Given a singleton class Employee with 2 methods
int getSalary()
void updateSalary(int increment)
Do I need to synchronize or lock both these functions or use atomic salary variable?
If yes then the question is that in this way we would have to synchronize all the functions that we define in multithreaded environment. So, why not just make synchronized a standard as today no real world application would be single threaded?
With Singleton, we always have to very careful because, singleton object being a single instance naturally, can be shared between threads. Making functions synchronized is one way, and it is not efficient way. We need to think about other aspect of concurrency, like immutability Atomic classes.
class Employee {
//singleton instantiation
private final AtomicInteger sal = new AtomicInteger(0);
int getSalary(){
return sla.get();
}
void updateSalary(int increment){
sla.add(increment);
}
}
This will solve, we do not need to synchronize every method of the singleton class.
We do not have to mark every function of every class to be synchronized, but always have to be careful if a function is modifying a state or reading a state and could be concurrently invoked, in such cases start thinking about synchronization. But, with singleton classes we always have to be careful.
When I read the Java Concurrency in Practice by Brian Goetz, I recall him saying "Immutable objects, on the other hand, can be safely accessed even when synchronization is not used to publish the object reference" in the chapter about visibility.
I thought that this implies that if you publish an immutable object, all fields(mutable final references included) are visible to other threads that might make use of them and at least up to date to when that object finished construction.
Now, I read in https://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html that
"Now, having said all of this, if, after a thread constructs an immutable object (that is, an object that only contains final fields), you want to ensure that it is seen correctly by all of the other thread, you still typically need to use synchronization. There is no other way to ensure, for example, that the reference to the immutable object will be seen by the second thread. The guarantees the program gets from final fields should be carefully tempered with a deep and careful understanding of how concurrency is managed in your code."
They seem to contradict each other and I am not sure which to believe.
I have also read that if all fields are final then we can ensure safe publication even if the object is not per say immutable.
For example, I always thought that this code in Brian Goetz's concurrency in practice was fine when publishing an object of this class due to this guarantee.
#ThreadSafe
public class MonitorVehicleTracker {
#GuardedBy("this")
private final Map<String, MutablePoint> locations;
public MonitorVehicleTracker(
Map<String, MutablePoint> locations) {
this.locations = deepCopy(locations);
}
public synchronized Map<String, MutablePoint> getLocations() {
return deepCopy(locations);
}
public synchronized MutablePoint getLocation(String id) {
MutablePoint loc = locations.get(id);
return loc == null ? null : new MutablePoint(loc);
}
public synchronized void setLocation(String id, int x, int y) {
MutablePoint loc = locations.get(id);
if (loc == null)
throw new IllegalArgumentException("No such ID: " + id);
loc.x = x;
loc.y = y;
}
private static Map<String, MutablePoint> deepCopy(
Map<String, MutablePoint> m) {
Map<String, MutablePoint> result =
new HashMap<String, MutablePoint>();
for (String id : m.keySet())
result.put(id, new MutablePoint(m.get(id)));
return Collections.unmodifiableMap(result);
}
}
public class MutablePoint { /* Listing 4.5 */ }
For example, in this code example, what if that final guarantee is false and a thread made an instance of this class and then the reference to that object is not null, but the field locations is null at the time another thread uses that class?
Once again, I don't know which is correct or if I happened to misinterpret both the article or Goetz
This question has been answered a few times before but I feel that many of those answers are inadequate. See:
https://stackoverflow.com/a/14617582
https://stackoverflow.com/a/35169705
https://stackoverflow.com/a/7887675
Effectively Immutable Object
etc...
In short, Goetz's statement in the linked JSR 133 FAQ page is more "correct", although not in the way that you are thinking.
When Goetz says that immutable objects are safe to use even when published without synchronization, he means to say that immutable objects that are visible to different threads are guaranteed to retain their original state/invariants, all else remaining the same. In other words, properly synchronized publication is not necessary to maintain state consistency.
In the JSR-133 FAQ, when he says that:
you want to ensure that it is seen correctly by all of the other thread (sic)
He is not referring to the state of the immutable object. He means that you must synchronize publication in order for another thread to see the reference to the immutable object. There's a subtle difference to what the two statements are talking about: while JCIP is referring to state consistency, the FAQ page is referring to access to a reference of an immutable object.
The code sample you provided has nothing, really, to do with anything that Goetz says here, but to answer your question, a correctly initializedfinal field will hold its expected value if the object is properly initialized (beware the difference between initialization and publication). The code sample also synchronizes access to the locations field so as to ensure updates to the final field are thread-safe.
