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Basics: All about Java threads

Multithreading

What are Java Threads?

A thread is a:

• Facility to allow multiple activities within a single process

• Referred as lightweight process

• A thread is a series of executed statements

• Each thread has its own program counter, stack and local variables

• A thread is a nested sequence of method calls

• Its shares memory, files and per-process state

Read: Multithreading in Java

Whats the need of a thread or why we use Threads?

• To perform asynchronous or background processing

• Increases the responsiveness of GUI applications

• Take advantage of multiprocessor systems

• Simplify program logic when there are multiple independent entities

What happens when a thread is invoked?

When a thread is invoked, there will be two paths of execution. One path will execute the thread and the other path will follow the statement after the thread invocation. There will be a separate stack and memory space for each thread.

Risk Factor

• Proper co-ordination is required between threads accessing common variables [use of synchronized and volatile] for consistence view of data

• overuse of java threads can be hazardous to program’s performance and its maintainability.

Threads in Java

Java threads facility and API is deceptively simple:

Every java program creates at least one thread [ main() thread ]. Additional threads are created through the Thread constructor or by instantiating classes that extend the Thread class.

Thread creation in Java

Thread implementation in java can be achieved in two ways:

1. Extending the java.lang.Thread class

2. Implementing the java.lang.Runnable Interface

Note: The Thread and Runnable are available in the   java.lang.* package

1) By extending thread class

• The class should extend Java Thread class.

• The class should override the run() method.

• The functionality that is expected by the Thread to be executed is written in the run() method.

void start(): Creates a new thread and makes it runnable.

void run(): The new thread begins its life inside this method.

Example:

public class MyThread extends Thread {

public void run(){

System.out.println("thread is running...");

}

public static void main(String[] args) {

MyThread obj = new MyThread();

obj.start();

}

2) By Implementing Runnable interface

• The class should implement the Runnable interface

• The class should implement the run() method in the Runnable interface

• The functionality that is expected by the Thread to be executed is put in the run() method

Example:

public class MyThread implements Runnable {

public void run(){

System.out.println("thread is running..");

}

public static void main(String[] args) {

Thread t = new Thread(new MyThread());

t.start();

}

Extends Thread class vs Implements Runnable Interface?

• Extending the Thread class will make your class unable to extend other classes, because of the single inheritance feature in  JAVA. However, this will give you a simpler code structure. If you implement Runnable, you can gain better object-oriented design and consistency and also avoid the single inheritance problems.

• If you just want to achieve basic functionality of a thread you can simply implement Runnable interface and override run() method. But if you want to do something serious with thread object as it has other methods like suspend(), resume(), ..etc which are not available in Runnable interface then you may prefer to extend the Thread class.

Thread life cycle in java

Read full article at: Thread life cycle in java

Ending Thread

A Thread ends due to the following reasons:

• The thread ends when it comes when the run() method finishes its execution.

• When the thread throws an Exception or Error that is not being caught in the program.

• Java program completes or ends.

• Another thread calls stop() methods.

Synchronization of Threads

• In many cases concurrently running threads share data and two threads try to do operations on the same variables at the same time. This often results in corrupt data as two threads try to operate on the same data.

• A popular solution is to provide some kind of lock primitive.  Only one thread can acquire a particular lock at any particular time. This can be achieved by using a keyword “synchronized” .

• By using the synchronize only one thread can access the method at a time and a second call will be blocked until the first call returns or wait() is called inside the synchronized method.

Deadlock

Whenever there is multiple processes contending for exclusive access to multiple locks, there is the possibility of deadlock. A set of processes or threads is said to be deadlocked when each is waiting for an action that only one of the others can perform.

In Order to avoid deadlock, one should ensure that when you acquire multiple locks, you always acquire the locks in the same order in all threads.

Guidelines for synchronization

• Keep blocks short. Synchronized blocks should be short — as short as possible while still protecting the integrity of related data operations.

• Don’t block. Don’t ever call a method that might block, such as InputStream.read(), inside a synchronized block or method.

• Don’t invoke methods on other objects while holding a lock. This may sound extreme, but it eliminates the most common source of deadlock.

Target keywords: Java threads, javathread example, create thread java, java Runnable

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Thread life cycle in java and thread scheduling

Multithreading

In previous post I have covered almost all the terms related to Java threads. Here we will learn Thread life cycle in java, we’ll also see thread scheduling.

Recommended Reads:

Multithreading in Java

Thread Life cycle in Java

• The start method creates the system resources, necessary to run the thread, schedules the thread to run, and calls the thread’s run method.

• A thread becomes “Not Runnable” when one of these events occurs:

o If sleep method is invoked.

o The thread calls the wait method.

o The thread is blocking on I/O.

• A thread dies naturally when the run method exits.

Below diagram clearly depicts the various phases of thread life cycle in java.

[pic]

2. Thread Scheduling

• Execution of multiple threads on a single CPU, in some order, is called scheduling.

