil() {
theExecutor = CreateThreadPoolExecutor();
}
private ExecutorService CreateThreadPoolExecutor() {
theQueue = new LinkedBlockingQueue<Runnable>();
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(10, 10, 900, java.util.concurrent.TimeUnit.SECONDS, theQueue);
threadPoolExecutor.allowCoreThreadTimeOut(true);
return threadPoolExecutor;
}
public static ExecutorUtil GetInstance() {
if (theInstance == null) {
synchronized(ExecutorUtil.class) {
if (theInstance == null) {
theInstance = new ExecutorUtil();
}
}
}
return theInstance;
}
public void SubmitTask(Runnable runnable) {
theExecutor.submit(runnable);
}
}
public static void main(String[] args) {
MainClass main = new MainClass();
main.MainMethod();
}
}
You might want to clean and rebuild your project. Otherwise it's working..
class hehe implements Runnable {
static int count = 0;
public synchronized void count() {
count++;
}
public void run() {
for (int i = 0; i < 10000; i++) {
count();
}
}
}
public class Sychronise {
public static void main(String[] args) {
Thread a1 = new Thread(new hehe());
Thread a2 = new Thread(new hehe());
a1.start();
a2.start();
try {
a1.join();
a2.join();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println(hehe.count);
}
}
The count should be 20000, but my output is still unstable.
Currently, your synchronized does nothing as it's synchronizing on each individual 'hehe' instance. To make it synchronize on the Class object (which owns the 'count' variable) make your count method static too:
public static synchronized void count(){
count++;
}
Or synchronize on an object:
public class hehe implements Runnable {
static int count= 0;
static Object lock = new Object();
public void count(){
synchronized(lock) {
count++;
}
}
public void run(){
for (int i=0;i<10000;i++){
count();
}
}
}
The issue here is that your count method is synchronized but there are 2 instances of the object Hehe, so each synchronization would be scoped to each instance, in this case I would suggest to use an AtomicInteger
First try with a static count method.
public static synchronized void count(){
count++;
}
public void run(){
for (int i=0;i<10000;i++){
Hehe.count();
}
}
I try to write some code about Lock and synchronized and to compare their performance difference.
Code:
public abstract class Task {
public abstract int getTotal();
}
// Lock test class
public class TaskWithLock extends Task implements Runnable {
private static int total = 0;
private final Lock lock = new ReentrantLock();
public void run() {
try {
lock.lock();
doSomething();
} finally {
lock.unlock();
}
}
private void doSomething() {
total++;
}
public int getTotal() {
return total;
}
}
// Synchronized test class
public class TaskWithSync extends Task implements Runnable {
private static int total = 0;
public void run() {
synchronized ("") {
doSomething();
}
}
private void doSomething() {
total++;
}
public int getTotal() {
return total;
}
}
// Test class
public class Test {
public static void main(String[] args) throws Exception {
int count = 100000;
runTasks(TaskWithLock.class, count);
runTasks(TaskWithSync.class, count);
}
public static void runTasks(Class<? extends Runnable> clazz, int count)
throws Exception {
List<Thread> list = new ArrayList<Thread>(count);
for (int i = 0; i < count; i++) {
list.add(new Thread(clazz.newInstance()));
}
for (int i = 0; i < count; i++) {
list.get(i).start();
}
for (int i = 0; i < count; i++) {
list.get(i).join();
}
System.out.println(clazz.getSimpleName() + "Total Result: "
+ ((Task) clazz.newInstance()).getTotal());
}
}
My understand is the above Lock and synchronized code block should be the same effect, but the result I run them are not same, synchronized code is right, it is always 100000, but lock code is always incorrect, sometimes 99995,or 99997, or other result, but it is not 100000.
Console:
TaskWithLock Result: 99991
TaskWithSync Result: 100000
I think my code should have some error, or my understand about Lock is wrong, or Lock can not be used like this.
Please point out what could be wrong.
In the lock-version, you are using one lock per instance. That means that every thread has its own lock, which ultimately renders the locks useless because no two threads use the same lock.
