How Synchronization works in Java ?

Synchronization
Synchronization in Java is an important concept since Java is a multi-threaded language where multiple threads run in parallel to complete program execution. In multi-threaded environment synchronization of java object or synchronization of java class becomes extremely important. Synchronization in Java is possible by using java keyword "synchronized" and "volatile”. Concurrent access of shared objects in Java introduces to kind of errors: thread interference and memory consistency errors and to avoid these errors you need to properly synchronize your java object to allow mutual exclusive access of critical section to two threads.

Why in Java

If your code is executing in multi-threaded environment you need synchronization for objects which are shared among multiple threads to avoid any corruption of state or any kind of unexpected behavior. Synchronization in Java will only be needed if shared object is mutable. if your shared object is read only or immutable object you don't need synchronization despite running multiple threads. Same is true with what threads are doing with object if all the threads are only reading value then you don't require synchronization in java. JVM guarantees that Java synchronized code will only be executed by one thread at a time.

• synchronized keyword in java provides locking which ensures mutual exclusive access of shared resource and prevent data race.
• synchronized keyword involve locking and unlocking. before entering into synchronized method or block thread needs to acquire the lock at this point it reads data from main memory than cache and when it release the lock it flushes write operation into main memory which eliminates memory inconsistency errors.
• You can have both static synchronized method and non static synchronized method and synchronized blocks in java but we can not have synchronized variable in java.

• Block synchronization in java is preferred over method synchronization in java because by using block synchronization you only need to lock the critical section of code instead of whole method. Since java synchronization comes with cost of performance we need to synchronize only part of code which absolutely needs to be synchronized.
  
  

  Example of synchronized method in Java

  
  

  Using synchronized keyword along with method is easy just apply synchronized keyword in front of method. What we need to take care is that static synchronized method locked on class object lock and non static synchronized method locks on current object (this). So it’s possible that both static and non static java synchronized method running in parallel.  This is the common mistake a naive developer do while writing java synchronized code.

  
  
  
  

public class Counter{
private static int count = 0;

public static synchronized int getCount(){
  return count;
}

public synchoronized setCount(int count){
   this.count = count;
}

}

In this example of java synchronization code is not properly synchronized because both getCount() and setCount() are not getting locked on same object and can run in parallel which results in getting incorrect count. Here getCount() will lock in Counter.class object while setCount() will lock on current object (this). To make this code properly synchronized in java you need to either make bothmethod static or non static or use java synchronized block instead of java synchronized method.
Example of synchronized block in Java

public class Singleton{
private static volatile Singleton _instance;

public static Singleton getInstance(){

   if(_instance == null){
            synchronized(Singleton.class){
              if(_instance == null)
              _instance = new Singleton();
            }

   }
   return _instance;

}

This is a classic example of double checked locking in Singleton. In this example of java synchronized code we have made only critical section (part of code which is creating instance of singleton) synchronized and saved some performance because if you make whole method synchronized every call of this method will be blocked while you only need to create instance on first call.

Important points of synchronized keyword in Java

1. Synchronized keyword in Java is used to provide mutual exclusive access of a shared resource with multiple threads in Java. Synchronization in java guarantees that no two threads can execute a synchronized method which requires same lock simultaneously or concurrently.
2. You can use java synchronized keyword only on synchronized method or synchronized block.
3. When ever a thread enters into java synchronized method or block it acquires a lock and whenever it leaves java synchronized method or block it releases the lock. Lock is released even if thread leaves synchronized method after completion or due to any Error or Exception.
4. Java Thread acquires an object level lock when it enters into an instance synchronized java method and acquires a class level lock when it enters into static synchronized java method.
5. java synchronized keyword is re-entrant in nature it means if a java synchronized method calls another synchronized method which requires same lock then current thread which is holding lock can enter into that method without acquiring lock.
6. Java Synchronization will throw NullPointerException if object used in java synchronized block is null e.g. synchronized (myInstance) will throws NullPointerException if myInstance is null.
7. One Major disadvantage of java synchronized keyword is that it doesn't allow concurrent read which you can implement using java.util.concurrent.locks.ReentrantLock.
8. One limitation of java synchronized keyword is that it can only be used to control access of shared object within the same JVM. If you have more than one JVM and need to synchronized access to a shared file system or database, the java synchronized keyword is not at all sufficient. You need to implement a kind of global lock for that.
10. Java synchronized keyword incurs performance cost. Synchronized method in Java is very slow and can degrade performance. So use synchronization in java when it absolutely requires and consider using java synchronized block for synchronizing critical section only.
11. Its possible that both static synchronized and non static synchronized method can run simultaneously or concurrently because they lock on different object.
12. Java synchronized code could result in deadlock or starvation while accessing by multiple thread if synchronization is not implemented correctly.
13. You cannot apply java synchronized keyword with variables and can not use java volatile keyword with method.
14. java synchronized keyword also synchronizes memory. In fact java synchronized synchronizes the whole of thread memory with main memory.
15. Do not synchronize on non final field on synchronized block in Java. because reference of non final field may change any time and then different thread might synchronizing on different objects i.e. no synchronization at all. example of synchronizing on non final field :
    private String lock = new String("lock");

synchronized(lock){
System.out.println("locking on :"  + lock);
}

16. Its not recommended to use String object as lock in java synchronized block because string is immutable object and literal string and interned string gets stored in String pool. so by any chance if any other part of code or any third party library used same String as there lock then they both will be locked on same object despite being completely unrelated which could result in unexpected behavior and bad performance. instead of String object its advised to use new Object() for Synchronization in Java on synchronized block.

private static final String LOCK = "lock";   //not recommended
private static final Object OBJ_LOCK = new Object(); //better

public void process() {
   synchronized(LOCK) {
      ........
   }
}