[Chapter 2] 2.4 Hello Web! IV: Netscape's Revenge

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| Chapter 2 A First Applet |

## 2.4 Hello Web! IV: Netscape's Revenge

We have explored quite a few features of Java with the first three versions of the `HelloWeb` applet. But until now, our applet has been rather passive; it has waited patiently for events to come its way and responded to the whims of the user. Now our applet is going to take some initiative--`HelloWeb4` will blink! The code for our latest version is shown below.

    import java.applet.Applet;
    import java.awt.*;
    import java.awt.event.*;
    public class HelloWeb4 extends Applet
        implements MouseMotionListener, ActionListener, Runnable {
        int messageX = 125, messageY = 95;
        String theMessage;
        Button theButton;
        int colorIndex = 0;
        static Color[] someColors = {
            Color.black, Color.red, Color.green, Color.blue, Color.magenta };
        Thread blinkThread;
        boolean blinkState;
        public void init() {
            theMessage = getParameter("message");
            theButton = new Button("Change Color");
            add(theButton);
            addMouseMotionListener(this);
            theButton.addActionListener(this);
        }

        public void paint( Graphics graphics ) {
            graphics.setColor( blinkState ? Color.white : currentColor() );
            graphics.drawString( theMessage, messageX, messageY );
        }
        public void mouseDragged( MouseEvent e ) {
            messageX = e.getX();
            messageY = e.getY();
            repaint();
        }
        public void mouseMoved( MouseEvent e ) { }
        public void actionPerformed( ActionEvent e ) {
            if ( e.getSource() == theButton ) {
                changeColor();
            }
        }
        synchronized private void changeColor() {
            if ( ++colorIndex == someColors.length )
                colorIndex = 0;
            theButton.setForeground( currentColor() );
            repaint();
        }
        synchronized private Color currentColor() {
            return someColors[ colorIndex ];
        }
        public void run() {
            while ( true ) {
                blinkState = !blinkState;
                repaint();
                try {
                    Thread.sleep(500);
                } catch (Exception e ) {
                    // Handle error condition here...
                }
            }
        }
        public void start() {
            if ( blinkThread == null ) {
                blinkThread = new Thread(this);
                blinkThread.start();
            }
        }
        public void stop() {
            if ( blinkThread != null ) {
                blinkThread.stop();
                blinkThread = null;
            }
        }
    }

If you create `HelloWeb4` as you have the other applets and then run it in a Java-enabled Web browser, you'll see that the text does in fact blink. My apologies if you don't like blinking text--I'm not overly fond of it either--but it does make for a simple, instructive example.

### Threads

All the changes we've made in `HelloWeb4` have to do with setting up a separate thread of execution to make the text of our applet blink. Java is a multithreaded language, which means there can be many threads running at the same time. A thread is a separate flow of control within a program. Conceptually, threads are similar to processes, except that unlike processes, multiple threads share the same address space, which means that they can share variables and methods (but they have their own local variables). Threads are also quite lightweight in comparison to processes, so it's conceivable for a single application to be running hundreds of threads concurrently.

Multithreading provides a way for an application to handle many different tasks at the same time. It's easy to imagine multiple things going on at the same time in an application like a Web browser. The user could be listening to an audio clip while scrolling an image, and in the background the browser is downloading an image. Multithreading is especially useful in GUI-based applications, as it can improve the interactive performance of these applications.

Unfortunately for us, programming with multiple threads can be quite a headache. The difficulty lies in making sure routines are implemented so they can be run by multiple concurrent threads. If a routine changes the value of a state variable, for example, then only one thread can be executing the routine at a time. Later in this section, we'll examine briefly the issue of coordinating multiple thread's access to shared data. In other languages, synchronization of threads can be an extremely complex and error-prone endeavor. You'll see that Java gives you a few simple tools that help you deal with many of these problems. Java threads can be started, stopped, suspended, and prioritized. Threads are preemptive, so a higher priority thread can interrupt a lower priority thread when vying for processor time. See Chapter 6, Threads for a complete discussion of threads.

The Java run-time system creates and manages a number of threads. I've already mentioned the AWT thread, which manages `repaint()` requests and event processing for GUI components that belong to the `java.awt` package. A Java-enabled Web browser typically has at least one separate thread of execution it uses to manage the applets it displays. Until now, our example has done most of its work from methods of the `Applet` class, which means that is has been borrowing time from these threads. Methods like `mouseDragged()` and `actionPerformed()` are invoked by the AWT thread and run on its time. Similarly, our `init()` method is called by a thread in the Web browser. This means we are somewhat limited in the amount of processing we do within these methods. If we were, for instance, to go into an endless loop in our `init()` method, our applet would never appear, as it would never finish initializing. If we want an applet to perform any extensive processing, such as animation, a lengthy calculation, or communication, we should create separate threads for these tasks.

