Multi-threaded applications SE-28111

SE-2811 Dr. Mark L. Hornick 2 What SE1011 students are told… When the main() method is called, the instructions within the method begin to execute in sequence The program terminates when the main() method finishes executing Is this really true? Sometimes it is….

SE-2811 Dr. Mark L. Hornick 3 The ugly truth… When the main() method is called, the instructions within the method begin to execute in sequence on the primary thread The program terminates when the primary thread, and any additional threads, finish executing

What’s a Thread? First, let’s define Process: A Process is most easily understood as a program or application running on your PC A process generally has a complete, private set of basic run-time resources, in particular: Its own memory space Execution priority A list of Threads that execute within it A set of credentials with which to execute (usually yours); this provides authorization to access various resources, such as files SE-28114

By default, a Process creates and executes a single primary Thread BUT: A Process can create and execute more than one Thread The JVM works with the OS to create Processes and Threads The underlying OS provides the essential multiprocessing support SE-28115

Modern operating systems are all capable of running multiple Processes simultaneously (On single-CPU PC’s) each Process runs individually for a discrete time period while one Process runs, other Processes sleep The Process currently executing changes very rapidly - every few milliseconds Operating systems use a Scheduler (basically, an ISR that executes on a timer interrupt) to distribute CPU time among Processes The net effect is that you (the user) observe all processes running simultaneously and continuously SE-28116

When you run a Java application, the JVM creates a Process and a Primary Thread The Primary Thread begins executing the main() method in the main class Note: other java programs, like applets, begin execution with an init() method If no other Threads are created, the Process terminates when the Primary Thread terminates That is, when there are no more instructions to execute on that Thread SE-28117

Threads wind their way through the code until they run out of instructions to execute public class App{ public static void main(String[] args) { App me = new App(); me.method_A(); } private void method_A() { // more code here method_B(); return; } private void method_B() { return; } private void method_C() { // more code here } SE-28118

Where do other Threads come from? 1. You implicitly create additional Threads when you write a Swing-based application Java applications that create and display windows cause Swing to create additional threads 2. You implicitly create additional Threads when you use various Java utility classes Using the Timer class causes a Thread to be created 3. You can explicitly create additional Threads and control their execution SE-28119

You already know how to create a multi-threaded app using Swing 1. Create a JFrame window containing JButtons, JTextField, etc. 2. Connect the JButtons etc to an ActionListener 3. Make the window visible Once the window is visible, a second Thread is created All calls to actionPerformed() occur on the second Thread The Event-Dispatching Thread SE-2811 Dr. Mark L. Hornick 10

Using a Timer is fairly straighforward: Timer timer = new Timer(timeoutPeriod, eventHandler); timer.start(); The eventHandler argument to the constructor is a reference to a class that implements Timer ActionListener That is, eventHandler contains an actionPerformed() method. This is similar to how Swing events are handled Whenever the Timer generates a timeout event, the JVM invokes actionPerformed() on another thread JVM uses the Event Dispatch thread when available; otherwise a “worker” thread is created SE-2811 Dr. Mark L. Hornick 11

Explicitly creating additional Threads is pretty easy: Thread t = new Thread( r ); t.start(); The r argument to the Thread constructor is a reference to a class that implements the Runnable interface Runnable declares a single method: public void run() When the Thread’s start() method is called, the instructions in the run() method begin executing on the new thread. The start() method returns essentially immediately; it does not wait for the started thread to finish execution. SE

The main class may implement the Runnable interface itself: public class App implements Runnable{ public static void main(String[] args) { App me = new App(); me.method_A(); } private void method_A() { // more code here method_B(); return; } private void method_B() { Thread t = new Thread(this); // App is runnable! t.start(); // start executing the run() method return; } public void run() { // more code here } SE

Both threads execute simultaneously and independently after the secondary thread is started public class App implements Runnable{ public static void main(String[] args) { App me = new App() me.method_A(); } private void method_A() { // more code here method_B(); return } private void method_B() { Thread t = new Thread(this); t.start();// execute run() on new thread return; } public void run() { // more code here } SE

