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Can't understand example of volatile in Java specification

I got general understanding what volatile means in Java. But reading
Java SE Specification 8.3.1.4 I have a problem understanding the text beneath that certain volatile example.
class Test {
static volatile int i = 0, j = 0;
static void one() { i++; j++; }
static void two() {
System.out.println("i=" + i + " j=" + j);
}
}
This allows method one and method two to be executed concurrently, but
guarantees that accesses to the shared values for i and j occur
exactly as many times, and in exactly the same order, as they appear
to occur during execution of the program text by each thread.
Therefore, the shared value for j is never greater than that for i,
because each update to i must be reflected in the shared value for i
before the update to j occurs. It is possible, however, that any given
invocation of method two might observe a value for j that is much
greater than the value observed for i, because method one might be
executed many times between the moment when method two fetches the
value of i and the moment when method two fetches the value of j.
How is
j never greater than i
, but at the same time
any given invocation of method two might observe a value for j that is
much greater than the value observed for i
??
Looks like contradiction.
I got j greater than i after running sample program. Why use volatile then? It gives almost the same result without volatile (also i can be greater than j, one of previous examples in specs). Why is this example here as an alternative to synchronized?

At any one time, then j is not greater than i.
This is different from what method two observes because it is accessing the variables i and j at different times. i is accessed first, and then j is accessed slightly later.
This isn't a direct alternative to the synchronized version because the behavior is different. One difference from not using volatile is that without volatile, values of 0 could always be printed. The increment doesn't ever need to be visible.
The example demonstrates the ordering of volatile accesses. An example that requires this could be something like:
volatile boolean flag = false;
volatile int value;
// Thread 1
if(!flag) {
value = ...;
flag = true;
}
// Thread 2
if(flag) {
System.out.println(value);
flag = false;
}
and thread 2 reads the value that thread 1 set rather than an old value.

I'd like to propose that it's a mistake and the examples were supposed to print j before i:
static void two() {
System.out.println("j=" + j + " i=" + i);
}
The novelty in the first example is that, due to update reordering, j can be greater than i even when observed first.
The final example now makes perfect sense with some minor edits to the explanation (edits and commentary in brackets):
This allows method one and method two to be executed concurrently, but guarantees that accesses to the shared values for i and j occur exactly as many times, and in exactly the same order, as they appear to occur during execution of the program text by each thread. Therefore, the shared value for j is never [observed to be] greater than that for i, because each update to i must be reflected in the shared value for i before the update to j occurs. It is possible, however, that any given invocation of method two might observe a value for [i] that is much greater than the value observed for [j], because method one might be executed many times between the moment when method two fetches the value of [j] and the moment when method two fetches the value of [i].
The key point here is that the second update will never be observed before the first update, when using volatile. The last sentence about the gap between the two reads is entirely parenthetical, and i and j were swapped to conform to the erroneous example.

I think the point of the example is to emphasize that you need to take care and ensure the order when using volatile; the behavior may be counter-intuitive and the example demonstrates it.
I agree that the wording there is a bit obscure and it is possible to provide more explicit and clear example for multiple cases, but there is no contradiction.
The shared value is the value at the same moment. If two threads read values of i and of j at exactly the same moment, the value of j will never be observed greater than i. volatile guarantees keeping order of reads and updates as in the code.
However, in the sample, print + i and + j are two different operations separated by an arbitrary amount of time; hence, j can be observed larger than i, because it can be updated arbitrary number of times after the read of i and before the read of j.
The point of using volatile is that when you concurrently update and access volatile variables with the right order, you can make assumptions that are not possible in principle without volatile.
In the sample above, the order of access in two() does not allow to conclude with a confidence which variable is greater or equal.
Consider, however, if the sample was changed to System.out.println("j=" + j + " i=" + i);
Here you can assert with a confidence that the printed value of j is never larger than the printed value of i. This assumption will not hold without volatile for two reasons.
First, updates i++ and j++ can be executed by compiler and hardware in an arbitrary order and in reality may execute as j++;i++. If from other thread you then access j and i after j++ but before i++, you can observe, say, j=1 and i=0, regardless of the access order. volatile guarantees that this will not happen and it will execute operations in the order that is written in your source.
Second, volatile guarantees that another thread will see most recent values changed by another thread, as long as it accesses it in the later point of time after the last update. Without volatile, there can be no assumptions about the observed value. In theory, the value can stay for another thread zero forever. The program may print two zeros, zero and an arbitrary number, etc. from past updates; the observed value in other threads may be less than the current value that the updater thread sees after an update. volatile guarantees that you will see the value in a second thread after the update in the first.
While the second guarantee may seem as a consequence of the first (the order guarantee), they are in fact orthogonal.
Regarding synchronized, it allows to execute a sequence of non-atomic operations, like i++;j++ as an atomic operation, e.g. if one thread does synchronized i++;j++ and another does synchronized System.out.println("i=" + i + " j=" + j);, the first thread may not perform increment sequence while the second prints and the result will be correct.
But this comes at a cost. First, synhronized has a performance penalty by itself. Second, more important, not always such behavior is required and the blocked thread wastes time, reducing the system throughput (e.g. you can do so many i++;j++; during System.out).