In fact, to elaborate further, I suggest that you look at JCIP listing 3.13 (VolatileCachedFactorizer). Notice that even though OneValueCache is immutable, that it is stored in a volatile field. To illustrate the FAQ statement, VolatileCachedFactorizer will not work correctly without volatile. "Synchronization" is referring to using a volatile field in order to ensure that updates made to it are visible to other threads.
A good way to illustrate the first JCIP statement is to remove volatile. In this case, the CachedFactorizer won't work. Consider this: what if one thread set a new cache value, but another thread tried to read the value and the field was not volatile? The reader might not see the updated OneValueCache. BUT, recalling that Goetz refers to the state of the immutable object, IF the reader thread happened to see an up-to-date instance of OneValueCache stored at cache, then the state of that instance would be visible and correctly constructed.
So although it is possible to lose updates to cache, it is impossible to lose the state of the OneValueCache if it is read, because it is immutable. I suggest reading the accompanying text stating that "volatile reference used to ensure timely visibility."
As a final example, consider a singleton that uses FinalWrapper for thread safety. Note that FinalWrapper is effectively immutable (depending on whether the singleton is mutable), and that the helperWrapper field is in fact non-volatile. Recalling the second FAQ statement, that synchronization is required for access the reference, how can this "correct" implementation possibly be correct!?
In fact, it is possible to do this here because it is not necessary for threads to immediately see the up-to-date value for helperWrapper. If the value that is held by helperWrapper is non-null, then great! Our first JCIP statement guarantees that the state of FinalWrapper is consistent, and that we have a fully initialized Foo singleton that can be readily returned. If the value is actually null, there are 2 possibilities: firstly, it is possible that it is the first call and it has not been initialized; secondly, it could just be a stale value.
In the case that it is the first call, the field itself is checked again in a synchronized context, as suggested by the second FAQ statement. It will find that this value is still null, and will initialize a new FinalWrapper and publish with synchronization.
In the case that it is just a stale value, by entering the synchronized block, the thread can setup a happens-before order with a preceding write to the field. By definition, if a value is stale, then some writer has already written to the helperWrapper field, and that the current thread just has not seen it yet. By entering into the synchronized block, a happens-before relationship is established with that previous write, since according to our first scenario, a truly uninitialized helperWrapper will be initialized by the same lock. Therefore, it can recover by rereading once the method has entered a synchronized context and obtain the most up-to-date, non-null value.
I hope that my explanations and the accompanying examples that I have given will clear things up for you.
Generally code samples use locks this way:
static readonly object lockForWorkingWithSharedObject = new object();
lock(lockForWorkingWithSharedObject)
{
// do something with shared object
}
This way we need many locks in a large class.
Is it good practice to use the shared object itself as the synchronization object ?
// of course here sharedObject is a reference type
lock(sharedObject)
{
// do something with sharedObject
}
In Java and .NET, the point of having each object lockable is precisely because language designers thought it would be useful to use the object itself as the lock; hence also the synchronized keyword in Java.
If you need much finer granularity of locking, I would assume that you better split the state of your object in multiple objects, grouped by things that semantically belong together, and thus likely also need to protected against concurrent access together.
I always had this specific scenario worry me for eons. Let's say my class looks like this
public class Person {
public Address Address{get;set;}
public string someMethod()
{}
}
My question is, I was told by my fellow developers that the Address propery of type Address, is not thread safe.
From a web request perspective, every request is run on a separate thread and every time
the thread processes the following line in my business object or code behind, example
var p = new Person();
it creates a new instance of Person object on heap and so the instance is accessed by the requesting thread, unless and otherwise I spawn multiple threads in my application.
If I am wrong, please explain to me why I am wrong and why the public property (Address) is not thread safe?
Any help will be much appreciated.
Thanks.
If the reference to your Person instance is shared among multiple threads then multiple threads could potentially change Address causing a race condition. However unless you are holding that reference in a static field or in Session (some sort of globally accessible place) then you don't have anything to be worried about.
If you are creating references to objects in your code like you have show above (var p = new Person();) then you are perfectly thread safe as other threads will not be able to access the reference to these objects without resorting to nasty and malicious tricks.
Your property is not thread safe, because you have no locking to prevent multiple writes to the property stepping on each others toes.
However, in your scenario where you are not sharing an instance of your class between multiple threads, the property doesn't need to be thread safe.
Objects that are shared between multiple threads, where each thread can change the state of the object, then all state changes need to be protected so that only one thread at a time can modify the object.
You should be fine with this, however there are a few things I'd worry about...
