I was asked at an interview to write java code which is guaranteed deadlock. I wrote a standard code which presents at every Java book, like create 2 threads and call synchronized methods at different order, sleep a little before call the 2nd.
Of course this stuff didn't satisfy the interviewers, so now I'm proceeding to figure the solution out.
I discovered a piece of code:
public class Lock implements Runnable {
static {
System.out.println("Getting ready to greet the world");
try {
Thread t = new Thread(new Lock());
t.start();
t.join();
} catch (InterruptedException ex) {
System.out.println("won't see me");
}
}
public static void main(String[] args) {
System.out.println("Hello World!");
}
public void run() {
try {
Thread t = new Thread(new Lock());
t.start();
t.join();
} catch (InterruptedException ex) {
System.out.println("won't see me");
}
}
}
But I'm not sure if this code satisfied them? Sure. The code never ends execution, but is it a true deadlock? Aren't deadlocks about synchronization? And, for example, I can also write an endless cycle, put a Thread.sleep inside and name it a "deadlock".
So the question is: is it possible to write a classic deadlock using synchronized methods but 100% guaranteed? (Please don't tell me about very, very, very likely deadlock cases. I know it.)
Thanks.
Create two resources, and have each thread try to get one before releasing the other, but in different orders. For instance:
CountDownLatch a = new CountDownLatch (1);
CountDownLatch b = new CountDownLatch (1);
void one() throws InterruptedException {
a.await();
b.countDown();
}
void two() throws InterruptedException {
b.await();
a.countDown();
}
The thread that runs one can't release b, because it's waiting for a. It'll wait forever, because the thread that runs two can't release a because it's waiting for b.
One or the classic deadlock scenarios is when you acquire locks in reverse order.
class Resource1 {
synchronized static void method1() {
try {
Thread.sleep(1);
} catch (InterruptedException e) {
}
Resource2.method1();
}
}
class Resource2 {
synchronized static void method1() {
Resource1.method1();
}
}
public class MultiThreadApp {
public static void main(String[] args) {
new Thread(new Runnable() {
public void run() {
Resource2.method1();
}
}).start();
Resource1.method1();
}
}
public class Deadlock {
public static void main(String[] args) {
String res1 = "a";
String res2 = "s";
new Thread(
() -> {
synchronized (res1) {
try {
Thread.sleep(2);
} catch (InterruptedException e) {
}
synchronized (res2) {
}
}
}
).start();
new Thread(
() -> {
synchronized (res2) {
try {
Thread.sleep(2);
} catch (InterruptedException e) {
}
synchronized (res1) {
}
}
}
).start();
}
}
Related
I cannot find The Problem Can Someone Help me.
public class Achterbahn {
private final Object monitor = new Object();
public synchronized void test() throws InterruptedException {
//monitor.wait();
System.out.println("car");
wait();
System.out.println("car");
}
public synchronized void Passagier() throws InterruptedException {
Thread.sleep(2000);
System.out.println("p");
notify();
//b.t1.notify();
}
public static void main(String []args) throws InterruptedException {
Thread t4 = new Thread(new Runnable() {
#Override
public void run() {
Achterbahn b = new Achterbahn();
try {
b.Passagier();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
});
Thread t5= new Thread(new Runnable() {
#Override
public void run() {
Achterbahn b = new Achterbahn();
try {
b.test();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
});
new Thread(t4).start();
new Thread(t5).start();
t5.join();
t4.join();
}
}
The output is:
car
p
it seems like notify is working i want print also car at the last but i donot konw why its not working
i hope Someone Help me. as soon as possible.
i have all methodes in the same class and i tried also sepreate classes but it didnt work
(I am guessing in this case that “it didn’t work” means the program hangs. Please be specific about what the issue you’re seeing is.)
There are 2 issues. One is that you are creating separate objects in each thread. The object that wait and notify are called on have to be the same, the monitor that is waited on is the one that needs to receive the notify. In this code the synchronized methods use the intrinsic lock on the instance that the methods are called on.
