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Can a thread-safe class contain any public instance fields?

Can a thread-safe class contain any public instance fields?

Access modifieres are irrelevant in this context of thread-safety. Of course you can have public fields in a thread safe class, the question you need to ask yourself is : Does this conform to my / a design pattern and what could I possibly achieve from doing this.

When people say that class C is "thread-safe", they usually mean that no interleaving of operations performed on a single instance of the class by multiple threads can leave the instance in an invalid state. (But as Marko says, that's not a formally agreed-upon definition.) So, what are the states of an instance of your class? Which states are valid and which are not valid? Is it possible to change a valid state into an invalid state by updating one of the public fields?
If there is any way that updating a public field can change the state from valid to invalid, then you can't say that the class is generally thread safe, but if that never happens in your application, then maybe the class is thread-safe in the limited context of your application.

Difference between class locking and object locking in Java

People tell about two types of multi-threaded locking - object and class. In my knowledge, locking is done on objects only.
Case 1: On objects we create using new or factory methods etc.
void synchronized myMethod(Type param) {
//will lock on the instance used to call this method
}
or
synchronized(this) {
//will lock on current object
}
or
synchronized(obj1) {
//will lock on specified obj1 object
}
Case 2: On java.lang.Class objects
This is called class lock, and can be used with static fields or methods or blocks, as they belong to class and shared among all the objects, and other class properties.
static void synchronized method() {
//will lock the Class object
}
or
static {
synchronized(SomeClass.class){
int a = 2;
}
}
Why I am thinking this also as an object locking because classes are loaded into the Method Area in the JVM, and all the static properties of the class are wrapped inside a java.lang.Class object created by JVM. So behind abstraction, its object locking and in the picture, we see Class locking.
So I can also infer one more thing. Just as objects locked by a thread can not be acquired by another thread as long as it is not released by first thread, class locking (the java.lang.Class instance) also works in same manner.
I want to know in case of synchronized static methods, lock on which class is acquired by the thread in following two cases:
This method is called from same class where it is defined.
This method is called from derived class with derived class name.
This is my understanding so far regarding the subject. Please add on or rectify.

People tell about two types of multi-threaded locking - object and class.
A Class is an Object. There is only one kind of locking in the Java language: Every Object (including every Class) has a mutex that can be locked by a synchronized block or a synchronized method. The Object to be locked is implicit in a synchronized method: It's the "this" instance for an instance method, and it's the Class object for a static method.
One of the most common newbie mistakes is to think that two different threads can't get into the same synchronized block at the same time. They can, and there's plenty of questions and answers here in StackOverflow that prove it. Another mistake is to think that if one thread is synchronized on some object, then other threads will not be able to modify the object. They can and they do.
Synchronization prevents two or more threads from synchronizing on the same Object at the same time. Which object is the right object? It's all about protecting your data. If the structure that you want to protect is a linked list, for example, then a good choice would be for any method that accesses the list to synchronize on the list header. If you want to protect global data (e.g., static variables), then you want to synchronize on a global object (e.g., the Class object that owns the variables.) The important thing is that, if you have read/write data (a.k.a., "mutable data") that are accessed by more than one thread, then every method that accesses the same data must synchronize on the same lock.
There is another kind of locking in Java, but it's not in the Java language; it's in the Java standard library. It's available through objects that implement the java.util.concurrent.locks.Lock interface. Of course a Lock object (like any Object) also implements the first kind of locking, but you should never, ever, synchronize on a Lock object unless you want to give people the impression that you are a clueless newbie.
The java.util.concurrent-style locking is more powerful than using synchronized blocks because of it's explicit lock() and unlock() methods. For example, it is possible for one method to lock a lock, and a different method to unlock it. That can lead to code that is tricky to understand, and I wouldn't do it unless I had a very good reason, but sometimes there are good reasons.

The only difference is that a static synchronized locks on the class instance and a non-static synchronized method locks on the instance.
People tell about two types of multi-threaded locking
There is object instance locks and Lock style locks. A Lock, confusingly, has both.
object and class
Not true as you have worked out already.
Just because people say stuff doesn't make it true. Often people say a lot of nonsense. In fact there is whole web sites devoted to non-sense about Java. :P

In Java, do methods that don't use static or class variables need to be synchronized?