How is j never greater than i?
Let's say you execute one() only once. During the execution of this method, i is always incremented before j as the increment operations happen one after the other.
If you are executing one() concurrently, each individual method call will wait for other methods in the execution queue to finish writing their values to i or j, depending on which variable the currently executing method is trying to increment. So, all writes to i happen one after the other, and all writes to j happen one after the other. And since within the method body itself i is incremented before j, at a given instant, j will never be greater than i.
any given invocation of method two might observe a value for j that is much greater than the value observed for i, how?
If method one() is being executed in the background while you call two(), between the time when i is read and then j is read, the method one can be executed multiple times. So, when the value of i is read it could be the result some invocation of one() that happened at time t=0, and when then value of j is read, it could be the result of an invocation of one() that happened later in time, for example at t=10. Hence, j can be greater than i in this case in the println statement.
Why use volatile in lieu of synchronized?
I will not list all the reasons why anyone should use volatile instead of a synchronized block. But bear in mind that volatile guarantees atomic access to that particular field alone, and does not ensure the atomic execution of a block of code that is not marked as synchronized. In this example, access to i and j are synchronized, but the overall operation {i++;j++} isn't synchronized hence it apparently (I use apparently since it is not exactly the same but looks similar) gives the same results as without using the volatile keyword.

How is
j never greater than i
, but at the same time
any given invocation of method two might observe a value for j that is much >>greater than the value observed for i
??
The first statement is always true at any given moment in the program's execution, and the second statement may be true for any given interval in the program's execution.
When a volatile variable is written to, writes to both it and everything before it must become visible to other threads (In Java 5+, at least. The explanation doesn't really change much for versions of Java before that, though). Thus, the increment of i must be visible by the time j is incremented, meaning that j can never appear greater than i to other threads.
The reads of i and j, though, are not guaranteed to occur at a single moment in the program execution. The read of i and j may appear to occur very close to each other to the thread executing two(), but in reality some arbitrary amount of time may have passed between the reads. For example, two() may read i when i = 5 and j = 5, but then get "frozen" while other threads execute, changing the values of i and j to, say, 20 and 19, respectively. When two() resumes, it picks up where it left off and reads j, which now has a value of 19. two() doesn't re-read i because as far as it is concerned there was no break in execution, so there is no need to undergo the extra work.
Why use volatile then?
While both volatile and synchronized provide visibility guarantees, the precise semantics are slightly different. volatile guarantees that changes made to the variable will be instantly visible to all threads, while synchronized guarantees that changes made in its block will be visible to all threads as long as they synchronize on the same lock. synchronized also provides additional atomicity guarantees that volatile does not.
Why is this example here as an alternative to synchronized?
volatile is a viable alternative to synchronized only if one() is executed by a single thread, which is the case here. In this case, only a single thread is ever writing to i and j, so there is no need for the atomicity guarantees synchronized provides. If one() were executed by multiple threads, volatile wouldn't work because the read-add-store operations that make up an increment must occur atomically, and volatile does not guarantee that.

