If we have 2 classes that operate on the same object under different threads and we want to avoid race conditions, we'll have to use synchronized blocks with the same monitor like in the example below:
class A {
private DataObject mData; // will be used as monitor
// thread 3
public setObject(DataObject object) {
mData = object;
}
// thread 1
void operateOnData() {
synchronized(mData) {
mData.doSomething();
.....
mData.doSomethingElse();
}
}
}
class B {
private DataObject mData; // will be used as monitor
// thread 3
public setObject(DataObject object) {
mData = object;
}
// thread 2
void processData() {
synchronized(mData) {
mData.foo();
....
mData.bar();
}
}
}
The object we'll operate on, will be set by calling setObject() and it will not change afterwards. We'll use the object as a monitor. However, intelliJ will warn about synchronization on a non-final field.
In this particular scenario, is the non-local field an acceptable solution?
Another problem with the above approach is that it is not guaranteed that the monitor (mData) will be observed by thread 1 or thread 2 after it is set by thread 3, because a "happens-before" relationship hasn't been established between setting and reading the monitor. It could be still observed as null by thread 1 for example. Is my speculation correct?
Regarding possible solutions, making the DataObject thread-safe is not an option. Setting the monitor in the constructor of the classes and declaring it final can work.
EDIT Semantically, the mutual exclusion needed is related to the DataObject. This is the reason that I don't want to have a secondary monitor. One solution would be to add lock() and unlock() methods on DataObject that need to be called before working on it. Internally they would use a Lock Object. So, the operateOnData() method becomes:
void operateOnData() {
mData.lock()
mData.doSomething();
.....
mData.doSomethingElse();
mData.unlock();
}
You may create a wrapper
class Wrapper
{
DataObject mData;
synchronized public setObject(DataObject mData)
{
if(this.mData!=null) throw ..."already set"
this.mData = mData;
}
synchronized public void doSomething()
{
if(mData==null) throw ..."not set"
mData.doSomething();
}
A wrapper object is created and passed to A and B
class A
{
private Wrapper wrapper; // set by constructor
// thread 1
operateOnData()
{
wrapper.doSomething();
}
Thread 3 also has a reference to the wrapper; it calls setObject() when it's available.
Some platforms provide explicit memory-barrier primitives which will ensure that if one thread writes to a field and then does a write barrier, any thread which has never examined the object in question can be guaranteed to see the effect of that write. Unfortunately, as of the last time I asked such a question, Cheapest way of establishing happens-before with non-final field, the only time Java could offer any guarantees of threading semantics without requiring any special action on behalf of a reading thread was by using final fields. Java guarantees that any references made to an object through a final field will see any stores which were performed to final or non-fields of that object before the reference was stored in the final field but that relationship is not transitive. Thus, given
class c1 { public final c2 f;
public c1(c2 ff) { f=ff; }
}
class c2 { public int[] arr; }
class c3 { public static c1 r; public static c2 f; }
If the only thing that ever writes to c3 is a thread which performs the code:
c2 cc = new c2();
cc.arr = new int[1];
cc.arr[0] = 1234;
c3.r = new c1(cc);
c3.f = c3.r.f;
a second thread performs:
int i1=-1;
if (c3.r != null) i1=c3.r.f.arr[0];
and a third thread performs:
int i2=-1;
if (c3.f != null) i2=c3.f.arr[0];
The Java standard guarantees that the second thread will, if the if condition yields true, set i1 to 1234. The third thread, however, might possibly see a non-null value for c3.f and yet see a null value for c3.arr or see zero in c3.f.arr[0]. Even though the value stored into c3.f had been read from c3.r.f and anything that reads the final reference c3.r.f is required to see any changes made to that object identified thereby before the reference c3.r.f was written, nothing in the Java Standard would forbid the JIT from rearranging the first thread's code as:
c2 cc = new c2();
c3.f = cc;
cc.arr = new int[1];
cc.arr[0] = 1234;
c3.r = new c1(cc);
Such a rewrite wouldn't affect the second thread, but could wreak havoc with the third.