You need to change this to one central lock for all threads. Add static to this line:
private final Lock lock = new ReentrantLock();
so it becomes
private static final Lock lock = new ReentrantLock();
Because your lock object is per instance and you are updating a static variable. So each Thread has it's own lock which is quite pointless to use it to protect a static variable.
package demo5;
class Process extends Thread {
static int counter = 0;
public static synchronized void increment() { counter++; }
public void run() {
for (int i = 0; i < 1000000; i++)
{
increment();
}
System.out.println("Done.");
}
}
public class App {
public static void main(String[] args) throws InterruptedException {
Process p1 = new Process();
Process p2 = new Process();
p1.start();
p2.start();
p1.join();
p2.join();
System.out.println("Value of count is :" + p1.counter);
}
}
if I declare the increment function as NON-STATIC function the value of the counter at the end will NOT be 2 million.
on the other hand, it works properly when the increment method is defined as static.
As far as I know there will be only ONE increment function for all the Process objects.. so why do I have to declare it as a static method..?
thanks
Declaring it static will cause the synchronized to lock on the Process.class instance. So all threads running will block on the Object within the increment method. Removing the static will cause each thread to only block on the Thread instance (which in your case there are two).
As a result your counter variable is being incremented in parallel and as noted many times, int incrementing is not thread-safe.
As far as I know there will be only ONE increment function for all the Process objects
There is one increment function for the Process class, but the synchronization is done on the Object and not the method for instance:
class Process{
public synchronized void increment() { counter++; }
}
Is equivallent to:
class Process{
public void increment() {
synchronized(this){
counter++;
}
}
Edit: To answer Rouki's question.
class Process{
public static synchronized void increment() { counter++; }
}
Is equivallent to
class Process{
public void increment() {
synchronized(Process.class){
counter++;
}
}
You may want to replace your int counter with an AtomicInteger counter - this way you can remove the synchronized keyword from the method, and it shouldn't matter if the method is a static or instance method.
I have two methods in two different classes, like this
public class ClassX implements Runnable {
public void methodAandB() {
for(int i=0;i<10;i++) {
System.out.println("This is A and B ");
}
}
#Override
public void run() {
methodAandB();
}
}
public class ClassY implements Runnable {
public void methodAorB() {
for(int i=0;i<10;i++) {
System.out.println("This is A or B");
}
}
#Override
public void run() {
methodAorB(a);
}
}
Thread t1 is calling methodAandB().
Thread t2 is calling methodAorB().
Can I switch between these two threads after each iteration of loop in methods?
I want to get output like this:
This is A and B
This is A or B
This is A and B
This is A or B
This is A and B
This is A or B
This is A and B
This is A or B
Best example of flip-flop between threads:
Given two int array (even and odd), 2 threads printing their numbers in natural order.
package com.rough;
public class ThreadsBehaviour {
static Object lock = new Object();
public static void main(String[] args) throws InterruptedException {
int a[] = {1,3,5,7,9};
int b[] = {2,4,6,8,10};
Thread odd = new Thread(new Looper(a, lock));
Thread even = new Thread(new Looper(b, lock));
odd.start();
even.start();
}
}
class Looper implements Runnable
{
int a[];
Object lock;
public Looper(int a[], Object lock)
{
this.a = a;
this.lock = lock;
}
#Override
public void run() {
for(int i = 0; i < a.length; i++)
{
synchronized(lock)
{
System.out.print(a[i]);
try
{
lock.notify();
if(i == (a.length - 1))
{
break;
}
lock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
You can achieve this simply by using the shared variables. I have implemented and verified the problem. code is below
class X
public class ClassX implements Runnable {
public void methodAandB() {
for(int i=0;i<10;i++) {
while(GlobalClass.isClassXdone)
{}
System.out.println("This is A and B ");
GlobalClass.isClassXdone = true;
GlobalClass.isClassYdone = false;
}}
#Override
public void run() {
methodAandB(); } }
ClassY
public class ClassY implements Runnable {
public void methodAorB() {
for(int i=0;i<10;i++) {
while(GlobalClass.isClassYdone)
{}
System.out.println("This is A or B ");
GlobalClass.isClassYdone = true;
GlobalClass.isClassXdone = false;}}
#Override
public void run() {
methodAorB();}}
Definition of the shared variable
public class GlobalClass {
public static boolean isClassXdone = false ;
public static boolean isClassYdone = false ;
}
You can just start your thread using t1.start and t2.start to get the desired output
Thread t1 = new Thread(new ClassX());
Thread t2 = new Thread(new ClassY());
t1.start();
t2.start();
This is probably more than needed to solve the problem, but, as it seems to be an introduction to concurrent programming exercise, it should be along the lines of what you'll encounter.
