In the following code fragment I'm using a semaphore to synchronize access to certain resources.
public void m () {
permit.acquire ();
while (!canFoo ()) {
permit.release ();
reticulateSpines ();
permit.acquire ();
}
doFoo ();
permit.release ();
}
It might be reasonable to enclose the acquire/release cycles in a try/finally. How can I do this, given the presence of the while loop?
On the principle of every acquire must be released I would suggest:
private final Semaphore permit = new Semaphore(8, true);
private final Random random = new Random();
private boolean canFoo() {
return random.nextBoolean();
}
private void doFoo() {
System.out.println("Foo done!");
}
private void reticulateSpines() {
System.out.println("Spines reticulated!");
}
public void m() throws InterruptedException {
permit.acquire();
try {
while (!canFoo()) {
permit.release();
try {
reticulateSpines ();
} finally {
permit.acquire();
}
}
doFoo();
} finally {
permit.release();
}
}
However - I am not sure of you are using Semaphores as they are intended. It looks much more like you are looking for something like ReentrantLock which would eliminate spinlock loop.
ReadWriteLock fooLock = new ReentrantReadWriteLock();
Lock fooReadLock = fooLock.readLock();
Lock fooWriteLock = fooLock.writeLock();
public void n() throws InterruptedException {
fooWriteLock.lock();
try {
doFoo();
} finally {
fooWriteLock.unlock();
}
}
or even perhaps
public void o() throws InterruptedException {
while (!fooWriteLock.tryLock()) {
reticulateSpines();
}
try {
doFoo();
} finally {
fooWriteLock.unlock();
}
}
Related
I want to call a method in java but synchronized and first-in-first-out.
Currently I call the method like so:
synchronized (synchronizeObject) {
executeMethode(object1, object2);
}
I found out, that synchronized does not actually care in what order something is added.
Is there an easy way to force FIFO?
I do not think ArrayBlockingQueue is applicable here, At least not in a way I see it, but I may be wrong
Here is a snippet:
public class AppTest {
#Test
void test() throws InterruptedException {
ExecutorService pool = Executors.newFixedThreadPool(8);
FairLockedMethod<Integer> method = new FairLockedMethod<>() {
#Override
protected void lockedMethod(Integer i) {
System.out.println(i);
try {
Thread.currentThread().wait(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
for (int i = 0; i < 16; i++) {
int n = i;
pool.submit(() ->method.run(n));
}
pool.shutdown();
pool.awaitTermination(10, TimeUnit.SECONDS);
}
public static abstract class FairLockedMethod<T> {
protected abstract void lockedMethod(T value);
private final ReentrantLock lock = new ReentrantLock(true);
private final Condition condition = lock.newCondition();
private final Queue<Thread> queue = new LinkedBlockingDeque<>();
public void run(T value) {
queue.offer(Thread.currentThread());
lock.lock();
try {
while (queue.peek() != Thread.currentThread()) {
condition.await();
}
queue.poll();
condition.signalAll();
lockedMethod(value);
} catch (InterruptedException e) {
throw new RuntimeException(e);
} finally {
lock.unlock();
}
}
}
}
It is not most efficient implementation, but simplest I can come with.
