Multithreading in java is a process of executing multiple threads
simultaneously.
Thread is basically a lightweight sub-process, a smallest unit of
processing. Multiprocessing and multithreading, both are used to achieve
multitasking.
But we use multithreading than
multiprocessing because threads share a common memory area. They don't allocate
separate memory area so saves memory, and context-switching between the threads
takes less time than process.
Java Multithreading is mostly
used in games, animation etc.
Advantages of Java Multithreading
1) It doesn't block the user because threads are independent and
you can perform multiple operations at same time.
2) You can perform many operations
together so it saves time.
3) Threads are independent so it doesn't affect other threads if
exception occur in a single thread.
Multitasking
Multitasking is a process of
executing multiple tasks simultaneously. We use multitasking to utilize the
CPU. Multitasking can be achieved by two ways:
- Process-based
Multitasking(Multiprocessing)
- Thread-based
Multitasking(Multithreading)
1) Process-based Multitasking (Multiprocessing)
- Each
process have its own address in memory i.e. each process allocates
separate memory area.
- Process
is heavyweight.
- Cost
of communication between the process is high.
- Switching
from one process to another require some time for saving and loading
registers, memory maps, updating lists etc.
2) Thread-based Multitasking (Multithreading)
- Threads
share the same address space.
- Thread
is lightweight.
- Cost
of communication between the thread is low.
Note: At least one process is required for each thread.
What is
Thread in java
A thread is a lightweight sub
process, a smallest unit of processing. It is a separate path of execution.
Threads are independent, if there
occurs exception in one thread, it doesn't affect other threads. It shares a
common memory area.
As shown in the above figure,
thread is executed inside the process. There is context-switching between the
threads. There can be multiple processes inside the OS and one process can have
multiple threads.
Note: At a time one thread is executed only.
Note: At a time one thread is executed only.
Life cycle of a Thread (Thread States)
A thread can be in one of the five states. According
to sun, there is only 4 states in thread life cycle in java new,
runnable, non-runnable and terminated. There is no running state.
But for better understanding the threads, we are
explaining it in the 5 states.
The
life cycle of the thread in java is controlled by JVM. The java thread states
are as follows:
- New
- Runnable
- Running
- Non-Runnable
(Blocked)
- Terminated
1) New
The
thread is in new state if you create an instance of Thread class but before
the invocation of start() method.
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2) Runnable
The thread is in runnable state after invocation of start() method, but the thread scheduler has not selected it to be the running thread.
3) Running
The thread is in running state if the thread scheduler has selected it.
4) Non-Runnable (Blocked)
This is the state when the thread is still alive, but is currently not eligible to run.
5) Terminated
A thread is in terminated or dead state when its run() method exits.
How to create thread
There are two ways to create a
thread:
- By
extending Thread class
- By
implementing Runnable interface
Thread class:
Thread
class provide constructors and methods to create and perform operations on a
thread. Thread class extends Object class and implements Runnable interface.
|
Commonly used Constructors of Thread class:
o Thread()
o Thread(String name)
o Thread(Runnable r)
o Thread(Runnable
r,String name)
|
Commonly used methods of Thread class:
1. public void
run(): is used to perform action for a
thread.
2. public void
start(): starts the execution of the thread.JVM
calls the run() method on the thread.
3. public void
sleep(long miliseconds): Causes the
currently executing thread to sleep (temporarily cease execution) for the
specified number of milliseconds.
4. public void
join(): waits for a thread to die.
5. public void
join(long miliseconds): waits for a
thread to die for the specified miliseconds.
6. public int
getPriority(): returns the
priority of the thread.
7. public int
setPriority(int priority): changes the
priority of the thread.
8. public String
getName(): returns the name of the thread.
9. public void
setName(String name): changes the
name of the thread.
10. public Thread
currentThread(): returns the
reference of currently executing thread.
11. public int
getId(): returns the id of the thread.
12. public
Thread.State getState(): returns the
state of the thread.
13. public
boolean isAlive(): tests if the
thread is alive.
14. public void
yield(): causes the currently executing thread
object to temporarily pause and allow other threads to execute.
15. public void
suspend(): is used to suspend the
thread(depricated).
16. public void
resume(): is used to resume the suspended
thread(depricated).
17. public void
stop(): is used to stop the
thread(depricated).
18. public
boolean isDaemon(): tests if the
thread is a daemon thread.