Do methods that only use local variables inside suffer any threading issues ?. Somewhere it was mentioned that the method with local variables are copied to each thread stack frame to work with and do not need to synchronized for multithreaded implementation unless it uses class level or static references/variables ?

If your method only operates on parameters and locally-defined (as opposed to class member) variables then there are zero synchronization problems to worry about.
But...
This means any mutable reference types you use must live and die only within the scope of your method. (Immutable reference types aren't a problem here.) For example this is no problem:
int doSomething(int myParameter)
{
MyObject working_set = new MyObject();
interim = working_set.doSomethingElse(myParameter);
return working_set.doSomethingElseAgain(interim);
}
A MyObject instance is created within your method, does all of its work in your method and is coughing up blood, waiting to be culled by the GC when you exit your method.
This, on the other hand, can be a problem:
int doSomething(int myParameter)
{
MyObject working_set = new MyObject();
interim = working_set.doSomethingElse(myParameter);
another_interim = doSomethingSneaky(working_set);
return working_set.doSomethingElseAgain(another_interim);
}
Unless you know for sure what's going on in doSomethingSneaky(), you may have a need for synchronization somewhere. Specifically you may have to do synchronization on the operations on working_set because doSomethingSneaky() could possibly store the reference to your local working_set object and pass that off to another thread while you're still doing stuff in your method or in the working_set's methods. Here you'll have to be more defensive.
If, of course, you're only working with primitive types, even calling out to other methods, passing those values along, won't be a problem.

Does methods that only use local variables inside, do not suffer any threading issues ?
True in a very simplistic sense, but lets be clear - I think this is only true if:
such a method uses only local variables that are primitives or references to mutable instances that cannot otherwise be accessed outside the method by any other means.
such a method invokes only methods that are thread-safe.
Some ways these rules could be violated:
A local variable could be initialized to point to an object that is also accessible outside the method. For example, a local variable could point to a singleton (Foo bar = Foo.getSingleton()).
A local instance held by a local variable could "leak" if the instance is passed as a argument to an external method that keeps a reference to the instance.
A class with no instance variables and with only a single method with no local variables could still call the static method of another class that is not thread-safe.

The question is very generic, so please do not expect any specificity from my answer.
1_ We need to more careful with static methods than say instance methods.
2_ #Justmycorrectopinion is about right, but some of the terms he described needs to be more elaborated to be perfect. ( Even if the static method, only works on local variable, there is still possibility of race condition.)
3_ For me there are simple rules that have helped me analyze thread safety.
Understand if each components encapsulated within it is shareable or not. So the simplest solution is to reduce the scope of all variable and only increase scope if absolutely necessary and if component perform mutation on a object, its usually not thread safe.
4_ Use tooling support to perform static code analysis on thread safety. (Idea has checkthread plugin).
5_ Never use static method to perform object mutation. If calling static variable causes object mutation, then the developer is just circumventing OOPS.
6_ Always document thread safety. Remember some method may not need to be synchronized when you develop, but can be made not thread safe very easily.
7_ Last but probably my most important point, make sure most of your objects are immutable. In my experience, most of the time, I never had to make many of my objects mutable. (In rare cases when object state needs to be changed, defensive copying / New Object Creation is almost always better. )

You do not need to worry about local variables. Instance variables however are something to care about.

Threads and synchronisation

I've problem understanding the following piece of code:-
public class SoCalledSigleton{
private final static boolean allDataLoaded = SoCalledSigleton();
private SoCalledSigleton(){
loadDataFromDB();
loadDataFromFile();
loadDataAgainFromDB();
}
}
Is this piece of code thread safe? If not then Why?

This will create an error in Java.
private final static boolean allDataLoaded = SoCalledSigleton();
You're assigning an object to a boolean variable.
You forgot to add new to instantiate the variable.
But if your code is like this
public class SoCalledSigleton{
private final static SoCalledSigleton allDataLoaded = new SoCalledSigleton();
private SoCalledSigleton(){
loadDataFromDB();
loadDataFromFile();
loadDataAgainFromDB();
}
}
It is thread-safe as static initialization and static attributes are thread-safe. They are initialized only once and exists throughout the whole life-cycle of the system.

