2. Thread Attributes

2. Thread Attributes
	Chapter 7. Threads

2.1. Thread class

It implements a system independent definition of Java threads.

2.2. Thread Body: The run() method

After a thread has been created and initialized, the runtime system calls its run() method.

2.2.1. Customizing the run() method

Subclassing the Thread class defined in the java.lang package and override the run() method.
Providing a class that implements the Runnable interface and giving to a new thread a handle to an instance of it.

When to use each?

If your class must subclass some other class, you should use Runnable as described in option #2.

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2.3. Thread States

Figure 7.1. Thread States

2.3.1. New Thread

Thread myThread = new MyThreadClass();

Definition: It is merely an empty Thread object. No system resources have been allocated for it yet.

2.3.2. Runnable

Thread myThread = new MyThreadClass();
myThread.start();

Definition: It is when the start() method creates the system resources necessary to run the thread and schedules the thread to run.

	Note
	The thread might not actually be running when it is in this state.

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2.3.3. Not Runnable

Definition: A thread enters the "Not Runnable" state when one of these four events occurs:

Someone invokes its sleep() method.
Someone invokes its suspend() method.
The thread uses its wait() method to wait on a condition variable.
The thread is blocking on I/O.

	Note
	Even if the processor becomes available a "Not Runnable" thread does not run.

Leaving the "Not Runnable" state:

If a thread has been put to sleep, then the specified number of milliseconds must elapse.
If a thread has been suspended, then someone must call its resume() method.
If a thread is waiting on a condition variable, whatever object owns the variable must relinquish it by calling either notify() or notifyAll().
If a thread is blocked on I/O, then the I/O must complete.

2.3.4. Dead

From natural causes:

A thread dies naturally when its run() method exits normally.

Being killed:

A thread is killed when its stop() method is called.

Thread myThread = new MyThreadClass();
myThread.start();
try {
    Thread.currentThread().sleep(10000);
} catch (InterruptedException e){
}
myThread.stop();

The stop() method throws a ThreadDeath object at the thread to kill it.

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2.4. Thread Priority

Scheduling: Execution of multiple threads on a single CPU, in some order.

Priority: The Java supports a very simple, deterministic scheduling algorithm known as fixed priority scheduling.

When a Java thread is created, it inherits its priority from the thread that created it.

Priority Modification: setPriority() method. Thread priorities are integers ranging between MIN_PRIORITY and MAX_PRIORITY.

Preemptive Scheduling: If a thread with a higher priority than the currently executing thread needs to execute, the higher priority thread is immediately scheduled.

Equal Priority Scheduling:

The scheduler chooses in a round-robin fashion. The chosen thread will run until one of the following conditions is true:

a higher priority thread becomes "Runnable"
it yields, or its run() method exits
on systems that support time-slicing, its time allotment has expired

Then the second thread is given a chance to run, and so on, until the interpreter exits.

	Note
	Use priority only to affect scheduling policy for efficiency purposes. Do not rely on thread priority for algorithm correctness.

Time-Slicing:

Time-sliced system divides the CPU into time slots and iteratively gives each of the equal-and-highest priority threads a time slot in which to run.

	Note
	The Java runtime does not implement (and therefore does not guarantee) time-slicing.

2.5. Daemon Threads

Daemon threads are those that provide a service for other threads in the system.
When the only remaining threads in a process are daemon threads, the interpreter exits.

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1. What is a Thread?		3. Thread Group