Thread _ Notes

Threads can be created by extending Thread and overriding the
public void run() method.
q Thread objects can also be created by calling the Thread constructor that
takes a Runnable argument. The Runnable object is said to be the target of
the thread.
q You can call start() on a Thread object only once. If start() is called
more than once on a Thread object, it will throw a RuntimeException.
q It is legal to create many Thread objects using the same Runnable object as
the target.
q When a Thread object is created, it does not become a thread of execution
until its start() method is invoked. When a Thread object exists but hasn't
been started, it is in the new state and is not considered alive.
Transitioning Between Thread States (Objective 4.2)
q Once a new thread is started, it will always enter the runnable state.
q The thread scheduler can move a thread back and forth between the
runnable state and the running state.
q For a typical single-processor machine, only one thread can be running at a
time, although many threads may be in the runnable state.
q There is no guarantee that the order in which threads were started
determines the order in which they'll run.
q There's no guarantee that threads will take turns in any fair way. It's up
to the thread scheduler, as determined by the particular virtual machine
implementation. If you want a guarantee that your threads will take turns
regardless of the underlying JVM, you can use the sleep() method. This
prevents one thread from hogging the running process while another thread
starves. (In most cases, though, yield() works well enough to encourage
your threads to play together nicely.)
q A running thread may enter a blocked/waiting state by a wait(), sleep(),
or join() call.
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q A running thread may enter a blocked/waiting state because it can't acquire
the lock for a synchronized block of code.
q When the sleep or wait is over, or an object's lock becomes available, the
thread can only reenter the runnable state. It will go directly from waiting to
running (well, for all practical purposes anyway).
q A dead thread cannot be started again.
Sleep, Yield, and Join (Objective 4.2)
q Sleeping is used to delay execution for a period of time, and no locks are
released when a thread goes to sleep.
q A sleeping thread is guaranteed to sleep for at least the time specified in
the argument to the sleep() method (unless it's interrupted), but there is
no guarantee as to when the newly awakened thread will actually return to
running.
q The sleep() method is a static method that sleeps the currently executing
thread's state. One thread cannot tell another thread to sleep.
q The setPriority() method is used on Thread objects to give threads
a priority of between 1 (low) and 10 (high), although priorities are not
guaranteed, and not all JVMs recognize 10 distinct priority levels—some
levels may be treated as effectively equal.
q If not explicitly set, a thread's priority will have the same priority as the
priority of the thread that created it.
q The yield() method may cause a running thread to back out if there are
runnable threads of the same priority. There is no guarantee that this will
happen, and there is no guarantee that when the thread backs out there
will be a different thread selected to run. A thread might yield and then
immediately reenter the running state.
q The closest thing to a guarantee is that at any given time, when a thread
is running it will usually not have a lower priority than any thread in the
runnable state. If a low-priority thread is running when a high-priority thread
enters runnable, the JVM will usually preempt the running low-priority
thread and put the high-priority thread in.
q When one thread calls the join() method of another thread, the currently
running thread will wait until the thread it joins with has completed. Think
of the join() method as saying, "Hey thread, I want to join on to the end
of you. Let me know when you're done, so I can enter the runnable state."
Two-Minute Drill 731
732 Chapter 9: Threads
Concurrent Access Problems and Synchronized Threads (Obj. 4.3)
q synchronized methods prevent more than one thread from accessing an
object's critical method code simultaneously.
q You can use the synchronized keyword as a method modifier, or to start a
synchronized block of code.
q To synchronize a block of code (in other words, a scope smaller than the
whole method), you must specify an argument that is the object whose lock
you want to synchronize on.
q While only one thread can be accessing synchronized code of a particular
instance, multiple threads can still access the same object's unsynchronized code.
q When a thread goes to sleep, its locks will be unavailable to other threads.
q static methods can be synchronized, using the lock from the
java.lang.Class instance representing that class.
Communicating with Objects by Waiting and Notifying (Obj. 4.4)
q The wait() method lets a thread say, "there's nothing for me to do now, so
put me in your waiting pool and notify me when something happens that I
care about." Basically, a wait() call means "wait me in your pool," or "add
me to your waiting list."
q The notify() method is used to send a signal to one and only one of the
threads that are waiting in that same object's waiting pool.
q The notify() method can NOT specify which waiting thread to notify.
q The method notifyAll() works in the same way as notify(), only it sends
the signal to all of the threads waiting on the object.
q All three methods—wait(), notify(), and notifyAll()—must be
called from within a synchronized context! A thread invokes wait() or
notify() on a particular object, and the thread must currently hold the lock
on that object.
Deadlocked Threads (Objective 4.4)
q Deadlocking is when thread execution grinds to a halt because the code is
waiting for locks to be removed from objects.
q Deadlocking can occur when a locked object attempts to access another
locked object that is trying to access the first locked object. In other words,
both threads are waiting for each other's locks to be released; therefore, the
locks will never be released!
q Deadlocking is bad. Don't do it.