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Is it possible to write a guaranteed classic deadlock with synchronized methods?

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();
}
}

Do not synchronize on objects that may be reused [duplicate]

public class Alternate {
static Boolean mutex = true;
public static void main(String[] args) {
Thread t1 = new Thread(new Odd(mutex));
Thread t2 = new Thread(new Even(mutex));
t1.start();
t2.start();
}
}
class Odd implements Runnable{
Boolean mutex;
Odd( Boolean mutex){
this.mutex=mutex;
}
#Override
public void run() {
try {
synchronized(mutex){
while(mutex){
mutex.wait();
}
System.out.println("odd");
mutex=true;
mutex.notifyAll();
Thread.sleep(500);
}
}catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class Even implements Runnable{
Boolean mutex;
Even( Boolean mutex){
this.mutex=mutex;
}
#Override
public void run() {
try {
synchronized(mutex){
while(!mutex){
mutex.wait();
}
System.out.println("even");
mutex=false;
mutex.notifyAll();
Thread.sleep(500);
}
}catch (InterruptedException e) {
e.printStackTrace();
}
}
}
The error is
java.lang.IllegalMonitorStateException
at java.lang.Object.notifyAll(Native Method)
at com.test.concurrency.Even.run(Alternate.java:55)
at java.lang.Thread.run(Thread.java:722)
I am not able to figure out the reason for the error. I am calling notifyAll() from synchronised context and calling it from the correct object.

You're changing the lock out from under your threads. Every time you set your boolean to something, that's a different object; the code
mutex=true;
mutex.notifyAll();
sets mutex to a different object from the one the thread synchronized on (so the thread hasn't acquired the monitor for it), then it calls notifyAll on the new object.
Use a single lock and don't change it.
Locking on Booleans, numeric wrappers, or Strings is too error-prone and should be avoided. Not only can you end up with the error you're seeing, but other unrelated parts of the application (maybe written by somebody else following the same practice) could be locking on the same object and causing mysterious problems. Booleans, number wrappers, and strings are available to everything in the JVM. It's better to use a lock that is constrained in scope so that nothing else in your application can acquire it.
Often it's best to use a dedicated lock, something you don't use for any other purpose. Overloading something with different uses can cause trouble too easily.

Corrected Code if anyone needs
import java.util.concurrent.atomic.AtomicInteger;
public class Alternate {
static final AtomicInteger mutex = new AtomicInteger(0);
public static void main(String[] args) {
Thread t1 = new Thread(new Odd());
Thread t2 = new Thread(new Even());
t1.start();
t2.start();
}
static class Odd implements Runnable{
#Override
public void run() {
try {
for(int i=0;i<10;i++){
synchronized(mutex){
while(mutex.get()==1){
mutex.wait();
}
System.out.println("odd");
mutex.compareAndSet(0, 1);
mutex.notifyAll();
}
}
}catch (InterruptedException e) {
e.printStackTrace();
}
}
}
static class Even implements Runnable{
#Override
public void run() {
try {
for(int i=0;i<10;i++){
synchronized(mutex){
while(mutex.get()==0){
mutex.wait();
}
System.out.println("even");
mutex.compareAndSet(1, 0);
mutex.notifyAll();
}
}
}catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}

Having troubles with threads and semaphors in JAVA

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.

Wait() / notify() synchronization

I'm trying to check how wait/notify works in java.
Code:
public class Tester {
public static void main(String[] args) {
MyRunnable r = new MyRunnable();
Thread t = new Thread(r);
t.start();
synchronized (t) {
try {
System.out.println("wating for t to complete");
t.wait();
System.out.println("wait over");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
class MyRunnable implements Runnable {
public void run() {
System.out.println("entering run method");
synchronized (this) {
System.out.println("entering syncronised block");
notify();
try {
Thread.currentThread().sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("leaving syncronized block");
}
System.out.println("leaving run method");
}
}
Output Returned
wating for t to complete
entering run method
entering syncronised block
//sleep called
leaving syncronized block
leaving run method
wait over
I was expecting when notify() is executed the wait will be over & System.out.println("wait over"); will get printed. But it seems it only gets printed when t finished its run().