If your Person object was to be modified or held some disposable resources, you could potentially find that one of the threads will be unable to read this variable. To prevent this, you will need to lock the object before read/writing it to ensure it won't be trampled on by other threads. The easiest way is by using the lock{} construct.
Recently I tried to Access a textbox from a thread (other than the UI thread) and an exception was thrown. It said something about the "code not being thread safe" and so I ended up writing a delegate (sample from MSDN helped) and calling it instead.
But even so I didn't quite understand why all the extra code was necessary.
Update:
Will I run into any serious problems if I check
Controls.CheckForIllegalCrossThread..blah =true
Eric Lippert has a nice blog post entitled What is this thing you call "thread safe"? about the definition of thread safety as found of Wikipedia.
3 important things extracted from the links :
“A piece of code is thread-safe if it functions correctly during
simultaneous execution by multiple threads.”
“In particular, it must satisfy the need for multiple threads to
access the same shared data, …”
“…and the need for a shared piece of data to be accessed by only one
thread at any given time.”
Definitely worth a read!
In the simplest of terms threadsafe means that it is safe to be accessed from multiple threads. When you are using multiple threads in a program and they are each attempting to access a common data structure or location in memory several bad things can happen. So, you add some extra code to prevent those bad things. For example, if two people were writing the same document at the same time, the second person to save will overwrite the work of the first person. To make it thread safe then, you have to force person 2 to wait for person 1 to complete their task before allowing person 2 to edit the document.
Wikipedia has an article on Thread Safety.
This definitions page (you have to skip an ad - sorry) defines it thus:
In computer programming, thread-safe describes a program portion or routine that can be called from multiple programming threads without unwanted interaction between the threads.
A thread is an execution path of a program. A single threaded program will only have one thread and so this problem doesn't arise. Virtually all GUI programs have multiple execution paths and hence threads - there are at least two, one for processing the display of the GUI and handing user input, and at least one other for actually performing the operations of the program.
This is done so that the UI is still responsive while the program is working by offloading any long running process to any non-UI threads. These threads may be created once and exist for the lifetime of the program, or just get created when needed and destroyed when they've finished.
As these threads will often need to perform common actions - disk i/o, outputting results to the screen etc. - these parts of the code will need to be written in such a way that they can handle being called from multiple threads, often at the same time. This will involve things like:
Working on copies of data
Adding locks around the critical code
Opening files in the appropriate mode - so if reading, don't open the file for write as well.
Coping with not having access to resources because they're locked by other threads/processes.
Simply, thread-safe means that a method or class instance can be used by multiple threads at the same time without any problems occurring.
Consider the following method:
private int myInt = 0;
public int AddOne()
{
int tmp = myInt;
tmp = tmp + 1;
myInt = tmp;
return tmp;
}
Now thread A and thread B both would like to execute AddOne(). but A starts first and reads the value of myInt (0) into tmp. Now for some reason, the scheduler decides to halt thread A and defer execution to thread B. Thread B now also reads the value of myInt (still 0) into it's own variable tmp. Thread B finishes the entire method so in the end myInt = 1. And 1 is returned. Now it's Thread A's turn again. Thread A continues. And adds 1 to tmp (tmp was 0 for thread A). And then saves this value in myInt. myInt is again 1.
So in this case the method AddOne() was called two times, but because the method was not implemented in a thread-safe way the value of myInt is not 2, as expected, but 1 because the second thread read the variable myInt before the first thread finished updating it.
Creating thread-safe methods is very hard in non-trivial cases. And there are quite a few techniques. In Java you can mark a method as synchronized, this means that only one thread can execute that method at a given time. The other threads wait in line. This makes a method thread-safe, but if there is a lot of work to be done in a method, then this wastes a lot of space. Another technique is to 'mark only a small part of a method as synchronized' by creating a lock or semaphore, and locking this small part (usually called the critical section). There are even some methods that are implemented as lock-less thread-safe, which means that they are built in such a way that multiple threads can race through them at the same time without ever causing problems, this can be the case when a method only executes one atomic call. Atomic calls are calls that can't be interrupted and can only be done by one thread at a time.
In real world example for the layman is
Let's suppose you have a bank account with the internet and mobile banking and your account have only $10.
You performed transfer balance to another account using mobile banking, and the meantime, you did online shopping using the same bank account.
If this bank account is not threadsafe, then the bank allows you to perform two transactions at the same time and then the bank will become bankrupt.
Threadsafe means that an object's state doesn't change if simultaneously multiple threads try to access the object.
