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1. Introduction

In this tutorial, we'll see several methods that we can use to download a file.

We'll cover examples ranging from the basic usage of Java IO to the NIO package, and some common libraries like Async Http Client and Apache Commons IO.

Finally, we'll talk about how we can resume a download if our connection fails before the whole file is read.

2. Using Java IO

The most basic API we can use to download a file is Java IO. We can use the URL class to open a connection to the file we want to download. To effectively read the file, we'll use the openStream() method to obtain an InputStream:

When reading from an InputStream, it's recommended to wrap it in a BufferedInputStream to increase the performance.

The performance increase comes from buffering. When reading one byte at a time using the read() method, each method call implies a system call to the underlying file system. When the JVM invokes the read() system call, the program execution context switches from user mode to kernel mode and back.

This context switch is expensive from a performance perspective. When we read a large number of bytes, the application performance will be poor, due to a large number of context switches involved.

For writing the bytes read from the URL to our local file, we'll use the write() method from the FileOutputStream class:

When using a BufferedInputStream, the read() method will read as many bytes as we set for the buffer size. In our example, we're already doing this by reading blocks of 1024 bytes at a time, so BufferedInputStream isn't necessary.

The example above is very verbose, but luckily, as of Java 7, we have the Files class which contains helper methods for handling IO operations. We can use the Files.copy() method to read all the bytes from an InputStream and copy them to a local file:

Our code works well but can be improved. Its main drawback is the fact that the bytes are buffered into memory.

Fortunately, Java offers us the NIO package that has methods to transfer bytes directly between 2 Channels without buffering.

We'll go into details in the next section.

3. Using NIO

The Java NIO package offers the possibility to transfer bytes between 2 Channels without buffering them into the application memory.

To read the file from our URL, we'll create a new ReadableByteChannel from the URL stream:

The bytes read from the ReadableByteChannel will be transferred to a FileChannel corresponding to the file that will be downloaded:

We'll use the transferFrom() method from the ReadableByteChannel class to download the bytes from the given URL to our FileChannel:

The transferTo() and transferFrom() methods are more efficient than simply reading from a stream using a buffer. Depending on the underlying operating system, the data can be transferred directly from the filesystem cache to our file without copying any bytes into the application memory.

On Linux and UNIX systems, these methods use the zero-copy technique that reduces the number of context switches between the kernel mode and user mode.

4. Using Libraries

We've seen in the examples above how we can download content from a URL just by using the Java core functionality. We also can leverage the functionality of existing libraries to ease our work, when performance tweaks aren't needed.

For example, in a real-world scenario, we'd need our download code to be asynchronous.

We could wrap all the logic into a Callable, or we could use an existing library for this.

4.1. Async HTTP Client

AsyncHttpClient is a popular library for executing asynchronous HTTP requests using the Netty framework. We can use it to execute a GET request to the file URL and get the file content.

First, we need to create an HTTP client:

The downloaded content will be placed into a FileOutputStream:

Next, we create an HTTP GET request and register an AsyncCompletionHandler handler to process the downloaded content:

Notice that we've overridden the onBodyPartReceived() method. The default implementation accumulates the HTTP chunks received into an ArrayList. This could lead to high memory consumption, or an OutOfMemory exception when trying to download a large file.

Instead of accumulating each HttpResponseBodyPart into memory, we use a FileChannel to write the bytes to our local file directly. We'll use the getBodyByteBuffer() method to access the body part content through a ByteBuffer.

ByteBuffers have the advantage that the memory is allocated outside of the JVM heap, so it doesn't affect out applications memory.

4.2. Apache Commons IO

Another highly used library for IO operation is Apache Commons IO. We can see from the Javadoc that there's a utility class named FileUtils that is used for general file manipulation tasks.

To download a file from a URL, we can use this one-liner:

From a performance standpoint, this code is the same as the one we've exemplified in section 2.

The underlying code uses the same concepts of reading in a loop some bytes from an InputStream and writing them to an OutputStream.

One difference is the fact that here the URLConnection class is used to control the connection timeouts so that the download doesn't block for a large amount of time:

5. Resumable Download

Considering internet connections fail from time to time, it's useful for us to be able to resume a download, instead of downloading the file again from byte zero.