A simple solution is to just define a public static final object to use as the lock. Declare it like this:
/**Used to sync access to the {#link #mData} field*/
public static final Object mDataLock = new Object();
Then in the program synchronize on mDataLock instead of mData.
This is very useful, because in the future someone may change mData such that it's value does change then your code would have a slew of weird threading bugs.
This method of synchronization removes that possibility. It also is really low cost.
Also having the lock be static means that all instances of the class share a single lock. In this case, that seems like what you want.
Note that if you have many instances of these classes, this could become a bottleneck. Since all of the instances are now sharing a lock, only a single instance can change any mData at a single time. All other instances have to wait.
In general, I think something like a wrapper for the data you want to synchronize is a better approach, but I think this will work.
This is especially true if you have multiple concurrent instances of these classes.
Related
I have a Java class, here's its code:
public class MyClass {
private AtomicInteger currentIndex;
private List<String> list;
MyClass(List<String> list) {
this.list = list; // list is initialized only one time in this constructor and is not modified anywhere in the class
this.currentIndex = new AtomicInteger(0);
}
public String select() {
return list.get(currentIndex.getAndIncrement() % list.size());
}
}
Now my question:
Is this class really thread safe thanks to using an AtomicInteger only or there must be an addional thread safety mechansim to ensure thread-safety (for example locks)?
The use of currentIndex.getAndIncrement() is perfectly thread-safe. However, you need a change to your code to make it thread-safe in all circumstances.
The fields currentIndex and list need to be made final to achieve full thread-safety, even on unsafe publication of the reference to your MyClass object.
private final AtomicInteger currentIndex;
private final List<String> list;
In practice, if you always ensure that your MyClass object itself is safely published, for example if you create it on the main thread, before any of the threads that use it are started, then you don't need the fields to be final.
Safe publication means that the reference to the MyClass object itself is done in a way that has a guaranteed multi-threaded ordering in the Java Memory Model.
It could be that:
All threads that use the reference get it from a field that was initialized by the thread that started them, before their thread was started
All threads that use the reference get it from a method that was synchronized on the same object as the code that set the reference (you have a synchronized getter and setter for the field)
You make the field that contains the reference volatile
It was in a final field if that final field was initialized as described in section 17.5 of the JLS.
A few more cases the are not easily used to publish references
I think your code contains two bugs.
First, normally when you receive an object from some unknown source like your constructor does, you make a defensive copy to be certain it is not modified outside of the class.
MyClass(List<String> list) {
this.list = new ArrayList<String>( list );
So if you do this, do you now need to mutate that list anywhere inside the class? If so, the method:
public String select() {
return list.get(currentIndex.getAndIncrement() % list.size());
isn't atomic. What could happen here is a thread call getAndIncrement() and then perform the modulus (%). Then at that point if it's swapped out with another thread that removes an item from the list, the old limit of list.size() will no longer be valid.
I think there's nothing for it but to add synchronized to the whole method:
public synchronized String select() {
return list.get(currentIndex.getAndIncrement() % list.size());
And the same with any other mutator.
(final as the other poster mentions is still required on the instance fields.)
The below example is from the book "Java Concurrency in Practice" by Brian Goetz, Chapter 3, Section 3.5.1. This is an example of Improper publication of objects:
class SomeClass {
public Holder holder;
public void initialize() {
holder = new Holder(42);
}
}
public class Holder {
private int n;
public Holder(int n) { this.n = n; }
public void assertSanity() {
if (n != n)
throw new AssertionError("This statement is false");
}
}
It says that the Holder could appear to another thread in an inconsistent state and another thread could observe a partially constructed object. How can this happen? Could you give a scenario using the above example?
Also it goes on to say that there are cases when a thread may see a stale value the first time it reads a field and then a more up to date value the next time, which is why the assertSanity can throw AssertionError. How can the AssertionError be thrown?
From further reading, one way to fix this problem is to make Holder immutable by making the variable n final. For now, let us assume that Holder is not immutable but effectively immutable.
