Synchronization in Java
Synchronization in java is
the capability of control the access of multiple threads to any shared
resource.
Java Synchronization is
better option where we want to allow only one thread to access the shared
resource.
Why use Synchronization
The synchronization is mainly
used to
To prevent thread
interference.
To prevent consistency
problem.
Types of Synchronization
There are two types of
synchronization
Process Synchronization
Thread Synchronization
Here, we will discuss only
thread synchronization.
Thread Synchronization
There are two types of thread
synchronization mutual exclusive and inter-thread communication.
Mutual Exclusive
1.
Synchronized
method.
2.
Synchronized
block.
3.
Static
synchronization.
Cooperation (Inter-thread communication
in java)
Mutual Exclusive
Mutual Exclusive helps keep
threads from interfering with one another while sharing data. This can be done
by three ways in java:
by synchronized method
by synchronized block
by static synchronization
Understanding the concept of Lock in Java
Synchronization is built
around an internal entity known as the lock or monitor. Every object has an
lock associated with it. By convention, a thread that needs consistent access
to an object's fields has to acquire the object's lock before accessing them,
and then release the lock when it's done with them.
From Java 5 the package
java.util.concurrent.locks contains several lock implementations.
Understanding the problem
without Synchronization
In this example, there is no
synchronization, so output is inconsistent. Let's see the example:
Class Table{
void printTable(int n){//method not synchronized
for(int i=1;i<=5;i++){
System.out.println(n*i);
try{
Thread.sleep(400);
}catch(Exception e){System.out.println(e);}
}
}
}
class MyThread1 extends Thread{
Table t;
MyThread1(Table t){
this.t=t;
}
public void run(){
t.printTable(5);
}
}
class MyThread2 extends Thread{
Table t;
MyThread2(Table t){
this.t=t;
}
public void run(){
t.printTable(100);
}
}
class Use{
public static void main(String args[]){
Table obj = new Table();//only one object
MyThread1 t1=new MyThread1(obj);
MyThread2 t2=new MyThread2(obj);
t1.start();
t2.start();
}
}
Output: 5
100
10
200
15
300
20
400
25
500
Java synchronized method
If you declare any method as
synchronized, it is known as synchronized method.
Synchronized method is used
to lock an object for any shared resource.
When a thread invokes a
synchronized method, it automatically acquires the lock for that object and
releases it when the thread completes its task.
//example of java synchronized method
Class Table{
synchronized void printTable(int n){//synchronized method
for(int i=1;i<=5;i++){
System.out.println(n*i);
try{
Thread.sleep(400);
}catch(Exception e){System.out.println(e);}
}
}
}
class MyThread1 extends Thread{
Table t;
MyThread1(Table t){
this.t=t;
}
public void run(){
t.printTable(5);
}
}
class MyThread2 extends Thread{
Table t;
MyThread2(Table t){
this.t=t;
}
public void run(){
t.printTable(100);
}
}
class Use{
public static void main(String args[]){
Table obj = new Table();//only one object
MyThread1 t1=new MyThread1(obj);
MyThread2 t2=new MyThread2(obj);
t1.start();
t2.start();
}
}
Output: 5
10
15
20
25
100
200
300
400
500
Synchronized block
Synchronized block can be
used to perform synchronization on any specific resource of the method.
Suppose you have 50 lines of
code in your method, but you want to synchronize only 5 lines, you can use
synchronized block.
If you put all the codes of
the method in the synchronized block, it will work same as the synchronized
method.
Points to remember for
Synchronized block
Synchronized block is used to
lock an object for any shared resource.
Scope of synchronized block
is smaller than the method.
Syntax to use synchronized
block
synchronized (object reference expression) {
//code block
}
Example of synchronized block
Let's see the simple example
of synchronized block.
class Table{
void printTable(int n){
synchronized(this){//synchronized block
for(int i=1;i<=5;i++){
System.out.println(n*i);
try{
Thread.sleep(400);
}catch(Exception e){System.out.println(e);}
}
}
}//end of the method
}
class MyThread1 extends Thread{
Table t;
MyThread1(Table t){
this.t=t;
}
public void run(){
t.printTable(5);
}
}
class MyThread2 extends Thread{
Table t;
MyThread2(Table t){
this.t=t;
}
public void run(){
t.printTable(100);
}
}
class Use{
public static void main(String args[]){
Table obj = new Table();//only one object
MyThread1 t1=new MyThread1(obj);
MyThread2 t2=new MyThread2(obj);
t1.start();
t2.start();
}
}
Output:5
10
15
20
25
100
200
300
400
500
Static synchronization
If you make any static method
as synchronized, the lock will be on the class not on object.