Let's rewrite the first example from earlier, to add this functionality.

The first thing we should know is that we can read the size of a file from a given URL without actually downloading it by using the HTTP HEAD method:

Now that we have the total content size of the file, we can check whether our file is partially downloaded. If so, we'll resume the download from the last byte recorded on disk:

What happens here is that we've configured the URLConnection to request the file bytes in a specific range. The range will start from the last downloaded byte and will end at the byte corresponding to the size of the remote file.

Another common way to use the Range header is for downloading a file in chunks by setting different byte ranges. For example, to download 2 KB file, we can use the range 0 – 1024 and 1024 – 2048.

Another subtle difference from the code at section 2. is that the FileOutputStream is opened with the append parameter set to true:

After we've made this change the rest of the code is identical to the one we've seen in section 2.

6. Conclusion

We've seen in this article several ways in which we can download a file from a URL in Java.

The most common implementation is the one in which we buffer the bytes when performing the read/write operations. This implementation is safe to use even for large files because we don't load the whole file into memory.

We've also seen how we can implement a zero-copy download using Java NIO Channels. This is useful because it minimized the number of context switches done when reading and writing bytes and by using direct buffers, the bytes are not loaded into the application memory.

Also, because usually downloading a file is done over HTTP, we've shown how we can achieve this using the AsyncHttpClient library.

The source code for the article is available over on GitHub.

This chapter shows you how you can paint your own custom drawing (such as graphs, charts, drawings and, in particular, computer game avatars) because you cannot find standard GUI components that meets your requirements. I shall stress that you should try to reuse the standard GUI components as far as possible and leave custom graphics as the last resort. Nonetheless, custom graphics is crucial in game programming.

Read 'Swing Tutorial' trail 'Performing Custom Painting'.

The java.awt.Graphics Class: Graphics Context and Custom Painting

A graphics context provides the capabilities of drawing on the screen. The graphics context maintains states such as the color and font used in drawing, as well as interacting with the underlying operating system to perform the drawing. In Java, custom painting is done via the java.awt.Graphics class, which manages a graphics context, and provides a set of device-independent methods for drawing texts, figures and images on the screen on different platforms.

The java.awt.Graphics is an abstract class, as the actual act of drawing is system-dependent and device-dependent. Each operating platform will provide a subclass of Graphics to perform the actual drawing under the platform, but conform to the specification defined in Graphics.

Graphics Class' Drawing Methods

The Graphics class provides methods for drawing three types of graphical objects:

1. Text strings: via the drawString() method. Take note that System.out.println() prints to the system console, not to the graphics screen.
2. Vector-graphic primitives and shapes: via methods drawXxx() and fillXxx(), where Xxx could be Line, Rect, Oval, Arc, PolyLine, RoundRect, or 3DRect.
3. Bitmap images: via the drawImage() method.

These drawing methods is illustrated below. The drawXxx() methods draw the outlines; while fillXxx() methods fill the internal. Shapes with negative width and height will not be painted. The drawImage() will be discussed later.

Graphics Class' Methods for Maintaining the Graphics Context

The graphic context maintains states (or attributes) such as the current painting color, the current font for drawing text strings, and the current painting rectangular area (called clip). You can use the methods getColor(), setColor(), getFont(), setFont(), getClipBounds(), setClip() to get or set the color, font, and clip area. Any painting outside the clip area is ignored.

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Graphics Class' Other Methods

Graphics Coordinate System

In Java Windowing Subsystem (like most of the 2D Graphics systems), the origin (0,0) is located at the top-left corner.

EACH component/container has its own coordinate system, ranging for (0,0) to (width-1, height-1) as illustrated.

You can use method getWidth() and getHeight() to retrieve the width and height of a component/container. You can use getX() or getY() to get the top-left corner (x,y) of this component's origin relative to its parent.

Custom Painting Template

Under Swing, custom painting is usually performed by extending (i.e., subclassing) a JPanel as the drawing canvas and override the paintComponent(Graphics g) method to perform your own drawing with the drawing methods provided by the Graphics class. The Java Windowing Subsystem invokes (calls back) paintComponent(g) to render the JPanel by providing the current graphics context g, which can be used to invoke the drawing methods. The extended JPanel is often programmed as an inner class of a JFrame application to facilitate access of private variables/methods. Although we typically draw on the JPanel, you can in fact draw on any JComponent (such as JLabel, JButton).

