We Keep Coding

Volatile and Synchronized to Solve Race Condition: Singleton Member Field

I was having some problem trying to understand and fix errors reported from Fortify scan. I have this class:
public class DaoImpl extends BaseDaoImpl {
private static volatile String sNric;
synchronized private void setInfo(InfoTO pers) {
sNric = pers.getNRIC();
}
synchronized public InfoTO getInfo() {
InfoTO pers = new InfoTO();
sNric = retrieveDetail();
pers.setNRIC(sNric);
}
synchronized private String retrieveDetail() {
// some logic to get info from database
}
}
My code was originally without the static volatile and synchronized keyword. And Fortify was reporting Race Condition: Singleton Member Field warning at the variable declaration of sNric as well as sNric = retrieveDetail();
I went to research and found this solution. However, I am not very sure on the concept of volatile with synchronized. Will the proposed solution above causing some deadlock issue?

The "concept" of volatile with synchronized is that you probably shouldn't do it.
If you use synchronized in all of the methods that access and update a shared variable (such as your sNric variable) then declaring the volatile is redundant and inefficient.
As to your question about deadlocks, I cannot see any way that you could get deadlocks based solely on the code above. However, you haven't shown us the code for the InfoTO or the code that uses these classes. It is not impossible for a deadlock to occur involving the DaoImpl instance lock and other locks.
If you are concerned that getInfo calling this.retrieveDetail might deadlock. There is only one (DaoImpl instance) lock involved here, and Java primitive locks are reentrant. (A thread will not be blocked if it tries to acquire a primitive lock that it already holds.)
Finally, you should if you are concerned about thread-safety, check that setNRIC and getNRIC are thread-safe. If they are not, I don't think that the above is handling the InfoTo objects safely.
Note that you cannot reason about the thread-safety of a class unless you take account of the other classes that it depends on AND the way that it used / intended to be used.

You have not really given enough information and code details for a certain Answer. But I will give it a shot.
AtomicReference
To quote the comment above by scottb:
Thread safety of shared variables is about visibility and atomicity of state transitions.
I often prefer the use of the Atomic… classes to address both visibility and atomicity. These classes can provide an alternative to volatile and synchronized.
In this case, we can use AtomicReference class to hold as its payload a reference to your current desired String value. Notice that we mark it final as the reference to the AtomicReference object itself will never change. Its payload, a reference (pointer) taking us to the desired String object, does change. At one moment it may point to the String value "dog" while a moment later it may point to the String value "cat". But the container of either String is always the very same AtomicReference object, a wrapper around that contained text, that contained String object.
If your InfoTO class looked like this:
package work.basil.example;
public class InfoTO
{
private String nric ;
public String getNric ( ) { return this.nric; }
public void setNric ( String nric ) { this.nric = nric; }
}
…then your DaoImpl might look something like this:
package work.basil.example;
import java.util.concurrent.atomic.AtomicReference;
public class DaoImpl
{
private AtomicReference < String > sNric;
private void setInfo ( InfoTO pers )
{
this.sNric.set( pers.getNric() );
}
public InfoTO getInfo ( )
{
InfoTO pers = new InfoTO();
String s = retrieveDetail();
pers.setNric( s );
return pers ;
}
private String retrieveDetail ( )
{
return this.sNric.get();
}
}
Your lines:
sNric = retrieveDetail();
pers.setNRIC(sNric);
…do not make sense to me. You use a field to hold what is a temporary value. So I substituted a local variable instead.
Your retrieveDetail method makes no sense to me. You seem to be returning from calls to a database the very same string value you are caching in the field sNric. So I changed that method to access the cached field sNric. This seems more consistent with your intended logic, and more importantly, shows both the getter and setter of the AtomicReference in action.
Of course, as others said, you may well have other thread-safety issues in the substantial code you did not show us.

java synchronised positives/negatives and usage

I've read all the theory I can on how synchronised methods operate, but I need a practical example.
What is the positive and negative or using synchronised like this:
public class Test {
public static void main(String[] args){
boolean sync = Boolean.valueOf(args[0]);
Person person1 = person2 = new Person();
person1.write(sync, args[1]);
person1.read(sync);
person2.write(sync, args[1]);
person2.read(sync);
}
}
write is naming the project: (either synchronized or not)
public static String project_name = "";
and read is printing the current name for the person (either synchronized or not)
So what is the difference when write and read are regular methods versus synchronised methods?
What could go wrong if I use the regular method?

