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Explaining ThreadPoolExecutor from java.util.concurrent package
newly introduced in 1.5 version of JDK.

As of JDK 1.5, ThreadPoolExecutor has Four constructors such as
ThreadPoolExecutor with five arguments in constructor, such as
 1. Core Pool Size with int data type.
    Number of sub threads available at any given point of time
    including all idle threads.
 2. Maximum Pool Size with data type as int.
    maximum number of threads allowed in this pool executor
 3. Keep thread alive time with data type as long.
    Any thread that has count more than the core pool size of this
    thread pool, and is idle/completed execution of any task,
    to be kept alive for this (keep alive time) period, before
    terminating/not alive state.
 4. Time unit as data type TimeUnit from java.util.concurrent package
    Time format to be used for keep alive time value, such as 
    Micro seconds, milli seconds, nano seconds, seconds.
 5. queue of all threads put in it and is having data type as BlockingQueue
    from java.util.concurrent package.
    This queue can hold all the runnable objects those will be executed
    along with any of the thread present inside the pool of threads in this thread
    pool executor.
 



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Question: Is it neccessary to have all the task provided to thread pool executor be
implementing Runnable?

Answer: No, Task can be implmenting Runnable or Callable (interface from 
java.util.concurrent package).

Question: What are the differences between Task implementing Runnable and Task 
implementing Callable interface?

Answer: As we know that Runnable interface has a method run with no argument
and that doesnot throw any checked exception, but Callable interface has a method
call that throws Exception (any checked exception) and has a return type as Object
(so any Object type can be returning with appropriate type casting).
This means by using Callable call method, one can have some return comming out of
Task to the caller program, can be used to return any desired status or return value
and the throwing of checked exception makes it easier for the Task to throw application
specific checked exception back to the caller program, in case there is any exceptional
condition arises.

Question: Do you mean to say that there will be difficulty in handing checked exception
in case of using Runnable Task with ThreadPoolExecutor?

Answer: No, there is a callback handler in ThreadPoolExecutor that notifies any
checked or unchecked or runtime exception happening during execution , but this happens
only after the Task execution completes/abruptly temninates because of any exception,
and on afterExecute callback method by passing the Task and any exception 
along with it to the caller program.



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One question was keep knocking my mind as to what would be starting point for Strategizing methods for laying down high level architectural guidelines for developing a Multi-Thread based Java application. Like for example if I am given a job of identifying Framework for a web application That should adhere to principles of MVC (Model View Controller paradigm), then what Will be yours/anyone who has already worked on projects/products using Struts Framework , recommendation, I guess it will be Struts or JSF. Then for a Java command line based application with multi-Thread capability, what Would be preferred Framework to start with (if any)??? Is a question came to my mind Sometime back till JDK 1.5 version released. Now with JDK 1.5 version, the way ThreadPoolExecutor provides some higher level of Abstraction in terms of handling low level API of managing lifecycle of child threads, Callback methods in case of specific events registered for, clean shutdown of sub-threads. And many more, I must say I would always recommend using ThreadPoolExecutor over traditionally handling Thread class and Runnable interfaces to start with. ThreadPoolExecutor as the name implies, has pool of threads and whose execution is controlled by this class. There is a variant or subclass of ThreadPoolExecutor, available for scheduling capability is named as ScheduledThreadPoolExecutor. Question: What is the condition that makes a ThreadPoolExecutor rejects a new task Thread? There are two parameters core pool size, and max pool size, passed to ThreadPoolExecutor as argument, and these two parametes values contributes in deciding factor for a new Task Thread to be rejected, as explained below: 1. if there are fewer task threads in ThreadPoolExecutor than value of core pool size, then the new task thread will be accepted. 2. If a new task thread is required , whose count is more than core pool size, but less than max pool size, then this new task thread is accepted. 3. But once all the running Task Thread count is equal to Max pool size, then this new Task thread will be rejected. If anyone is having any difference of opinion, please let me know by providing your valuable comment here.

Question while using ThreadPoolExecutor, what are the ways by which all the Tasks and active threads can be shutdown in an orderlt manner? Answer There are two methods available in ThreadPoolExecutor, such as shutdown and ShutdownNow, shutdown method waits for all the previously submitted Tasks and related threads to complete their tasks and then upon completion, initiates shut down process for these Tasks, while stops accepting any new Task. shutdownNow initiates or attempts to initiate process of shut down of all actively running or executing Tasks and halts processing of any waiting Tasks in queue. This method returns list of all the waiting Tasks, those were not processed.

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