The code is unusable in its current form, so any notions of thread safety are irrelevent.
What public interface would users use to get an instance of the singleton?

(I assume that allDataLoaded is meant to be a SoCalledSigleton and boolean is just a typo :-)
If the class has no other constructors, or the loadData* methods don't do funny business (such as publishing this), its initialization is thread safe, because the initialization of final static data members is guarded by the JVM. Such members are initialized by the class loader when the class is first loaded. During this, there is a lock on the class so the initialization process is thread safe even if multiple threads try to access the class in parallel. So the constructor of the class is guaranteed to be called only once (per classloader - thanks Visage for the clarification :-).
Note that since you don't show us the rest of the class (I suppose it should have at least a static getInstance method, and probably further nonstatic members), we can't say anything about whether the whole implementation of the class is thread safe or not.

From what we can see, there are no specific issues - it's guaranteed that the constructor will only ever by called once (so by definition can't be run multithreaded), which I presume is what you were concerned about.
However, there are still possible areas for problems. Firstly, if the loadData... methods are public, then they can be called by anyone at any time, and quite possibly could lead to concurrency errors.
Additionally, these methods are presumably modifying some kind of collection somewhere. If these collections are publically accessible before the constructor returns, then you can quite easily run into concurrency issues again. This could be an issue with anything exception updating instance-specific fields (static fields may or may not exhibit this problem depending where they are defined in the file).
Depending on the way the class is used, simply writing all of the data single-threaded may not be good enough. Collection classes are not necessarily safe for multi-threaded access even if read-only, so you'll need to ensure you're using the thread-safe data structures if multiple threads might access your singleton.
There are possibly other issues too. Thread-safety isn't a simple check-list; you need to think about what bits of code/data might be accessed concurrently, and ensure that appropriate action is taken (declaring methods synchronized, using concurrent collections, etc.). Thread-safety also isn't a binary thing (i.e. there's no such thing as "thread safe" per se); it depends how many threads will be accessing the class at once, what combinations of methods are thread-safe, whether sequences of operations will continue to function as one would expect (you can make a class "thread safe" in that is doesn't crash, but certain return values are undefined if pre-empted), what monitors threads need to hold to guarantee certain invariants etc.
I guess what I'm trying to say is that you need to think about and understand how the class is used. Showing people a snapshot of half a file (which doesn't even compile), and asking them to give a yes/no answer, is not going to be beneficial. At best they'll point out some of the issues for you if there are any; at worst you'll get a false sense of confidence.

Yeah, it's thread safe. The "method" is the constructor, and it will be called when the class is loaded, i.e. exactly once.
But looking at the stuff being done, I think it's probably a lousy idea to call it from the class loader. Essentially, you'll end up doing your DB connection and stuff at the point in time when something in your code touches the SoCalledSingleton. Chances are, this will not be inside some well-defined sequence of events where, if there's an error you have catch blocks to take you to some helpful GUI message handling or whatever.
The "cleaner" way is to use a synchronized static getInstance() method, which will construct your class and call its code exactly when getInstance() is called the first time.
EDIT: As The Elite Gentleman pointed out, there's a syntax error in there. You need to say
private final static SoCalledSingleton allDataLoaded = new SoCalledSigleton();

Java: Rationale of the Object class not being declared abstract

Why wasn't the java.lang.Object class declared to be abstract ?
Surely for an Object to be useful it needs added state or behaviour, an Object class is an abstraction, and as such it should have been declared abstract ... why did they choose not to ?

An Object is useful even if it does not have any state or behaviour specific to it.
One example would be its use as a generic guard that's used for synchronization:
public class Example {
private final Object o = new Object();
public void doSomething() {
synchronized (o) {
// do possibly dangerous stuff
}
}
}
While this class is a bit simple in its implementation (it isn't evident here why it's useful to have an explicit object, you could just declare the method synchronized) there are several cases where this is really useful.