To safely publish this object, do we have to make holder initialization static and declare it as volatile (both static initialization and volatile or just volatile)?
Something like this:
public class SomeClass {
public static volatile Holder holder = new Holder(42);
}
You can imagine creation of an object has a number of non-atomic functions. First you want to initialize and publish Holder. But you also need to initialize all the private member fields and publish them.
Well, the JMM has no rules for the write and publication of the holder's member fields to happen before the write of the holder field as occurring in initialize(). What that means is that even though holder is not null, it is legal for the member fields to not yet be visible to other threads.
You may end up seeing something like
public class Holder {
String someString = "foo";
int someInt = 10;
}
holder may not be null but someString could be null and someInt could be 0.
Under an x86 architecture this is, from what I know, impossible to happen but may not be the case in others.
So next question may be "Why does volatile fix this?" The JMM says that all writes that happen prior to the volatile store are visible to all subsequent threads of the volatile field.
So if holder is volatile and you see holder is not null, based on volatile rules, all of the fields would be initialized.
To safely publish this object, do we have to make holder
initialization static and declare it as volatile
Yes, because as I mentioned if the holder variable is not null then all writes would be visible.
How can the AssertionError be thrown?
If a thread notices holder not to be null, and invokes AssertionError upon entering the method and reading n the first time may be 0 (the default value), the second read of n may now see the write from the first thread.
public class Holder {
private int n;
public Holder(int n) { this.n = n; }
public void assertSanity() {
if (n!=n)
throw new AssertionError("This statement is false");
}
}
Say one thread creates an instance of Holder, and passes the reference to another thread, which calls assertSanity.
The assignment to this.n in the constructor occurs in one thread. And two reads of n occur in another thread. The only happens-before relation here is between the two reads. There is no happens-before relation involving the assignment and any of the reads.
Without any happens-before relations, statements can be reordered in various ways, so from the perspective of one thread, this.n = n can occur after the constructor has returned.
This means that the assignment can appear to occur in the second thread after the first read and before the second, resulting in inconsistent values. The can be prevented by making n final, which guarantees that the value is assigned before the constructor finishes.
The problem which you ask about is caused by JVM optimizations and the fact that simple object creation:
MyClass obj = new MyClass()
isn't always done by steps:
Reserve memory for new instance of MyClass on the Heap
Execute constructor to set internal properties values
Set 'obj' reference to address on the Heap
For some optimization purposes JVM can do it by steps:
Reserve memory for new instance of MyClass on the Heap
Set 'obj' reference to address on the Heap
Execute constructor to set internal properties values
So, imagine if two threads want to access MyClass object. First one creates it but due to JVM it executes 'optimized' set of steps. If it will execute only step 1 and 2 (but wont do 3) than we can have a serious problem. If second thread uses this object (it wont be null because it already points to reserved part of memory on the Heap) than it's properties will be incorrect which can lead to nasty things.
This optimization wont happen if reference will be volatile.
The Holder class is OK, but the class someClass can appear in an inconsisten state - between creation and the call to initialize() the holder instance variable is null.