Create the object once in the main method, each thread needs to reference the same object.
The second issue, once you fix the first issue, will be a race condition. If the notify performed by one thread occurs first then when the wait executes the notify has already happened and the wait keeps waiting forever.
Add a condition variable to remember whether the notify occurred.
In general the pattern is to check the condition in a loop, see this question: Why we must use "while" for checking race condition not "if". The post has an example of using a variable to see if a condition occurred, here it
synchronized(obj)
{
while (condition_not_matched)
{
obj.wait();
}
//continue
dosomething();
}
You're doing several things wrong.
only start a single instance of C. Then use that instance to invoke your methods. Different instances don't share monitors within synchronized methods
You're starting two new threads when you start them. Just start them as follows:
t4.start();
t5.start();
The primary problem is that t4 starts first and immediately sleeps. So t5 won't start until the sleep finishes. But by that time, the notify() for the wait in t4 has been issued before the wait() is invoked in t5 Thus the wait will never see it. So you need to give t4 a chance to start before the sleep occurs. There are several ways to fix this. One is to use a flag to signal that the other method is ready. But do not use a tight while loop. Put a sleep inside it for a small amount of time. I have provided an example below. I also assigned names to your threads to match your variables.
public class C {
boolean ready = false;
public synchronized void test() throws InterruptedException {
System.out.println("Current thread = " + Thread.currentThread().getName());
ready = true;
System.out.println("car");
wait();
System.out.println("car");
}
public synchronized void Passagier() throws InterruptedException {
Thread.sleep(4000);
System.out.println("Current thread = " + Thread.currentThread().getName());
System.out.println("p");
notify();
}
public static void main(String[] args)
throws InterruptedException {
C b = new C();
Thread t4 = new Thread(new Runnable() {
#Override
public void run() {
try {
while(!b.ready) {
Thread.sleep(100);
}
b.Passagier();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
},"t4");
Thread t5 = new Thread(new Runnable() {
#Override
public void run() {
try {
b.test();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
},"t5");
System.out.println("Starting t4");
t4.start();
System.out.println("Starting t5");
t5.start();
//
t5.join();
t4.join();
}
}
This Code work for me I have now the while loop
public class C {
int i = 34;
public synchronized void test() throws InterruptedException {
System.out.println("car");
while(i == 34) {
wait();
}
notify();
System.out.println("car");
}
public synchronized void Passagier() throws InterruptedException {
i = 55;
System.out.println("p");
notify();
}
public static void main(String[] args)
throws InterruptedException {
C b = new C();
Thread t4 = new Thread(new Runnable() {
#Override
public void run() {
try {
b.Passagier();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
});
Thread t5 = new Thread(new Runnable() {
#Override
public void run() {
try {
b.test();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
});
t4.start();
t5.start();
t4.join();
t5.join();
}
}
I am trying to learn java concurrency programming. Kindly check my sample code and help me understanding why I'm getting "java.lang.IllegalMonitorStateException" even though I have called wait() and notify in a synchronized context.
public class Test {
public static void main(String[] args) throws Exception {
Test t1 = new Test();
t1.m1();
}
private void m1() {
Example ex = new Example();
Thread t1 = new Thread(ex);
t1.start();
synchronized (ex) {
System.out.println("waiting");
try {
wait();
} catch (Exception e) {
e.printStackTrace();
}
}
}
public class Example implements Runnable {
#Override
public void run() {
System.out.println("Running");
notifyMethod();
}
private void notifyMethod() {
System.out.println("Notifying");
synchronized (this) {
try {
Thread.sleep(1000);
} catch (Exception ex) {
ex.printStackTrace();
}
notify();
}
}
}
}
I expect out of "waiting,running,notifying" but the actual output is:
waiting
Running
java.lang.IllegalMonitorStateException
Notifying
at java.lang.Object.wait(Native Method)
at java.lang.Object.wait(Object.java:502)
at examples.Test.m1(Test.java:18)
at examples.Test.main(Test.java:8)
First of, I think synchonizing on a Runnable isn't a good idea (Example in your case). You either synchonize on this or, even better, on a dedicated Object, that is lock in my example. Edit: Synchronizing on a Runnable is the same as using this but for me it looks better. Guts tell me there might be something more to that, but I'm not an expert in this field. Dedicated lock Object is always better, read this article if you want to find out more on the topic.
