I have to create a program that simulate a bomb... The user has 5 seconds to digit the right code, if he can't, the bomb explodes.
class Codice implements Runnable{
String code;
#Override
public void run() {
code = JOptionPane.showInputDialog("Inserire codice disinnesco:");
if(code.equals(Bomba.check)) {
Bomba.s = "true";
JOptionPane.showMessageDialog(null, "Bomba disinnescata");
}
System.out.println(Bomba.s);
}
}
class Esplosione implements Runnable{
#Override
public void run() {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if(!Bomba.s.equals("true")) {
JOptionPane.showMessageDialog(null, "BOOM!");
}
}
}
public class Bomba {
static String s = "false";
static String check = "123456";
public static void main(String[] args) throws IOException {
Codice c = new Codice();
Esplosione ex = new Esplosione();
Thread t1 = new Thread(c);
Thread t2 = new Thread(ex);
t1.start();
t2.start();
}
}
With this code i can insert the code, and if it's right the bomb doesn't explodes and the program finish. If 5 seconds passes, the message "boom" appears but the first thread doeasn't stop... How can i do?
There are many ways to coordinate between threads; interrupting is one way. Here is an example using Thread.interrupt. It dispenses with your state variable s, which becomes unnecessary:
import javax.swing.*;
import java.io.*;
class Codice implements Runnable {
String code;
Thread other;
Codice(Thread other) {
this.other = other;
}
#Override
public void run() {
code = JOptionPane.showInputDialog("Inserire codice disinnesco:");
if(code.equals(Bomba.check)) {
other.interrupt();
JOptionPane.showMessageDialog(null, "Bomba disinnescata");
}
}
}
class Esplosione implements Runnable {
#Override
public void run() {
try {
Thread.sleep(5000);
JOptionPane.showMessageDialog(null, "BOOM!");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public class Bomba {
static String check = "123456";
public static void main(String[] args) throws IOException {
Esplosione ex = new Esplosione();
Thread t2 = new Thread(ex);
Codice c = new Codice(t2);
Thread t1 = new Thread(c);
t1.start();
t2.start();
}
}
I visited interview some recently. Interviewer asked me to write guaranteed deadlock.
I have wrote following:
public class DeadLockThreadSleep {
private static class MyThread implements Runnable {
private Object o1;
private Object o2;
#Override
public void run() {
try {
test(o1, o2);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public MyThread(Object o1, Object o2) {
this.o1 = o1;
this.o2 = o2;
}
public void test(Object o1, Object o2) throws InterruptedException {
synchronized (o1) {
System.out.println("1.acquired: " + o1);
Thread.sleep(1000);
synchronized (o2) {
System.out.println("2.acquired: " + o2);
}
}
}
}
public static void main(String[] args) {
Object o1 = new Object();
Object o2 = new Object();
new Thread(new MyThread(o1, o2)).start();
new Thread(new MyThread(o2, o1)).start();
}
}
Then he asked if I sure that it is guaranted. I rememebered that Thread.sleep nothing guaranteed.
Then I wrote this code:
public static void main(String[] args) {
final Thread mainThread = Thread.currentThread();
Thread thread = new Thread(new Runnable() {
#Override
public void run() {
try {
mainThread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
thread.start();
try {
thread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
this answer was accepted.
Also he asked to write analog via wait/notify. I thought a lot and I cannot imagine how to write this.
Is it possible?
This may be done by creating a cycle where one thread holds a resource and waits for another resource whereas the other thread does the same but in reverse order.
Thread tholds resourceOne and waits for resourceTwo , whereas t1holds resourceTwo and waits for resourceOne
Below is a sample code:
public class WaitNotifyLock {
boolean isONHold = false;
public synchronized void hold(){
while(isONHold){
try {
wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
isONHold = true;
System.out.println(Thread.currentThread().getId() + " : Holded");
}
public synchronized void unHold(){
while(!isONHold){
try {
wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println(Thread.currentThread().getId() + " : Produced");
isONHold = false;
notify();
}
public static void main(String[] args) {
WaitNotifyLock resourceOne = new WaitNotifyLock();
WaitNotifyLock resourceTwo = new WaitNotifyLock();
Thread t = new Thread(new Runnable() {
#Override
public void run() {
// TODO Auto-generated method stub
resourceOne.hold();
try {
Thread.sleep(2);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
resourceTwo.hold();
resourceOne.unHold();
resourceTwo.unHold();
}
});
Thread t1 = new Thread(new Runnable() {
#Override
public void run() {
resourceTwo.hold();
try {
Thread.sleep(2);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
resourceOne.hold();
resourceTwo.unHold();
resourceOne.unHold();
}
});
t.start();
t1.start();
}
}
A deadlock is a so-called liveness hazard (others are starvation, poor responsiveness, or livelocks), where two main types can be considered:
Lock-ordering deadlocks
Resource deadlocks
However, the Java documentation simplifies this as follows:
Deadlock describes a situation where two or more threads are blocked forever, waiting for each other.
