Threads in JAVA

Threads : Java
Prepared By Tuhin Kundu

What is a Thread?
facility to allow multiple activities to coexist within a
single process.
Represents a separate path of execution of a group of
statements
Java is the first language to include threading within the
language, rather than treating it as a facility of the OS
 Video Game example
1.one thread for graphics
2.one thread for user interaction
3.one thread for networking
 Server Example
1.Do various jobs
2.Handle Several Clients

Main Thread
Default Thread in any Java Program
JVM uses to execute program statements
o Program To Find the Main Thread
Class Current
{
public static void main(String args[])
{
Thread t=Thread.currentThread();
System.out.println(“Current Thread: “+t);
System.out.println(“Name is: “+t.getName());
}
}

Threads in Java
Creating threads in Java:
Extend java.lang.Thread class
run() method must be overridden (similar to main method
of sequential program)
run() is called when execution of the thread begins
A thread terminates when run() returns
start() method invokes run()
OR
Implement java.lang.Runnable interface
4

 New
The thread is in new state if you create an instance
of Thread class but before the invocation of start()
method.
 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.
 Running
The thread is in running state if the thread scheduler has
selected it.
 Non-Runnable (Blocked)
This is the state when the thread is still alive, but is currently not
eligible to run.
 Terminated
A thread is in terminated or dead state when its run() method
exits.

Thread Priority
 Each thread is assigned a default priority of
Thread.NORM_PRIORITY (constant of 5).
 You can reset the priority using setPriority(int priority).
 Some constants for priorities include:
o Thread.MIN_PRIORITY
o Thread.MAX_PRIORITY
o Thread.NORM_PRIORITY
 By default, a thread has the priority level of the thread that
created it.
7

 Thread Synchronization8
A shared resource may be corrupted if it is accessed
simultaneously by multiple threads.
Example: two unsynchronized threads accessing the same
bank account may cause conflict.
 Known as a race condition in multithreaded programs.
A thread-safe class does not cause a race condition in the
presence of multiple threads.

Synchronized
Problem : race conditions
Solution : give exclusive access to one thread at a time to code
that manipulates a shared object.
Synchronization keeps other threads waiting until the object is
available.
The synchronized keyword synchronizes the method so that only one
thread can access the method at a time.
public synchronized void xMethod() {
// method body
}
Obj t1 ( Enters the object )
t2—wait until t1 comes out
9

Deadlock :
a part of multithreading
can occur 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

Preventing Deadlock
Deadlock can be easily avoided by resource ordering.
With this technique, assign an order on all the objects
whose locks must be acquired and ensure that the
locks are acquired in that order.
Example:
Thread 1:
lock A lock B
Thread 2:
wait for A lock C(when A is locked)
Thread 3:
wait for A wait for B wait for C
11

Advantages of Threads:
easier to program
provide better performance
allow any program to perform multiple tasks at once.
multiple threads can share resources
an Internet-aware language such as Java, this is a very
important tool

References
 http://www.slideshare.net/parag/multithreading-in-java
 https://code.google.com/p/googleappengine/wiki/SdkForJavaReleaseNotes
 http://stackoverflow.com/questions/2213340/what-is-daemon-thread-in-java
 https://github.com/orientechnologies/orientdb/wiki/Java-Multi-Threading
 http://www.javatpoint.com/creating-thread
 http://www.tutorialspoint.com/java/java_multithreading.htm
 http://tutorials.jenkov.com/java-concurrency/creating-and-starting-threads.html
 http://docs.oracle.com/javase/tutorial/essential/concurrency/runthread.html
 http://www.javabeginner.com/learn-java/java-threads-tutorial
 http://www.geeksforgeeks.org/java/
 http://www.javacodegeeks.com/2014/08/java-concurrency-tutorial-visibility-between-
threads.html

Threads in JAVA

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