I read now book Thinking in Java, chapter about critical sections and I cannot understand an example, because I got exceptions which are not described in the book. An example looks like below:
class Pair {
private int x, y;
public Pair(int x, int y) {
this.x = x;
this.y = y;
}
public Pair() {
this(0, 0);
}
public int getX() { return x; }
public int getY() { return y; }
public void incrementX() { x++; }
public void incrementY() { y++; }
public class PairValuesNotEqualException extends RuntimeException {
public PairValuesNotEqualException() {
super("Values are not equal: " + Pair.this);
}
}
public void checkState() {
if (x != y) {
throw new PairValuesNotEqualException();
}
}
}
abstract class PairManager {
AtomicInteger checkCounter = new AtomicInteger(0);
protected Pair p = new Pair();
public synchronized Pair getPair() {
// Make copies to protect the original
return new Pair(p.getX(), p.getY());
}
public abstract void increment();
}
// synchronization of the whole method
class PairManager1 extends PairManager {
#Override
public synchronized void increment() {
p.incrementX();
p.incrementY();
}
}
// Critical section
class PairManager2 extends PairManager {
#Override
public void increment() {
synchronized (this) {
p.incrementX();
p.incrementY();
}
}
}
class PairManipulator implements Runnable {
private PairManager pairManager;
public PairManipulator(PairManager pairManager) {
this.pairManager = pairManager;
}
#Override
public void run() {
while (true)
pairManager.increment();
}
}
class PairChecker implements Runnable {
private PairManager pairManager;
public PairChecker(PairManager pairManager) {
this.pairManager = pairManager;
}
#Override
public void run() {
while (true) {
pairManager.checkCounter.incrementAndGet();
pairManager.getPair().checkState();
}
}
}
public class CriticalSection {
static void testApproaches(PairManager pman1, PairManager pman2) {
ExecutorService exec = Executors.newCachedThreadPool();
PairManipulator
pm1 = new PairManipulator(pman1),
pm2 = new PairManipulator(pman2);
PairChecker
pcheck1 = new PairChecker(pman1),
pcheck2 = new PairChecker(pman2);
exec.execute(pm1);
exec.execute(pm2);
exec.execute(pcheck1);
exec.execute(pcheck2);
try {
TimeUnit.MILLISECONDS.sleep(500);
} catch (InterruptedException e) {
System.out.println("InterruptedException");
}
System.out.println("pm1: " + pm1 + "\npm2: " + pm2);
System.exit(0);
}
public static void main(String[] args) {
PairManager
pman1 = new PairManager1(),
pman2 = new PairManager2();
testApproaches(pman1, pman2);
}
}
An example output:
pm1: Pair: Pair{x=364, y=364} counter = 471421
pm2: Pair: Pair{x=365, y=365} counter = 1015604598
This example executed without exception.
In above example I understand how does it work but the problem is in example with explicit locks.
Example with explicit lock from book:
class ExplicitPairManager1 extends PairManager {
private Lock lock = new ReentrantLock();
// why synchronized ??
public synchronized void increment() {
lock.lock();
try {
p.incrementX();
p.incrementY();
} finally {
lock.unlock();
}
}
}
class ExplicitPairManager2 extends PairManager {
private Lock lock = new ReentrantLock();
public void increment() {
lock.lock();
try {
p.incrementX();
p.incrementY();
} finally {
lock.unlock();
}
}
}
public class ExplicitCriticalSection {
public static void main(String[] args) throws Exception {
PairManager
pm1 = new ExplicitPairManager1(),
pm2 = new ExplicitPairManager2();
CriticalSection.testApproaches(pm1, pm2);
}
}
Output:
Exception in thread "pool-1-thread-4" critical.sections.Pair$PairValuesNotEqualException: Values are not equal: Pair{x=2, y=1}
at critical.sections.Pair.checkState(CriticalSection.java:49)
at critical.sections.PairChecker.run(CriticalSection.java:133)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
pm1: Pair: Pair{x=1024, y=1024} counter = 3
pm2: Pair: Pair{x=1025, y=1025} counter = 1499445
First what I don't understand why author use synchronized in ExplicitPairManager1#increment if he use also Lock object? Is that the mistake in the book?
Second problem is that I don't understand why I got exception?
Excpetion was thrown in:
class PairChecker implements Runnable {
private PairManager pairManager;
public PairChecker(PairManager pairManager) {
this.pairManager = pairManager;
}
#Override
public void run() {
while (true) {
pairManager.checkCounter.incrementAndGet();
pairManager.getPair().checkState(); // here was thrown an exception
}
}
}
Why I got excpetions and author dont? Is that possible JVM behavior is different on different systems? I use Ubuntu 16.04 LTS and Java 8.