Hence, IMHO you could simply enforce a deadlock with this:
public class DeadlockDemo {
public static void main(String[] args) {
Object a = new Object();
Object b = new Object();
new Thread(() -> waitLeftNotifyRight(a, b)).start();
waitLeftNotifyRight(b, a);
}
public static void waitLeftNotifyRight(Object left, Object right) {
synchronized (left) {
try {
System.out.println("Wait");
left.wait();
} catch (InterruptedException e) { /* NOP */ }
}
synchronized (right) {
System.out.println("Notify");
right.notify();
}
}
}
This demo never terminates because the created thread waits on a's monitor, whereas the main thread waits on b's monitor. As a result, the corresponding notify() methods aren't invoked (which would terminate the program).
let's say i have 3 classes:
1. Storage which contains just one integer.
2. Counter which contains a thread inside who's responsible for counting (0,1,..,k) and stores each iteration of the loop index in Storage class.
3.Printer which contains a thread who's responsible for reading the value in class Storage and print it.
now i have to create a main class which creates these 3 objects runs the threads of Counter and Printer , and everynumber from(0,1,..,k) has to be printed just once and in the right order.
how do i synchronize the access to my Storage class so first i put a number inside Storage with Counter ,than print it with my Printer class ?
here's what i've wrote so far:
public class Storage {
private int num;
public Storage(){
}
public synchronized void setNum(int num){
this.num = num;
}
public synchronized int getNum(){
return num;
}
public class Counter implements Runnable {
Storage s;
public Counter(Storage t){
s = t;
}
#Override
public void run() {
int i = 0;
while(true){
s.setNum(i++);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
public class Printer implements Runnable {
Storage s;
public Printer(Storage s){
this.s= s;
}
#Override
public void run() {
while(true){
System.out.println(s.getNum());
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
public class mainProg {
public static void main(String[] args){
Storage s = new Storage();
Counter c = new Counter(s);
Printer p = new Printer(s);
Thread c1 = new Thread(c);
Thread p2 = new Thread(p);
c1.start();
p2.start();
}
}
EDIT: i found out a solution, here it is:
public class Storage {
private int num;
private boolean available = false;
public Storage(){
}
public synchronized void setNum(int num){
while(available){
try {
wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
available = true;
notifyAll();
this.num = num;
}
public synchronized int getNum(){
while(!available){
try {
wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
available = false;
notifyAll();
return num;
}
}
This approach won't work, because it's not guaranteed that for every cycle of Counter a cycle of Printer will be executed in a parallel thread. You need to be able to store more than a one value in your Storage.
You can use BlockingQueue here and rewrite your Storage class like this:
public class Storage {
private BlockingQueue<Integer> numbers = new LinkedBlockingQueue<Integer>();
public void setNum(int num) {
try {
this.numbers.put(num);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
public int getNum() {
try {
return numbers.take();
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
}
Note that if BlockingQueue is empty and Printer wants to get a new value, it will wait while a new element occurrs in the queue.
Odd even number printing using thread.Create one thread class, two instance of the thread. One will print the odd number and the other will print the even number.