This program does guarantee that method two() observes j >= i-1 (not considering overflow).
Without volatile, the observed values of i,j could be all over the place.
The statement
the shared value for j is never greater than that for i
is very informal, because it means "at the same time", which is not a defined concept in JMM.
The core principle of JMM is about "sequential consistency". The driving motivation of JMM is
JLS#17 - If a program is correctly synchronized, then all executions of the program will appear to be sequentially consistent
In the following program
void f()
{
int x=0, y=0;
x++;
print( x>y );
y++
}
x>y will always be observed as true. It has to be, if we follow the sequence of actions. Otherwise, there is really no way for us to reason about any (imperative) code. That is "sequential consistency".
"Sequential consistency" is an observed property, it doesn't have to coincide with "real" actions (whatever that means). It is entirely possible that x>y is evaluated to be true by JVM before x is actually incremented (or at all). As long as JVM can guarantee observed sequential consistency, it can optimize actual execution anyway it can, e.g. execute code out of order.
But this is for a singlet thread. If multiple threads are reading/writing shared variables, such optimizations of course will completely wreck sequential consistency. We cannot reason about program behavior by thinking of interleaving actions from multiple threads (with actions in the same thread following intra-thread sequence).
If we want to guarantee inter-thread sequential consistency of any multi-thread code, we must abandon the optimization techniques developed for single thread. That is going to have severe performance penalty for most programs. And it is also uncalled for -- data exchange among threads is rather rare.
Therefore, special instructions are created just for establishing inter-thread sequential consistency when it is needed. Volatile reads and writes are such actions. All volatile reads and writes obey inter-thread sequential consistency. In this case, it guarantees that j >= i-1 in two().

All dependes on how you are using it. The volatile keyword in Java is used as an indicator to Java compiler and Thread that do not cache value of this variable and always read it from main memory. So if you want to share any variable in which read and write operation is atomic by implementation e.g. read and write in an int or a boolean variable then you can declare them as volatile variable.
From Java 5 along with major changes like Autoboxing, Enum, Generics and Variable arguments , Java introduces some change in Java Memory Model (JMM), Which guarantees visibility of changes made from one thread to another also as "happens-before" which solves the problem of memory writes that happen in one thread can "leak through" and be seen by another thread.
The Java volatile keyword cannot be used with method or class and it can only be used with a variable. Java volatile keyword also guarantees visibility and ordering, after Java 5 write to any volatile variable happens before any read into the volatile variable. By the way use of volatile keyword also prevents compiler or JVM from the reordering of code or moving away them from synchronization barrier.
Important points on Volatile keyword in Java
The volatile keyword in Java is only application to a variable and using volatile keyword with class and method is illegal.
volatile keyword in Java guarantees that value of the volatile variable will always be read from main memory and not from Thread's local cache.
In Java reads and writes are atomic for all variables declared using Java volatile keyword (including long and double variables).
Using the volatile keyword in Java on variables reduces the risk of memory consistency errors because any write to a volatile variable in Java establishes a happens-before relationship with subsequent reads of that same variable.

What is the difference between Atomic Integer and Normal immutable Integer class in Java?

As Integer class is also immutable class and we know that immutable class is thread-safe what is the need of Atomic Integer.
I am confused .
Is it the reason that reads and write of immutable objects need not be atomic whereas read and write of atomic integer is atomic .
That means atomic classes are also thread-safe.

AtomicInteger is used in multithreaded environments when you need to make sure that only one thread can update an int variable. The advantage is that no external synchronization is requried since the operations which modify it's value are executed in a thread-safe way.
Consider the followind code:
private int count;
public int updateCounter() {
return ++count;
}
If multiple threads would call the updateCounter method, it's possible that some of them would receive the same value. The reason it that the ++count operation isn't atomical since isn't only one operation, but made from three operations: read count, add 1 to it's value and write it back to it. Multiple calling threads could see the variable as unmodified to it's latest value.
The above code should be replaced with this:
private AtomicInteger count = new AtomicInteger(0);
public int updateCounter() {
return count.incrementAndGet();
}
The incrementAndGet method is guaranteed to atomically increment the stored value and return it's value without using any external synchonization.
If your value never changes, you don't have to use the AtomicInteger, it's enought to use int.

