Java How to Program Fourth Edition - DCC

888 Multithreading Chapter 15
• A running thread enters the blocked state when the thread issues an input/output request. A
blocked thread becomes ready when the I/O it is waiting for completes. A blocked thread cannot
use a processor even if one is available.
• When a running method calls wait, the thread enters a waiting state for the particular object in
which the thread was running. A thread in the waiting state for a particular object becomes ready
on a call to notify issued by another thread associated with that object.
• Every thread in the waiting state for a given object becomes ready on a call to notifyAll by
another thread associated with that object.
• Every Java thread has a priority in the range Thread.MIN_PRIORITY (a constant of 1) and
Thread.MAX_PRIORITY (a constant of 10). By default, each thread is given priority
Thread.NORM_PRIORITY (a constant of 5).
• Some Java platforms support a concept called timeslicing and some do not. Without timeslicing,
threads of equal priority run to completion before their peers get a chance to execute. With
timeslicing, each thread receives a brief burst of processor time called a quantum during which
that thread can execute. At the completion of the quantum, even if that thread has not finished
executing, the processor is taken away from that thread and given to the next thread of equal priority,
if one is available.
• The job of the Java scheduler is to keep a highest priority thread running at all times and, if
timeslicing is available, to ensure that several equally high-priority threads each execute for a
quantum in round-robin fashion.
• A thread’s priority can be adjusted with the setPriority method. Method getPriority
returns the thread’s priority.
• A thread can call the yield method to give other threads a chance to execute.
• Every object that has synchronized methods has a monitor. The monitor lets only one thread
at a time execute a synchronized method on the object.
• A thread executing in a synchronized method may determine that it cannot proceed, so the
thread voluntarily calls wait. This removes the thread from contention for the processor and
from contention for the object.
• A thread that has called wait is awakened by a thread that calls notify. The notify acts as
a signal to the waiting thread that the condition the waiting thread has been waiting for is now (or
could be) satisfied, so it is acceptable for that thread to reenter the monitor.
• A daemon thread serves other threads. When only daemon threads remain in a program, Java will
exit. If a thread is to be a daemon, it must be set as such before its start method is called.
• To support multithreading in a class derived from some class other than Thread, implement the
Runnable interface in that class.
• Implementing the Runnable interface gives us the ability to treat the new class as a Runnable
object (just like inheriting from a class allows us to treat our subclass as an object of its superclass).
As with deriving from the Thread class, the code that controls the thread is placed in the
run method.
• A thread with a Runnable class is created by passing to the Thread class constructor a reference
to an object of the class that implements the Runnable interface. The Thread constructor
registers the run method of the Runnable object as the method to be invoked when the thread
begins execution.
• Class ThreadGroup contains the methods for creating and manipulating groups of related
threads in a program.