[java] What is the difference between atomic / volatile / synchronized?

How do atomic / volatile / synchronized work internally?

What is the difference between the following code blocks?

Code 1

private int counter;

public int getNextUniqueIndex() {
    return counter++; 
}

Code 2

private AtomicInteger counter;

public int getNextUniqueIndex() {
    return counter.getAndIncrement();
}

Code 3

private volatile int counter;

public int getNextUniqueIndex() {
    return counter++; 
}

Does volatile work in the following way? Is

volatile int i = 0;
void incIBy5() {
    i += 5;
}

equivalent to

Integer i = 5;
void incIBy5() {
    int temp;
    synchronized(i) { temp = i }
    synchronized(i) { i = temp + 5 }
}

I think that two threads cannot enter a synchronized block at the same time... am I right? If this is true then how does atomic.incrementAndGet() work without synchronized? And is it thread-safe?

And what is the difference between internal reading and writing to volatile variables / atomic variables? I read in some article that the thread has a local copy of the variables - what is that?

The Java volatile modifier is an example of a special mechanism to guarantee that communication happens between threads. When one thread writes to a volatile variable, and another thread sees that write, the first thread is telling the second about all of the contents of memory up until it performed the write to that volatile variable.

Atomic operations are performed in a single unit of task without interference from other operations. Atomic operations are necessity in multi-threaded environment to avoid data inconsistency.

Declaring a variable as volatile means that modifying its value immediately affects the actual memory storage for the variable. The compiler cannot optimize away any references made to the variable. This guarantees that when one thread modifies the variable, all other threads see the new value immediately. (This is not guaranteed for non-volatile variables.)

Declaring an atomic variable guarantees that operations made on the variable occur in an atomic fashion, i.e., that all of the substeps of the operation are completed within the thread they are executed and are not interrupted by other threads. For example, an increment-and-test operation requires the variable to be incremented and then compared to another value; an atomic operation guarantees that both of these steps will be completed as if they were a single indivisible/uninterruptible operation.

Synchronizing all accesses to a variable allows only a single thread at a time to access the variable, and forces all other threads to wait for that accessing thread to release its access to the variable.

Synchronized access is similar to atomic access, but the atomic operations are generally implemented at a lower level of programming. Also, it is entirely possible to synchronize only some accesses to a variable and allow other accesses to be unsynchronized (e.g., synchronize all writes to a variable but none of the reads from it).

Atomicity, synchronization, and volatility are independent attributes, but are typically used in combination to enforce proper thread cooperation for accessing variables.

Addendum (April 2016)

Synchronized access to a variable is usually implemented using a monitor or semaphore. These are low-level mutex (mutual exclusion) mechanisms that allow a thread to acquire control of a variable or block of code exclusively, forcing all other threads to wait if they also attempt to acquire the same mutex. Once the owning thread releases the mutex, another thread can acquire the mutex in turn.

Addendum (July 2016)

Synchronization occurs on an object. This means that calling a synchronized method of a class will lock the this object of the call. Static synchronized methods will lock the Class object itself.

Likewise, entering a synchronized block requires locking the this object of the method.

This means that a synchronized method (or block) can be executing in multiple threads at the same time if they are locking on different objects, but only one thread can execute a synchronized method (or block) at a time for any given single object.

I know that two threads can not enter in Synchronize block at the same time

Two thread cannot enter a synchronized block on the same object twice. This means that two threads can enter the same block on different objects. This confusion can lead to code like this.

private Integer i = 0;

synchronized(i) {
   i++;
}

This will not behave as expected as it could be locking on a different object each time.

if this is true than How this atomic.incrementAndGet() works without Synchronize ?? and is thread safe ??

yes. It doesn't use locking to achieve thread safety.

If you want to know how they work in more detail, you can read the code for them.

And what is difference between internal reading and writing to Volatile Variable / Atomic Variable ??

Atomic class uses volatile fields. There is no difference in the field. The difference is the operations performed. The Atomic classes use CompareAndSwap or CAS operations.

i read in some article that thread has local copy of variables what is that ??

I can only assume that it referring to the fact that each CPU has its own cached view of memory which can be different from every other CPU. To ensure that your CPU has a consistent view of data, you need to use thread safety techniques.

This is only an issue when memory is shared at least one thread updates it.

Synchronized Vs Atomic Vs Volatile:

• Volatile and Atomic is apply only on variable , While Synchronized apply on method.
• Volatile ensure about visibility not atomicity/consistency of object , While other both ensure about visibility and atomicity.
• Volatile variable store in RAM and it’s faster in access but we can’t achive Thread safety or synchronization whitout synchronized keyword.
• Synchronized implemented as synchronized block or synchronized method while both not. We can thread safe multiple line of code with help of synchronized keyword while with both we can’t achieve the same.
• Synchronized can lock the same class object or different class object while both can’t.

Please correct me if anything i missed.

A volatile + synchronization is a fool proof solution for an operation(statement) to be fully atomic which includes multiple instructions to the CPU.

Say for eg:volatile int i = 2; i++, which is nothing but i = i + 1; which makes i as the value 3 in the memory after the execution of this statement. This includes reading the existing value from memory for i(which is 2), load into the CPU accumulator register and do with the calculation by increment the existing value with one(2 + 1 = 3 in accumulator) and then write back that incremented value back to the memory. These operations are not atomic enough though the value is of i is volatile. i being volatile guarantees only that a SINGLE read/write from memory is atomic and not with MULTIPLE. Hence, we need to have synchronized also around i++ to keep it to be fool proof atomic statement. Remember the fact that a statement includes multiple statements.

Hope the explanation is clear enough.

volatile:

volatile is a keyword. volatile forces all threads to get latest value of the variable from main memory instead of cache. No locking is required to access volatile variables. All threads can access volatile variable value at same time.

Using volatile variables reduces the risk of memory consistency errors, because any write to a volatile variable establishes a happens-before relationship with subsequent reads of that same variable.

This means that changes to a volatile variable are always visible to other threads. What's more, it also means that when a thread reads a volatile variable, it sees not just the latest change to the volatile, but also the side effects of the code that led up the change.

When to use: One thread modifies the data and other threads have to read latest value of data. Other threads will take some action but they won't update data.

AtomicXXX:

AtomicXXX classes support lock-free thread-safe programming on single variables. These AtomicXXX classes (like AtomicInteger) resolves memory inconsistency errors / side effects of modification of volatile variables, which have been accessed in multiple threads.

When to use: Multiple threads can read and modify data.

synchronized:

synchronized is keyword used to guard a method or code block. By making method as synchronized has two effects:

1. First, it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.

2. Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.

When to use: Multiple threads can read and modify data. Your business logic not only update the data but also executes atomic operations

AtomicXXX is equivalent of volatile + synchronized even though the implementation is different. AmtomicXXX extends volatile variables + compareAndSet methods but does not use synchronization.

Related SE questions:

Difference between volatile and synchronized in Java

Volatile boolean vs AtomicBoolean

Good articles to read: ( Above content is taken from these documentation pages)

https://docs.oracle.com/javase/tutorial/essential/concurrency/sync.html

https://docs.oracle.com/javase/tutorial/essential/concurrency/atomic.html

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/atomic/package-summary.html