AtomicInteger is thread safe (in fact, all classes from java.util.concurrent.atomic package are thread safe), while normal integers are NOT threadsafe.
You would require 'synchronized' & 'volatile' keywords, when you are using an 'Integer' variable in multi-threaded environment (to make it thread safe) where as with atomic integers you don't need 'synchronized' & 'volatile' keywords as atomic integers take care of thread safety.
Also, I would recommend the below helpful tutorial on the same subject:
http://tutorials.jenkov.com/java-concurrency/compare-and-swap.html
Please refer below oracle doc for more information on 'atomic' package:
https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/atomic/package-summary.html

While immutable objects are thread-safe by definition, mutable objects can be thread safe too.
That is precisely the purpose of the Atomic... classes (AtomicInteger, AtomicBoolean, and so on).
The various ...get... and ...set... methods allow thread-safe access and mutation of the object.
Not surprisingly, the class is declared in the java.util.concurrent package.
You only have to browse the API for the java.util.concurrent.atomic package:
A small toolkit of classes that support lock-free thread-safe programming on single variables.

Consider a variable
int myInt = 3;
AtomicInteger relates to myInt.
Integer relates to 3.
in other words, your variable is mutable and can change it's value. While the value 3 is an integer literal, a constant, an immutable expression.
Integers are object representations of literals and are therefore immutable, you can basically only read them.
AtomicIntegers are containers for those values. You can read and set them. Same as asigning a value to variable. But different to changing the value of int variable, operations on an AtomicInteger are atomic.
For example this is not atomic
if(myInt == 3) {
myInt++;
}
This is atomic
AtomicInteger myInt = new AtomicInteger(3);
//atomic
myInt.compareAndSet(3, 4);

I think the main difference between AtomicInteger and normal immutable Integer will come into the picture, once we understand why even immutable Integers are not thread-safe.
Let's see with an example.
Suppose, we have a value of int count = 5, which is being shared by two different threads named T1 and T2 with both reading and writing at the same time.
We know that, if there is any value being reassigned into an immutable object, the old object remains at the pool and the new one takes over.
Now, when T1 and T2 are updating their values into count variable, Java might take this value into some cache and will do the set operations there and we won't know when JVM will write the updated value into main memory, so there might be a possibility that one of the threads may be updating the value into a totally stale value.
This brings us to the volatile keyword.
Volatile - This keyword ensures that all the I/O operations on any variable will take place on the main memory so that all the threads are working with the most updated value.
Consider, if 1 Thread is writing and all other threads are reading then, volatile will solve our problem, but if all the threads are reading and writing on the same variable at the same time, then we need synchronizing to ensure thread-safety.
Volatile keyword does not ensure thread-safety.
Now, coming to why AtomicIntegers. Even if are using syncrhonized keyword to ensure thread-safety, the actual update operation of count variable will be a three step process.
get updated value of count variable
increment the value by 1
set the value to count variable
This is why it takes a slightly longer time to update any value for normal Integers to update values once the thread safety is taken into consideration.
**AtomicIntegers solve this problem furthermore of thread safety and also faster updates by an optimized lock-free algorithm called Compare-And-Swap (CAS method).
They perform all the update operations atomically as a single-step process. **

Some doubts about volatile and Atomic classes?

i am going thru Java threads book. I came across this statement
Statement 1:- "volatile variables can be safely used only for single load or store operation and can't be
applied to long or double variales. These restrictions make the use of volatile variables uncommon"
I did not get what does single load or store operation mean here? why volatile can't be
applied to long or double variales?
Statement 2:- "A Volatile integer can not be used with the ++ operator because ++ operator contains
multiple instructions.The AtomicInteger class has a method that allows the integer it holds to be
incremented atomically."
Why Volatile integer can not be used with the ++ operator and how AtomicInteger addresses it?

Statement 1:- "volatile variables can be safely used only for single load or store operation and can't be applied to long or double variales. These restrictions make the use of volatile variables uncommon"
What?! I believe this is simply flat-out wrong. Maybe your book is out of date.
Statement 2:- "A Volatile integer can not be used with the ++ operator because ++ operator contains multiple instructions.The AtomicInteger class has a method that allows the integer it holds to be incremented atomically."
Exactly what it says. The ++ operator actually translates to machine code like this (in Java-like pseudocode):
sync_CPU_caches();
int processorRegister = variable;
processorRegister = processorRegister + 1;
variable = processorRegister;
sync_CPU_caches();
This is not thread-safe, because even though it has a memory barrier, and reads atomically, and writes atomically, it is not guaranteed that you won't get a thread switch in the middle, and processor registers are local to a CPU core (think of them as like "local variables" inside the CPU core). But an AtomicInteger is thread-safe - it probably is implemented using special machine code instructions such as compare-and-swap.