Quick answer:
A non-synchronous method being accessed by multiple sources will generally cause undefined behaviour, but a synchronised method will work every time.
Longer answer:
I don't think you fully understand what a synchronised method is, because your code does not demonstrate it at all.
If there really is a possibility of 100 people accessing the same method then you will have undefined behaviour when the same variable is being written to and read from.
However, if that method is accessed synchronously then each method call will be added to a queue and will happen in order.
For example:
100 different threads(People?) could call SynchronizedProjectName.renameProject("exampleName"); and/or SynchronizedProjectName.projectName(); on the code below, and no error would occur, and no read/write would happen at the same time.
public class SynchronizedProjectName {
private string project_name = "";
public synchronized void renameProject(String newProjectName) {
project_name = newProjectName;
}
public synchronized string projectName() {
return project_name;
}
}
You should always always use some sort of thread safe strategy when dealing with multiple threads/users, and if you don't, then you should expect your code to misbehave and probably crash.
See here for a little bit of extra info: https://docs.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html

Ensuring safe publication and thread safety in java by means of static factories

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.

Java, lazily initialized field without synchronization

Sometimes when I need lazily initialized field, I use following design pattern.
class DictionaryHolder {
private volatile Dictionary dict; // some heavy object
public Dictionary getDictionary() {
Dictionary d = this.dict;
if (d == null) {
d = loadDictionary(); // costy operation
this.dict = d;
}
return d;
}
}
It looks like Double Checking idion, but not exactly. There is no synchronization and it is possible for loadDictionary method to be called several times.
I use this pattern when the concurrency is pretty low. Also I bear in mind following assumptions when using this pattern:
loadDictionary method always returns the same data.
loadDictionary method is thread-safe.
My questions:
Is this pattern correct? In other words, is it possible for getDictionary() to return invalid data?
Is it possible to make dict field non-volatile for more efficiency?
Is there any better solution?

I personally feel that the Initialization on demand holder idiom is a good fit for this case. From the wiki:
public class Something {
private Something() {}
private static class LazyHolder {
private static final Something INSTANCE = new Something();
}
public static final Something getInstance() {
return LazyHolder.INSTANCE;
}
}
Though this might look like a pattern intended purely for singleton control, you can do many more cool things with it. For e.g. the holder class can invoke a method which in turn populates some kind of data.
Also, it seems that in your case if multiple threads queue on the loadDictionary call (which is synchronized), you might end up loading the same thing multiple times.

The simplest solution is to rely on the fact that a class is not loaded until it is needed. i.e. it is lazy loaded anyway. This way you can avoid having to do those checks yourself.
public enum Dictionary {
INSTANCE;
private Dictionary() {
// load dictionary
}
}
There shouldn't be a need to make it any more complex, certainly you won't make it more efficient.
EDIT: If Dictionary need to extend List or Map you can do.
public enum Dictionary implements List<String> { }
OR a better approach is to use a field.
public enum Dictionary {
INSTANCE;
public final List<String> list = new ArrayList<String>();
}
OR use a static initialization block
public class Dictionary extends ArrayList<String> {
public static final Dictionary INSTANCE = new Dictionary();
private Dictionary() { }
}

Your code is correct. To avoid loading more than once, synchronized{} would be nice.
You can remove volatile, if Dictionary is immutable.
private Dictionary dict; // not volatile; assume Dictionary immutable
public Dictionary getDict()
if(dict==null)
dict = load()
return dict;
If we add double checked locking, it's perfect
public Dictionary getDict()
if(dict==null)
synchronized(this)
if(dict==null)
dict = load()
return dict;
Double checked locking works great for immutable objects, without need of volatile.
Unfortunately the above 2 getDict() methods aren't theoretically bullet proof. The weak java memory model will allow some spooky actions - in theory. To be 100% correct by the book, we must add a local variable, which clutters our code:
public Dictionary getDict()
Dictionary local = dict;
if(local==null)
synchronized(this)
local = dict;
if(local==null)
local = dict = load()
return local;

1.Is this pattern correct? In other words, is it possible for getDictionary() to return invalid data?
Yes if it's okay that loadDictionary() can be called by several threads simultaneously and thus different calls to getDictionary() can return different objects. Otherwise you need a solution with syncronization.
2.Is it possible to make dict field non-volatile for more efficiency?
No, it can cause memory visibility problems.
3.Is there any better solution?
As long as you want a solution without syncronization (either explicit or implicit) - no (as far as I understand). Otherwise, there are a lot of idioms such as using enum or inner holder class (but they use implicit synchronization).

