Concurrency - SCJP Sun Certified Programmer for Java Platform SE6 - page 351

Starting a Thread Twice Is Not Valid

A new thread cannot be started twice. For example, the following code compiles fi ne:

23. File source = new File(“somedata.txt“);
24. ReadAFile target = new ReadAFile(source);
25. Thread t = new Thread(target);
26. t.start();
27. t.start();

However, the call to start on line 27 throws an IllegalThreadStateException at
runtime:

Exception in thread “main” java.lang.IllegalThreadStateException
at java.lang.Thread.start(Thread.java:595)
at ReadAFile.main(ReadAFile.java:27)

Keep an eye out for this scenario on the exam.

Runnable Threads

A runnable thread is a thread that is either executing or waiting to be scheduled. The JVM
schedules which thread to execute based on the thread priority . The priority is an integer
value, and a thread inherits its priority from the thread that started it. Priority can be
changed at any time using the setPriority method of the Thread class:

public final void setPriority(int p)

Typically you set a thread's priority to be MIN_PRIORITY , NORM_PRIORITY , or
MAX_PRIORITY , static fi elds in the Thread class. The setPriority method throws
an IllegalArgumentException if the argument is not between MIN_PRIORITY and
MAX_PRIORITY .
Java programmers should assume the scheduler uses preemptive scheduling , meaning
that if a thread is executing and another thread of a higher priority becomes runnable, it
preempts the lower-priority thread, as Figure 5.2 shows. (Preemptive scheduling is not an
absolute guarantee, so your algorithm logic should not rely on it.)

FIGURE 5.2 The JVM scheduler determines which thread to schedule on a CPU.

preempted

runnable
threads

CPU

scheduled