You need to synchronize on the same object if you want to establish a critical section for multiple threads.
Your exception is getting thrown for pair modified in ExplicitPairManager2.
Let's see how possible exception-causing flow looks like:
ExplicitPairManager2.lock.lock() gets acquired
ExplicitPairManager2.p.incrementX() happens
PairChecker calls getPair()
PairChecker acquires pairManager's internal (this) monitor, but it is different than ExplicitPairManager2.lock
the result of getPair() therefore has x != y
so in the end there is no critical section.
In other words, while modifying, you were using two different objects to synchronize:
ExplicitPairManager2.lock to write
internal monitor of ExplicitPairManager2 (this) to create a copy for checking state
I have two threads. The first changes the value of variable Data. And second one print the value if its value has changed. I am trying to do that second thread just print each time that the variable's value changed, but I don't reach success. Someone can help me?
thread 1
class someservice{
volatile int data;
Boolean Flag = false;
public void mymethod(){
flag = true;
for (Integer i = 1; i < sheet.getRows(); i++) {
data = someMethod(); //this method when called return a new
//value
}
flag = false;
...
}
}
thread 2
Promise p = task {
try {
while (true) {
if (engineService.getFlag()) {
print(someservice.data);
}else{
break;
}
}
} catch(Throwable t) {
...
}
}
Since you mention Promises, I infer you are familiar with future/ promise in +C++11
in java there is a similar approach, with future callable...
public class HW5 {
public static void main(String[] argv) throws InterruptedException, ExecutionException {
FutureTask<Boolean> myFutureTask = new FutureTask<>(new Callable<Boolean>() {
#Override
public Boolean call() throws Exception {
// implement the logic here and return true if everything was
// ok, false otherwise.
Thread.sleep(5000);
System.out.println("dddd");
return System.currentTimeMillis() % 2 == 0;
}
});
ExecutorService executor = Executors.newFixedThreadPool(1);
executor.execute(myFutureTask);
Boolean result = myFutureTask.get();
System.out.println("Done!");
}
}
FutureTask in a class that takes a callable which can return an Object after its job is done... in Order to execute the Future task you can use a Executor service, especifically calling the method execute, since you need to wait for the thread to do the job then is necessary that you call Future.get, that will basically blocks the main thread until the future is done, to verify the result, just read the variable result..
You could use the notify() and notifyAll() methods within thread. Check out this link: https://docs.oracle.com/javase/tutorial/essential/concurrency/guardmeth.html
public synchronized void guardedJoy() {
// This guard only loops once for each special event, which may not
// be the event we're waiting for.
while(!joy) {
try {
wait();
} catch (InterruptedException e) {}
}
System.out.println("Joy and efficiency have been achieved!");
}
public synchronized notifyJoy() {
joy = true;
notifyAll();
}
You have to look up more data about Concurrent programming,I can tell you now some basics,well,not so so basic,but i will do my best:
Here,you have a Monitor,it is an abstract concept,in resume,a Monitor is a
class with all it's
method using"syncronized"
as modifier, it means,
that only
one thread
can access
the method
at once.So,
in the
monitor is
the variable
that you
want to print,
and the"flag",
that tells you if
the variable
was modified.Finally,
you can
see the
most important thing,the"wait()"and"notify()"methods,
those method
stops the thread,or"play"
them again.
You ask
here in
the printValue() method, if your variable was changed, if the variable was'nt change, put the thead to sleep with the wait() method, and when the other
method changeValue() is executed, the value is modified, and the notify() method is called, waking up the thread, so, doing all this, you can guarantee three things:
Safety: meaning that the threads will do that you want
Absence of deadlock: meaning that the thread that is put to sleep, will be awake in the future.
