<Java><Multi-thread>

Overview

• Notify () VS Notifyall ()

From a basic example

• One of the most classic producer consumer models:

• public synchronized void put (String value) throws interruptedexception{WH            Ile (buffer.size () = = capacity) {System.out.println (Thread.CurrentThread (). GetName () + "is waiting ...");        Wait ();        } buffer.add (value);        System.out.println (Thread.CurrentThread (). GetName () + "has produced" + value);    Notify (); Synchronized String get () throws interruptedexception{while (buffer.size () = = 0) {SYSTEM.O            Ut.println (Thread.CurrentThread (). GetName () + "is waiting ...");        Wait ();        } String val = buffer.remove (0);        Notify ();        System.out.println (Thread.CurrentThread (). GetName () + "has consumed" + val);    return Val; }

• First, the first question is, why is the get and put internally used while loops ? Answer: We need a while loop in case we get this situation:
  • C1 into the critical section, found that size is empty, so wait;
  • P1 enters the critical zone, adding buffer, and notify;
  • At the same time, C2 is also waiting to enter the critical zone (block blocking state);
  • C1 was notified, but C1 not preemption to lock, because C2 entered the critical area, consumption of buffer, then buffer is empty;
  • After that, C1 has a lock, but at this point the buffer is empty, so there must be a while loop to judge again.
• Then, why do I need Notifyall ()?
  • We all know that notify () is informing a thread, while Notifyall () is informing all threads. What's the difference, specifically?
  • Consider a queue with buffer 1 (for simplicity):

  • STEP 1:
    -P1 puts 1 char into the buffer

    STEP 2:
    -P2 attempts put -checks wait Loop-already a char-waits "P2 in Wait"

    STEP 3:
    -P3 attempts put -checks wait Loop-already a char-waits "P2, P3 in Wait"

    STEP 4:
    -C1 attempts to get 1 char
    -C2 attempts to get 1 char-blocks in entry to get the method "C2 block"
    -C3 attempts to get 1 char-blocks in entry to get the method "C2, C3 block"

    STEP 5:
    -C1 is executing the get method-gets the char, calls notify , exits method
    -The notify wakes up P2
    -but, C2 enters method before P2 can (P2 must reacquire the lock), so P2 blocks on entry to the put method "P3 Wait" "P2, C3 Blocking
    -C2 checks wait loop, no more chars in buffer, so Waits "P3, C2 Waiting" "P2, C3 block"
    -C3 enters method after C2, but before P2, checks wait loop, no more chars in buffer, so Waits "C3, C2, P3 Wait" "P2 block"

    STEP 6:
    -Now:there is P3, C2, and C3 waiting!
    -finally P2 acquires the lock, puts a char in the buffer, calls notify, exits method "P2 finally grabbed the lock"

    STEP 7:
    -P2 ' s notification wakes P3 (remember any thread can be woken) "P3 was notified"
    -P3 checks the wait loop condition, there is already a char in the buffer, so waits. "P3, C2, C3 Wait" "Block queue is empty"
    -NO more THREADS to call NOTIFY and three THREADS permanently suspended!

  • The solution to this deadlock problem is to Replace all notify () with Notifyall (). This makes it possible to notifyall all waiting threads into a blocking state every time.
• Above, is actually caused the difference between the two states of blocked and wait.

FYI

• StackOverflow

<Java><Multi-thread>