19. public void
setDaemon(boolean b): marks the
thread as daemon or user thread.
20. public void
interrupt(): interrupts the thread.
21. public
boolean isInterrupted(): tests if the
thread has been interrupted.
22. public static boolean interrupted(): tests if the current thread has been interrupted.
Runnable interface:
The
Runnable interface should be implemented by any class whose instances are
intended to be executed by a thread. Runnable interface have only one method
named run().
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public void run(): is used to perform action for a thread.
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Starting a thread:
start()
method of Thread class is
used to start a newly created thread. It performs following tasks:
o A new thread starts (with
new callstack).
o The thread moves from
New state to the Runnable state.
o When the thread gets a
chance to execute, its target run() method will run.
|
1) Java Thread Example by extending Thread class
class Multi extends Thread{
public void run(){
System.out.println("thread is running...");
}
public static void main(String args[]){
Multi t1=new Multi();
t1.start();
}
}
Output:thread is running...
2) Java Thread Example by implementing Runnable interface
class Multi3 implements Runnable{
public void run(){
System.out.println("thread is running...");
}
public static void main(String args[]){
Multi3 m1=new Multi3();
Thread t1 =new Thread(m1);
t1.start();
}
}
Output:thread is running...
If you are
not extending the Thread class,your class object would not be treated as a
thread object. So you need to explicitly create Thread class object. We are
passing the object of your class that implements Runnable so that your class
run() method may execute.
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Thread Scheduler in Java
Thread scheduler in java is the part of the JVM that decides
which thread should run.
There is no guarantee that
which runnable thread will be chosen to run by the thread scheduler.
Only one thread at a time can
run in a single process.
The thread scheduler mainly
uses preemptive or time slicing scheduling to schedule the threads.
Difference between preemptive scheduling and time slicing
Under preemptive scheduling,
the highest priority task executes until it enters the waiting or dead states
or a higher priority task comes into existence. Under time slicing, a task
executes for a predefined slice of time and then reenters the pool of ready
tasks. The scheduler then determines which task should execute next, based on
priority and other factors.
Sleep method in java
The sleep() method of Thread
class is used to sleep a thread for the specified amount of time.
Syntax of sleep() method in java
The Thread class provides two
methods for sleeping a thread:
- public
static void sleep(long miliseconds)throws InterruptedException
Example of sleep method in java
class TestSleepMethod1 extends Thread{
public void run(){
for(int i=1;i<5;i++){
try{Thread.sleep(500);}catch(InterruptedException e){System.out.println(e);}
System.out.println(i);
}
}
public static void main(String args[]){
TestSleepMethod1 t1=new TestSleepMethod1();
TestSleepMethod1 t2=new TestSleepMethod1();
t1.start();
t2.start();
}
}
Output:
1
1
2
2
3
3
4
4
As you know well that at a time
only one thread is executed. If you sleep a thread for the specified time, the
thread scheduler picks up another thread and so on.
Can we start a thread twice
No. After starting a thread, it
can never be started again. If you does so, an IllegalThreadStateException is thrown. In such case, thread will
run once but for second time, it will throw exception.
Let's understand it by the
example given below:
public class TestThreadTwice1 extends Thread{
public void run(){
System.out.println("running...");
}
public static void main(String args[]){
TestThreadTwice1 t1=new TestThreadTwice1();
t1.start();
t1.start();
}
}
Output:
running
Exception in thread "main" java.lang.IllegalThreadStateException
What if we call run() method directly instead
start() method?
o Each thread starts in
a separate call stack.
o Invoking the run()
method from main thread, the run() method goes onto the current call stack
rather than at the beginning of a new call stack.
|
class TestCallRun1 extends Thread{
public void run(){
System.out.println("running...");
}
public static void main(String args[]){
TestCallRun1 t1=new TestCallRun1();
t1.run();//fine, but does not start a separate call stack
}
}
Output:running...
Problem
if you direct call run() method
class TestCallRun2 extends Thread{
public void run(){
for(int i=1;i<5;i++){
try{Thread.sleep(500);}catch(InterruptedException e)
{System.out.println(e);}
System.out.println(i);
}
}
public static void main(String args[]){
TestCallRun2 t1=new TestCallRun2();
TestCallRun2 t2=new TestCallRun2();
t1.run();
t2.run();
}
}
Output:1
2
3
4
5
1
2
3
4
5
As you can see in the above program that there is no
context-switching because here t1 and t2 will be treated as normal object not
thread object.