The custom painting code template is as follows:

Dissecting the Program

• Custom painting is performed by extending a JPanel (called DrawCanvas) and overrides the paintComponent(Graphics g) method to do your own drawing with the drawing methods provided by the Graphics class.
• DrawCanvas is designed as an inner class of this JFrame application, so as to facilitate access of the private variables/methods.
• Java Windowing Subsystem invokes (calls back) paintComponent(g) to render the JPanel, with the current graphics context in g, whenever there is a need to refresh the display (e.g., during the initial launch, restore, resize, etc). You can use the drawing methods (g.drawXxx() and g.fillXxx()) on the current graphics context g to perform custom painting on the JPanel.
• The size of the JPanel is set via the setPreferredSize(). The JFrame does not set its size, but packs the components contained via pack().
• In the main(), the constructor is called in the event-dispatch thread via static method javax.swing.SwingUtilities.invokeLater() (instead of running in the main thread), to ensure thread-safety and avoid deadlock, as recommended by the Swing developers.

(Advanced) Anonymous Inner Class for Drawing Canvas

Instead of a named-inner class called DrawCanvas in the previous example, you can also use an anonymous inner class for the drawing canvas, if the painting code is short. For example,

(Advanced) Getting the Graphics Context

You can retrieve the Graphics context of a JComponent via the getGraphics() method. This is, however, not commonly used. For example,

Custom Painting in AWT (Obsolete)

Under AWT, you can perform custom painting by extending java.awt.Canvas, and override the paint(Graphics g) method, in a java.awt.Frame application. Similarly, you can explicitly invoke repaint() to update the graphics.

Refreshing the Display via repaint()

At times, we need to explicitly refresh the display (e.g., in game and animation). We shall NOT invoke paintComponent(Graphics) directly. Instead, we invoke the JComponent's repaint() method. The Windowing Subsystem will in turn call back the paintComponent() with the current Graphics context and execute it in the event-dispatching thread for thread safety. You can repaint() a particular JComponent (such as a JPanel) or the entire JFrame. The children contained within the JComponent will also be repainted.

Colors and Fonts

java.awt.Color

The class java.awt.Color provides 13 standard colors as named-constants. They are: Color.RED, GREEN, BLUE, MAGENTA, CYAN, YELLOW, BLACK, WHITE, GRAY, DARK_GRAY, LIGHT_GRAY, ORANGE, and PINK. (In JDK 1.1, these constant names are in lowercase, e.g., red. This violates the Java naming convention for constants. In JDK 1.2, the uppercase names are added. The lowercase names were not removed for backward compatibility.)

You can use the toString() to print the RGB values of these color (e.g., System.out.println(Color.RED)):

You can also use the RGB values or RGBA value (A for alpha to specify transparency/opaque) to construct your own color via constructors:

For example:

To retrieve the individual components, you can use getRed(), getGreen(), getBlue(), getAlpha(), etc.

To set the background and foreground (text) color of a component/container, you can invoke:

To set the color of the Graphics context g (for drawing lines, shapes, and texts), use g.setColor(color):

(Advanced) JColorChooser Example

This example uses the javax.swing.JColorChooser to set the background color of the JPanel.

java.awt.Font

The class java.awt.Font represents a specific font face, which can be used for rendering texts. You can use the following constructor to construct a Font instance:

You can use the setFont() method to set the current font for the Graphics context g for rendering texts. For example,

Font's Family Name vs. Font Name

A font could have many faces (or style), e.g., plain, bold or italic. All these faces have similar typographic design. The font face name, or font name for short, is the name of a particular font face, like 'Arial', 'Arial Bold', 'Arial Italic', 'Arial Bold Italic'. The font family name is the name of the font family that determines the typographic design across several faces, like 'Arial'. For example,

Logical Font vs. Physical Font

JDK supports these logical font family names: 'Dialog', 'DialogInput', 'Monospaced', 'Serif', or 'SansSerif'. JDK 1.6 provides these String constants: Font.DIALOG, Font.DIALOG_INPUT, Font.MONOSPACED, Font.SERIF, Font.SANS_SERIF.