I did the following coding. But it comes to dead lock state. Can some one please explain what might be the reason for that?
public class NumberPrinter implements Runnable{
private String type;
private static boolean oddTurn=true;
public NumberPrinter(String type){
this.type=type;
}
public void run() {
int i=type.equals("odd")?1:2;
while(i<10){
if(type.equals("odd"))
printOdd(i);
if(type.equals("even"))
printEven(i);
i=i+2;
}
}
private synchronized void printOdd(int i){
while(!oddTurn){
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(type + i);
oddTurn=false;
notifyAll();
}
private synchronized void printEven(int i){
while(oddTurn){
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(type + i);
oddTurn=true;
notifyAll();
}
public static void main(String[] s){
Thread odd=new Thread(new NumberPrinter("odd"));
Thread even=new Thread(new NumberPrinter("even"));
odd.start();
even.start();
}
}
Out Put:
odd1
even2
then comes to deadlock!!!!!!
Thanks for your help.
You're waiting and notifying different objects (monitors).
The idea is that you can call obj.wait() to wait for someone to do obj.notify(), while you're doing objA.wait() and objB.notify().
Change your printOdd method to something like
private void printOdd(int i) {
synchronized (lock) { // <-------
while (!oddTurn) {
try {
lock.wait(); // <-------
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(type + i);
oddTurn = false;
lock.notifyAll(); // <-------
}
}
and the printEven method similarly.
Then provide the NumberPrinter with a lock object:
Object lock = new Object();
Thread odd = new Thread(new NumberPrinter("odd", lock));
Thread even = new Thread(new NumberPrinter("even", lock));
Output:
odd1
even2
odd3
even4
odd5
even6
odd7
even8
odd9
There are a lot of bugs in the code.
First of all, the synchronized statements have no effect whatsoever. You create two thread instances, and each calls only its own methods. synchronized is only useful if another thread can call a method.
Then notifyAll() has no effect for the same reasons. odd.notifyAll() doesn't reach even hanging in the wait().
So what you need is another object which contains the state and which both threads can see and use. Use synchronized, wait() and notifyAll() on that third instance.
The same can be solved using Lock interface:
NaturalOrder.java
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class NaturalOrder {
public int currentNumber = 1;
public boolean evenOdd = false;
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
public static void main(String[] args) {
NaturalOrder naturalOrder = new NaturalOrder();
Thread t1 = new Thread(new OddNumberLock(naturalOrder, naturalOrder.lock, naturalOrder.condition));
Thread t2 = new Thread(new EvenNumberLock(naturalOrder, naturalOrder.lock, naturalOrder.condition));
t1.start();
t2.start();
}
}
OddNumberLock.java
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
public class OddNumberLock implements Runnable {
NaturalOrder naturalOrder;
Lock lock;
Condition condition;
public OddNumberLock(NaturalOrder naturalOrder, Lock lock, Condition condition) {
this.naturalOrder = naturalOrder;
this.lock = lock;
this.condition = condition;
}
#Override
public void run() {
lock.lock();
while (naturalOrder.currentNumber < 20) {
while (naturalOrder.evenOdd != false) {
try {
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
try {
Thread.sleep(ThreadLocalRandom.current().nextInt(1000));
} catch (InterruptedException e) {
e.printStackTrace();
}
if (naturalOrder.currentNumber % 2 != 0) {
System.out.println(naturalOrder.currentNumber);
}
naturalOrder.currentNumber++;
naturalOrder.evenOdd = true;
condition.signalAll();
}
lock.unlock();
}
}
EvenNumberLock.java
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
public class EvenNumberLock implements Runnable {
NaturalOrder naturalOrder;
Lock lock;
Condition condition;
public EvenNumberLock(NaturalOrder naturalOrder, Lock lock, Condition condition) {
this.naturalOrder = naturalOrder;
this.lock = lock;
this.condition = condition;
}
#Override
public void run() {
lock.lock();
while (naturalOrder.currentNumber < 20) {
while (naturalOrder.evenOdd != true) {
try {
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
try {
Thread.sleep(ThreadLocalRandom.current().nextInt(1000));
} catch (InterruptedException e) {
e.printStackTrace();
}
if (naturalOrder.currentNumber % 2 == 0) {
System.out.println(naturalOrder.currentNumber);
}
naturalOrder.currentNumber++;
naturalOrder.evenOdd = false;
condition.signalAll();
}
lock.unlock();
}
}
I think the problem might be that printOdd and printEven synchronize on different lock (the Thread's object instance locks). Therefor you have not guaranteed that the change on the static variable oddTurn will be visible in the other thread. Try to make the oddTurn volatile for the start.