The join() method
The join() method waits for a
thread to die. In other words, it causes the currently running threads to stop
executing until the thread it joins with completes its task.
Syntax:
public
void join()throws InterruptedException
|
public
void join(long milliseconds)throws InterruptedException
|
Example of join() method
class TestJoinMethod1 extends Thread{
public void run(){
for(int i=1;i<=5;i++){
try{
Thread.sleep(500);
}catch(Exception e){System.out.println(e);}
System.out.println(i);
}
}
public static void main(String args[]){
TestJoinMethod1 t1=new TestJoinMethod1();
TestJoinMethod1 t2=new TestJoinMethod1();
TestJoinMethod1 t3=new TestJoinMethod1();
t1.start();
try{
t1.join();
}catch(Exception e){System.out.println(e);}
t2.start();
t3.start();
}
}
Output:1
2
3
4
5
1
1
2
2
3
3
4
4
5
5
As you can see in the
above example, when t1 completes its task then t2 and t3 starts executing.
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Example
of join(long miliseconds) method
class TestJoinMethod2 extends Thread{
public void run(){
for(int i=1;i<=5;i++){
try{
Thread.sleep(500);
}catch(Exception e){System.out.println(e);}
System.out.println(i);
}
}
public static void main(String args[]){
TestJoinMethod2 t1=new TestJoinMethod2();
TestJoinMethod2 t2=new TestJoinMethod2();
TestJoinMethod2 t3=new TestJoinMethod2();
t1.start();
try{
t1.join(1500);
}catch(Exception e){System.out.println(e);}
t2.start();
t3.start();
}
}
Output:1
2
3
1
4
1
2
5
2
3
3
4
4
5
5
In the above example, when t1 is completes its task for 1500
miliseconds(3 times) then t2 and t3 starts executing.
The
currentThread() method:
The
currentThread() method returns a reference to the currently executing thread
object.
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Syntax:
public
static Thread currentThread()
|
Example of currentThread() method
class CurrentThreadEx extends Thread{
public void run(){
System.out.println(Thread.currentThread().getName());
}
}
public static void main(String args[]){
TestJoinMethod4 t1=new TestJoinMethod4();
TestJoinMethod4 t2=new TestJoinMethod4();
t1.start();
t2.start();
}
}
Output:Thread-0
Thread-1
Naming Thread and Current Thread
Naming Thread
The Thread class provides
methods to change and get the name of a thread. By default, each thread has a
name i.e. thread-0, thread-1 and so on. By we can change the name of the thread
by using setName() method. The syntax of setName() and getName() methods are
given below:
- public String getName(): is used to return the name of a
thread.
- public void setName(String name): is used to change the name of a
thread.
Example of naming a thread
class TestMultiNaming1 extends Thread{
public void run(){
System.out.println("running...");
}
public static void main(String args[]){
TestMultiNaming1 t1=new TestMultiNaming1();
TestMultiNaming1 t2=new TestMultiNaming1();
System.out.println("Name of t1:"+t1.getName());
System.out.println("Name of t2:"+t2.getName());
t1.start();
t2.start();
t1.setName("Sindhu
");
System.out.println("After changing name of t1:"+t1.getName());
}
}
Output:
Output:Name of t1:Thread-0
Name of t2:Thread-1
id of t1:8
running...
After changeling name of t1: Sindhu
running...
Priority of a Thread (Thread Priority):
Each
thread have a priority. Priorities are represented by a number between 1 and
10. In most cases, thread scheduler schedules the threads according to their
priority (known as preemptive scheduling). But it is not guaranteed because
it depends on JVM specification that which scheduling it chooses.
|
3 constants defiend in Thread class:
1. public static int
MIN_PRIORITY
2. public static int
NORM_PRIORITY
3. public static int
MAX_PRIORITY
|
Default
priority of a thread is 5 (NORM_PRIORITY). The value of MIN_PRIORITY is 1 and
the value of MAX_PRIORITY is 10.
