How Synchronization works in Java ? Example of synchronized block

In this Java synchronization tutorial we will see what is meaning of Synchronization in Java, Why do we need Synchronization in java, what is java synchronized keyword, example of using java synchronized method and blocks and important points about synchronization in Java.  

 
 

Example of Synchronization in Java using synchronized method and block

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.

This Java Synchronization tutorial is in continuation of my article How HashMap works in Java  and difference between HashMap and Hashtable in Java  if you haven’t read already you may find some useful information based on my experience in Java Collections.
 

Why do we need Synchronization 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.

In Summary Java Synchronized Keyword provides following functionality essential for concurrent programming :

1) synchronized keyword in java provides locking which ensures mutual exclusive access of shared resource and prevent data race.
2) synchronized keyword also prevent reordering of code statement by compiler which can cause subtle concurrent issue if we don't use synchronized or volatile keyword.
3) 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.

Synchronized keyword in Java

Prior to Java5 synchronized keyword in java was only way to provide synchronization of shared object. Any code written in synchronized block in java will be mutual exclusive and can only be executed by one thread at a time. 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. Using synchronized keyword with variable is illegal and will result in compilation error. Instead of java synchronized variable you can have java volatile variable, which will instruct JVM threads to read value of volatile variable from main memory and don’t cache it locally. 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 both method static or non static or use java synchronized block instead of java synchronized method.

Example of synchronized block in Java

Using synchronized block in java is also similar to using synchronized keyword in methods. Only important thing to note here is that if object used to lock synchronized block of code, Singleton.class in below example is null then java synchronized block will throw a NullPointerException.

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. To read more about Singleton in Java see here.

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.

9. 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.

10. Java synchronized block is better than java synchronized method in java because by using synchronized block you can only lock critical section of code and avoid locking whole method which can possibly degrade performance. A good example of java synchronization around this concept is getInstance() method Singleton class. See here.

11. Its possible that both static synchronized and non static synchronized method can run simultaneously or concurrently because they lock on different object.

12. From java 5 after change in Java memory model reads and writes are atomic for all variables declared using volatile keyword (including long and double variables) and simple atomic variable access is more efficient instead of accessing these variables via synchronized java code. But it requires more care and attention from the programmer to avoid memory consistency errors.

13. Java synchronized code could result in deadlock or starvation while accessing by multiple thread if synchronization is not implemented correctly. To know how to avoid deadlock in java see here.

14. According to the Java language specification you can not use java synchronized keyword with constructor it’s illegal and result in compilation error. So you can not synchronized constructor in Java which seems logical because other threads cannot see the object being created until the thread creating it has finished it.

15. You cannot apply java synchronized keyword with variables and can not use java volatile keyword with method.

16. Java.util.concurrent.locks extends capability provided by java synchronized keyword for writing more sophisticated programs since they offer more capabilities e.g. Reentrancy and interruptible locks.

17. java synchronized keyword also synchronizes memory. In fact java synchronized synchronizes the whole of thread memory with main memory.

18. Important method related to synchronization in Java are wait(), notify() and notifyAll() which is defined in Object class.

19. 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);

}
any if you write synchronized code like above in java you may get warning "Synchronization on non-final field"  in IDE like Netbeans and InteliJ

20. 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) {
      ........
   }
}

21. From Java library Calendar and SimpleDateFormat classes are not thread-safe and requires external synchronization in Java to be used in multi-threaded environment.  

Probably most important point about synchronization in Java is that in the absence of synchronized keyword or construct compiler, JVM and hardware are free to make optimization, assumption, reordering or caching of code and variable which can cause subtle concurrency bugs in code. By introducing synchronization may be either using volatile or synchronized keyword we instruct compiler or JVM to not to do that

If you like to read UNIX command tips you may find  10 tips of using find command in Linux ,  10 tips to increase speed on Unix command and  10 basic networking Commands in Unix useful.

Update: Recently I have been reading several java synchronization and concurrency articles in internet and I come across jeremymanson's blog which works in google and has worked on JSR 133 Java Memory Model, I would recommend some of this blog post for every java developer, he has covered certain details about concurrent programming , synchronization and volatility in simple and easy to understand language, here is the link atomicity, visibility and ordering. 

Other Java Threading tutorial you may like:

Difference between wait and Sleep in Java

How to create Thread Safe Class in Java

Top 15 multi-threading and Concurrency Interview Questions asked in Investment banks 

What is Race condition, two example of race condition in java

Why wait and notify method are defined in Object Class 

Difference between Thread and Runnable in Java

Why wait and notify method needs to be called from synchronized context

How to solve Producer Consumer problem using blocking Queue in Java

Please share with your friends if like this article