15.1 Threads and Multi- threading

Understanding threads and multi-threading In general, modern computers perform one task at a time It is often desirable to perform several tasks simultaneously Some operating systems can load several programs at once, then give them shared access to the CPU This is multi-threading, and gives the appearance that the programs are running simultaneously

Thread class An instance of a program that runs simultaneously with other programs is a thread The java.lang.Thread class models a thread of execution To create a new thread of execution, extend Thread, then put your code in Thread’s run() method To use it, instantiate the class and call it’s start() method (NOT run())

Runnable interface If you do not wish to extend Thread, create a class that implements the Runnable interface Define your run() method in this class Create a new Thread, passing your Runnable class as an argument, then call the Thread’s start() method

15.3 Creating and Running Threads Create a class that extends Thread –Override the run() method with your code –Instantiate your new class –Call it’s start() method Create a class that implements Runnable –Create a run() method that holds your code –Create an instance of Thread, passing your class to the constructor –Call the Thread’s start() method

Running threads The only way to start a Thread is by calling the Thread’s start() method –Not its run() method –Not a start() method of the class implementing the Runnable interface

Thread states Born –The Thread object has been created Ready –The Thread’s start() method has been called, but it has not yet been allocated CPU time Running –The Thread’s run() method is being executed by the JVM Blocked –The Thread is waiting for an event to occur Dead –The Thread’s run method has completed

Blocked state Blocked Threads allow other Threads to access the CPU Thread.sleep() places the calling Thread into a blocked state for an amount of time Thread.join() cause a Thread to wait until the completion of the called Thread Thread.yield() allows other Threads of the same priority to access the CPU

Thread scheduling Obtain a reference to the current Thread using the Thread.currentThread() static method Use the isAlive() method to determine if a Thread is dead or not Use the setPriority() method to control the Thread’s priority

Stopping threads Older methods that stopped Threads are unreliable Use a flag in the run() method to indicate when the Thread should end

Synchronization and deadlocks Sometimes, different Threads can attempt to access the same data Two Threads may be executing the same method at the same time The contents of local variables may becoming unpredictable

Synchronization and deadlocks Blocks of statements can be declared synchronized Only one Thread at a time can access a synchronized block When two Threads try to access the same synchronized block, one must wait Every Object has a flag or lock A Thread must obtain the object’s lock before it can access the synchronized code

Synchronization and deadlocks A Thread waits until the object’s lock becomes available It is possible that two Threads could prevent the release of the lock the other is waiting for Neither Thread can progress; this is called deadlock Java does not have any mechanisms to detect or prevent deadlock

Communication between threads Threads can be used to query the status of an object Polling is an inefficient means of querying An alternative is to force the querying Thread to wait When the object’s status changes, another Thread calls notify for that object All waiting Threads then become eligible to continue