Java threads

THREADS
Mr.V.M.Prabhakaran,
Department of CSE,
KIT- Coimbatore

INTRODUCTION
• A thread is an independent path of execution
within a program.
• Many threads can run concurrently within a
program.
• Every thread in Java is created and controlled
by the java.lang.Thread class.
• A Java program can have many threads, and
these threads can run concurrently, either
asynchronously or synchronously.

• Thread is a lightweight sub
process
• 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.
• 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.

Multitasking and Multithreading
• Multitasking:
– refers to a computer's ability to perform multiple jobs
concurrently
– more than one program are running concurrently, e.g., UNIX
• Multithreading:
– A thread is a single sequence of execution within a program
– refers to multiple threads of control within a single program
– each program can run multiple threads of control within it,
e.g., Web Browser
4

Concurrency vs. Parallelism
5
CPU CPU1 CPU2

What are Threads Good For?
• To maintain responsiveness of an application during a
long running task
• To enable cancellation of separable tasks
• Some problems are intrinsically parallel
• To monitor status of some resource (e.g., DB)
• Some APIs and systems demand it (e.g., Swing)
6

Advantage 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.

Life cycle of a Thread
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

Life cycle of a Thread (Contd)
• New: A new thread begins its life cycle in the new state. It
remains in this state until the program starts the thread. It is also
referred to as a born thread.
• Runnable: After a newly born thread is started, the thread
becomes runnable. A thread in this state is considered to be
executing its task.
• Waiting: Sometimes, a thread transitions to the waiting state
while the thread waits for another thread to perform a task. A
thread transitions back to the runnable state only when another
thread signals the waiting thread to continue executing.
• Timed waiting: A runnable thread can enter the timed waiting
state for a specified interval of time. A thread in this state
transitions back to the runnable state when that time interval
expires or when the event it is waiting for occurs.
• Terminated ( Dead ): A runnable thread enters the terminated
state when it completes its task or otherwise terminates.

Daemon thread
• Daemon thread is a low priority thread (in
context of JVM) that runs in background to
perform tasks such as garbage collection.
• JVM terminates itself when all user threads
(non-daemon threads) finish their execution,
JVM does not care whether Daemon thread is
running or not,

Thread Priority
• When a Java thread is created, it inherits its
priority from the thread that created it.
• You can modify a thread’s priority at any time
after its creation using the setPriority method.
• Thread priorities are integers ranging between
• MIN_PRIORITY (1)
• NORM_PRIORITY (5)
• MAX_PRIORITY (10)

Creating Threads
• There are two ways to create our own Thread
object
– By extending Thread class
– By implementing Runnable interface.
• In both cases the run() method should be
implemented

Commonly used Constructors of
Thread class
• Thread()
• Thread(String name)
• Thread(Runnable r)
• Thread(Runnable r,String name)

Extending Thread
SYNTAX
class Mythread extends Thread
{
public void run(){
--------------
--------------
--------------
--------------
}
}
PROGRAM
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...

Thread Methods
void start()
– Creates a new thread and makes it runnable
– This method can be called only once
void run()
– The new thread begins its life inside this method
void stop() (deprecated)
– The thread is being terminated

Thread Methods
void yield()
– Causes the currently executing thread object to
temporarily pause and allow other threads to
execute
– Allow only threads of the same priority to run
void sleep(int m) or sleep(int m, int n)
– The thread sleeps for m milliseconds, plus n
nanoseconds

Thread Methods
• public void start()
• public void run()
• public final void
setName(String name)
setPriority(int priority)
setDaemon(boolean on)
• public final void join(long
millisec)
• public void interrupt()
• public final boolean
isAlive()
• public static void yield()
• public static void sleep(long
millisec)
• public static boolean
holdsLock(Object x)
• public static Thread
currentThread()
• public static void
dumpStack()

Implementing Runnable
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...

Java threads

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