I did this way
public class OddEven{
public static void main(String[] args){
Print o=new Print();
Thread even=new Thread(new MyRunnable(2,o));
Thread odd=new Thread(new MyRunnable(1,o));
even.start();
odd.start();
}
}
class MyRunnable implements Runnable{
int start;
Print ob;
MyRunnable(int s,Print o){
start=s;
ob=o;
}
public void run(){
for(int i=start;i<=20;i+=2)
ob.display(i);
}
}
class Print{
int rem=0;
synchronized void display(int n){
while(n%2==rem)
try{
wait();
}
catch(Exception e){System.out.println("Display interrupted");}
System.out.print(n+" ");
rem=n%2;
notify();
}
}
You're missing volatile keyword within oddTurn variable. Without it there are no guarantees the threads see the actual value.
i Used a shared object to control the order of execution
class Counter implements Runnable {
int count;
static Class cl = Counter.class;
public synchronized void increment() {
String tname = Thread.currentThread().getName();
System.out.printf("%s: %d\n", tname, count++);
}
#Override
public void run() {
String tname = Thread.currentThread().getName();
while (true) {
increment();
synchronized (Counter.class) {
try {
cl.notify();
cl.wait();
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
}
public class WaitNotify {
public static void main(String[] args) {
Counter c = new Counter();
Thread t1 = new Thread(c, "thread1");
Thread t2 = new Thread(c, "thread2");
t1.start();
t2.start();
}
}
Here's my solution without any waits or notify.
wait() and notify()/notifyAll() ,
I dont see any reason to use them for this problem statement.
package threading;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class EvenOddPrinting {
int count=0;
boolean isOdd = false;
public static void main(String[] args) {
ExecutorService exec = Executors.newCachedThreadPool();
EvenOddPrinting obj = new EvenOddPrinting();
exec.submit(new EvenPrinter(obj));
exec.submit(new OddPrinter(obj));
exec.shutdown();
}
}
class EvenPrinter implements Runnable{
EvenOddPrinting obj;
public EvenPrinter(EvenOddPrinting obj) {
this.obj=obj;
}
#Override
public void run() {
while(obj.count < 100){
if(!obj.isOdd){
System.out.println("Even:"+obj.count);
obj.count++;
obj.isOdd = true;
}
}
}
}
class OddPrinter implements Runnable{
EvenOddPrinting obj;
public OddPrinter(EvenOddPrinting obj) {
this.obj = obj;
}
#Override
public void run() {
while(obj.count < 100){
if(obj.isOdd){
System.out.println("Odd:"+obj.count);
obj.count++;
obj.isOdd = false;
}
}
}
}
Your code corrected with using Lock interface:
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class NumberPrinter implements Runnable {
private Lock lock;
private Condition condition;
private String type;
private static boolean oddTurn = true;
public NumberPrinter(String type, Lock lock, Condition condition) {
this.type = type;
this.lock = lock;
this.condition = condition;
}
public void run() {
int i = type.equals("odd") ? 1 : 2;
while (i <= 10) {
if (type.equals("odd"))
printOdd(i);
if (type.equals("even"))
printEven(i);
i = i + 2;
}
}
private void printOdd(int i) {
// synchronized (lock) {
lock.lock();
while (!oddTurn) {
try {
// lock.wait();
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(type + " " + i);
oddTurn = false;
// lock.notifyAll();
condition.signalAll();
lock.unlock();
}
// }
private void printEven(int i) {
// synchronized (lock) {
lock.lock();
while (oddTurn) {
try {
// lock.wait();
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(type + " " + i);
oddTurn = true;
// lock.notifyAll();
condition.signalAll();
lock.unlock();
}
// }
public static void main(String[] args) {
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
Thread odd = new Thread(new NumberPrinter("odd", lock, condition));
Thread even = new Thread(new NumberPrinter("even", lock, condition));
odd.start();
even.start();
}
}
public class Number_Thread extends Thread {
String thread;
int limit;
public Number_Thread(String thread,int limit){
this.thread=thread;
this.limit=limit;
}
Object lock=new Object();
public void run()
{
synchronized (lock)
{
//------------------- "print even"--------------------//
if(thread.equals("even"))
{
for (int i = 2; i <=limit; i+=2)
{
System.out.println(thread+" thread "+i);
try {
lock.wait(1000);
continue;
}
catch (InterruptedException e) {}
}
lock.notifyAll();
}
//------------------- "print odd"--------------------//
if(thread.equals("odd"))
{
for (int i = 1; i <=limit; i+=2)
{
System.out.println(thread+" thread "+i);
try {
lock.wait(1000);
continue;
}
catch (InterruptedException e) {}
}
lock.notifyAll();
}
}
}
}
//------------------thread creater class------------------//
import java.util.Scanner;
public class Main_Thread {
private static Scanner s;
public static void main(String[] args) throws InterruptedException {
System.out.print("enter limit:\t ");
s=new Scanner(System.in);
int n=s.nextInt();
s.close();
Thread t1=new Number_Thread("even",n);
Thread t2=new Number_Thread("odd",n);
t2.start();
Thread.sleep(100);
t1.start();
}
}
output for limit 5:
enter limit: 5
odd thread 1
even thread 2
odd thread 3
even thread 4
odd thread 5
I have implemented in such a way, based on the argument, no of threads will be spawned and will the respective no in round robin manner.