• In general, the runnable thread with the highest priority is active (running)

• Java is priority-preemptive

o If a high-priority thread wakes up, and a low-priority thread is running

o Then the high-priority thread gets to run immediately

• Allows on-demand processing

• Efficient use of CPU

2.1 Types of scheduling

• Waiting and Notifying

o Waiting [wait()] and notifying [notify(), notifyAll()] provides means of communication between threads that synchronize on the same object.

• wait(): when wait() method is invoked on an object, the thread executing that code gives up its lock on the object immediately and moves the thread to the wait state.

• notify(): This wakes up threads that called wait() on the same object and moves the thread to ready state.

• notifyAll(): This wakes up all the threads that called wait() on the same object.

• Running and Yielding

o Yield() is used to give the other threads of the same priority a chance to execute i.e. causes current running thread to move to runnable state.

• Sleeping and Waking up

o nSleep() is used to pause a thread for a specified period of time i.e. moves the current running thread to Sleep state for a specified amount of time, before moving it to runnable state. Thread.sleep(no. of milliseconds);

2.2 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) and MAX_PRIORITY (10) . The higher the integer, the higher the priority.Normally the thread priority will be 5.

2.3 isAlive() and join() methods

• isAlive() method is used to determine if a thread is still alive. It is the best way to determine if a thread has been started but has not yet completed its run() method. final boolean isAlive();

• The nonstatic join() method of class Thread lets one thread “join onto the end” of another thread. This method waits until the thread on which it is called terminates. final void join();

3. Blocking Threads

• When reading from a stream, if input is not available, the thread will block

• Thread is suspended (“blocked”) until I/O is available

• Allows other threads to automatically activate

• When I/O available, thread wakes back up again

o Becomes “runnable” i.e. gets into ready state

4. Grouping of threads

• Thread groups provide a mechanism for collecting multiple threads into a single object and manipulating those threads all at once, rather than individually.

• To put a new thread in a thread group the group must

• be explicitly specified when the thread is created

o – public Thread(ThreadGroup group, Runnable runnable)

o – public Thread(ThreadGroup group, String name)

o – public Thread(ThreadGroup group, Runnable runnable, String name)

• A thread can not be moved to a new group after the thread has been created.

• When a Java application first starts up, the Java runtime system creates a ThreadGroup named main.

• Java thread groups are implemented by the java.lang.ThreadGroup class.

Target keywords: thread life cycle in java, java threading tutorial, using threads in java, javathread run.

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Multithreading in java with examples

Multithreading

In this tutorial we will learn what is multithreaded environment? How to implement it in Java? and what are the advantages of doing so?

• The main purpose of multithreading is to provide simultaneous execution of two or more parts of a program to maximum utilize the CPU time. A multithreaded program contains two or more parts that can run concurrently. Each part of such a program called a thread. Each thread has a separate path of its execution. So this way a single program can perform two or more tasks simultaneously.

• Threads are lightweight processes; they share the same address space. In Multithreaded environment, programs make maximum use of CPU so that the idle time can be kept to minimum.

• Handling of multithreading in java is quite simple. We will learn that in this post.

• There are several thread states, A thread can be in any one of the state at a particular point of time. It can be running state. It can be ready to run state as soon as it gets CPU time. A running thread can be suspended. A suspended thread can be resumed. A thread can be blocked when waiting for a resource. At any time a thread can be terminated.

Recommended Reads:

1. Java threads

2. Thread life cycle in Java

The Thread class and Runnable Interface

A thread can be created in two ways: 1)By extending Thread class 2) By implementing Runnable interface.

Before we learn how to create the thread. Lets have a look at the methods that helps in managing the threads.

• getName(): It is used for Obtaining a thread’s name

• getPriority(): Obtain a thread’s priority

• isAlive(): Determine if a thread is still running

• join(): Wait for a thread to terminate

• run(): Entry point for the thread

• sleep(): suspend a thread for a period of time

• start(): start a thread by calling its run() method

Method 1: Thread creation by implementing Runnable Interface

• One way of creating a thread is to create a class that implements the Runnable interface. We must need to give the definition of run() method.

• This run method is the entry point for the thread and thread will be alive till run method finishes its execution.

• Once the thread is created it will start running when start() method gets called. Basically start() method calls run() method implicitly.

A Simple Example

class MultithreadingDemo implements Runnable{

public void run(){

System.out.println("My thread is in running state.");

}

public static void main(String args[]){

MultithreadingDemo obj=new MultithreadingDemo();

Thread tobj =new Thread(obj);

tobj.start();

}

}

Output:

My thread is in running state.

Example Program 2:

Observe the output of this program and try to understand what is happening in this program. If you have understood the usage of each thread method then you would not face any issue, understanding this example.

class Count implements Runnable

{

Thread mythread ;

Count()

{

mythread = new Thread(this, "my runnable thread");

System.out.println("my thread created" + mythread);

mythread.start();

}

public void run()

{

try

{

for (int i=0 ;i ................
................

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