Ande, I think you are approaching this -- pun NOT intended -- with an unnecessary degree of abstraction. I think this (IMHO) unnecessary level of abstraction is what is causing the "problem" here. You are perhaps approaching this from a mathematical theoretical approach, where many of us are approaching this from a "programmer trying to solve problems" approach. I believe this difference in approach is causing the disagreements.
When programmers look at practicalities and how to actually implement something, there are a number of times when you need some totally arbitrary Object whose actual instance is totally irrelevant. It just cannot be null. The example I gave in a comment to another post is the implementation of *Set (* == Hash or Concurrent or type of choice), which is commonly done by using a backing *Map and using the Map keys as the Set. You often cannot use null as the Map value, so what is commonly done is to use a static Object instance as the value, which will be ignored and never used. However, some non-null placeholder is needed.
Another common use is with the synchronized keyword where some Object is needed to synchronize on, and you want to ensure that your synchronizing item is totally private to avoid deadlock where different classes are unintentionally synchronizing on the same lock. A very common idiom is to allocate a private final Object to use in a class as the lock. To be fair, as of Java 5 and java.util.concurrent.locks.Lock and related additions, this idiom is measurably less applicable.
Historically, it has been quite useful in Java to have Object be instantiable. You could make a good point that with small changes in design or with small API changes, this would no longer be necessary. You're probably correct in this.
And yes, the API could have provided a Placeholder class that extends Object without adding anything at all, to be used as a placeholder for the purposes described above. But -- if you're extending Object but adding nothing, what is the value in the class other than allowing Object to be abstract? Mathematically, theoretically, perhaps one could find a value, but pragmatically, what value would it add to do this?
There are times in programming where you need an object, some object, any concrete object that is not null, something that you can compare via == and/or .equals(), but you just don't need any other feature to this object. It exists only to serve as a unique identifier and otherwise does absolutely nothing. Object satisfies this role perfectly and (IMHO) very cleanly.
I would guess that this is part of the reason why Object was not declared abstract: It is directly useful for it not to be.

Does Object specify methods that classes extending it must implement in order to be useful? No, and therefor it needn't be abstract.
The concept of a class being abstract has a well defined meaning that does not apply to Object.

You can instantiate Object for synchronization locks:
Object lock = new Object();
void someMethod() {
//safe stuff
synchronized(lock) {
//some code avoiding race condition
}
}
void someOtherMethod() {
//safe code
synchronized(lock) {
//some other stuff avoiding race condition
}
}

I am not sure this is the reason, but it allows (or allowed, as there are now better ways of doing it) for an Object to be used as a lock:
Object lock = new Object();
....
synchronized(lock)
{
}

How is Object any more offensive than null?
It makes a good place marker (as good as null anyway).
Also, I don't think it would be good design to make an object abstract without an abstract method that needs to go on it.
I'm not saying null is the best thing since sliced bread--I read an article the other day by the "Inventor" discussing the cost/value of having the concept of null... (I didn't even think null was inventable! I guess someone somewhere could claim he invented zero..) just that being able to instantiate Object is no worse than being able to pass null.

You never know when you might want to use a simple Object as a placeholder. Think of it as like having a zero in a numerical system (and null doesn't work for this, since null represents the absence of data).

There should be a reason to make a class abstract. One is to prevent clients from instantiating the class and force them into using only subclasses (for whatever reasons). Another is if you wish to use it as an interface by providing abstract methods, which subclasses must implement. Probably, the designers og Java saw no such reasons, so java.lang.Object remains concrete.

As always, Guava comes to help: with http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Optional.html
Stuff here can be used to kill nulls / Object instances for "a not-null placeholder" from the code.
There are entirely seperated questions here:
why did not they make Object abstract?
how much disaster comes after if they decide to make it abstract in a future release?

I'll just throw in another reason that I've found Object to useful to instantiate on its own. I have a pool of objects I've created that has a number of slots. Those slots can contain any of a number of objects, all that inherit from an abstract class. But what do I put in the pool to represent "empty". I could use null, but for my purpose, it made more sense to insure that there was always some object in each slot. I can't instantiate the abstract class to put in there, and I wouldn't have wanted to. So I could have created a concrete subclass of my abstract class to represent "not a useful foo", but that seemed unnecessary when using an instance of Object was just as good..in fact better, as it clearly says that what's in the slot has no functionality. So when I initialize my pool, I do so by creating an Object to assign to each slot as the initial condition of the pool.
I agree that it might have made sense for the original Java crew to have defined a Placeholder object as a concrete subclass of Object, and then made Object abstract, but it doesn't rub me wrong at all that they went the way they did. I would then have used Placeholder in place of Object.