i.e., If thread count is 3, thread 1 will print 1,4 ...; thread 2 will print 2,5,... and thread 3 will print 3,6...
public class ThreadSynchronizer
{
public static void main(String[] args)
{
// BASED ON THE ARGUMENT MULTIPLE THREADS WILL BE CREATED AND EACH WILL PRINT ITS RESPECTIVE NO
// IE, IF THREAD COUNT IS 3, THREAD 1 WILL PRINT 1,4 ..., THREAD2 WILL PRINT 2,5,... AND THREAD3 WILL PRINT 3,6...
// LIMITED THE DISPLAY TO 1000 NOS
int threadCnt = Integer.parseInt(args[0]);
ReentrantLock lckArray[] = new ReentrantLock[threadCnt + 1];
for (int i = 0; i < threadCnt + 1; i++)
{
ReentrantLock lck = new ReentrantLock();
lck.lock();
lckArray[i] = lck;
}
for (int i = 0; i < threadCnt; i++)
{
Thread th = new Thread(new Printer(lckArray, i + 1));
th.start();
}
for (int i = 1; i < threadCnt + 1; i++)
{
lckArray[i].unlock();
while (!lckArray[i].isLocked())
{
}
}
lckArray[0].unlock();
}
}
class Printer implements Runnable
{
private ReentrantLock[] lckArray;
private int index;
Printer(ReentrantLock[] lckArray, int startValue)
{
this.lckArray = lckArray;
this.index = startValue;
}
#Override public void run()
{
ReentrantLock prevLock = null;
int printCounter = index;
for (int counter = 0; printCounter <= 1000; counter++)
{
int remCounter = counter % lckArray.length;
int incCounter = lckArray.length - remCounter;
int indexPostion = index + incCounter;
int curElementIndex = indexPostion % lckArray.length;
lckArray[curElementIndex].lock();
if (prevLock != null)
prevLock.unlock();
prevLock = lckArray[curElementIndex];
if (curElementIndex == 0)
{
System.out.println("Printed by Thread " + index + " " + printCounter);
printCounter = printCounter + lckArray.length - 1;
}
}
if (prevLock != null)
{
if (prevLock.isHeldByCurrentThread())
prevLock.unlock();
}
}
}
Program for Two Threads Alternatively Print Odd and Even Numbers.
#Implemented Using "Object Lock" Concept.