The class below is meant to be immutable (but see edit):
public final class Position extends Data {
double latitude;
double longitude;
String provider;
private Position() {}
private static enum LocationFields implements
Fields<Location, Position, List<Byte>> {
LAT {
#Override
public List<byte[]> getData(Location loc, final Position out) {
final double lat = loc.getLatitude();
out.latitude = lat;
// return an arrayList
}
#Override
public void parse(List<Byte> list, final Position pos)
throws ParserException {
try {
pos.latitude = listToDouble(list);
} catch (NumberFormatException e) {
throw new ParserException("Malformed file", e);
}
}
}/* , LONG, PROVIDER, TIME (field from Data superclass)*/;
}
// ========================================================================
// Static API (factories essentially)
// ========================================================================
public static Position saveData(Context ctx, Location data)
throws IOException {
final Position out = new Position();
final List<byte[]> listByteArrays = new ArrayList<byte[]>();
for (LocationFields bs : LocationFields.values()) {
listByteArrays.add(bs.getData(data, out).get(0));
}
Persist.saveData(ctx, FILE_PREFIX, listByteArrays);
return out;
}
public static List<Position> parse(File f) throws IOException,
ParserException {
List<EnumMap<LocationFields, List<Byte>>> entries;
// populate entries from f
final List<Position> data = new ArrayList<Position>();
for (EnumMap<LocationFields, List<Byte>> enumMap : entries) {
Position p = new Position();
for (LocationFields field : enumMap.keySet()) {
field.parse(enumMap.get(field), p);
}
data.add(p);
}
return data;
}
/**
* Constructs a Position instance from the given string. Complete copy
* paste just to get the picture
*/
public static Position fromString(String s) {
if (s == null || s.trim().equals("")) return null;
final Position p = new Position();
String[] split = s.split(N);
p.time = Long.valueOf(split[0]);
int i = 0;
p.longitude = Double.valueOf(split[++i].split(IS)[1].trim());
p.latitude = Double.valueOf(split[++i].split(IS)[1].trim());
p.provider = split[++i].split(IS)[1].trim();
return p;
}
}
Being immutable it is also thread safe and all that. As you see the only way to construct instances of this class - except reflection which is another question really - is by using the static factories provided.
Questions :
Is there any case an object of this class might be unsafely published ?
Is there a case the objects as returned are thread unsafe ?
EDIT : please do not comment on the fields not being private - I realize this is not an immutable class by the dictionary, but the package is under my control and I won't ever change the value of a field manually (after construction ofc). No mutators are provided.
The fields not being final on the other hand is the gist of the question. Of course I realize that if they were final the class would be truly immutable and thread safe (at least after Java5). I would appreciate providing an example of bad use in this case though.
Finally - I do not mean to say that the factories being static has anything to do with thread safety as some of the comments seem(ed) to imply. What is important is that the only way to create instances of this class is through those (static of course) factories.
Yes, instances of this class can be published unsafely. This class is not immutable, so if the instantiating thread makes an instance available to other threads without a memory barrier, those threads may see the instance in a partially constructed or otherwise inconsistent state.
The term you are looking for is effectively immutable: the instance fields could be modified after initialization, but in fact they are not.
Such objects can be used safely by multiple threads, but it all depends on how other threads get access to the instance (i.e., how they are published). If you put these objects on a concurrent queue to be consumed by another thread—no problem. If you assign them to a field visible to another thread in a synchronized block, and notify() a wait()-ing thread which reads them—no problem. If you create all the instances in one thread which then starts new threads that use them—no problem!
But if you just assign them to a non-volatile field and sometime "later" another thread happens to read that field, that's a problem! Both the writing thread and the reading thread need synchronization points so that the write truly can be said to have happened before the read.
Your code doesn't do any publication, so I can't say if you are doing it safely. You could ask the same question about this object:
class Option {
private boolean value;
Option(boolean value) { this.value = value; }
boolean get() { return value; }
}
If you are doing something "extra" in your code that you think would make a difference to the safe publication of your objects, please point it out.
Position is not immutable, the fields have package visibility and are not final, see definition of immutable classes here: http://www.javapractices.com/topic/TopicAction.do?Id=29.
Furthermore Position is not safely published because the fields are not final and there is no other mechanism in place to ensure safe publication. The concept of safe publication is explained in many places, but this one seems particularly relevant: http://www.ibm.com/developerworks/java/library/j-jtp0618/
There are also relevant sources on SO.
In a nutshell, safe publication is about what happens when you give the reference of your constructed instance to another thread, will that thread see the fields values as intended? the answer here is no, because the Java compiler and JIT compiler are free to re-order the field initialization with the reference publication, leading to half baked state becoming visible to other threads.