Problem without static
synchronization
Suppose there are two objects
of a shared class(e.g. Table) named object1 and object2.In case of synchronized
method and synchronized block there cannot be interference between t1 and t2 or
t3 and t4 because t1 and t2 both refers to a common object that have a single
lock.But there can be interference between t1 and t3 or t2 and t4 because t1
acquires another lock and t3 acquires another lock.I want no interference
between t1 and t3 or t2 and t4.Static synchronization solves this problem.
Example of static
synchronization
In this example we are
applying synchronized keyword on the static method to perform static
synchronization.
class Table{
synchronized static void printTable(int n){
for(int i=1;i<=10;i++){
System.out.println(n*i);
try{
Thread.sleep(400);
}catch(Exception e){}
}
}
}
class MyThread1 extends Thread{
public void run(){
Table.printTable(1);
}
}
class MyThread2 extends Thread{
public void run(){
Table.printTable(10);
}
}
class MyThread3 extends Thread{
public void run(){
Table.printTable(100);
}
}
class MyThread4 extends Thread{
public void run(){
Table.printTable(1000);
}
}
class Use{
public static void main(String t[]){
MyThread1 t1=new MyThread1();
MyThread2 t2=new MyThread2();
MyThread3 t3=new MyThread3();
MyThread4 t4=new MyThread4();
t1.start();
t2.start();
t3.start();
t4.start();
}
}
Output: 1
2
3
4
5
6
7
8
9
10
10
20
30
40
50
60
70
80
90
100
100
200
300
400
500
600
700
800
900
1000
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Synchronized block on a class lock:
The block synchronizes on the
lock of the object denoted by the reference .class name .class. A static
synchronized method printTable(int n) in class Table is equivalent to the
following declaration:
static void printTable(int n) {
synchronized (Table.class) { // Synchronized block on class A
// ...
}
}
Deadlock:
|
Deadlock can occur in a
situation when a thread is waiting for an object lock, that is acquired by
another thread and second thread is waiting for an object lock that is
acquired by first thread. Since, both threads are waiting for each other to
release the lock, the condition is called deadlock.
|
Example of Deadlock in java:
public class DeadlockExample {
public static void main(String[] args) {
final String resource1 = "ratan jaiswal";
final String resource2 = "vimal jaiswal";
// t1 tries to lock resource1 then resource2
Thread t1 = new Thread() {
public void run() {
synchronized (resource1) {
System.out.println("Thread 1: locked resource 1");
try { Thread.sleep(100);} catch (Exception e) {}
synchronized (resource2) {
System.out.println("Thread 1: locked resource 2");
}
}
}
};
// t2 tries to lock resource2 then resource1
Thread t2 = new Thread() {
public void run() {
synchronized (resource2) {
System.out.println("Thread 2: locked resource 2");
try { Thread.sleep(100);} catch (Exception e) {}
synchronized (resource1) {
System.out.println("Thread 2: locked resource 1");
}
}
}
};
t1.start();
t2.start();
}
}
Output: Thread 1: locked
resource 1
Thread 2: locked resource 2
Inter-thread communication in
Java
Inter-thread
communication or Co-operation is all about allowing synchronized
threads to communicate with each other.
Cooperation (Inter-thread
communication) is a mechanism in which a thread is paused running in its
critical section and another thread is allowed to enter (or lock) in the same
critical section to be executed.It is implemented by following methods
of Object class:
wait()
notify()
notifyAll()
1) wait() method
Causes current thread to
release the lock and wait until either another thread invokes the notify()
method or the notifyAll() method for this object, or a specified amount of time
has elapsed.
The current thread must own
this object's monitor, so it must be called from the synchronized method only
otherwise it will throw exception.
|
Method
|
Description
|
|
public final void
wait()throws InterruptedException
|
waits until object is
notified.
|
|
public final void wait(long
timeout)throws InterruptedException
|
waits for the specified
amount of time.
|
2) notify() method
Wakes up a single thread that
is waiting on this object's monitor. If any threads are waiting on this object,
one of them is chosen to be awakened. The choice is arbitrary and occurs at the
discretion of the implementation. Syntax:
public final void notify()
3) notifyAll() method
Wakes up all threads that are
waiting on this object's monitor. Syntax:
public final void notifyAll()
Understanding the process of
inter-thread communication
The point to point
explanation of the above diagram is as follows:
Threads enter to acquire
lock.
Lock is acquired by on
thread.
Now thread goes to waiting
state if you call wait() method on the object. Otherwise it releases the lock
and exits.
If you call notify() or
notifyAll() method, thread moves to the notified state (runnable state).
Now thread is available to
acquire lock.