class Increment{
private int count;
public void increment(){
count++;
System.out.println(Thread.currentThread().getName()+"::::::::::::::::::"+count);
}
}
class SimpleThread extends Thread{
Increment obj = null;
SimpleThread(Increment obj){
this.obj=obj;
}
public void run(){
try {
Thread.sleep(100);
while(true){
synchronized(obj){
obj.increment();
Thread.sleep(1000);
obj.notify();
obj.wait();
}
}
} catch(InterruptedException ie) {
ie.printStackTrace();
}
}
}
public class Main
{
public static void main(String[] args) {
Increment increment = new Increment();
SimpleThread t1 = new SimpleThread(increment);
SimpleThread t2 = new SimpleThread(increment);
t1.start();
t2.start();
System.out.println(Thread.currentThread().getName()+"::::::::::::::"+"Hello World");
System.out.println(Runtime.getRuntime().availableProcessors()+"::::::::::::::"+"CORE SIZE");
}
}
I implemented it in a very simple way, from 1 to 40>
public class EvenOddProblem {
public static void main(String[] args) {
Printer p = new Printer();
EvenThread enenThread = new EvenThread(p);
OddThread oddThread = new OddThread(p);
new Thread(enenThread).start();
new Thread(oddThread).start();
}
}
class EvenThread implements Runnable {
private Printer printer;
public EvenThread(Printer p) {
printer = p;
}
#Override
public void run() {
try {
int i = 0;
while (true) {
if (i == 20)
break;
i++;
printer.evenPrintEven();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class OddThread implements Runnable {
private Printer printer;
public OddThread(Printer p) {
printer = p;
}
#Override
public void run() {
int i = 0;
try {
while (true) {
if (i == 20)
break;
i++;
printer.evenPrintOdd();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class Printer {
private static volatile Integer i = 1;
public synchronized void evenPrintOdd() throws InterruptedException {
while (i % 2 == 0) {
wait();
}
System.out.println(i);
i++;
notifyAll();
}
public synchronized void evenPrintEven() throws InterruptedException {
while (!(i % 2 == 0)) {
wait();
}
System.out.println(i);
i++;
notifyAll();
}
}
So I have been working on a simple wait/notify example in Java and for some reason I have not been able to get it to run properly. If anyone is able to see what might be the issue It would be very appreciated!
public class ThreadDemonstration
{
private String str = null;
Thread stringCreator = new Thread(new Runnable()
{
public void run()
{
synchronized(this)
{
str = "I have text";
notify();
}
}
});
private Thread stringUser = new Thread(new Runnable()
{
public void run()
{
synchronized(this)
{
if(str == null)
{
try {
System.out.println("str is null, I need help from stringCreator");
wait();
System.out.println(str);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
}
});
public static void main (String [] args)
{
ThreadDemonstration td = new ThreadDemonstration();
td.stringUser.start();
td.stringCreator.start();
}
}
My current output is:
str is null, I need help from stringCreator
So for some reason the thread stringCreator does not wake up the stringUser or am I missing something else entirely?
Thank you!
Your blocks are synchronized over different objects. They should be synchronized over a common object, for example the monitor object below:
public class ThreadDemonstration
{
private String str = null;
private final Object monitor = new Object();
Thread stringCreator = new Thread(new Runnable()
{
public void run()
{
synchronized(monitor)
{
str = "I have text";
monitor.notify();
}
}
});
private Thread stringUser = new Thread(new Runnable()
{
public void run()
{
synchronized(monitor)
{
while(str == null) //changed from if to while. This allows you to wait again if the thread gets woken up by something other than the appropriate notify.
{
try {
System.out.println("str is null, I need help from stringCreator");
monitor.wait();
//removed print statement from here
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
System.out.println(str); //added print statement here. str is guaranteed to not be null here.
}
}
});
In order to avoid creating a separate object for synchronization, you can use synchronized(ThreadDemonstration.this) or synchronized(ThreadDemonstration.class) for example.
Try this :
private Thread stringUser = new Thread(new Runnable() {
//-----
System.out.println("str is null, I need help from stringCreator");
notify();
wait(100);
System.out.println(str);
//----
});
You need to use the wait and notify of the same instance in order for it to work. Since you create two different objects (2 instances of Runnable) it will not work. I've written a simple example using two different classes using the main class' instance for the intrinsic lock. You could also us a 'dummy object' (Object lock = new Object) for this.
public class ThreadDemonstration {
private static String text;
public ThreadDemonstration(){
Thread user = new Thread(new StringUser(this));
Thread creator = new Thread(new StringCreator(this));
user.start();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
creator.start();
}
public class StringCreator implements Runnable{
private Object lock;
StringCreator(Object lock){
this.lock = lock;
}
#Override
public void run() {
synchronized(lock){
text = "Yeeeehaaaaa";
lock.notify();
}
}
}
public class StringUser implements Runnable{
private Object lock;
StringUser(Object lock){
this.lock = lock;
}
#Override
public void run() {
synchronized(lock){
if((text == null)){
System.out.println("I need help!");
try {
lock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(text);
}
}
}
public static void main(String[] args){
new ThreadDemonstration();
}
}