The main purpose of volatile variables is not to cause immediate thread-safe access to that variable, but to ensure a so called happens-before safety.
Theoretically a call to
volatile int i = 0;
and
int i = 0;
has no difference, as a 32-bit word is written atomically anyways (on 32 bit and higher machines to be correct). Since pointers are 32/64 bit ints as well internally, there is basically only one operation that volatile makes atomically, and that is if you use a 64 bit long in a 32 bit environment.
The happens-before however is something that actually messes up the above example. To understand this you need to know that threads don't use the actual memory of the variable in question but might make copies of it to speed up execution and can re-order the statements for optimization. Now if you have something like:
Thread A: value = 1; doIt = true;
Thread B: if (doIt) { doDoIt(value); }
It is possible that in Thread B doIt is true, but value is not yet 1, because the order of execution might have been changed by the JVM, or the new value has just not yet been broadcasted to the copy of Thread B's value.
If doIt is declared volatile instead, then at the moment of accessing it, the JVM ensures that all code before that access has already been executed and broadcasted. So the above example is the actual reason to use volatile.

Volatile Vs Atomic [duplicate]

This question already has answers here:
What is the difference between atomic / volatile / synchronized?
(7 answers)
Closed 9 years ago.
I read somewhere below line.
Java volatile keyword doesn't means atomic, its common misconception
that after declaring volatile, ++ operation will be atomic, to make
the operation atomic you still need to ensure exclusive access using
synchronized method or block in Java.
So what will happen if two threads attack a volatile primitive variable at same time?
Does this mean that whosoever takes lock on it, that will be setting its value first. And if in meantime, some other thread comes up and read old value while first thread was changing its value, then doesn't new thread will read its old value?
What is the difference between Atomic and volatile keyword?

The effect of the volatile keyword is approximately that each individual read or write operation on that variable is made atomically visible to all threads.
Notably, however, an operation that requires more than one read/write -- such as i++, which is equivalent to i = i + 1, which does one read and one write -- is not atomic, since another thread may write to i between the read and the write.
The Atomic classes, like AtomicInteger and AtomicReference, provide a wider variety of operations atomically, specifically including increment for AtomicInteger.

Volatile and Atomic are two different concepts. Volatile ensures, that a certain, expected (memory) state is true across different threads, while Atomics ensure that operation on variables are performed atomically.
Take the following example of two threads in Java:
Thread A:
value = 1;
done = true;
Thread B:
if (done)
System.out.println(value);
Starting with value = 0 and done = false the rule of threading tells us, that it is undefined whether or not Thread B will print value. Furthermore value is undefined at that point as well! To explain this you need to know a bit about Java memory management (which can be complex), in short: Threads may create local copies of variables, and the JVM can reorder code to optimize it, therefore there is no guarantee that the above code is run in exactly that order. Setting done to true and then setting value to 1 could be a possible outcome of the JIT optimizations.
volatile only ensures, that at the moment of access of such a variable, the new value will be immediately visible to all other threads and the order of execution ensures, that the code is at the state you would expect it to be. So in case of the code above, defining done as volatile will ensure that whenever Thread B checks the variable, it is either false, or true, and if it is true, then value has been set to 1 as well.
As a side-effect of volatile, the value of such a variable is set thread-wide atomically (at a very minor cost of execution speed). This is however only important on 32-bit systems that i.E. use long (64-bit) variables (or similar), in most other cases setting/reading a variable is atomic anyways. But there is an important difference between an atomic access and an atomic operation. Volatile only ensures that the access is atomically, while Atomics ensure that the operation is atomically.
Take the following example:
i = i + 1;
No matter how you define i, a different Thread reading the value just when the above line is executed might get i, or i + 1, because the operation is not atomically. If the other thread sets i to a different value, in worst case i could be set back to whatever it was before by thread A, because it was just in the middle of calculating i + 1 based on the old value, and then set i again to that old value + 1. Explanation:
Assume i = 0
Thread A reads i, calculates i+1, which is 1
Thread B sets i to 1000 and returns
Thread A now sets i to the result of the operation, which is i = 1
Atomics like AtomicInteger ensure, that such operations happen atomically. So the above issue cannot happen, i would either be 1000 or 1001 once both threads are finished.