Mutex: meaning that only one thread is executing the critical code, for example, the op. "++" is not atomic, is Subdivided inside in more the one action, create a local var, read the var, sum, and asign, so, if more than one thread are in the game, the value may not be consecutive, example:
i = 0;
i ++;
output: 1;
output: 2;
output: 3;
output: 5;
output: 4;
output: 7;
That could happen, and even so, that will happen in the next code, because there a more than one thread executing. Well, this is the way to program with several threads, more or less
public class Monitor {
private int value = 0;
public static boolean valueHasChanged = false;
public synchronized int changeValue(int newValue){
this.value = newValue;
Monitor.valueHasChanged = true;
this.notify();
return this.value + 1;
}
public synchronized void printValue(){
while(!Monitor.valueHasChanged){
try {
this.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println(this.value);
Monitor.valueHasChanged = false;
}
public static void main(String[] args) {
Monitor ac = new Monitor();
BClass t1 = new BClass(ac);
AClass t2 = new AClass(ac);
t1.start();
t2.start();
}
public int getValue() {
return this.value;
}
}
Now the threads:
public class AClass extends Thread{
private Monitor ac;
public AClass(Monitor ac) {
this.ac = ac;
}
#Override
public void run() {
while(true){
this.ac.printValue();
}
}
}
And finally:
public class BClass extends Thread{
private Monitor ac;
public BClass(Monitor ac) {
this.ac = ac;
}
#Override
public void run() {
int v = 0;
while(true){
this.ac.changeValue(v);
v++; // this sum is not secure, if you want to print an
// ascending order, the code is diferent, I will show in
// above.
}
}
Now, if you want an ordered print:
the monitor will look like:
public class Monitor {
private int value = 0;
public boolean valueHasChanged = false;
private boolean hasPrint = true;
public synchronized void changeValue(int newValue) {
this.value = newValue;
this.valueHasChanged = true;
this.notify();
}
public synchronized void changeValuePlusOne() {
while (!hasPrint) {
try {
this.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
this.value++;
this.valueHasChanged = true;
this.hasPrint = false;
this.notifyAll();
}
public synchronized void printValue() {
while (!this.valueHasChanged) {
try {
this.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(this.value);
this.valueHasChanged = false;
this.hasPrint = true;
this.notifyAll();
}
public static void main(String[] args) {
Monitor ac = new Monitor();
BClass t1 = new BClass(ac);
AClass t2 = new AClass(ac);
t1.start();
t2.start();
}
public int getValue() {
return this.value;
}
}
And the Threads:
public class BClass extends Thread{
private Monitor ac;
public BClass(Monitor ac) {
this.ac = ac;
}
#Override
public void run() {
while(true){
this.ac.changeValuePlusOne();
}
}
}
The other Thread look equals:
public class AClass extends Thread{
private Monitor ac;
public AClass(Monitor ac) {
this.ac = ac;
}
#Override
public void run() {
while(true){
this.ac.printValue();
}
}
}
I am trying to create a basic Semaphore implementation using Queue. The idea is, there is a database, and there are 10 writers. Writers can only write to the database in mutual exclusion. I am using Queue because I want to implement First In First Out and Last In First Out.
Using Semaphore, I can't notify a specific thread to wake up. So my idea is what I am doing is for every Writer, I create an object and tell the Writer to wait on that object. Puts that object in a queue. Then remove the object from the queue and notify the Thread that is waiting on that object. In this way, I think I can make a FIFO or LIFO implementation.
I need help on the actual code implementation:
1. I run the code below, it gave me a lot of IllegalMonitorStateException.
2. FIFO and LIFO code (my FIFO code seems incorrect, while for LIFO code, I'm thinking to use Stack instead of Queue).
public class Test {
public static void main(String [] args) {
Database db = new Database();
for (int i = 0; i < 10; i++)
(new Thread(new Writer(db))).start();
}
}
public class Writer implements Runnable {
private Database database;
public Writer(Database database) {
this.database = database;
}
public void run() {
this.database.acquireWriteLock();
this.database.write();
this.database.releaseWriteLock();
}
}
public class Database {
private Semaphore lockQueue;
public Database() {
this.lockQueue = new Semaphore();
}
public void write() {
try {
Thread.sleep(1000);
} catch (InterruptedException ie) {}
}
public void acquireWriteLock() {
lockQueue.acquire();
}
public void releaseWriteLock() {
lockQueue.release();
}
}
import java.util.Queue;
import java.util.LinkedList;
public class Semaphore {
private Queue<Object> queue;
public Semaphore() {
this.queue = new LinkedList<Object>();
}
public synchronized void acquire() {
Object object = new Object();
try {
if (this.queue.size() > 0) {
object.wait();
this.queue.add(object);
}
} catch (InterruptedException ie) {}
this.queue.add(object);
}
public synchronized void release() {
Object object = this.queue.remove();
object.notify();
}
}
You need to acquire the lock of the object before you can use wait() and notify().
Try to check if the following code will work:
public class Semaphore {
private Queue<Object> queue;
private int state;
public Semaphore() {
this.queue = new LinkedList<Object>();
}
public void acquire() {
Object object = new Object();
synchronized (object) {
try {
if (this.state > 0) {
this.queue.add(object);
object.wait();
} else {
state++;
}
} catch (InterruptedException ie) {
}
}
}
public void release() {
Object object = this.queue.poll();
state--;
if(null == object) {
return;
}
synchronized (object) {
object.notify();
}
}
}
I have that code:
Main class:
public class myTest {
public static void main(String[] args) {
try {
Thread t1 = new myThreadClass("thread 1");
t1.start();
} catch (UnknownHostException ex) {
Logger.getLogger(glownyTest.class.getName()).log(Level.SEVERE, null, ex);
} catch (IOException ex) {
Logger.getLogger(glownyTest.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
My Thread class
public class myThreadClass extends Thread {
private HashSet<String> texts = new HashSet<String>();
public myThreadClass(String id) throws UnknownHostException, IOException {}
#Override
public void run() {
... collecting Strings into my hashSet ....
}
public HashSet<String> getTexts() {
return texts;
}
}
My Thread class is watching for network traffic, so I just cant call once
t1.getTexts()
whenever I want, because my hashSet can be empty (there are delays and latency in this network). How can I watch this texts hashSet and when some String will be added into hashSet - I i want my MAIN CLASS know about it? I just want to watch my Thread resources from Main class in smart way :)
If it will still be empty after my thread timeout, I want to know about it too.
You can use condition variables for this. Try something like:
class Monitor {
private final ConcurrentMap<String,String> data = new ConcurrentHashMap<String,String>();
private final Object mutex = new Object();
/* Private to the monitoring thread. Wakes up other
* threads, which may be waiting for data to arrive
*/
public void addElement(String key) {
data.put(key, key);
synchronized (mutex) { mutex.notifyAll(); }
}
public void removeElement(String key) {
data.remove(key);
synchronized (mutex) { mutex.notifyAll(); }
}
public Set<String> getElements() {
return data.keySet();
}
/* Can be called from any thread. Will wait at most "timeout"
* milliseconds
*/
public boolean waitForChanges(long timeout) throws InterruptedException {
final long then = System.currentTimeMillis() + timeout;
long left = timeout;
synchronized (mutex) {
while (data.isEmpty() && left > 0) {
mutex.wait(left);
left = then - System.currentTimeMillis();
}
return !data.isEmpty();
}
}
}
class MonitoringTask extends Runnable {
private final Monitor monitor;
MonitoringTask(Monitor m) {
this.monitor = m;
}
public void run() {
while (true) {
if (somethingHasHappened()) {
monitor.addElement("foo");
}
}
}
}
class Main {
public static void main(String[] args) {
final Monitor monitor = new Monitor();
final MonitoringTask task = new MonitoringTask(monitor);
final Thread thread = new Thread(task);
thread.setName("Monitor Thread");
thread.start();
if (monitor.waitForChanges(1500)) {
final Set<String> elts = monitor.getElements();
...
} else {
// Time-out
}
}
}
(I haven't tried to present this to a Java compiler, so watch out for all kinds of mistakes).