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Example of priority of a Thread:
class TestMultiPriority1 extends Thread{
public void run(){
System.out.println("running thread name is:"+Thread.currentThread().getName()) ;
System.out.println("running thread priority is:"+Thread.currentThread().getPriorit y());
}
public static void main(String args[]){
TestMultiPriority1 m1=new TestMultiPriority1();
TestMultiPriority1 m2=new TestMultiPriority1();
m1.setPriority(Thread.MIN_PRIORITY);
m2.setPriority(Thread.MAX_PRIORITY);
m1.start();
m2.start();
}
}
Output:
Output:running thread name is:Thread-0
running thread priority is:10
running thread name is:Thread-1
running thread priority is:1
Daemon Thread in Java
Daemon thread in java is a service provider thread that provides
services to the user thread. Its life depend on the mercy of user threads i.e.
when all the user threads dies, JVM terminates this thread automatically.
There are many java daemon
threads running automatically e.g. gc, finalizer etc.
You can see all the detail by
typing the jconsole in the command prompt. The jconsole tool provides
information about the loaded classes, memory usage, running threads etc.
Points to remember for Daemon Thread in Java
- It
provides services to user threads for background supporting tasks. It has
no role in life than to serve user threads.
- Its
life depends on user threads.
- It is a low priority thread.
Why JVM terminates the daemon thread if there is no user
thread?
The sole purpose of the daemon
thread is that it provides services to user thread for background supporting
task. If there is no user thread, why should JVM keep running this thread. That
is why JVM terminates the daemon thread if there is no user thread.
Methods for Java Daemon thread by Thread class
The java.lang.Thread class
provides two methods for java daemon thread.
S.No
|
Method
|
Description
|
1
|
public void setDaemon(boolean status)
|
is used to mark the current thread as daemon thread or user
thread.
|
2
|
public boolean isDaemon()
|
is used to check that current is daemon.
|
Simple example of Daemon thread in java
File:
MyThread.java
public class TestDaemonThread1 extends Thread{
public void run(){
if(Thread.currentThread().isDaemon()){//checking for daemon thread
System.out.println("daemon thread work");
}
else{
System.out.println("user thread work");
}
}
public static void main(String[] args){
TestDaemonThread1 t1=new TestDaemonThread1();//creating thread
TestDaemonThread1 t2=new TestDaemonThread1();
TestDaemonThread1 t3=new TestDaemonThread1();
t1.setDaemon(true);//now t1 is daemon thread
t1.start();//starting threads
t2.start();
t3.start();
}
}
Output
daemon thread work
user thread work
user thread work
Note: If
you want to make a user thread as Daemon, it must not be started otherwise it
will throw IllegalThreadStateException.
File: MyThread.java
class TestDaemonThread2 extends Thread{
public void run(){
System.out.println("Name: "+Thread.currentThread().getName());
System.out.println("Daemon: "+Thread.currentThread().isDaemon());
}
public static void main(String[] args){
TestDaemonThread2 t1=new TestDaemonThread2();
TestDaemonThread2 t2=new TestDaemonThread2();
t1.start();
t1.setDaemon(true);//will throw exception here
t2.start();
}
}
Output:exception
in thread main: java.lang.IllegalThreadStateException
How to perform single task by multiple
threads?
If you
have to perform single task by many threads, have only one run() method.For
example:
|
Program of performing single task by multiple
threads
class TestMultitasking1 extends Thread{
public void run(){
System.out.println("task one");
}
public static void main(String args[]){
TestMultitasking1 t1=new TestMultitasking1();
TestMultitasking1 t2=new TestMultitasking1();
TestMultitasking1 t3=new TestMultitasking1();
t1.start();
t2.start();
t3.start();
}
}
Output:task
one
task one
task one
Program
of performing single task by multiple threads
class TestMultitasking2 implements Runnable{
public void run(){
System.out.println("task one");
}
public static void main(String args[]){
Thread t1 =new Thread(new TestMultitasking2());
Thread t2 =new Thread(new TestMultitasking2());
t1.start();
t2.start();
}
}
Output:task
one
task one
How to perform multiple tasks by multiple threads
(multitasking in multithreading)?
