Threads Doing Several Things at Once. Threads n What are Threads? n Two Ways to Obtain a New Thread...
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Transcript of Threads Doing Several Things at Once. Threads n What are Threads? n Two Ways to Obtain a New Thread...
Threads
Doing Several Things at Once
Threads
What are Threads? Two Ways to Obtain a New Thread The Lifecycle of a Thread Four Kinds of Thread Programming
What are Threads?
A Thread is a unit of program execution that runs independently from other threads.- lightweight process
Threads behave as if they were running on different CPUs
Extend the concept of time-sharing. Garbage collectors and listeners run in
separate threads
Thread Context
Context switched when JVM switched to a different thread
Not as costly as switching processes Threads can make a program run faster Threads allow a program to do more
that one thing at a time- one processor?
• the system handles the sharing
Two Ways to Obtain a New Thread
Extend the java.lang.Thread Class
Implement the “Runnable” interface- Applets extend the class Applet by
definition. an applet must always use the second
technique; extends is taken by (J)Applet
Extension of java.lang.Thread
Create a class extending Thread Class Put the code for the task into the run
method of the new class Create an instance of that class Call the start method for that instance The start method will call the run method The thread executes until the run method
terminates.
Extending the Thread Class
Class AThread extends Thread
{
…
public void run()
{
// your code here
}
}
Running a Thread In another thread, create the thread
AThread t1 = new AThread(); Start the thread by calling its start method
t1.start(); The call to start creates and schedules the
thread to execute. The run method is called by the JVM when it is the thread’s turn to execute
See Java Documentation on Thread See:
- NameThread.java- ThreadTest.java
The Runnable Interface
public interface Runnable
{ // Must write run()
public abstract void run();
}
Implement Runnable Create a class ARunnable which implements the
“runnable” interface.public class ARunnable implements Runnable
{
public void run() { … }
}
Use that class in a call to the Thread constructorThread t = new Thread( new ARunnable() );
t.start(); // start the thread
See Java Doc on Runnable See NameUsingThread.java
Notes on Runnable
Statements within the Runnable Interface implementation of run() can’t invoke Thread methods like “sleep()”, “getName()”, etc. because no “this” object is available in Runnable.- the object isn’t the thread
Remarks
A call to currentThread() can appear anywhere in Java code. Any of the methods of that thread can be called via the thread returned in currentThread().
Runnable vs Thread: Use Runnable when only the “run()” method will be overridden. Classes should not be extended unless there is a fundamental enhancement of behavior.
Terminating Threads
A thread terminates when its run method returns – the normal way
A thread can be interrupted (never stopped) by using the thread.interrupt() method.
A thread’s run method should occasionally check for the interrupt signal
Interrupt Coding
public void run(){
try // DO NOT PUT try in a loop, but loop in try{ // do some work}catch (InterruptedException e){ // do whatever needs to be done}// clean up
}
Parameters to Threads
The run method cannot have parameters
The constructor can have parameters The Thread constructor can have
certain parameters- see API for java.lang.Thread
Thread Lifecycle
A thread is created, then “started” The thread “dies” when
- The run method terminates:• normally• through an exception or return
- A thrown exception will cause the thread to terminate but not the parent
Priorities A thread runs at the priority of its parent unless its
priority is changed. A higher priority executes first if they are both
ready to run Java threads
- can be preempted.- may or may not be time-sliced
Computationally intensive threads should “yield” periodically
Raising a thread’s priority does not affect other (heavyweight) processes, only parent, sibling, children threads (competing lightweight processes)
Four Kinds of Thread Programming
1. Unrelated Threads
2. Related Unsynchronized Threads
3. Mutually-Exclusive Threads
4. Communicating Mutually-Exclusive Threads
Thread Diagram
A ClassA Class
An ObjectAn Object
An ObjectAn Object
An Object has MethodsAn Object has Methods
An Object has MethodsAn Object has Methods
Thread runs in a methodThread runs in a method
Unrelated Threads
The simplest thread programs involves threads that do different things
Don’t interact with one another Coffee_Tea Example
- Change sleep time; run on PC and Suns to see difference
Related, Unsynchronized Threads
The problem is partitioned into subproblems A thread solves each subproblem The threads don’t interact They don’t work on shared data
- Example: testPrime.java A server connection for each socket
connection is a good example A “Work-to-order” thread is called a daemon
- always on
Mutally-Exclusive Threads
Threads that access common data concurrently need to be controlled so that the correct state of the data is preserved at all times. Such a situation occurs frequently in the real world.
