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Thread Life Cycle in Java
Life cycle of Thread tells the various information of thread form born to terminate. Thread life cycle contains the several stages and at a time any thread can be present in a single state.
States of Thread life cycle
- Born
- Ready
- Running
- Blocked
- Sleep
- Wait
- Dead
Ready: In this state thread is ready to execute but not running.
When the start() method calls thread enters from born to ready state.
Running: When a run() method executes highest priority ready thread assigned a processor and enter in a running state for execution.
Thread can visit more than ones in a running state.
Blocked: When a running thread has a input-output issue it enters in a blocked state from running state and when input-output issue solves it again comes in a running thread.
When sleep time expires then thread moves to ready state.
When a high priority thread execution completes it calls
Dead: A running thread is enter in a dead state when its execution completes (run() method completes) or terminate for any reason.
Thread isAlive() and join() Methods in Java
There are two ways to determine whether a thread has finished or not.
- isAlive()
- join()
isAlive(): isAlive() method is used to check whether a thread is in the running or not .
or
In a another word, this method determines thread is live or dead.
isAlive( )method returns true if the thread is still running otherwise it returns false.
join(): This method wait for the specified thread to terminate.
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import java.lang.*; public class Demo implements Runnable { public void run() { Thread t = Thread.currentThread(); // tests if this thread is alive System.out.println("thread is live = " + t.isAlive()); // tests if this thread is alive } public static void main(String args[]) throws Exception { Thread t = new Thread(new Demo()); t.start(); // this will call run() function t.join(); // waits for this thread to die System.out.println("thread is live = " + t.isAlive()); // tests if this thread is alive } } |
Result
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thread is live = true thread is live = false |
Thread suspend() and resume() method in Java
suspend() : This method
resume() : This method revives the suspended thread.
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class A extends Thread { public void run() { for (int i = 1; i <= 5; i++) { System.out.println("A" + i); } System.out.println("Exit from A"); } } class B extends Thread { public void run() { for (int i = 1; i <= 5; i++) { System.out.println("B" + i); } System.out.println("Exit from B"); } } public class TestMain { public static void main(String args[]) { A a = new A(); B b = new B(); a.start(); b.start(); a.suspend(); a.resume(); } } |
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A1 A2 A3 A4 A5 Exit from A B1 B2 B3 B4 B5 Exit from B |
Thread stop method in Java
In java, stop() method kills/terminates the currently executing thread.
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class A extends Thread { public void run() { for (int i = 1; i <= 5; i++) { if (i == 3) stop(); System.out.println("A" + i); } System.out.println("Exit from A"); } } public class TestMain { public static void main(String args[]) { A a = new A(); a.start(); } } |
Result
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A1 A2 |
Description: in the above program, when
Thread Yield method in Java
In
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class A extends Thread { public void run() { for (int i = 1; i <= 5; i++) { if (i == 2) yield(); System.out.println("A" + i); } System.out.println("Exit from A"); } } class B extends Thread { public void run() { for (int i = 1; i <= 5; i++) { System.out.println("B" + i); } System.out.println("Exit from B"); } } public class xyz { public static void main(String args[]) { A a = new A(); B b = new B(); a.start(); b.start(); } } |
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A1 A2 A3 A4 A5 Exit from A B1 B2 B3 B4 B5 Exit from B |
Producer Consumer Problem in Java
In computing,
In the
The producer’s job is to generate data, put it into the buffer and start again.
At the same time, the consumer’s job is consuming the data, one piece at a time and removing it from the buffer.
The producer–consumer problem occurs when the producer try to add data into the buffer if it’s full and the same consumer try to remove data from an empty buffer.
Solution of Producer & Consumer
The solution for the producer must go to sleep if the buffer is full and the consumer must consume data.
Once the data is consumed it removes from the buffer by consumer process. Consumer process also notifies the producer, to starts fill the buffer again.
Similarly, the consumer process must go to sleep if it finds the buffer is empty and the producer puts data into the buffer.
Ones the data is filled into buffer by producer process it also notifies the Consumer, to starts consume the data from buffer.
