Java ThreadPoolExecutor, API, Java Thread example, Thread Pool Executor

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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.
 











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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