There are two important concepts in multithreading environment:
atomicity
visibility
The volatile keyword eradicates visibility problems, but it does not deal with atomicity. volatile will prevent the compiler from reordering instructions which involve a write and a subsequent read of a volatile variable; e.g. k++.
Here, k++ is not a single machine instruction, but three:
copy the value to a register;
increment the value;
place it back.
So, even if you declare a variable as volatile, this will not make this operation atomic; this means another thread can see a intermediate result which is a stale or unwanted value for the other thread.
On the other hand, AtomicInteger, AtomicReference are based on the Compare and swap instruction. CAS has three operands: a memory location V on which to operate, the expected old value A, and the new value B. CAS atomically updates V to the new value B, but only if the value in V matches the expected old value A; otherwise, it does nothing. In either case, it returns the value currently in V. The compareAndSet() methods of AtomicInteger and AtomicReference take advantage of this functionality, if it is supported by the underlying processor; if it is not, then the JVM implements it via spin lock.

As Trying as indicated, volatile deals only with visibility.
Consider this snippet in a concurrent environment:
boolean isStopped = false;
:
:
while (!isStopped) {
// do some kind of work
}
The idea here is that some thread could change the value of isStopped from false to true in order to indicate to the subsequent loop that it is time to stop looping.
Intuitively, there is no problem. Logically if another thread makes isStopped equal to true, then the loop must terminate. The reality is that the loop will likely never terminate even if another thread makes isStopped equal to true.
The reason for this is not intuitive, but consider that modern processors have multiple cores and that each core has multiple registers and multiple levels of cache memory that are not accessible to other processors. In other words, values that are cached in one processor's local memory are not visisble to threads executing on a different processor. Herein lies one of the central problems with concurrency: visibility.
The Java Memory Model makes no guarantees whatsoever about when changes that are made to a variable in one thread may become visible to other threads. In order to guarantee that updates are visisble as soon as they are made, you must synchronize.
The volatile keyword is a weak form of synchronization. While it does nothing for mutual exclusion or atomicity, it does provide a guarantee that changes made to a variable in one thread will become visible to other threads as soon as it is made. Because individual reads and writes to variables that are not 8-bytes are atomic in Java, declaring variables volatile provides an easy mechanism for providing visibility in situations where there are no other atomicity or mutual exclusion requirements.

The volatile keyword is used:
to make non atomic 64-bit operations atomic: long and double. (all other, primitive accesses are already guaranteed to be atomic!)
to make variable updates guaranteed to be seen by other threads + visibility effects: after writing to a volatile variable, all the variables that where visible before writing that variable become visible to another thread after reading the same volatile variable (happen-before ordering).
The java.util.concurrent.atomic.* classes are, according to the java docs:
A small toolkit of classes that support lock-free thread-safe
programming on single variables. In essence, the classes in this
package extend the notion of volatile values, fields, and array
elements to those that also provide an atomic conditional update
operation of the form:
boolean compareAndSet(expectedValue, updateValue);
The atomic classes are built around the atomic compareAndSet(...) function that maps to an atomic CPU instruction. The atomic classes introduce the happen-before ordering as the volatile variables do. (with one exception: weakCompareAndSet(...)).
From the java docs:
When a thread sees an update to an atomic variable caused by a
weakCompareAndSet, it does not necessarily see updates to any other
variables that occurred before the weakCompareAndSet.
To your question:
Does this mean that whosoever takes lock on it, that will be setting
its value first. And in if meantime, some other thread comes up and
read old value while first thread was changing its value, then doesn't
new thread will read its old value?
You don't lock anything, what you are describing is a typical race condition that will happen eventually if threads access shared data without proper synchronization. As already mentioned declaring a variable volatile in this case will only ensure that other threads will see the change of the variable (the value will not be cached in a register of some cache that is only seen by one thread).
What is the difference between AtomicInteger and volatile int?
AtomicInteger provides atomic operations on an int with proper synchronization (eg. incrementAndGet(), getAndAdd(...), ...), volatile int will just ensure the visibility of the int to other threads.