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class Item { int n; boolean condition = false; synchronized int get() { while(!condition) try { wait(); } catch(InterruptedException e) { System.out.println("Exception caught"); } System.out.println("Got: " + n); condition = false; notify(); return n; } synchronized void put(int x) { while(condition) try { wait(); } catch(InterruptedException e) { System.out.println("Exception caught"); } n = x; condition = true; System.out.println("Put: " + n); notify(); } } class Producer implements Runnable { Item q; Producer(Item q1) { q = q1; new Thread(this, "Producer").start(); } public void run() { int i = 0; while(true) { q.put(i++); } } } class Consumer implements Runnable { Item q; Consumer(Item q1) { q = q1; new Thread(this, "Consumer").start(); } public void run() { while(true) { q.get(); } } } public class PCDemo { public static void main(String args[]) { Item q = new Item(); new Producer(q); new Consumer(q); System.out.println("Press Control-C to stop."); } } |
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Press Control-C to stop. Put: 0 Got: 0 Put: 1 Got: 1 Put: 2 Got: 2 Put: 3 Got: 3 Put: 4 Got: 4 Put: 5 Got: 5 Put: 6 Got: 6 Put: 7 Got: 7 Put: 8 Got: 8 |
Description : In the above program there are four classes: Item, Producer, Consumer and
Item: the class is trying to synchronize.
Producer: produces the data.
Consumer: consumes the data produced by producer class.
In the above program we can see that, producer process producess the data and consumed by consuner process, then again a producer process producess a new data and consumed by consumer process and so on.
To stop the execution we have to pess ”Ctrl+C”.
Thread Synchronization in Java
When two or more threads try to access the same resource, they need somehow to ensure that the resource will be used by only one thread at a time.
The process by which this
The monitor allows one thread at a time to execute a Synchronized method. Only one thread at a time may hold a lock on a monitor.
For a thread can lock or unlock each object in Java is associated with a monitor.
Example: Write a program to synchronize a thread.
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class Demo { void write(String msg) { System.out.print("[" + msg); try { Thread.sleep(1000); } catch (InterruptedException e) { System.out.println("Interrupted"); } System.out.println("]"); } } class Caller implements Runnable { String msg; Demo obj; Thread t; public Caller(Demo obj1, String s) { // constructor obj = obj1; msg = s; t = new Thread(this); t.start(); } public void run() { obj.write(msg); } } public class SynchronizedMain { public static void main(String args[]) { Demo obj = new Demo(); Caller ob1 = new Caller(obj, "Hello"); Caller ob2 = new Caller(obj, "Hi"); Caller ob3 = new Caller(obj, "Students"); // wait for threads to end try { ob1.t.join(); ob2.t.join(); ob3.t.join(); } catch (InterruptedException e) { System.out.println("Interrupted"); } } } |
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[Hi[Hello[Students] ] ] |
In the above program, nothing exist to stop all three threads from entering write () method while other thread is using it.
These three threads are racing each other to complete the method and this condition is known as a race condition.
To fix this program, we must serialize access to write () method and restrict its access to only one thread at a time.
To do this, we simply need to write synchronized keyword before write () method definition. It is known as a synchronized method.
Synchronized Method
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// synchronize calls to call() synchronized void write(String msg) { ..... } |
above method write() can be changed as below
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synchronized void write(String msg) { System.out.print("[" + msg); try { Thread.sleep(1000); } catch (InterruptedException e) { System.out.println("Interrupted"); } System.out.println("]"); } } |
Synchronized block: this technique is also used for thread synchronization
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//synchronize calls to call() void write(String msg) { synchronized(msg) { ............... } } |
using synchronization block we can change write method as below
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synchronized void write(String msg) { System.out.print("[" + msg); try { Thread.sleep(1000); } catch (InterruptedException e) { System.out.println("Interrupted"); } System.out.println("]"); } } |
This
Output:
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[Hi] [Students] [Hello] |
Thread Priorities in Java
In java, when two or more than two thread is computing for CPU time, every thread is assigned a priority value. A highest Priority thread get preference over lower priority thread.
All Java threads have a priority in the range 1-10.
Top priority is 10, lowest priority is 1.Normal priority ie. priority by default is 5.
Thread.MIN_PRIORITY – 1 minimum thread priority
Thread.MAX_PRIORITY – 10 maximum thread priority
Thread.NORM_PRIORITY – 5 maximum thread priority
Whenever a new Java thread is created it has the same priority as the thread which created it.
Thread priority can be changed by the setPriority() method.
Example: Write a program to demonstrate the thread priority.
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class ThreadPriority extends Thread { public void run(){ for(int i=1;i<=5;i++){ try{ System.out.println("Thread "+Thread.currentThread().getName()+ " priority "+Thread.currentThread().getPriority()); Thread.sleep(1000); }catch(InterruptedException e){ System.out.println("InterruptedException "+e.getMessage()); } } } } public class ThreadPriorityMain{ public static void main(String s[]){ ThreadPriority t1=new ThreadPriority(); ThreadPriority t2=new ThreadPriority(); ThreadPriority t3=new ThreadPriority(); t1.setName("Thread 1"); t2.setName("Thread 2"); t3.setName("Thread 3"); t2.setPriority(6); t3.setPriority(Thread.MAX_PRIORITY); t1.start(); t2.start(); t3.start (); } } |
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Thread Thread 3 priority 10 Thread Thread 1 priority 5 Thread Thread 2 priority 6 Thread Thread 3 priority 10 Thread Thread 2 priority 6 Thread Thread 1 priority 5 Thread Thread 3 priority 10 Thread Thread 2 priority 6 Thread Thread 1 priority 5 Thread Thread 3 priority 10 Thread Thread 2 priority 6 Thread Thread 1 priority 5 Thread Thread 3 priority 10 Thread Thread 2 priority 6 Thread Thread 1 priority 5 |
Thread Creation in Java
We can create a thread by two ways:
- By implementing Runnable interface
- By extending Thread
By Implementing Runnable Interface
- We can create a thread by creating a class that implements the Runnable interface.