The Pressure Gauge
Consider reading and setting a pressure gauge. The gauge should not be set above 20 psi. But the pressure is set by pressure setting objects, independent of one another. Each must check the gauge and if it is safe to do so, increase the pressure by a fixed amount. - Example:
• BadPressure.java
BadPressure output
>java BadPressure
Gauge reads 150, safe limit is 20
What happened?- code appeared to restrict gauge value, but didn’t
• Setting the gauge is a critical section– Mutual exclusion required
Mutual Exclusion Simultaneous reading and setting of the
pressure gauge by different threads- set followed check; all did check, THEN did
change• check-change must be atomic
one object needs to lock others out until it is finished
can be done at the class level, method level, or on a block of code
The keyword synchronized
This keyword obtains a mutex (mutual exclusion) lock for the executing thread on the named object. The code is then executed, and the lock is released. If another object already holds the lock, the thread is suspended until the lock is released. Lock-competing threads are queued.
Some considerations
Java programmers don’t need to do the low level details of creating, acquiring, and releasing locks- Unix semaphores, locks
Specify the portion of code (critical section) for mutual exclusion, and the object that must be exclusively locked
The region of code must be as small as possible
Mutual Exclusion over an Entire Class apply the keyword synchronized to a class method.
- e.g. static synchronized void RaisePressure()
Only one static synchronized method for a given class can be running at any given time in the JVM, regardless of how many objects there are of that class
the class object is used to synchronize the threads There is one lock for static synchronized methods, and a
different lock for synchronized methods. One thread could have the static lock while another thread has a method lock, while other threads could be running other unsynchronized methods- Example: SetPressure.java
Class Mutual Exclusion
From SetPressure.java
// Only one thread may be executing in herestatic synchronized void raisePressure() { if(BadPressure.pressureGauge < BadPressure.SafetyLimit - 15) { ....
>java SetPressureGauge reads 15, safe limit is 20
Mutual Exclusion over a Block
use the keyword synchronized before a block of code
Use a static object for the lock any available, convenient object will do cannot be a local object or instance
variable
Block Mutual Exclusion
class pressure extends Thread{ static Object lock = new Object(); void raisePressure() { synchronized(lock); Mutex ‘til end { if(SetPressure.pressureGauge < SetPressure.SafetyLimit - 15) { ... } ...}
Mutual Exclusion over a Method use synchronized keyword on the instance
method. guarantees that only one of the perhaps
many synchronized instance methods will be executing at any one time on a given instance of that class
equivalent to synchronized(this) over a block- Examples:
• raisePressure• Monitor in MessagePass.java
Method Synchronization
synchronized void raisePressure() { if(p.pressureGauge < p.SafetyLimit - 15) { ...same as void raisePressure() { synchronized(this) // on this object { if(p.pressureGauge < p.SafetyLimit - 15) { ... } }
Communicating Mutually Exclusive Threads
Threads that need to access common data, but also communicate with one another
the hardest kind of thread programming Producer/Consumer type of problem I produce, you consume. Don’t try to produce
until there is something to consume. Don’t produce too much, until something is consumed
Wait/Notify is the solution
Wait / Notify
Wait says “I have the lock, but there is nothing to consume. I give up the lock and wait
Notify says “I just produced something, I will place it in common area, release the lock and wait”
Producer Pseudo-code
// producer thread
enter synchronized code // get lock
while(buffer_full)
wait();
produce_items()
notify() // tell waiting consumers
leave synchronized code // release lock
consumerPseudo-code
// consumer thread
enter synchronized code // get lock
while(no_items)
wait();
consume_items()
leave synchronized code
Notes: Examples
- plum.java• inelegant dependant producer-consumer
- Operator Examples• MessagePass.java
– Operator with Monitor
• Operator.java– Operator w/o monitor (inelegant, too)
- Tally Examples• TallyWrong.java
• TallyRight.java– Note how inner classes compile
- PutGet.java • vary relative wait times and see what happens
- Deadlock.java• oh, oh; somewhat inelegant – works with zero buffer
Examples, etc;
CommunicatingThreads.java- applet with piped communication
between two threads Exercise
- Try the PutGet example with multiple producers/consumers. Change the constructors so that you know who is producing what and who is consuming what.