Object monitor locks need to be performed a single reference of the same lock...
In your example you are waiting on an instance of the Thread, but using notify from the Runnable. Instead, you should use a single, common lock object...for example
public class Tester {
public static final Object LOCK = new Object();
public static void main(String[] args) {
MyRunnable r = new MyRunnable();
Thread t = new Thread(r);
t.start();
synchronized (LOCK) {
try {
System.out.println("wating for t to complete");
LOCK.wait();
System.out.println("wait over");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static class MyRunnable implements Runnable {
public void run() {
System.out.println("entering run method");
synchronized (LOCK) {
System.out.println("entering syncronised block");
LOCK.notify();
try {
Thread.currentThread().sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("leaving syncronized block");
}
System.out.println("leaving run method");
}
}
}
Output...
wating for t to complete
entering run method
entering syncronised block
leaving syncronized block
wait over
leaving run method
wait over and leaving run method could change positions depending on the thread scheduling.
You could try putting the sleep out side the synchronized block. This will release the monitor lock allowing the wait section to continue running (as it can't start until the lock is released)
public static class MyRunnable implements Runnable {
public void run() {
System.out.println("entering run method");
synchronized (LOCK) {
System.out.println("entering syncronised block");
LOCK.notify();
System.out.println("leaving syncronized block");
}
try {
Thread.currentThread().sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("leaving run method");
}
}

Answer to updated code :
From Thread.sleep() javadoc:
Causes the currently executing thread to sleep (temporarily cease execution) for the
specified number of milliseconds, subject to the precision and accuracy of system timers
and schedulers. The thread does not lose ownership of any monitors.
If you call Thread.sleep while inside a synchronized block, other threads won't be able to enter the synchronized block. You should never do time consuming tasks while in a synchronized block to avoid this.

Note (as others pointed out as well) that you have to use the same object for locking/synchronizing in both threads.
If you want your main thread to continue immediately after notify is called, you have to relinquish the lock temporarily. Otherwise wait will get called only after the secondary thread leaves the synchronized block. And it's never a good idea to keep a lock in a long running computation!
One way how to achieve is to use wait(int) on the lock instead of sleep, because wait releases the synchronization lock temporarily:
public class Tester {
private static final Object lock = new Object();
public static void main(String[] args) {
Thread t = new Thread(new MyRunnable());
t.start();
synchronized (lock) {
try {
System.out.println("wating for t to complete");
lock.wait();
System.out.println("wait over");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
static class MyRunnable implements Runnable {
public void run() {
System.out.println("entering run method");
synchronized (lock) {
System.out.println("entering syncronised block");
lock.notify();
try {
lock.wait(1000); // relinquish the lock temporarily
} catch (InterruptedException ex) {
System.out.println("got interrupted");
}
System.out.println("leaving syncronized block");
}
System.out.println("leaving run method");
}
}
}
However, using these low-level primitives can be very error prone and I'd discourage from using them. Instead, I'd suggest you to use Java's high-level primitives for that. For example, you can use CountDownLatch which lets one thread wait until other threads count down to zero:
import java.util.concurrent.*;
public class TesterC {
private static final CountDownLatch latch = new CountDownLatch(1);
public static void main(String[] args) {
Thread t = new Thread(new MyRunnable());
t.start();
System.out.println("wating for t to complete");
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("wait over");
}
static class MyRunnable implements Runnable {
public void run() {
System.out.println("entering run method");
try {
latch.countDown();
Thread.sleep(1000);
} catch (InterruptedException ex) {
System.out.println("got interrupted");
}
System.out.println("leaving run method");
}
}
}
Here you don't have to synchronize anything, the latch does everything for you. There are many other primitives you can use - semaphores, an exchanger, thread-safe queues, etc. Explorer the java.util.concurrent package.
Perhaps even better solution is to use even higher level API, such as Akka provides. There you work with Actors or Software transactional memory, which can be composed easily and spare you of most of concurrency issues.