You can get more explanation from the book "Java Concurrency in Practice":
A class is thread‐safe if it behaves correctly when accessed from multiple threads, regardless of the scheduling or interleaving of the execution of those threads by the runtime environment, and with no additional synchronization or other coordination on the part of the calling code.
A module is thread-safe if it guarantees it can maintain its invariants in the face of multi-threaded and concurrence use.
Here, a module can be a data-structure, class, object, method/procedure or function. Basically scoped piece of code and related data.
The guarantee can potentially be limited to certain environments such as a specific CPU architecture, but must hold for those environments. If there is no explicit delimitation of environments, then it is usually taken to imply that it holds for all environments that the code can be compiled and executed.
Thread-unsafe modules may function correctly under mutli-threaded and concurrent use, but this is often more down to luck and coincidence, than careful design. Even if some module does not break for you under, it may break when moved to other environments.
Multi-threading bugs are often hard to debug. Some of them only happen occasionally, while others manifest aggressively - this too, can be environment specific. They can manifest as subtly wrong results, or deadlocks. They can mess up data-structures in unpredictable ways, and cause other seemingly impossible bugs to appear in other remote parts of the code. It can be very application specific, so it is hard to give a general description.
Thread safety: A thread safe program protects it's data from memory consistency errors. In a highly multi-threaded program, a thread safe program does not cause any side effects with multiple read/write operations from multiple threads on same objects. Different threads can share and modify object data without consistency errors.
You can achieve thread safety by using advanced concurrency API. This documentation page provides good programming constructs to achieve thread safety.
Lock Objects support locking idioms that simplify many concurrent applications.
Executors define a high-level API for launching and managing threads. Executor implementations provided by java.util.concurrent provide thread pool management suitable for large-scale applications.
Concurrent Collections make it easier to manage large collections of data, and can greatly reduce the need for synchronization.
Atomic Variables have features that minimize synchronization and help avoid memory consistency errors.
ThreadLocalRandom (in JDK 7) provides efficient generation of pseudorandom numbers from multiple threads.
Refer to java.util.concurrent and java.util.concurrent.atomic packages too for other programming constructs.
Producing Thread-safe code is all about managing access to shared mutable states. When mutable states are published or shared between threads, they need to be synchronized to avoid bugs like race conditions and memory consistency errors.
I recently wrote a blog about thread safety. You can read it for more information.
You are clearly working in a WinForms environment. WinForms controls exhibit thread affinity, which means that the thread in which they are created is the only thread that can be used to access and update them. That is why you will find examples on MSDN and elsewhere demonstrating how to marshall the call back onto the main thread.
Normal WinForms practice is to have a single thread that is dedicated to all your UI work.
I find the concept of http://en.wikipedia.org/wiki/Reentrancy_%28computing%29 to be what I usually think of as unsafe threading which is when a method has and relies on a side effect such as a global variable.
For example I have seen code that formatted floating point numbers to string, if two of these are run in different threads the global value of decimalSeparator can be permanently changed to '.'
//built in global set to locale specific value (here a comma)
decimalSeparator = ','
function FormatDot(value : real):
//save the current decimal character
temp = decimalSeparator
//set the global value to be
decimalSeparator = '.'
//format() uses decimalSeparator behind the scenes
result = format(value)
//Put the original value back
decimalSeparator = temp
To understand thread safety, read below sections:
4.3.1. Example: Vehicle Tracker Using Delegation
As a more substantial example of delegation, let's construct a version of the vehicle tracker that delegates to a thread-safe class. We store the locations in a Map, so we start with a thread-safe Map implementation, ConcurrentHashMap. We also store the location using an immutable Point class instead of MutablePoint, shown in Listing 4.6.
Listing 4.6. Immutable Point class used by DelegatingVehicleTracker.
class Point{
public final int x, y;
public Point() {
this.x=0; this.y=0;
}
public Point(int x, int y) {
this.x = x;
this.y = y;
}
}
Point is thread-safe because it is immutable. Immutable values can be freely shared and published, so we no longer need to copy the locations when returning them.
DelegatingVehicleTracker in Listing 4.7 does not use any explicit synchronization; all access to state is managed by ConcurrentHashMap, and all the keys and values of the Map are immutable.