Then, while synchonizing on a lock, you have to call wait() on that same object: lock.wait(). If you synchronize on this, then you call this.wait() or just wait().
When you want to notify the waiting thread, you again have to synchronize on the lock and call notify() on that object: lock.notify(). Both the monitor in the synchronize and the object on which you call notify() have to be the exact same object you have called wait() on.
Here is a code that works:
public class Test {
public static final Object lock = new Object();
public static void main(String[] args) throws Exception {
Test t1 = new Test();
t1.m1();
}
private void m1() {
Example ex = new Example();
Thread t1 = new Thread(ex);
t1.start();
synchronized (lock) {
System.out.println("waiting");
try {
lock.wait();
} catch (Exception e) {
e.printStackTrace();
}
}
}
public static class Example implements Runnable {
#Override
public void run() {
System.out.println("Running");
notifyMethod();
}
private void notifyMethod() {
System.out.println("Notifying");
synchronized (lock) {
try {
Thread.sleep(1000);
} catch (Exception ex) {
ex.printStackTrace();
}
lock.notify();
}
}
}
}
So, i apologize for the title. It's quite hard to explain in one sentence what i would like to do if you have no idea on how it is called.
So assume i can only use primitive thread functions (wait, notify, no concurrent package)
The program has 3 threads, all of them are the same and are called by the main thread. They behave normally until one of the three get an exception and so it must wait for the end of the remaining 2 threads in order to start a recovery process.
I was thinking about a static variable but I'm not really sure about it, i would love to keep it as simple as possible.
Each thread starts at the same time.
I don't see any reason why you can't use a static variable like you suggest. Here's how I would do it with an inner class...
private static boolean running = true;
public void test26546397() {
while (true) {
Thread t1 = new Thread(new MyRunnable());
Thread t2 = new Thread(new MyRunnable());
Thread t3 = new Thread(new MyRunnable());
t1.start();
t2.start();
t3.start();
try {
t1.join();
t2.join();
t3.join();
} catch (InterruptedException ex) {
ex.printStackTrace();
}
running = true;
// Do recovery
}
}
public class MyRunnable implements Runnable {
#Override
public void run() {
while (running) {
try {
// doStuff
} catch (Exception ex) {
running = false;
}
}
}
}
I would of course replace the while (true) with something a little more suitable.
I think you need java.concurrent.CountdownLatch, however if the java.concurrent package is not available to you can code this yourself using Object.wait/notify and synchronized blocks.
The latch can then be decremented in a finally {} on each Thread, this will be run if the Thread completes, or an exception occurs.
Your main program then just needs to wait for count to become 0.
public class StackOverflow26546397 {
static class CountdownLatch {
private int count;
private Object monitor = new Object();
public CountdownLatch(int count) {
this.count = count;
}
public void countDown() {
synchronized (monitor) {
count--;
monitor.notifyAll();
}
}
public void await() throws InterruptedException {
synchronized (monitor) {
while (count > 0) {
monitor.wait();
}
}
}
}
static class Job implements Runnable {
private CountdownLatch latch;
public Job(CountdownLatch latch) {
this.latch = latch;
}
#Override
public void run() {
try {
// do work.