The secondary thread may execute a method defined in another class that implements Runnable public class App { public static void main(String[] args) { App me = new App(); me.method_A(); } private void method_A() { ThreadRunner tr = new ThreadRunner(); Thread t = new Thread(tr); t.start(); return; } private class ThreadRunner implements Runnable { // inner class public void run() { // more code here } SE

An anonymous inner class is another typical approach… public class App { public static void main(String[] args) { App me = new App(); me.method_A(); } private void method_A() { Thread t = new Thread( new Runnable() { public void run() { // more code here } } ); t.start(); return; } SE

An application may be designed to execute the same instructions on more than one thread at the same time public class App implements Runnable { public static void main(String[] args) { App me = new App() me.method_A(); } private void method_A() { ThreadRunner tr = new ThreadRunner(); Thread t = new Thread(tr); t.start(); // execute run() on new secondary thread method_C(); // execute method_C on the primary thread } private void method_C() { // more code here } public void run() { // some other instructions here method_C(); // execute method_C on the secondary thread method_C(); // execute method_C on the secondary thread } SE

Question: Is it a good idea to let two (or more) threads execute the same code at the same time? SE

Fortunately, Java supports several mechanisms for synchronizing the execution of multiple threads SE Keeping code thread-safe

The Thread class’s join() method is one way of synchronizing two threads: Thread t = new Thread(cref); t.start(); t.join(); // wait for 2 nd thread to finish… method_C(); // …before executing next inst. The second Thread starts executing the instructions in the run() method. The current thread (the launcher of the second thread) waits until the second thread finishes SE

If a method is declared to be synchronized, it will only be run on a single thread at a time public class App implements Runnable { public static void main(String[] args) { App me = new App() me.method_A(); } private void method_A() { ThreadRunner tr = new ThreadRunner(); Thread t = new Thread(tr); t.start(); method_C(); // run method_C on the primary thread } private synchronized void method_C() { // More code here } public void run() { // some other instructions here method_C(); // run method_C on the secondary thread method_C(); // run method_C on the secondary thread } SE

Once a thread enters a synchronized method, no other threads can enter until the first thread completes execution of the method, and exits the method. private synchronized void method_C() { } SE Thread x Thread yThread z When Thread x leaves the method, the Scheduler arbitrarily allows one of the other threads to enter the method. When the second thread exits, the Scheduler allows another waiting thread to enter. If all threads get to the method at the same time, the thread that gets to enter the method first is determined arbitrarily by the Scheduler.

Is synchronized the solution to everything? Can you think of any disadvantage to making a method synchronized? SE

If only a few statements within a method need to be guarded against simultaneous execution, use a synchronized block instead of making the entire method synchronized. private void method_C() { synchronizedsync_object synchronized( ) { } } SE Thread x Thread yThread z When Thread x leaves the block, the Scheduler arbitrarily allows one of the other threads to enter.

The synchronizing object can be any object Java’s Object class incorporates the concept of something called a Monitor Monitors are used to guard the gates of synchronized blocks Monitors only become active within a synchronized block SE

Since every class derives from Object, the class containing a synchronized block can act as the Monitor for the block: private void method_C() { synchronized( this ) { // gate down }// gate up }// gate up } SE Thread x Thread yThread z

Or any generic Object can act as a Monitor private Object guard = new Object(); private Object guard = new Object(); private void method_C() { guard synchronized( guard ) { // gate down } // gate up } SE Thread x Thread yThread z

Consider the following code. Suppose all threads reach the for() loop simultaneously. How do the threads compete to run the for() loop? private Object guard = new Object(); private void method_C() { for( int i=0; i<100; i++ ) { synchronized( guard ) { // gate down } // gate up } // gate up } // end for } SE Thread x Thread yThread z

After each thread executes the synchronized section, it can notifiy the Monitor that another thread can be allowed to enter the synchronized block as soon as it relinquishes ownership of the synchronized section by entering a wait (or exiting the synchronized section) private Object guard = new Object(); private void method_C() { for( int i=0; i<100; i++ ) { synchronized( guard ) { // gate down guard.notify(); // signal waiting threads guard.wait(); // wait for other threads guard.wait(); // wait for other threads } } // end for } SE Thread x Thread yThread z