If you
have to perform multiple tasks by multiple threads, have multiple run()
methods. For example:
|
Program of performing two tasks by two threads
class Simple1 extends Thread{
public void run(){
System.out.println("task one");
}
}
class Simple2 extends Thread{
public void run(){
System.out.println("task two");
}
}
class TestMultitasking3{
public static void main(String args[]){
Simple1 t1=new Simple1();
Simple2 t2=new Simple2();
t1.start();
t2.start();
}
}
Output:task one
task two
Same example as above by annonymous class that extends
Thread class:
Program of performing two tasks by two threads
class TestMultitasking4{
public static void main(String args[]){
Thread t1=new Thread(){
public void run(){
System.out.println("task one");
}
};
Thread t2=new Thread(){
public void run(){
System.out.println("task two");
}
};
t1.start();
t2.start();
}
}
Output:task one
task two
Same example as above by annonymous class that implements
Runnable interface:
Program of performing two tasks by two threads
class TestMultitasking5{
public static void main(String args[]){
Runnable r1=new Runnable(){
public void run(){
System.out.println("task one");
}
};
Runnable r2=new Runnable(){
public void run(){
System.out.println("task two");
}
};
Thread t1=new Thread(r1);
Thread t2=new Thread(r2);
t1.start();
t2.start();
}
}
Output:task one
task two
Java Garbage Collection
In java, garbage means
unreferenced objects.
Garbage Collection is process
of reclaiming the runtime unused memory automatically. In other words, it is a
way to destroy the unused objects.
To do so, we were using free()
function in C language and delete() in C++. But, in java it is performed
automatically. So, java provides better memory management.
Advantage of Garbage Collection
- It
makes java memory efficient because garbage collector removes
the unreferenced objects from heap memory.
- It is automatically done by the garbage collector(a part of JVM) so we don't need to make extra efforts.
How can an object be unreferenced?
There are many ways:
- By
nulling the reference
- By
assigning a reference to another
- By
annonymous object etc.
1) By nulling a reference:
Employee e=new Employee();
e=null;
2) By assigning a reference to another:
Employee e1=new Employee();
Employee e2=new Employee();
e1=e2;//now the first object referred by e1 is available for garbage collection
3) By annonymous object:
new Employee();
finalize() method
The finalize() method is
invoked each time before the object is garbage collected. This method can be
used to perform cleanup processing. This method is defined in Object class as:
protected void finalize(){} .
gc() method
The gc() method is used to
invoke the garbage collector to perform cleanup processing. The gc() is found
in System and Runtime classes.
public static void gc(){}
Note:
Garbage collection is performed by a daemon thread called Garbage
Collector(GC). This thread calls the finalize() method before object is garbage
collected.
Simple Example of garbage collection in java
public class TestGarbage1{
public void finalize(){System.out.println("object is garbage collected");}
public static void main(String args[]){
TestGarbage1 s1=new TestGarbage1();
TestGarbage1 s2=new TestGarbage1();
s1=null;
s2=null;
System.gc();
}
}
object is garbage collected
object is garbage collected
Note:
Neither finalization nor garbage collection is guaranteed.
Synchronization in Java
Synchronization in java is the
capability to 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
- Synchronized
method.
- Synchronized
block.
- 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
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.
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 TestSynchronization1{
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);
}
}
public class TestSynchronization2{
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
Example of synchronized method by using annonymous class
In this program, we have
created the two threads by annonymous class, so less coding is required.
//Program of synchronized method by using annonymous class
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);}
}
}
}
public class TestSynchronization3{
public static void main(String args[]){
final Table obj = new Table();//only one object
Thread t1=new Thread(){
public void run(){
obj.printTable(5);
}
};
Thread t2=new Thread(){
public void run(){
obj.printTable(100);
}
};
t1.start();
t2.start();
}
}
Output: 5
10
15
20
25
100
200
300
400
500
Synchronized block in java
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.
Program 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);
}
}
public class TestSynchronizedBlock1{
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
Same Example of synchronized block by using annonymous class:
//Program of synchronized block by using
annonymous class
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
}
public class TestSynchronizedBlock2{
public static void main(String args[]){
final Table obj = new Table();//only one object
Thread t1=new Thread(){
public void run(){
obj.printTable(5);
}
};
Thread t2=new Thread(){
public void run(){
obj.printTable(100);
}
};
t1.start();
t2.start();
}
}
Output:5
10
15
20
25
100
200
300
400
500
Deadlock in java
Deadlock in java is a part of
multithreading. 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 TestDeadlockExample1 {
public static void main(String[] args) {
final String resource1 = "Sachin";
final String resource2 = "Sindhu";
// 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.
2) notify() method
S.No
|
Method
|
Description
|
1
|
public
final void wait()throws InterruptedException
|
waits
until object is notified.
|
2
|
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
3) notifyAll() method
Wakes up all threads that are
waiting on this object's monitor. Syntax:
public final void notifyAll()
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
|
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
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