### The Thread Class

As you might have guessed, threads are created and controlled as `Thread` objects. We have added a new instance variable, `blinkThread`, to our example to hold the `Thread` that handles our blinking activity:

    Thread blinkThread;

An instance of the `Thread` class corresponds to a single thread. It contains methods to start, control, and stop the thread's execution. Our basic plan is to create a `Thread` object to handle our blinking code. We call the `Thread`'s `start()` method to begin execution. Once the thread starts, it continues to run until we call the `Thread`'s `stop()` method to terminate it.

But Java doesn't allow pointers to methods, so how do we tell the thread which method to run? Well, the `Thread` object is rather picky; it always expects to execute a method called `run()` to perform the action of the thread. The `run()` method can, however, with a little persuasion, be located in any class we desire.

We specify the location of the `run()` method in one of two ways. First, the `Thread` class itself has a method called `run()`. One way to execute some Java code in a separate thread is to subclass `Thread` and override its `run()` method to do our bidding. In this case, we simply create an instance of this subclass and call its `start()` method.

But it's not always desirable or possible to create a subclass of `Thread` to contain our `run()` method. In this case, we need to tell the `Thread` which object contains the `run()` method it should execute. The `Thread` class has a constructor that takes an object reference as its argument. If we create a `Thread` object using this constructor and call its `start()` method, the `Thread` executes the `run()` method of the target object, rather than its own. In order to accomplish this, Java needs to have a guarantee that the object we are passing it does indeed contain a compatible `run()` method. We already know how to make such a guarantee: we use an interface. Java provides an interface named `Runnable` that must be implemented by any class that wants to become a `Thread`.

### The Runnable Interface

The second technique I described for creating a `Thread` object involved passing an object that implements the `Runnable` interface to the `Thread` constructor. The `Runnable` interface specifies that the object contains a `run()` method that takes no arguments and returns no value. This method is called automatically when the system needs to start the thread.

Sticking with our technique for implementing our applet in a single class, we have opted to add the `run()` method for `blinkThread` to our `HelloWeb4` class. This means that `HelloWeb4` needs to implement the `Runnable` interface. We indicate that the class implements the interface in our class declaration:

    public class HelloWeb4 extends Applet
        implements MouseMotionListener, ActionListener, Runnable {...}

At compile time, the Java compiler checks to make sure we abide by this statement. We have carried through by adding an appropriate `run()` method to our applet. Our `run()` method has the task of changing the color of our text a couple of times a second. It's a very short routine, but I'm going to delay looking at it until we tie up some loose ends in dealing with the `Thread` itself.

### start( ) and stop( )

Now that we know how to create a `Thread` to execute our applet's `run()` method, we need to figure out where to actually do it. The `start()` and `stop()` methods of the `Applet` class are similar to `init()`. The `start()` method is called when an applet is first displayed. If the user then leaves the Web document or scrolls the applet off the screen, the `stop()` method is invoked. If the user subsequently returns, the `start()` method is called again, and so on. Unlike `init()`, `start()` and `stop()` can be called repeatedly during the lifetime of an applet.

The `start()` and `stop()` methods of the `Applet` class have absolutely nothing to do with the `Thread` object, except that they are a good place for an applet to start and stop a thread. An applet is responsible for managing the threads that it creates. It would be considered rude for an applet to continue such tasks as animation, making noise, or performing extensive calculations long after it's no longer visible on the screen. It's common practice, therefore, to start a thread when an applet becomes visible and stop it when the applet is no longer visible.

Here's the `start()` method from `HelloWeb4`:

    public void start() {
        if ( blinkThread == null ) {
            blinkThread = new Thread(this);
            blinkThread.start();
        }
    }

The method first checks to see if there is an object assigned to `blinkThread`; recall that an uninitialized instance variable has a default value of `null`. If not, the method creates a new instance of `Thread`, passing the target object that contains the `run()` method to the constructor. Since `HelloWeb4` contains our `run()` method, we pass the special variable `this` to the constructor to let the thread know where to find the `run()` method it should run. `this` always refers to our object. Finally, after creating the new `Thread`, we call its `start()` method to begin execution.

Our `stop()` method takes the complimentary position:

    public void stop() {
        if ( blinkThread != null ) {
            blinkThread.stop();
            blinkThread = null;
        }
    }

This method checks to see if `blinkThread` is empty. If not, it calls the thread's `stop()` method to terminate its execution. By setting the value of `blinkThread` back to `null`, we have eliminated all references to the thread object we created in the `start()` method, so the garbage collector can dispose of the object.

### run( )

Our `run()` method does its job by setting the value of the variable `blinkState`. We have added `blinkState`, a `boolean` value, to represent whether we are currently blinking on or off:

    boolean blinkState;

The `setColor()` line of our `paint()` method has been modified slightly to handle blinking. The call to `setColor()` now draws the text in white when `blinkState` is `true`:

    gc.setColor( blinkState ? Color.white : currentColor() );

Here we are being somewhat terse and using the C-like ternary operator to return one of two alternate color values based on the value of `blinkState`.

Finally, we come to the `run()` method itself:

    public void run() {
        while ( true ) {
            blinkState = !blinkState;
            repaint();
            try {
                Thread.sleep(500);
            }
            catch (InterruptedException e ) {
            }
        }
    }

At its outermost level, `run()` uses an infinite `while` loop. This means the method will run continuously until the thread is terminated by a call to the controlling `Thread` object's `stop()` method.