Listing 4.7. Delegating Thread Safety to a ConcurrentHashMap.
public class DelegatingVehicleTracker {
private final ConcurrentMap<String, Point> locations;
private final Map<String, Point> unmodifiableMap;
public DelegatingVehicleTracker(Map<String, Point> points) {
this.locations = new ConcurrentHashMap<String, Point>(points);
this.unmodifiableMap = Collections.unmodifiableMap(locations);
}
public Map<String, Point> getLocations(){
return this.unmodifiableMap; // User cannot update point(x,y) as Point is immutable
}
public Point getLocation(String id) {
return locations.get(id);
}
public void setLocation(String id, int x, int y) {
if(locations.replace(id, new Point(x, y)) == null) {
throw new IllegalArgumentException("invalid vehicle name: " + id);
}
}
}
If we had used the original MutablePoint class instead of Point, we would be breaking encapsulation by letting getLocations publish a reference to mutable state that is not thread-safe. Notice that we've changed the behavior of the vehicle tracker class slightly; while the monitor version returned a snapshot of the locations, the delegating version returns an unmodifiable but “live” view of the vehicle locations. This means that if thread A calls getLocations and thread B later modifies the location of some of the points, those changes are reflected in the Map returned to thread A.
4.3.2. Independent State Variables
We can also delegate thread safety to more than one underlying state variable as long as those underlying state variables are independent, meaning that the composite class does not impose any invariants involving the multiple state variables.
VisualComponent in Listing 4.9 is a graphical component that allows clients to register listeners for mouse and keystroke events. It maintains a list of registered listeners of each type, so that when an event occurs the appropriate listeners can be invoked. But there is no relationship between the set of mouse listeners and key listeners; the two are independent, and therefore VisualComponent can delegate its thread safety obligations to two underlying thread-safe lists.
Listing 4.9. Delegating Thread Safety to Multiple Underlying State Variables.
public class VisualComponent {
private final List<KeyListener> keyListeners
= new CopyOnWriteArrayList<KeyListener>();
private final List<MouseListener> mouseListeners
= new CopyOnWriteArrayList<MouseListener>();
public void addKeyListener(KeyListener listener) {
keyListeners.add(listener);
}
public void addMouseListener(MouseListener listener) {
mouseListeners.add(listener);
}
public void removeKeyListener(KeyListener listener) {
keyListeners.remove(listener);
}
public void removeMouseListener(MouseListener listener) {
mouseListeners.remove(listener);
}
}
VisualComponent uses a CopyOnWriteArrayList to store each listener list; this is a thread-safe List implementation particularly suited for managing listener lists (see Section 5.2.3). Each List is thread-safe, and because there are no constraints coupling the state of one to the state of the other, VisualComponent can delegate its thread safety responsibilities to the underlying mouseListeners and keyListeners objects.
4.3.3. When Delegation Fails
Most composite classes are not as simple as VisualComponent: they have invariants that relate their component state variables. NumberRange in Listing 4.10 uses two AtomicIntegers to manage its state, but imposes an additional constraint—that the first number be less than or equal to the second.
Listing 4.10. Number Range Class that does Not Sufficiently Protect Its Invariants. Don't do this.
public class NumberRange {
// INVARIANT: lower <= upper
private final AtomicInteger lower = new AtomicInteger(0);
private final AtomicInteger upper = new AtomicInteger(0);
public void setLower(int i) {
//Warning - unsafe check-then-act
if(i > upper.get()) {
throw new IllegalArgumentException(
"Can't set lower to " + i + " > upper ");
}
lower.set(i);
}
public void setUpper(int i) {
//Warning - unsafe check-then-act
if(i < lower.get()) {
throw new IllegalArgumentException(
"Can't set upper to " + i + " < lower ");
}
upper.set(i);
}
public boolean isInRange(int i){
return (i >= lower.get() && i <= upper.get());
}
}
NumberRange is not thread-safe; it does not preserve the invariant that constrains lower and upper. The setLower and setUpper methods attempt to respect this invariant, but do so poorly. Both setLower and setUpper are check-then-act sequences, but they do not use sufficient locking to make them atomic. If the number range holds (0, 10), and one thread calls setLower(5) while another thread calls setUpper(4), with some unlucky timing both will pass the checks in the setters and both modifications will be applied. The result is that the range now holds (5, 4)—an invalid state. So while the underlying AtomicIntegers are thread-safe, the composite class is not. Because the underlying state variables lower and upper are not independent, NumberRange cannot simply delegate thread safety to its thread-safe state variables.
NumberRange could be made thread-safe by using locking to maintain its invariants, such as guarding lower and upper with a common lock. It must also avoid publishing lower and upper to prevent clients from subverting its invariants.
If a class has compound actions, as NumberRange does, delegation alone is again not a suitable approach for thread safety. In these cases, the class must provide its own locking to ensure that compound actions are atomic, unless the entire compound action can also be delegated to the underlying state variables.
If a class is composed of multiple independent thread-safe state variables and has no operations that have any invalid state transitions, then it can delegate thread safety to the underlying state variables.