16. Synchronized block

1. What is Synchronized block in Thread

1.       If very few lines of code required synchronization then it's not recommended to declare method as synchronized we have to enclose those few line of the code by using synchronized block.

2.       The main advantage of synchronized block over synchronized method is it reduces waiting time of thread and improves performance of the system.

3.       Syntax of synchronized block

Synchronized(this){}

4.       We can declare synchronized block as following ways

To get lock of current object	To get lock of particular object	To get class level lock
Synchronized(this){ ......................... }	Synchronized(b){ ......................... }	Synchronized(Display.class){ ......................... }
If a thread got lock of current object, then only it allows to execute this area	If a thread got lock of object ‘b’ , then only it allows to execute this area	If a thread get class level lock Of ‘Display’ class then only its Allowed to execute this area.

5.       Synchronized block contains an object or class , means for current thread executing Synchronized block first it will lock the object or lock the class.

6.       For next thread before entering into Synchronized block it will check lock is available or not, if not available it will go to wait state.

Synchronized(new String(“a”)){ Sop(name); }	Synchronized(“a”){ Sop(name); }	Synchronized(String.class){ Sop(name); }
For each thread t1, t2 checks object has locked or not. All threads will enter same time because every time new object Will be created and checks the Lock is available or not.	For each thread t1, t2 checks object has locked or not. At a time single thread will Execute because “a” is mutable object , so if t2 came already Lock on “a” object	For each thread t1, t2 checks object has locked or not. At a time single thread will Execute because class level Lock , for t2 it will check Class level lock is available or Not, if not it will go to wait State.
Multiple Threads enter (all threads )	Single thread enter at a time	Single Thread enter at a time

Synchronized(this) Sop(name); }	Synchronized(d){ Sop(name); }	Synchronized(Display.class){ Sop(name); }
Single thread enter at a time, Current object instance Synchronized methods also Will not execute	Single thread enter at a time, Lock on d object, so d instance Synchronized methods also Not execute	Single Thread enter at a time, Lock on Display class, so Synchronized static methods Will not execute
If two threads t1, t2 are calling D1.wish(); - this = d1 D2.wiah(); - this = d2 T1 need lock of d1, t2 need lock Of d2 , So both threads will execute In this case.	If two threads t1, t2 are calling D1.wish(); - d = d1 D2.wiah(); - d = d2 T1 need lock of d1, t2 need lock Of d2 , So both threads will execute In this case.	If two threads t1, t2 are calling D1.wish(); - class = Display D2.wiah(); - class = Display T1 need lock of Display class , t2 need lock Of same Display class , So one thread will execute At a time.

2. Types of locks in Thread

1.       In Threads they are 2 types of locks available

a.       Object level Lock

b.      Class level Lock

2.       Lock is available for objects and class but not applicable for primitive data types.

Work	Not Work (compile error)	Not Work (compile error)
Integer i = 10 Synchronized(i) Sop(name); }	Int i=10 Synchronized(i){ Sop(name); }	Synchronized(10){ Sop(name); }