Life cycle of thread in java

• Its recommended to learn about life cycle of Thread before you start programming on Thread.
• Threads exists in different type of states.


• Thread having below states.

1. New State
2. Ready State
3. Running State
4. Dead State
5. Non Runnable States 

Life cycle of thread in java with diagram

1.New State:

• A thread has been created but not started yet. A thread will be started by calling its start() method.

2.Runnable State:

• This state is also called ready to run stage also called queue. A thread starts in runnable state by calling start() method.
• The Thread scheduler decides which thread runs and how long.

3.Running State:

• If a Thread is executing that means Thread is in Running stage.

4.Dead State:

• Once a Thread reached dead state it can not run again.

5. Non runnable States:

• A Running Thread transit to one of the non runnable states, depending upon the circumstances.
• A Thread remains non runnable until a special transition occurs.
• A Thread does not go directly to the running state from non runnable state.
• But transits first to runnable state.

1. Sleeping: The Threas sleeps for specified amount of time.
2. Blocked for I/O: The Thread waits for a blocking operation to complete.
3. Blocked for join completion: The Thread waits for completion of another Thread.
4. Waiting for notification: The Thread waits for notification another Thread.
5. Blocked for lock acquisition: The Thread waits to acquire the lock of an object.

• JVM executes the Thread based on their priority and scheduling.

Thread Scheduler:

• Schedulers in JVM implementations usually employ one of these two Strategies.
• Preemptive Scheduling
• Time Sliced or Round robin Scheduling

• Thread schedulers are implementation and platform independent, therefore how thread will scheduled is unpredictable

Thread priority:

• JVM will assign a priority for every Thread created in it.
• 0- will be the minimum priority
• 5- will be the normal priority
• 10- will be the maximum priority

• To hold all these values Thread class has below three corresponding variables
• public static final int MIN_PRIORITY
• public static final int NORM_PRIORITY
• public static final int MAX_PRIORITY 

•  A thread inherits the priority of its parent Thread. The default priority of the every thread is normal priority 5, because main thread priority is 5.
• We can set the priority of a thread by using setPriority(int priority) method
• public final void setPriority(int priority)
• public void getPriority();
• User defined thread created with default name  Thread+<index>, where index is the integer number starts from 0.
• The name of a thread can be change using setName(String name) method.
• Get by using getName() method.
• public final void setName(String name)
• public final String getName().

Life cycle of thread in java program

1. package com.instanceofjava;
2.  
3. class UserThread{
4.      
5. UserThread(){
6.         super();
7.  }
8.  
9.  UserThread(String name){
10.         UserThread(name);        
11.  }
12.  
13.  public void run()
14.  {
15.       System.out.println("thread started running..");
16.  }
17.  
18. public static void main(String [] args){ 
19. 
    
20.   UserThread thread1 = new UserThread("Thread1");
21.   UserThread thread2 = new UserThread("Thread2");        
22. 
    
23.          System.out.println("Thread 1 initial name and priority");
24.          System.out.println("name:"+thread1.getName());
25.          System.out.println("priority:"+thread1.getPriority());
26.  
27.       System.out.println("Thread 2 initial name and priority");
28.       System.out.println("name:"+thread2.getName());
29.       System.out.println("priority:"+thread2.getPriority());
30.  
31.       thread1.setPriority(6);
32.       thread2.setPriority(9);
33.  
34.       System.out.println("Thread 1 initial name and priority");
35.       System.out.println("name:"+thread1.getName());
36.       System.out.println("priority:"+thread1.getPriority())
37.  
38.       System.out.println("Thread 2 initial name and priority");
39.       System.out.println("name:"+thread2.getName());
40.       System.out.println("priority:"+thread2.getPriority());
41.  
42.       thread1.start();
43.       thread2.start();
44.  
45.       for (int i = 0; i < 5; i++) {
46.           System.out.println("main method i value:"+i);
47.     }
48. }
49. }

Output:

1. Thread 1 initial name and priority
2. name:Thread1
3. priority:5
4. Thread 2 initial name and priority
5. name:Thread2
6. priority:5
7. Thread 1 initial name and priority
8. name:Thread1
9. priority:6
10. Thread 2 initial name and priority
11. name:Thread2
12. priority:9
13. Thread1i:0
14. Thread1i:1
15. Thread1i:2
16. Thread2i:0
17. Thread1i:3
18. Thread1i:4
19. Thread2i:1
20. Thread2i:2
21. Thread2i:3
22. Thread2i:4
23. main method i value:0
24. main method i value:1
25. main method i value:2
26. main method i value:3
27. main method i value:4

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