```
package programs;
public class TestThreads {
public static void main(String[] args) {
ThreadOne t1 = new ThreadOne();
ThreadTwo t2 = new ThreadTwo();
Thread one = new Thread(t1);
Thread two = new Thread(t2);
one.start();
two.start();
}
}
class Accum{
private static Accum a = new Accum();
private int counter = 0;
private Accum() {
}
public static Accum getAccum() {
return a;
}
public void updateCounter(int add) {
counter +=add;
}
public int getCount() {
return counter;
}
}
class ThreadOne implements Runnable{
Accum a = Accum.getAccum();
#Override
public void run() {
for(int x=0;x<98;x++) {
a.updateCounter(1000);
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("one "+ a.getCount());
}
}
class ThreadTwo implements Runnable{
Accum a = Accum.getAccum();
#Override
public void run() {
for(int x=0;x<99;x++) {
a.updateCounter(1);
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("two "+ a.getCount());
}
}
```
The expected output should be as below
One 98098
Two 98099
But I am getting values same for both One and Two.
Is this expected or both should result in different values?
When it comes to thread priorities eventhough the jvm scheduler takes the role to choose the turn of which thread should execute first, what about the results of this program where two void run programs of for loop with 98 and 99 which should result in two different values else the same
This is because shared data is NOT synchronized.
To resolve this problem, use synchronized on methods those touch the shared data, to make Accum class Thread-Safe.
example:
class Accum{
private static Accum a = new Accum();
private int counter = 0;
private Accum() {
}
public static Accum getAccum() {
return a;
}
public synchronized void updateCounter(int add) {
counter +=add;
}
public synchronized int getCount() {
return counter;
}
}
Maybe make the methods and fields in the Accum class static, because then it is saved in a general place and not an instance...
Code:
class Accum{
private static Accum a = new Accum();
private static int counter = 0;
private Accum() {
}
public static Accum getAccum() {
return a;
}
public static void updateCounter(int add) {
counter +=add;
}
public static int getCount() {
return counter;
}
}
class ThreadTwo implements Runnable{
#Override
public void run() {
for(int x=0;x<99;x++) {
Accum.updateCounter(1);
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("two "+ Accum.getCount());
}
}
class ThreadOne implements Runnable{
#Override
public void run() {
for(int x=0;x<98;x++) {
Accum.updateCounter(1000);
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("one "+ Accum.getCount());
}
}
I am not very experienced in Java yet, so this might not work...
[EDIT] I tested it and I believe it works
I have a singleton class and a method with an endless loop. I would like to call that a hundred times with 100 threads inside getInstance. The threads are created, but I only have 60-70 object which are created by the infinite loop.
I'm open to any idea.
I've already tried with newCachedThreadPool and newFixedThreadPool.
public final class MyClass {
private static MyClass instance = null;
public static boolean stop;
private static Map<Integer, CustomBean> pieces = new HashMap<>();
public static MyClass getInstance() {
if (instance == null) {
instance = new MyClass();
for (int i = 0; i < 100; i++) {
Executors.newSingleThreadExecutor().execute(() -> {
try {
endlessMagic();
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
}
return instance;
}
public static void endlessMagic() throws InterruptedException {
while (!stop) {
// where magic happens
pieces.put(something);
Thread.sleep(20);
}
}
}
public class MyClassServletContextListener implements ServletContextListener {
#Override public void contextInitialized(ServletContextEvent servletContextEvent) {
MyClass.getInstance();
}
#Override public void contextDestroyed(ServletContextEvent servletContextEvent) {
MyClass.stop = true;
}
}
I was doing some practice assignments and dabbling around with some dummy code for trying to develop a better understanding of the concepts of threads and locks. Following is a piece of code that (sometimes) goes into deadlock.
A.java
public class A {
private B b;
public void setB(B b) {
this.b = b;
}
public synchronized void foo(boolean callBar) {
System.out.println("foo");
if (callBar) {
b.bar(false);
}
}
}
B.java
public class B {
private A a;
public void setA(A a) {
this.a = a;
}
public synchronized void bar(boolean callFoo) {
System.out.println("bar");
if (callFoo) {
a.foo(false);
}
}
}
Demo.java
public class Demo {
public static void main(String[] args) {
A a = new A();
B b = new B();
a.setB(b);
b.setA(a);
new Thread(() -> {
a.foo(true);
}).start();
new Thread(() -> {
b.bar(true);
}).start();
}
}
Solution: I used Locks instead of synchronized.
A.java
public class A {
private final ReentrantLock lock = new ReentrantLock();
private B b;
public void setB(B b) {
this.b = b;
}
public ReentrantLock lock() {
return lock;
}
public boolean impendingExecute() {
Boolean thisLock = false;
Boolean otherLock = false;
try {
thisLock = lock.tryLock();
otherLock = b.lock().tryLock();
} finally {
if (!(thisLock && otherLock)) {
if (thisLock) {
lock.unlock();
}
if (otherLock) {
b.lock().unlock();
}
}
}
return thisLock && otherLock;
}
public void foo(boolean callBar) {
System.out.println("foo");
if (callBar && impendingExecute()) {
try {
b.bar(false);
} finally {
lock.unlock();
b.lock().unlock();
}
}
}
}
B.java
public class B {
private final ReentrantLock lock = new ReentrantLock();
private A a;
public void setA(A a) {
this.a = a;
}
public ReentrantLock lock() {
return lock;
}
public boolean impendingExecute() {
Boolean thisLock = false;
Boolean otherLock = false;
try {
thisLock = lock.tryLock();
otherLock = a.lock().tryLock();
} finally {
if (!(thisLock && otherLock)) {
if (thisLock) {
lock.unlock();
}
if (otherLock) {
a.lock().unlock();
}
}
}
return thisLock && otherLock;
}
public void bar(boolean callFoo) {
System.out.println("bar");
if (callFoo && impendingExecute()) {
try {
a.foo(false);
} finally {
lock.unlock();
a.lock().unlock();
}
}
}
}
After making the above changes, the code doesn't lead to a deadlock. Is it the proper way to implement this (Basically, I want the impendingExecute() method to be reviewed.)? Also, (deviating a bit from the review) are there any real world scenarios I can encounter this?
Note: I had posted this question on Code Review but it seems reviewal of dummy code is off-topic.
You can use java.util.concurrent.locks.ReentrantLock. This design permits the the method to attempt to acquire the locks of both the classes, to release the locks if it fails, and to try again later if necessary. If you are required to attempt until successful, then you need to put a it in a loop and terminate with some policy.
while (true) {
if (this.lock.tryLock()) {
try {
if (ba.lock.tryLock()) {
try {
//some logic
break;
} finally {
ba.lock.unlock();
}
}
} finally {
this.lock.unlock();
}
}
int n = number.nextInt(1000);
int TIME = 1000 + n; // 1 second + random delay to prevent livelock
Thread.sleep(TIME);
}
or you can use this solution which ensures that multiple locks are acquired and released in the same order:
if (compareTo(ba) < 0) {
former = this;
latter = ba;
} else {
former = ba;
latter = this;
}
synchronized (former) {
synchronized (latter) {
//Some logic
}
}
}
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 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();
}
}