- To implement Runnable, a class only need to implement a single method called run().
- Inside run () we will define the code that constitute the new thread.
- When a new thread creates, it do not executes until we call its start () method. When start () method executes it call to run() method.
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class Demo implements Runnable { Thread t; Demo() { // creating a new, second thread t = new Thread(this, "DemoThread"); System.out.println("Child Thread= " + t); t.start(); // start a thread & call run() } public void run() { try { for (int i = 5; i > 0; i--) { System.out.println("Child Thread: " + i); Thread.sleep(500); // Let the thread sleep for a while. } } catch (InterruptedException e) { System.out.println("Child exception "); } System.out.println("exit child thread"); } } public class CreateThread { public static void main(String args[]) { new Demo(); try { for (int i = 5; i > 0; i--) { System.out.println(" Main Thread: " + i); Thread.sleep(1000); // Let the thread sleep for a while. } } catch (InterruptedException e) { System.out.println("main exception "); } System.out.println("exit main thread"); } } |
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Child Thread= Thread[DemoThread,5,main] Child Thread: 5 Main Thread: 5 Child Thread: 4 Main Thread: 4 Child Thread: 3 Child Thread: 2 Main Thread: 3 Child Thread: 1 exit child thread Main Thread: 2 Main Thread: 1 exit main thread |
Description:
Passing “this” as
In this program, main thread
This cause the child thread terminates earlier than main thread.
By Extending Thread
- To create a thread we will create a thread a class that extends Thread class.
- To extend Thread class, a class only needs to implement a single method called run (). This extending class must override the run() method
- Inside run () we will define the code that constitute the new thread.
- When a new thread creates, it do not executes until we call its start () method. When start () method executes it call to run() method.
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class Demo extends Thread { Demo() { super("DemoThread"); // creating a new, second thread System.out.println("Child Thread= " + this); start(); // start a thread } public void run() // entry point for new Thread { try { for (int i = 5; i > 0; i--) { System.out.println("Child Thread: " + i); Thread.sleep(500); // thread sleep for a while. } } catch (InterruptedException e) { System.out.println("Child exception "); } System.out.println("exit child thread"); } } public class Test { public static void main(String args[]) { new Demo(); // call constructor try { for (int i = 5; i > 0; i--) { System.out.println(" Main Thread: " + i); Thread.sleep(1000); // thread sleep for a while. } } catch (InterruptedException e) { System.out.println("main exception "); } System.out.println("exit main thread"); } } |
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Child Thread= Thread[DemoThread,5,main] Main Thread: 5 Child Thread: 5 Child Thread: 4 Main Thread: 4 Child Thread: 3 Child Thread: 2 Main Thread: 3 Child Thread: 1 exit child thread Main Thread: 2 Main Thread: 1 exit main thread |
Main Thread in Java
In Java programming, when a program begins the first thread that starts running is called main thread.
Other threads(child thread) can be spawned from this main thread.
When program terminates, the main thread must be the last thread in the program to end. When the main thread stops, the program stops running.
Main thread is created automatically, but it can be controlled by the program by using a Thread object.
By default name of main thread is “main”
By default Group name of main thread is “main”
By default priority of main thread is “5”.
Q.1 WAP to print the main thread and change the name of the main thread.
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public class MainTherad { public static void main(String a[]) { Thread obj = Thread.currentThread(); System.out.println("current = "+obj); obj.setName("NewMainThread"); System.out.println("after = "+obj); } } |
Output:
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current = Thread[main,5,main] after = Thread[NewMainThread,5,main] |
Q.2 WAP to print the 1 to 5 using thread and illustrate the use of sleep().
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public class DemoTherad { public static void main(String a[]) { Thread obj = Thread.currentThread(); try { for (int i = 5; i > 0; i--) { System.out.println(i); obj.sleep(1000); } } catch (InterruptedException e) { System.out.println("Exception " + e.getMessage()); } } } |
Output:
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5 4 3 2 1 |