Physical font names are actual font libraries such as 'Arial', 'Times New Roman' in the system.

GraphicsEnvironment's getAvailableFontFamilyNames() and getAllFonts()

You can use GraphicsEnvironment's getAvailableFontFamilyNames() to list all the font famiy names; and getAllFonts() to construct all Font instances (with font size of 1 pt). For example,

Font's deriveFont()

You can use Font's deriveFont() to derive a new Font instance from this Font with varying size, style and others.

For example,

(Advanced) java.awt.FontMetrics

The java.awt.FontMetrics class can be used to measure the exact width and height of the string for a particular font face, so that you can position the string as you desire (such as at the center of the screen).

To create a FontMetrics, use getFontMetrics() methods of the Graphics class, as follows:

The most commonly-used function for FontMetrics is to measure the width of a given String displayed in a certain font.

To centralize a string on the drawing canvas (e.g., JPanel):

Custom Graphics Examples

Example 1: Moving an Object via Key/Button Action

This example illustrates how to re-paint the screen in response to a KeyEvent or ActionEvent.

The display consists of two JPanel in a JFrame, arranged in BorderLayout. The top panel is used for custom painting; the bottom panel holds two JButton arranged in FlowLayout. Clicking the 'Move Right' or 'Move Left' buttons moves the line. The JFrame listens to the 'Left-arrow' and 'Right-arrow' keys, and responses by moving the line left or right.

Dissecting the Program

• To do custom painting, you have to decide which superclass to use. It is recommended that you use a JPanel (or a more specialized Swing component such as JButton or JLabel). In this example, we extend the JPanel to do our custom painting, in an inner class, as follows:
• The paintComponent() method is overridden to provide the custom drawing codes. We use the drawLine() method to draw a line from (x1,y1) to (x2, y2).
• The paintComponent() method cannot be called directly from your code, because it requires a Graphics object as argument.
• paintComponent() is a so-called 'call-back' method. The Windowing subsystem invokes this method and provides a pre-configured Graphics object to represent its state (e.g., current color, font, clip area and etc). There are two kinds of painting: system-triggered painting and application-triggered painting. In a system-trigger painting, the system request a component to render its content when the component is first made visible on the screen, or the component is resized, or the component is damaged that needs to be repaint. In an application-triggered painting, the application invokes a repaint() request. Under both cases, the Windowing subsystem will call-back the paintComponent() to render the contents of the component with a proper Graphics object as argument.
• In this example, the application requests for a repaint() in the KeyEvent and MouseEvent handlers, which triggers the paintComponent() with an appropriate Graphics object as the argument.
• To be precise, when you invoke the repaint() method to repaint a JComponent, the Windowing subsystem calls-backpaint() method. The paint() method then calls-back three methods: paintComponent(), paintBorder() and paintChilden().
• In the overridden paintComponent() method, we call super.paintComponent() to paint the background of the JComponent. If this call is omitted, you must either paint the background yourself (via a fillRect() call) or use setOpaque(false) to make the JComponent transparent. This will inform Swing system to paint those JComponents behind the transparent component.
• We choose the JFrame as the source of the KeyEvent. JFrame shall be 'in focus' when the key is pressed. The requestFocus() method (of 'this' JFrame) is invoked to request for the keyboard focus.

[TODO]: may need to revise.

Try

Modifying the program to move a ball in response to up/down/left/right buttons, as well as the 4 arrow and 'wasd' keys , as shown:

Example 2: Moving Sprites

In game programming, we have moving game objects called sprites. Each sprite is usually modeled in its own class, with its own properties, and it can paint itself.

Sprite.java

This class models a sprite, with its own properties, and it can paint itself via the paint() method provided given a Graphics context. A rectangle is used here.

MoveASprite.java

Instead of repainting the entire display, we only repaint the affected areas (clips), for efficiency, via the repaint(x, y, width, height) method. In moveLeft() and moveRight(), we save the states, move the object, repaint the saved clip-area with the background color, and repaint the new clip-area occupied by the sprite. Repainting is done by asking the sprite to paint itself at the new location, and erase from the old location.