This last point is crucial, from the OP comment to one of the answers below he appears to believe static methods somehow work differently from other methods, that is not the case. A static method can get inlined much like any other method, and the same is true for constructors (the exception being final fields in constructors post Java 1.5). To be clear, while the JMM doesn't guarantee the construction is safe, it may well work fine on certain or even all JVMs. For ample discussion, examples and industry expert opinions see this discussion on the concurrency-interest mailing list: http://jsr166-concurrency.10961.n7.nabble.com/Volatile-stores-in-constructors-disallowed-to-see-the-default-value-td10275.html
The bottom line is, it may work, but it is not safe publishing according to JMM. If you can't prove it is safe, it isn't.
The fields of the Position class are not final, so I believe that their values are not safely published by the constructor. The constructor is therefore not thread-safe, so no code (such as your factory methods) that use them produce thread-safe objects.
I have a code like the one below where an object is shared among two threads (the main thread and the Monitor thread). Do I have to declare MyObject globally and make it volatile to ensure it will be pushed to memory? Otherwise the if statement can print "Not null" if MyObject is only locally accessed by the thread and is not declared volatile, right?
public static void main(String[] args) {
MyObject obj = MyObjectFactory.createObject();
new Monitor(obj).start();
Thread.sleep(500);
if(obj == null)
System.out.println("Null");
else
System.out.println("Not null");
}
public void doSomethingWithObject(MyObject obj) {
obj = null;
}
private class Monitor extends Thread {
public Monitor(MyObject obj) {
this.obj=obj;
}
public void run() {
doSomethingWithObject(obj);
}
}
Note: The code example may not compile since I wrote it myself here on Stackoverflow. Consider it as a mix of pseudo code and real code.
The instance is shared but the references to it are not. Example:
String a = "hello";
String b = a;
b = null; // doesn't affect a
a and b are references to the same instance; changing one reference has no effect on the instance or any other references to the same instance.
So if you want to share state between threads, you will have to create a field inside MyObject which has to be volatile:
class MyObject { public volatile int shared; }
public void doSomethingWithObject(MyObject obj) {
obj.shared = 1; // main() can see this
}
Note that volatile just works for some types (references and all primitives except long). Since this is easy to get wrong, you should have a look at types in java.util.concurrent.atomic.
[EDIT] What I said above isn't correct. Instead, using volatile with long works as expected for Java 5 and better. This is the only way to ensure atomic read/writes for this type. See this question for references: Is there any point in using a volatile long?
Kudos go to Affe for pointing that out. Thanks.
You would rather have to synchronize on the object to ensure it will be set to null before the if check. Setting it to volatile only means changes will be "seen" immediately by other threads, but it is very likely that the if check will be executed before the doSomethingWithObject call.
If you want your object to go through a read-update-write scheme atomically, volatile won't cut it. You have to use synchronisation.
Volatility will ensure that the variable will not be cached in the current thread but it will not protect the variable from simultaneous updates, with the potential for the variable becoming something unexpected.
IBM's developerWorks has a useful article on the subject.
Your example consists only one thread, Monitor, which is created and run in main().
"make it volatile to ensure it will be pushed to memory?" - on the contrary, when you declare a variable as volatile - it ensures that it's NOT being "pushed" (cached) to the thread-local memory, cause there might be other threads that will change the value of the variable.
In order to make sure you print the correct value of a variable you should synchronize the method doSomethingWithObject (change the signature of the method to):
public synchronized void doSomethingWithObject(MyObject obj)
or create synchronized blocks around:
obj = null;
and
this.obj=obj;
I am confused about synchronizing an instance method and a static method.
I want to write a thread safe class as follow :
public class safe {
private final static ConcurrentLinkedQueue<Object> objectList=
new ConcurrentLinkedQueue<Object>();
/**
* retrieves the head of the object and prints it
*/
public synchronized static void getHeadObject() {
System.out.println(objectList.peek().toString());
}
/**
* creates a new object and stores in the list.
*/
public synchronized void addObject() {
Object obj=new Object();
objectList.add(obj);
}
}
Synchronizing on a static method will lock on safe.class lock and synchronizing on a instance method will lock on this .and hence an inconsistent state will be reached.