So what will happen if two threads attack a volatile primitive variable at same time?
Usually each one can increment the value. However sometime, both will update the value at the same time and instead of incrementing by 2 total, both thread increment by 1 and only 1 is added.
Does this mean that whosoever takes lock on it, that will be setting its value first.
There is no lock. That is what synchronized is for.
And in if meantime, some other thread comes up and read old value while first thread was changing its value, then doesn't new thread will read its old value?
Yes,
What is the difference between Atomic and volatile keyword?
AtomicXxxx wraps a volatile so they are basically same, the difference is that it provides higher level operations such as CompareAndSwap which is used to implement increment.
AtomicXxxx also supports lazySet. This is like a volatile set, but doesn't stall the pipeline waiting for the write to complete. It can mean that if you read a value you just write you might see the old value, but you shouldn't be doing that anyway. The difference is that setting a volatile takes about 5 ns, bit lazySet takes about 0.5 ns.

Is volatile read happens-before volatile write?

I try to understand why this example is a correctly synchronized program:
a - volatile
Thread1:
x=a
Thread2:
a=5
Because there are conflicting accesses (there is a write to and read of a) so in every sequential consistency execution must be happens-before relation between that accesses.
Suppose one of sequential execution:
1. x=a
2. a=5
Is 1 happens-before 2, why?

Is 1 happens-before 2, why?
I'm not 100% sure I understand your question.
If you have a volatile variable a and one thread is reading from it and another is writing to it, the order of those accesses can be in either order. It is a race condition. What is guaranteed by the JVM and the Java Memory Model (JMM) depends on which operation happens first.
The write could have just happened and the read sees the updated value. Or the write could happen after the read. So x could be either 5 or the previous value of a.
every sequential consistency execution must be happens-before relation between that accesses
I'm not sure what this means so I'll try to be specific. The "happens before relation" with volatile means that all previous memory writes to a volatile variable prior to a read of the same variable are guaranteed to have finished. But this guarantee in no way explains the timing between the two volatile operations which is subject to the race condition. The reader is guaranteed to have seen the write, but only if the write happened before the read.
You might think this is a pretty weak guarantee, but in threads, whose performance is dramatically improved by using local CPU cache, reading the value of a field might come from a cached memory segment instead of central memory. The guarantee is critical to ensure that the local thread memory is invalidated and updated when a volatile read occurs so that threads can share data appropriately.
Again, the JVM and the JMM guarantee that if you are reading from a volatile field a, then any writes to the same field that have happened before the read, will be seen by it -- the value written will be properly published and visible to the reading thread. However, this guarantee in no way determines the ordering. It doesn't say that the write has to happen before the read.

No, a volatile read before (in synchronization order) a volatile write of the same variable does not necessarily happens-before the volatile write.
This means they can be in a "data race", because they are "conflicting accesses not ordered by a happens-before relationship". If that's true pretty much all programs contain data races:) But it's probably a spec bug. A volatile read and write should never be considered a data race. If all variables in a program are volatile, all executions are trivially sequentially consistent. see http://cs.oswego.edu/pipermail/concurrency-interest/2012-January/008927.html

Sorry, but you cannot say correctly how the JVM will optimize the code depending on the 'memory model' of the JVM. You have to use the high level tools of Java for defining what you want.
So volatile means only that there will be no "inter-thread cache" used for the variables.
If you want a stricter order, you have to use synchronized blocks.
http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html

Volatile and happens-before is only useful when the read of the field drives some condition. For example:
volatile int a;
int b =0;
Thread-1:
b = 5;
a = 10;
Thread-2
c = b + a;
In this case there is no happens-before, a can be either 10 or 0 and b can be either 5 or 0, so as a result c could be either 0, 5, 10 or 15. If the read of a implies some other condition then the happens-before is established for instance:
int b = 0;
volatile int a = 0;
Thread-1:
b = 5
a = 10;
Thread 2:
if(a == 10){
c = b + a;
}
In this case you will ensure c = 15 because the read of a==10 implies that the write of b = 5 happens-before the write of a = 10
Edit: Updating addition order as noted the inconsistency by Gray