27. Daemon thread and green thread

1. What is Daemon thread.

1. The threads which are executing on background is called daemon threads
2. Examples of daemon threads     
1. Garbage collector
2. Signal dispatcher
3. Attach listener
4. Reference Handler
5. Finalizer
6. DestroyJavaVM
3. The main objective of daemon thread is to provide support for non daemon threads (main-thread).
4. If main thread runs with low memory then JVM runs garbage collector to destroy useless objects. So that number of bytes of free memory will be improved with this free memory main thread can continue its execution.
5. Daemon threads are running with low priority normally, but if any need JVM will increase priority.
6. Usually daemon thread having low priority but based on requirement, daemon threads can run with high priority also.
7. Methods related to daemon thread in Thread class
1. Public boolean    isDaemon()
2. Public   void    setDaemon(Boolean b)
8. We can check daemon nature of a thread by using isDaemon() of Thread class.
9. We can change daemon nature of a thread by using setDaemon(..).
10. Changing daemon nature is possible before starting of a thread only.
11. After starting a thread if we are trying to change daemon nature then we will De get runtime exception  “IllegalThreadStateException”.
12. By default main thread is always non daemon and for all remaining threads daemon nature will be inherited from parent to child.
13. If parent thread is daemon then automatically child thread is also daemon.
14. If parent thread is non daemon then automatically child thread is also non daemon.

2. Is possible to change daemon nature of Main thread.

1. No, it is impossible to change daemon nature of main thread because it is already started by JVM at beginning.
2. We can change daemon nature for threads created by us , before start.

Class X{   public synchronized void m1(){         sop(Thread.currentThread().isDaemon());//false         Thread.currentThread().setPriority(true);//exception   MyThread t1 = new MyThread();   Sop(t1.isDaemon());//false   T1.setDaemon(true); Sop(t1.isDaemon());//true   } }

3. Life of daemon thread.

1. Whenever last non daemon thread completes its execution, then automatically all daemon threads are terminated its execution irrespective their position.

Class MyThread extends Thread{   Public void run(){         For(int i = 0; i<10; i++){               Sop(“child thread”+i);               Thread.sleep(2000l);         }   } } Class MainDemo{   public static void main(String[] args)throws Exception{         MyThread thread1 = new MyThread(d,”Rama”);         thread1.setDaemon(true);         thread1.start();         Sop(“main thread end”);   } }

Output(based on jvm it will change)

child thread 1

main thread end

Note : In above example, main thread is non daemon and child thread is daemon hence whenever main thread terminates automatically child thread will be terminated, in this case output is end of main thread followed by child thread or end of main thread directly.

4. What is green thread.

1. Java multithreading concept is implemented by using the following 2 models
1. Green Thread model
2. Native OS model
2. The thread which is managed completely by JVM without taking underlying  OS support is called Green Thread.
3. Very few operating systems like Sun Salaries provide support for green thread.
4. Any way Green thread model is deprecated  

5. What is Native OS model in thread.

1. The thread which is managed by JVM with the help of underlying OS, is called native OS model.
2. All windows based OS provide support for native OS model.