An Ultimate Tutorial to Synchronization in Java

Why Do You Need to Use Synchronization in Java?

1. To prevent thread interleaving or interference
2. To provide consistency to the program

How Is It Carried Out?

Synchronization in Java is done by using the synchronized keyword. This keyword can be used on top of a method or a block.

Types of Synchronization

There are two types of Synchronization in Java, as mentioned below-

• Process Synchronization

When multiple threads are executed simultaneously process synchronization ensures that they reach a particular state and agree to a certain set of actions.

• Thread Synchronization

When multiple threads want to access the same resource, thread synchronization makes sure that only one thread gains access at a time.

Depicted below is an example of code to understand Thread Synchronization-

Output

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Mutual Exclusive

A mutex (short for mutual exclusion) is a type of object that is used to control access to a shared resource. Threads will request mutex access, and the mutex can only allow it access provided no other thread owns the mutex. When the mutex is released, other threads waiting in line may obtain access. If a thread already owns a mutex, and requests access again, it will be given continued access immediately. However, the thread must release the mutex the same number of times before other threads can obtain ownership.

In short, it helps in keeping the thread from interfering while sharing data. You can achieve mutual Exclusion in Java in the following three ways-

• Synchronized Method

The general syntax of using the synchronized keyword in a method is given below-

<access modifier> synchronized method (parameter)

{

//code

}

First, look at an example of code where synchronization is not done.

Output

In the above code, as you can see, both the statements are running simultaneously, which creates an inconsistency in the program. Just using the synchronized keyword before the method can easily solve this problem, as shown below.

Output

When you add the keyword synchronized, it executes the first statement first and the second statement after that, and so on.

• Synchronized Block

A synchronized block is simply a block that uses the synchronized keyword.

It is important to understand the difference between a synchronized method and a synchronized block.

Take an example where there is a method called method1 containing some instructions.

void method1

{

// This method contains some instructions

………

……..

……..

………

}

Now, if all these instructions present in method1 are not shareable by multiple threads simultaneously, then you must use the synchronized method as in the code above. 

But what if there is a set of instructions that are not shareable between multiple threads at a time, and the remaining instructions could be shared? 

Here comes the need to use the synchronized block. In this situation, the synchronized keyword should be added before those particular sets of instructions that are not shareable.

void method1

{

// ……… (these set of instructions are shareable between multiple threads at a given time)

………..

………..

synchronized …….. (these set of instructions are not shareable)

}

Here is an example of code to understand the synchronized block.

Output

• Static Synchronization

In static synchronization, the lock and monitor belong to the class instead of an object. The memory for static methods is only assigned once during class loading. The whole class is blocked when a static method is being executed. As a result, other static synchronized methods are also obstructed. If one thread attempts to execute a static synchronized method, all other threads attempting to execute any static synchronized methods will be blocked.

Example for static synchronization-

Output

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Inter Thread Communication

Inter thread communication in Java is a method that allows several threads to communicate with one another. It provides an effective way for multiple threads to communicate with each other by minimizing CPU idle time. 

CPU idle time is a procedure that prevents CPU cycles from being wasted. When many threads are running at the same time, they may need to communicate with one another by sharing information. A thread exchanges data before or after changing its state. Communication between threads is vital in a variety of cases. 

For example; Assume there are two threads: A and B. Thread B executes its task using the data developed by Thread A. Thread B would waste several CPU cycles while it waits for Thread A to create data. On the other hand, threads A and B do not have to wait and check each other's status every time they complete their tasks if they engage with each other after they have finished their tasks, As a result, you would not waste CPU cycles either.

Locks in Java

A lock is a thread synchronization mechanism similar to synchronized blocks, except that locks can be more complex than Java's synchronized blocks. Locks are formed using synchronized blocks, so you can't get rid of the synchronized keyword entirely.

Multi-Threading Without Synchronization

In the following example, you do not use synchronization and instead create multiple threads that access the display method and generate random output.

Output

Multi-Threading With Synchronization

Output

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Conclusion

We hope this tutorial on the synchronization in Java helped you understand how it works and the examples gave you a practical walk-through that you’ll need.

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