Example 3: Paint

MyPaint.java

PolyLine.java

Dissecting the Program

[TODO]

Drawing Images

javax.swing.ImageIcon

The javax.swing.ImageIcon class represents an icon, which is a fixed-size picture, typically small-size and used to decorate components. To create an ImageIcon:

Graphics Class' drawImage()

ImageIcon is fixed-in-sized and cannot be resized in display. You can use Graphics's drawImage() to resize a source image in display.

The java.awt.Graphics class declares 6 overloaded versions of abstract method drawImage().

The coordinates involved is shown in the above diagram. The ImageObserver receives notification about the Image as it is loaded. In most purposes, you can set it to null or this.

The drawImage() method requires an Image instance, which can be obtained via ImageIcon's getImage() method; or via static method ImageIO.read() (read 'Reading Images into your program'). For example,

Example

Images:

This example places absolute numbers in the draw methods, which is hard to maintain and reuse. You should define name-constants such as CELL_WIDTH, BORDER_WIDTH, etc, and compute the numbers based on these constants.

Animation

Animation using javax.swing.Timer

Creating an animation (such as a bouncing ball) requires repeatedly running an updating task at a regular interval. Swing provides a javax.swing.Timer class which can be used to fire ActionEvent to its registered ActionListeners at regular interval.

The Timer class has one constructor:

You are required to override the actionPerformed() method of the ActionListener to specify your task's behavior. The Timer fires an ActionEvent to the ActionListener after the (initial) delay, and then at regular interval after delay.

You can start and stop the Timer via the Timer's start() and stop() methods. For example,

You can use method setRepeats(false) to set the Timer to fire only once, after the delay. You can set the initial delay via setInitialDelay() and regular delay via setDelay().

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A Timer can fire the ActionEvent to more than one ActionListeners. You can register more ActionListeners via the addActionListener() method.

The actionPerformed() runs on the event-dispatching thread, just like all the event handlers. You can be relieved of the multi-threading issues.

JDK 1.3 introduced another timer class called java.util.Timer, which is more general, but javax.swing.Timer is sufficient (and easier) to run animation in Swing application.

Example: A Bouncing Ball

javax.swing.Timer does not provide very accurate timing due to the overhead of event-handling. It probaly cannot be used for real-time application such as displaying a clock.

[TODO] Stop the Timer after x steps

(Advanced) Animation using a new Thread

Animation usually involves multi-threading, so that the GUI refreshing operations does not interfere with the programming logic. Multi-threading is an advanced topics. Read 'Multithreading & Concurrent Programming'

In the previous example, we use javax.swing.Timer, which run the updating task at regular interval on the event-dispatching thread. In this example, we shall create a new thread to run the update.

To create a new thread, define a (anonymous and inner) subclass of Thread and override the run() method to specify the behavior of the task. Create an instance and start the instance via the start() method, which calls back the run() defined earlier.

To ensure the new thread does not starve the other threads, in particular the event-dispatching thread, the thread shall yield control via the sleep(mills) method, which also provides the necessary delay.

• To update the display regularly, we explicitly invoke the repaint() method of the JFrame, which will callback the paintComponent(g) of all the components contained in this JFrame.
• The display refreshing code is run in its own thread, so as to avoid the infamous unresponsive user interface problem. It is programmed as an anonymous inner class, extends class Thread, by overriding the run() method to provide the programmed operations (i.e., repaint()). The start() method is use to start the thread, which will callback the run().
• Inside the overridden run(), the repaint() is programmed inside an infinite loop, followed by a Thread.sleep(milliseconds) method, which suspends the thread for the given milliseconds. This operation provides the necessary delay and also yield control to other thread to perform their intended operations.

[TODO] Stopping the thread after x steps

(Advanced) A Closer Look at repaint()

Reference: 'Painting in AWT and Swing' @ http://www.oracle.com/technetwork/java/painting-140037.html. I summarize some of the important points here.

