Synchronization in java | Why, what, how | Java Tutorials

As we have seen that how Multithreading in java works. In such an environment, sometimes the deadlock condition might occur. Deadlock means a condition when two or more thread tries to work or acquire the lock on the object.

To overcome this scenario, we use the concept of synchronization in java. Using synchronization, we can restrict the access of the resource to one thread at a time. The second thread can only use the resource once the lock is released by the first thread.

What is synchronization in java?

Synchronization is a mechanism using which we can develop a way by which we can restrict the access of a resource by multiple threads thereby reducing the risk of deadlock conditions.

Lock in java

Synchronization uses locking technique which is built around an internal entity known as lock or monitor. In synchronization, whenever a thread needs access to a resource, it must acquire the lock on the object. After the process is completed the thread must release the lock.

Why we use synchronization?

Synchronization is used to

        1.     Prevent thread interference

        2.     Prevent consistency programming.

To understand this let us create a program which will print the numbers in increment order by using two threads.

In the above example, we can see that there is only one object of class sequence which is shared among the two threads and as there is no synchronization the output will be not as what is desired.

output

5

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500

Now, to overcome this issue we must implement synchronization into the program. Let us see how we can achieve synchronization and in how many ways.

Types of Synchronization

        1.     Process Synchronization

        2.     Thread Synchronization

Thread Synchronization

Thread Synchronization is divided into two parts

        1.     Mutual Exclusive

a.      Synchronized method

b.     Synchronized block

c.      Static Synchronization

        2.     Inter-thread communication

Let’s discuss this one by one.

        1.    Mutual Exclusive

This technique of Synchronization restricts the other threads to access the already locked resource. This can be achieved in three ways:

a.     Synchronized method

If we use the Synchronized keyword before any normal method, then that method will be said to be a synchronized method.

By making the method as synchronized it is assured that the thread which calls the synchronized method will acquire the object lock and will keep the object lock until it’s execution is completed.

Let’s take the above example and try to reconstruct it using method synchronization and see what will be the output

Here the method printSequence is synchronized, hence one thread will execute the method completely and after that only it will release the lock and gives the turn to next thread. We will get the desired output.

Output

5

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25

100

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500

b.     Synchronized block

Suppose we have 60 lines of code inside a method but, we want to synchronize only a few resources in that method. In such a case, synchronizing the whole method will not be feasible. Hence, we have the concept of synchronized block.

Important points

        1.     The synchronized block has less scope as compared to method synchronization.

        2.     Block level synchronization is more preferable than method level synchronization because making the whole method synchronized rather than a particular resource will slow the performance.

        3.     The scope of lock will be block level.

Let us take the same example of sequence class and this time we will make use of block synchronization.

Observe in the above example we have created a synchronized block and removed the synchronized method. The output will be the same, but the performance will be much faster.

Output

5

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25

100

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500

c.      Static synchronization

Non Static synchronization

Consider the scenario where there are two objects of a shared class. During method synchronization and block synchronization, there will be no interference between thread1 and thread2, thread3 and thread4. But between thread1 and thread3 there can be some interference similar is the case with thread2 and thread4.

To overcome this situation static synchronization comes into the picture. The static synchronization will acquire the lock on class and not on the object. Consider the following example below. 

        2.    Inter-thread communication

For this topic, I have a separate description. Please check out Multithreading concept in java.

That is all for this tutorial. I recommend reading the Multithreading concept after or before this article. Also, you can check out the related articles below. Please feel free to fill the gaps in this article if any.