Just a quick stab at this but what about...
class DictionaryHolder {
private volatile Dictionary dict; // some heavy object
public Dictionary getDictionary() {
Dictionary d = this.dict;
if (d == null) {
synchronized (this) {
d = this.dict;
if (d == null) { // gated test for null
this.dict = d = loadDictionary(); // costy operation
}
}
return d;
}
}

Is it possible to make dict field non-volatile for more efficiency?
No. That would hurt visibility, i.e. when one thread initializes dict, other threads may not see the updated reference in time (or at all). This in turn would results in multiple heavy initializations, thus lots of useless work , not to mention returning references to multiple distinct objects.
Anyway, when dealing with concurrency, micro-optimizations for efficiency would be my last thought.

Initialize-on-demand holder class idiom
This method relies on the JVM only
intializing the class members upon
first reference to the class. In this
case, we have a inner class that is
only referenced within the
getDictionary() method. This means
DictionaryHolder will get initialized
on the first call to getDictionary().
public class DictionaryHolder {
private DictionaryHolder ()
{
}
public static Dictionary getDictionary()
{
return DictionaryLazyHolder.instance;
}
private static class DictionaryLazyHolder
{
static final DictionaryHolder instance = new DictionaryHolder();
}
}

How do you ensure multiple threads can safely access a class field?

When a class field is accessed via a getter method by multiple threads, how do you maintain thread safety? Is the synchronized keyword sufficient?
Is this safe:
public class SomeClass {
private int val;
public synchronized int getVal() {
return val;
}
private void setVal(int val) {
this.val = val;
}
}
or does the setter introduce further complications?

If you use 'synchronized' on the setter here too, this code is threadsafe. However it may not be sufficiently granular; if you have 20 getters and setters and they're all synchronized, you may be creating a synchronization bottleneck.
In this specific instance, with a single int variable, then eliminating the 'synchronized' and marking the int field 'volatile' will also ensure visibility (each thread will see the latest value of 'val' when calling the getter) but it may not be synchronized enough for your needs. For example, expecting
int old = someThing.getVal();
if (old == 1) {
someThing.setVal(2);
}
to set val to 2 if and only if it's already 1 is incorrect. For this you need an external lock, or some atomic compare-and-set method.
I strongly suggest you read Java Concurrency In Practice by Brian Goetz et al, it has the best coverage of Java's concurrency constructs.

In addition to Cowan's comment, you could do the following for a compare and store:
synchronized(someThing) {
int old = someThing.getVal();
if (old == 1) {
someThing.setVal(2);
}
}
This works because the lock defined via a synchronized method is implicitly the same as the object's lock (see java language spec).

From my understanding you should use synchronized on both the getter and the setter methods, and that is sufficient.
Edit: Here is a link to some more information on synchronization and what not.

If your class contains just one variable, then another way of achieving thread-safety is to use the existing AtomicInteger object.
public class ThreadSafeSomeClass {
private final AtomicInteger value = new AtomicInteger(0);
public void setValue(int x){
value.set(x);
}
public int getValue(){
return value.get();
}
}
However, if you add additional variables such that they are dependent (state of one variable depends upon the state of another), then AtomicInteger won't work.
Echoing the suggestion to read "Java Concurrency in Practice".

For simple objects this may suffice. In most cases you should avoid the synchronized keyword because you may run into a synchronization deadlock.
Example:
public class SomeClass {
private Object mutex = new Object();
private int val = -1; // TODO: Adjust initialization to a reasonable start
// value
public int getVal() {
synchronized ( mutex ) {
return val;
}
}
private void setVal( int val ) {
synchronized ( mutex ) {
this.val = val;
}
}
}
Assures that only one thread reads or writes to the local instance member.
Read the book "Concurrent Programming in Java(tm): Design Principles and Patterns (Java (Addison-Wesley))", maybe http://java.sun.com/docs/books/tutorial/essential/concurrency/index.html is also helpful...

Synchronization exists to protect against thread interference and memory consistency errors. By synchronizing on the getVal(), the code is guaranteeing that other synchronized methods on SomeClass do not also execute at the same time. Since there are no other synchronized methods, it isn't providing much value. Also note that reads and writes on primitives have atomic access. That means with careful programming, one doesn't need to synchronize the access to the field.
Read Sychronization.
Not really sure why this was dropped to -3. I'm simply summarizing what the Synchronization tutorial from Sun says (as well as my own experience).
Using simple atomic variable access is
more efficient than accessing these
variables through synchronized code,
but requires more care by the
programmer to avoid memory consistency
errors. Whether the extra effort is
worthwhile depends on the size and
complexity of the application.