Understanding Java Wait and Notify methods

I have a following program:
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class SimpleWaitNotify implements Runnable {
final static Object obj = new Object();
static boolean value = true;
public synchronized void flag() {
System.out.println("Before Wait");
try {
obj.wait();
} catch (InterruptedException e) {
System.out.println("Thread interrupted");
}
System.out.println("After Being Notified");
}
public synchronized void unflag() {
System.out.println("Before Notify All");
obj.notifyAll();
System.out.println("After Notify All Method Call");
}
public void run() {
if (value) {
flag();
} else {
unflag();
}
}
public static void main(String[] args) throws InterruptedException {
ExecutorService pool = Executors.newFixedThreadPool(4);
SimpleWaitNotify sWait = new SimpleWaitNotify();
pool.execute(sWait);
SimpleWaitNotify.value = false;
SimpleWaitNotify sNotify = new SimpleWaitNotify();
pool.execute(sNotify);
pool.shutdown();
}
}
When I wait on obj, I get the following exception Exception in thread "pool-1-thread-1" java.lang.IllegalMonitorStateException: current thread not owner for each of the two threads.
But if I use SimpleWaitNotify's monitor then the program execution is suspended. In other words, I think it suspends current execution thread and in turn the executor. Any help towards understanding what's going on would be duly appreciated.
This is an area1 where the theory and javadoc seem straightforward, and since there aren't many examples, conceptually left a big gap in me.

You're calling wait and notifyAll on obj, but you're synchronizing on this (because you've got synchronized methods).
In order to wait or notify, you need to "own" the monitor first. Unsynchronize the methods, and synchronize on obj instead:
public void flag() {
System.out.println("Before Wait");
synchronized (obj) {
try {
obj.wait();
} catch (InterruptedException e) {
System.out.println("Thread interrupted");
}
}
System.out.println("After Being Notified");
}
public void unflag() {
System.out.println("Before Notify All");
synchronized (obj) {
obj.notifyAll();
}
System.out.println("After Notify All Method Call");
}

Either synchronize on obj, or call wait and notify on this. The calling thread must hold the monitor of the same object on which these methods are called.
For example,
synchronized void flag() {
System.out.println("Before Wait");
try {
wait();
} catch (InterruptedException e) {
System.out.println("Thread interrupted");
}
System.out.println("After Being Notified");
}
In this example, the lock is held on this (when the modifier synchronized is used on a instance method, the monitor of the instance is acquired). So, the wait() method may be invoked on the implied instance this.
In order to coordinate the two threads, they need to share the same lock. The original version had a static obj that could be used as a lock, but it wasn't used in the synchronized blocks. Here is a better example:
class SimpleWaitNotify implements Runnable {
private final Object lock;
private final boolean wait;
SimpleWaitNotify(Object lock, boolean wait) {
this.lock = lock;
this.wait = wait;
}
public void flag() {
synchronized (lock) {
System.out.println("Before Wait");
try {
lock.wait();
System.out.println("After Being Notified");
} catch (InterruptedException ex) {
System.out.println("Thread interrupted");
}
}
}
public void unflag() {
synchronized(lock) {
System.out.println("Before Notify All");
lock.notifyAll();
System.out.println("After Notify All Method Call");
}
}
public void run() {
if (wait) {
flag();
} else {
unflag();
}
}
public static void main(String[] argv) throws Exception {
ExecutorService pool = Executors.newFixedThreadPool(4);
Object shared = new Object();
SimpleWaitNotify sWait = new SimpleWaitNotify(shared, true);
pool.execute(sWait);
SimpleWaitNotify sNotify = new SimpleWaitNotify(shared, false);
pool.execute(sNotify);
pool.shutdown();
}
}

Rather putting the direct statement pool.shutdown(), try as below.
while (!service.isTerminated())
{
service.shutdown();
}
So it will wait for till all the threads execution completes.