Thread.sleep((long) (Math.random() * 3000d));
} catch (InterruptedException e) {
//
} finally {
latch.countDown();
}
}
}
public static void main(String[] args) throws InterruptedException {
CountdownLatch latch = new CountdownLatch(3);
new Thread(new Job(latch)).start();
new Thread(new Job(latch)).start();
new Thread(new Job(latch)).start();
latch.await();
System.out.println("All threads finished");
}
}
Not sure what you are trying to do but this is as simple as I can think of (just native concurrency):
Create a static or shared volatile boolean
private static volatile boolean exceptionOccured=false
Set the above to 'true' when exception occurs:
....}catch(Exception e){
exceptionOccured=true;
}
Check this periodically in you normal thread flow:
if (exceptionOccured)
//enter you synchronized call here
the synchronized method could look something like:
public synchronized void checkAndRecover(){
//decrement a counter or other logic to identify which is the last Thread and then
//perform any recovery logic
}
I am new to threading and semaphors, and I have some problem in synchronizing threads. For example, in the following code I want to do a pretty simple thing. To let one thread run, while other waits. For example, if it starts with the first thread, I want the second to wait for the first one to finish and then start. I really don't know what am I doing wrong.
Here is the code :
import java.io.*;
import java.util.concurrent.Semaphore;
public class ThreadTest {
public static void main(String[] args) throws InterruptedException {
Semaphore binaren = new Semaphore(1);
Runnable t1 = new T2(binaren);
Thread a = new Thread(t1);
Thread a2 = new T1(binaren);
System.out.println(binaren.availablePermits());
a.start();
a2.start();
}
}
class Work {
private static int a = 4;
public synchronized static void QQR(String s1)
{
for(int i=0;i<100;i++)
System.out.println(s1+" : "+(a++));
}
}
class T1 extends Thread
{
Semaphore sem;
public T1(Semaphore s1)
{
sem=s1;
}
public void run()
{
synchronized(this) {
if(!sem.tryAcquire()){
try {
wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
Work.QQR("T1");
sem.release();
notifyAll();
}
}
}
class T2 extends Thread
{
Semaphore sem;
public T2(Semaphore s1)
{
sem=s1;
}
#Override
public void run() {
synchronized(this) {
if(!sem.tryAcquire()){
try {
wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
Work.QQR("T2");
sem.release();
notifyAll();
}
}
}
The problem is that notify and notifyAll only wake up threads holding locks on the monitor being notified. But the t1 and t2 instances are waiting on themselves and are never awoken. You can have them wait on the semaphore for this simple test or introduce a new shared object to see how it works.
Use
sem.wait();
and
sem.notifyAll();
You can use Thread.join() on the first thread so that second thread will wait till the execution of this instance is not completed.
Code snippet:
class Counter implements Runnable {
Object s = new Object();
#Override
public void run() {
try {
synchronized (s) {
s.wait(10000);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
//...do Something
}
public void stopCounter() {
synchronized (s) {
s.notifyAll();
}
}
}
Irrespective of whether i call stopCounter or not, the ...do Something code always executes only after the wait interval. Even after notify it still waits for 10 secs.
I cannot tell from your example what you are trying to achieve. If it is to try and replace some sort of polling then consider the BlockingQueue interface that was released in Java 5. Since that has appeared I have had no need for wait/notify. It's a lot more simple to use and java behind the scenes does the equivalent of the wait/notify for you.
It depends of the way you use it. I have just tried it by adding a main method and running it and it seems like the wait / notify mechanism is working fine, not the way you described it. Please try it yourself:
public static void main(String[] args) {
Counter c = new Counter();
new Thread(c).start();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
c.stopCounter();
}
My guess is that you call the run and stopCounter methods on different instances of your Counter class. They therefore use different monitors (your s = new Object()) and the call to stop won't notify the other Counter.
For example, this would behave similarly to what you describe (unless you get a spurious wakeup):
public static void main(String[] args) throws InterruptedException {
Counter c = new Counter();
new Thread(c).start();
Thread.sleep(200);
new Counter().stopCounter();
}
static class Counter implements Runnable {
Object s = new Object();
#Override
public void run() {
try {
System.out.println("in");
synchronized (s) {
s.wait(10000);
}
System.out.println("out");
} catch (InterruptedException e) {
e.printStackTrace();
}
//...do Something
}
public void stopCounter() {
synchronized (s) {
s.notifyAll();
}
System.out.println("notified");
}
}