If I want to achieve a consistent state for a below code snippet how can that be achieved?
First, ConcurrentLinkedQueue does not require explicit synchronization. See this answer.
Second, you always can synchronize object you are accessing:
public class safe {
private final static ConcurrentLinkedQueue<Object> objectList=
new ConcurrentLinkedQueue<Object>();
/**
* retrieves the head of the object and prints it
*/
public static void getHeadObject() {
synchronized(objectList){
System.out.println(objectList.peek().toString());
}
}
/**
* creates a new object and stores in the list.
*/
public void addObject() {
Object obj=new Object();
synchronized(objectList){
objectList.add(obj);
}
}
}
EDIT: I'm assuming you meant Queue<Object> objectList instead of ConcurrentLinkedQueue<Object> objectList. ConcurrentLinkedQueue<Object> already does all of your thread safety for you, meaning you can call objectList.peek() all you want without worrying about race conditions. This is great if you're developing multi-threaded programs but not so great for learning about thread safety.
Your methods need not be synchronized, assuming you have one thread operating on one instance of the object at a time, but however if you need to have multiple instances of the class that all refer to the same static class variable, you need to synchronized over the class variable like so:
public static void getHeadObject() {
synchronized(safe.objectList) {
System.out.println(objectList.peek().toString());
}
}
This locks the objectList and does not allow it to be read or written to in any other thread as soon as the program is inside the synchronization block. Do the same for all other methods to be synchronized.
NOTE:
However, since you are doing only one simple get operation List.peek(), you really don't need to synchronize over the objectList since in a race condition, it'll get either one value of the List or another. The problem with race conditions is when multiple complex read/write operations are performed, with the value changing in between them.
For example, if you had a class PairInt with a PairInt.x and PairInt.y fields, with the constraint that x = 2y, and you wanted to do
System.out.println(myIntPair.x.toString() + ", " + myIntPair.y.toString());
and another thread was updating the value of x and y at the same time,
myIntPair.y = y + 3;
myIntPair.x = 2 * y;
And the write thread modified myIntPair in between your read thread's myIntPair.x.toString() and myIntPair.y.toString() you might get an output that looks like (10, 8), which means if you are operating on the assumption that x == 2 * y could crash your program.
In that case, your read needs to use a synchronized, but for more simpler things like peek() on a simple object that is being added or deleted, not modified while in the queue, the synchronized can, in most cases be dropped. In fact, for string, int, bool, and the like, the synchronized condition for a simple read should be dropped.
However, writes should always be synchronized on operations that are not explicitly thread safe, i.e. already handled by java. And as soon as you acquire more than one resource, or require that your resource stay the same throughout the operation as you do multiple lines of logic to it, then you MUST USE synchronized
A few comments:
Java conventions:
class names should be in CamelCase (i.e. call your class Safe, not safe)
static comes before synchronized in methods declaration
static comes before final in fields declaration
as others have already said, ConcurrentLinkedQueue is already thread safe, so there is no need for synchronization in the example you give.
mixing static and non static methods the way you do looks weird.
assuming that your actual use case is more complicated and you need a method to run atomic operations, then your code does not work, as you pointed out, because the 2 synchronized methods don't synchronize on the same monitor:
public static synchronized getHeadObject(){} //monitor = Safe.class
public static synchronized addObject(){} //monitor = this
So to answer your specific question, you could use a separate static object as a lock:
public class Safe {
private static final ConcurrentLinkedQueue<Object> objectList =
new ConcurrentLinkedQueue<Object>();
// lock must be used to synchronize all the operations on objectList
private static final Object lock = new Object();
/**
* retrieves the head of the object and prints it
*/
public static void getHeadObject() {
synchronized (lock) {
System.out.println(objectList.peek().toString());
}
}
/**
* creates a new object and stores in the list.
*/
public void addObject() {
synchronized (lock) {
Object obj = new Object();
objectList.add(obj);
}
}
}