The body of the loop does three things on each pass:

- Flips the value of `blinkState` to its opposite value using the `not` operator, `!`.

- Calls `repaint()` so that our `paint()` method can have an opportunity to redraw the text in accordance with `blinkState`.

- Uses a `try/catch` statement to trap for an error in our call to the `sleep()` method of the `Thread` class. `sleep()` is a static method of the `Thread` class. The method can be invoked from anywhere and has the effect of putting the current thread to sleep for the specified number of milliseconds. The effect here is to give us approximately two blinks per second.

### Exceptions

The `try/catch` statement in Java is used to handle special conditions called exceptions. An exception is a message that is sent, normally in response to an error, during the execution of a statement or a method. When an exceptional condition arises, an object is created that contains information about the particular problem or condition. Exceptions act somewhat like events. Java stops execution at the place where the exception occurred, and the exception object is said to be thrown by that section of code. Like events, an exception must be delivered somewhere and handled. The section of code that receives the exception object is said to catch the exception. An exception causes the execution of the instigating section of code to abruptly stop and transfers control to the code that receives the exception object.

The `try/catch` construct allows you to catch exceptions for a section of code. If an exception is caused by a statement inside of a `try` clause, Java attempts to deliver the exception to the appropriate `catch` clause. A `catch` clause looks like a method declaration with one argument and no return type. If Java finds a `catch` clause with an argument type that matches the type of the exception, that `catch` clause is invoked. A `try` clause can have multiple `catch` clauses with different argument types; Java chooses the appropriate one in a way that is analogous to the selection of overloaded methods.

If there is no `try/catch` clause surrounding the code, or a matching `catch` clause is not found, the exception is thrown up the call stack to the calling method. If the exception is not caught there, it's thrown up another level, and so on until the exception is handled. This provides a very flexible error-handling mechanism, so that exceptions in deeply nested calls can bubble up to the surface of the call stack for handling. As a programmer, you need to know what exceptions a particular statement can generate, so methods in Java are required to declare the exceptions they can throw. If a method doesn't handle an exception itself, it must specify that it can throw that exception, so that the calling method knows that it may have to handle it. See Chapter 4, The Java Language for a complete discussion of exceptions and the `try/catch` clause.

So, why do we need a `try/catch` clause around our `sleep()` call? What kind of exception can `Thread`'s `sleep()` method throw and why do we care about it, when we don't seem to check for exceptions anywhere else? Under some circumstances, `Thread`'s `sleep()` method can throw an `InterruptedException`, indicating that it was interrupted by another thread. Since the `run()` method specified in the `Runnable` interface doesn't declare it can throw an `InterruptedException`, we must catch it ourselves, or the compiler will complain. The `try/catch` statement in our example has an empty `catch` clause, which means that it handles the exception by ignoring it. In this case, our thread's functionality is so simple it doesn't matter if it's interrupted. All of the other methods we have used either handle their own exceptions or throw only general-purpose exceptions that are assumed to be possible everywhere and don't need to be explicitly declared.

### A Word About Synchronization

At any given time, there can be a number of threads running in the Java interpreter. Unless we explicitly coordinate them, these threads will be executing methods without any regard for what the other threads are doing. Problems can arise when these methods share the same data. If one method is changing the value of some variables at the same time that another method is reading these variables, it's possible that the reading thread might catch things in the middle and get some variables with old values and some with new. Depending on the application, this situation could cause a critical error.

In our `HelloWeb` examples, both our `paint()` and `mouseDrag()` methods access the `messageX` and `messageY` variables. Without knowing the implementation of our particular Java environment, we have to assume that these methods could conceivably be called by different threads and run concurrently. `paint()` could be called while `mouseDrag()` is in the midst of updating `messageX` and `messageY`. At that point, the data is in an inconsistent state and if `paint()` gets lucky, it could get the new x value with the old y value. Fortunately, in this case, we probably would not even notice if this were to happen in our application. We did, however, see another case, in our `changeColor()` and `currentColor()` methods, where there is the potential for a more serious "out of bounds" error to occur.

The `synchronized` modifier tells Java to acquire a lock for the class that contains the method before executing that method. Only one method can have the lock on a class at any given time, which means that only one synchronized method in that class can be running at a time. This allows a method to alter data and leave it in a consistent state before a concurrently running method is allowed to access it. When the method is done, it releases the lock on the class.

Unlike synchronization in other languages, the `synchronized` keyword in Java provides locking at the language level. This means there is no way that you can forget to unlock a class. Even if the method throws an exception or the thread is terminated, Java will release the lock. This feature makes programming with threads in Java much easier than in other languages. See Chapter 6, Threads for more details on coordinating threads and shared data.

Whew! Now it's time to say goodbye to `HelloWeb`. I hope that you have developed a feel for the major features of the Java language, and that this will help you as you go on to explore the details of programming with Java.

Hello Web! III: The Button Strikes! |  | Tools of the Trade