After completion of the task,
thread releases the lock and exits the monitor state of the object.
Why wait(), notify() and
notifyAll() methods are defined in Object class not Thread class?
It is because they are
related to lock and object has a lock.
Difference between wait and
sleep?
Let's see the important
differences between wait and sleep methods.
|
wait()
|
sleep()
|
|
wait() method releases the
lock
|
sleep() method doesn't
release the lock.
|
|
is the method of Object
class
|
is the method of Thread
class
|
|
is the non-static method
|
is the static method
|
|
is the non-static method
|
is the static method
|
|
should be notified by
notify() or notifyAll() methods
|
after the specified amount
of time, sleep is completed.
|
Example of inter thread
communication in java
Let's see the simple example
of inter thread communication.
class Customer{
int amount=10000;
synchronized void withdraw(int amount){
System.out.println("going to withdraw...");
if(this.amount<amount){
System.out.println("Less balance; waiting for deposit...");
try{wait();}catch(Exception e){}
}
this.amount-=amount;
System.out.println("withdraw completed...");
}
synchronized void deposit(int amount){
System.out.println("going to deposit...");
this.amount+=amount;
System.out.println("deposit completed... ");
notify();
}
}
class Test{
public static void main(String args[]){
final Customer c=new Customer();
new Thread(){
public void run(){c.withdraw(15000);}
}.start();
new Thread(){
public void run(){c.deposit(10000);}
}.start();
}}
Output: going to withdraw...
Less balance; waiting for deposit...
going to deposit...
deposit completed...
withdraw completed
Interrupting a Thread:
|
If any thread is in
sleeping or waiting state (i.e. sleep() or wait() is invoked), calling the
interrupt() method on the thread, breaks out the sleeping or waiting state
throwing InterruptedException. If the thread is not in the sleeping or
waiting state, calling the interrupt() method performs normal behaviour and
doesn't interrupt the thread but sets the interrupt flag to true. Let's first
see the methods provided by the Thread class for thread interruption.
|
The 3 methods provided by the
Thread class for interrupting a thread
|
public void interrupt()
public static boolean
interrupted()
public boolean isInterrupted()
|
Example of interrupting a
thread that stops working
|
In this example, after
interrupting the thread, we are propagating it, so it will stop working. If
we don't want to stop the thread, we can handle it where sleep() or wait()
method is invoked. Let's first see the example where we are propagating the
exception.
|
class A extends Thread{
public void run(){
try{
Thread.sleep(1000);
System.out.println("task");
}catch(InterruptedException e){
throw new RuntimeException("Thread interrupted..."+e);
}
}
public static void main(String args[]){
A t1=new A();
t1.start();
try{
t1.interrupt();
}catch(Exception e){System.out.println("Exception handled "+e);}
}
}
Output:Exception in
thread-0
java.lang.RuntimeException: Thread
interrupted...
java.lang.InterruptedException: sleep
interrupted
at A.run(A.java:7)
Example of interrupting a
thread that doesn't stop working
|
In this example, after
interrupting the thread, we handle the exception, so it will break out the
sleeping but will not stop working.
|
class A extends Thread{
public void run(){
try{
Thread.sleep(1000);
System.out.println("task");
}catch(InterruptedException e){
System.out.println("Exception handled "+e);
}
System.out.println("thread is running...");
}
public static void main(String args[]){
A t1=new A();
t1.start();
t1.interrupt();
}
}
Output:Exception handled
java.lang.InterruptedException: sleep
interrupted
thread is running...
Example of interrupting
thread that behaves normally
|
If thread is not in
sleeping or waiting state, calling the interrupt() method sets the
interrupted flag to true that can be used to stop the thread by the java
programmer later.
|
class A extends Thread{
public void run(){
for(int i=1;i<=5;i++)
System.out.println(i);
}
public static void main(String args[]){
A t1=new A();
t1.start();
t1.interrupt();
}
}
Output:1
2
3
4
5
What about isInterrupted and interrupted
method?
|
The isInterrupted() method
returns the interrupted flag either true or false. The static interrupted()
method returns the interrupted flag afterthat it sets the flag to false if it
is true.
|
class InterruptedDemo extends Thread{
public void run(){
for(int i=1;i<=2;i++){
if(Thread.interrupted()){
System.out.println("code for interrupted thread");
}
else{
System.out.println("code for normal thread");
}
}//end of for loop
}
public static void main(String args[]){
InterruptedDemo t1=new InterruptedDemo();
InterruptedDemo t2=new InterruptedDemo();
t1.start();
t1.interrupt();
t2.start();
}
}
Output:Code for interrupted
thread
code for normal thread
code for normal thread
code for normal thread
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