You should probably have a shared object that both your threads know, so that they may synchronize through it. Like so:
public class MyMutex {
private int whoGoes;
private int howMany;
public MyMutex(int first, int max) {
whoGoes = first;
howMany = max;
}
public synchronized int getWhoGoes() { return whoGoes; }
public synchronized void switchTurns() {
whoGoes = (whoGoes + 1) % howMany;
notifyAll();
}
public synchronized void waitForMyTurn(int id) throws
InterruptedException {
while (whoGoes != id) { wait(); }
}
}
Now, your classes should receive their respective identifier, and this shared object.
public class ClassX implements Runnable {
private final int MY_ID;
private final MyMutex MUTEX;
public ClassX(int id, MyMutex mutex) {
MY_ID = id;
MUTEX = mutex;
}
public void methodAandB() {
for(int i = 0; i < 10; i++) {
try {
MUTEX.waitForMyTurn(MY_ID);
System.out.println("This is A and B ");
MUTEX.switchTurns();
} catch (InterruptedException ex) {
// Handle it...
}
}
}
#Override
public void run() { methodAandB(); }
}
ClassY should do the same. Wait for its turn, do its action, and then yield the turn to the other.
I know it's a little late to answer this. But it's yesterday only I have come across this question. So I guess it's never too late.. ;)
Solution, as #afsantos mentioned is having a shared object between the two threads and implementing mutual exclusion on the shared object. The shared object could be alternatively locked by the two threads. Two possible implementations are as follows. This is actually more like an extension of #afsantos solution. His work is hereby acknowledged.
Solution 1:
Blueprint of the object that will be shared is as follows.
public class MutEx {
public int whoGoes, howMany;
public MutEx(int whoGoes, int howMany) {
this.whoGoes = whoGoes;
this.howMany = howMany;
}
public synchronized void switchTurns(){
this.whoGoes = (this.whoGoes + 1) % 2;
notifyAll();
}
public synchronized void waitForTurn(int id) throws InterruptedException{
while(this.whoGoes != id)
wait();
}
}
Then, you could implement the ClassX as follows.
public class ClassX implements Runnable {
private final int MY_ID;
private final MutEx MUT_EX;
public ThreadOne(int MY_ID, MutEx MUT_EX) {
this.MY_ID = MY_ID;
this.MUT_EX = MUT_EX;
}
#Override
public void run(){
this.doTheWork();
}
public void doTheWork(){
for(int i = 0; i < 10; i++){
try {
MUT_EX.waitForMyTurn(MY_ID);
System.out.println("This is A and B");
MUT_EX.switchTurns();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
ClassY also will be the same, with whatever the differences you need to be there. Then, in the invocation (i.e. in the main method),
public static void main(String[] args) {
MutEx mutEx = new MutEx(0, 2);
Thread t1 = new Thread(new ClassX(0, mutEx);
Thread t2 = new Thread(new ClassY(1, mutEx));
t1.start();
t2.start();
}
Voila! You have two threads, alternating between each as you need.
Solution 2: Alternatively, you could implement the ClassX & ClassY as follows.
public class ClassX extends Thread{
Here, you are subclassing the java.lang.Thread to implement your requirement. For this to be invoked, change the main method as follows.
public static void main(String[] args) {
MutEx mutEx = new MutEx(0, 2);
ClassX t1 = new ClassX(0, mutEx);
ClassY t2 = new ClassY(1, mutEx);
t1.start();
t2.start();
}
Run this, and you have the same result.
If you don't need to use Thread try this code:
for (int i = 0; i < 20; i++) {
if (i % 2 == 0) {
methodAandB();
} else {
methodAorB();
}
}