Heavyweight AWT Components vs. Lightweight Swing Components

The original AWT components are heavyweight components. 'Heavyweight' means that the component has it's own opaque native window. Heavyweight components, such as java.awt.Button, are mapped to the platform-specific components. It relies on the windowing subsystem in each native platform to take care of details such as damage detection, clip calculation, and z-ordering. On the other hand, the newer Swing JComponents (such as javax.swing.JButton) are lightweight components. A 'lightweight' component does not own its screen resources but reuses the native window of its closest heavyweight ancestor. Swing JComponents do not rely on the native platform and are written purely in Java, . The top-level containers, such as JFrame, JApplet and JDialog, which are not subclass of JComponent, remain heavyweight. It is because the lightweight Swing JComponents need to attach to a heavyweight ancestor.

Painting Mechanism

Painting is carried out via a 'call-back' mechanism. A program shall put its painting codes in a overridden method (paint() for AWT components or paintComponent() for Swing component), and the windowing subsystem will call back this method when it's time to paint.

System-triggered vs. Application-triggered Painting Requests

There are two types of paint (or repaint) requests:

1. System-triggered: e.g., the component is first made visible, the componet is resized, etc. The windowing subsystem will schedule paint() or paintComponent() on the event-dispatching thread.
2. Application-triggered: application has modified the appearance of the component and requested to repaint the component. However, Application shall not invoke paint() or paintComponent() directly. Instead, it shall invoke a special method called repaint(), which will in turn invoke paint() or paintComponent(). Multiple repaint() requests may be collapsed into a single paint() call.

Instead of issuing repaint() to paint the entire component, for efficiency, you can selectively repaint a rectangular clip area. You can also specify a maximum time limit for painting to take place.

Painting the Lightweight Swing Components

A lightweight needs a heavyweight somewhere up the containment hierarchy in order to have a place to paint, as only heavyweight components have their own opaque window. When this heavyweight ancestor is asked to paint its window, it must also paint all of its lightweight descendants. This is handled by java.awt.Container's paint() method, which calls paint() on any of its visible, lightweight children which intersect with the rectangle to be painted. Hence, it is crucial for all Container subclasses (lightweight or heavyweight) that override paint() to place a super.paint() call in the paint() method. This super.paint() call invoke Container's (super) paint() method, which in turn invoke paint() on all its descendants. If the super.paint() call is missing, some of the lightweight descendants will be shown up.

Opaque and Transparent

Lightweight components does not own its opaque window and 'borrow' the screen real estate of its heavyweight ancestor. As a result, they could be made transparent, by leaving their background pixels unpainted to allow the underlying component to show through.

To improve performance of opaque components, Swing adds a property called opaque to all JComponents. If opaque is set to true, the component agrees to paint all of the pixels contained within its rectangular bounds. In order words, the windowing subsystem does not have to do anything within these bounds such as painting its ancestors. It opaque is set to false, the component makes no guarantees about painting all the bits within its rectangular bounds, and the windowing subsystem has more work to do.

Swing further factor the paint() method into three methods, which are invoked in the following order:

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Swing programs should override paintComponent() instead of paint().

Most of the standard Swing components (in particular, JPanel) have their look and feel implemented by separate look-and-feel objects (called 'UI delegates') for Swing's Pluggable look and feel feature. This means that most or all of the painting for the standard components is delegated to the UI delegate and this occurs in the following way:

1. paint() invokes paintComponent().
2. If the ui property is non-null, paintComponent() invokes ui.update().
3. If the component's opaque property is true, ui.udpate() fills the component's background with the background color and invokes ui.paint().
4. ui.paint() renders the content of the component.

This means that subclasses of Swing components which have a UI delegate (such as JPanel), should invoke super.paintComponent() within their overridden paintComponent(), so that ui.update() fills the background (of the superclass such as JPanel) provided opaque is true.

Try removing the super.paintComponent() from a Swing program that does animation (e.g., bouncing ball). The background will not be painted, and the previous screen may not be cleared. You can also paint the background yourself by filling a Rectangle with background color.

Furthermore, if you set the opaque to false (via setOpaque(false)) for the subclass of JPanel, the super.paintComponent(g) does not fill the background.

REFERENCES & RESOURCES

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• 'The Swing Tutorial' @ http://docs.oracle.com/javase/tutorial/uiswing/, in particular, the section on 'Performing Custom Graphics'.
• 'Painting in AWT and Swing' @ http://www.oracle.com/technetwork/java/painting-140037.html.