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.
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
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.
