1 / 40

Java Applet

Kdot :)

FebertGuiang
Download Presentation

Java Applet

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Dhea.cess.pat.sthan

  2. DEFINING APPLET • The word applet is meant to suggest a small application. • Applets were intended to be small programs run over the Internet: • However, there are no size constraints on applets. • Applets can be viewed over the Internet, or without any connection to the internet. • An applet is similar to a Swing GUI: • In fact, almost all of the Swing techniques can be used in applets.

  3. An applet class is normally defined as a derived class of the class Japplet. • The class JApplet is in the package javax.swing. • There is also an older class, Applet, which has been superseded by the JApplet class.

  4. An applet class can be designed as a derived class of JApplet in much the same way that regular Swing GUIs are defined as derived classes of Jframe. • However, an applet normally defines no constructors. • The method in it performs the initializations that would be performed in a constructor for a regular Swing GUI • Components can be added to an applet in the same way that a component is added to a JFrame • The method add is used to add components to an applet in the same way that components are added to a JFrame

  5. JApplethas a method named setJMenuBar that behaves the same as the setJMenuBar method of a JFrame • JApplet can also have menu bars added to a JApplet or to a panel that is part of the JApplet using the add method

  6. Inheritance Hierarchy of the Applet Class • Every applet is implemented by creating a subclass of the Applet class.

  7. HOW APPLET DIFFER FROM SWING GUI • Some of the items included in a Swing GUI are not included in an applet • Applets do not contain a main or setVisible method • Applets are displayed automatically by a Web page or an applet viewer • Applets do not have titles • Therefore, they do not use the setTitle method • They are normally embedded in an HTML document, and the HTML document can add any desired title

  8. Applets do not use the setSize method • The HTML document takes care of sizing the applet • Applets do not have a close-window button • Therefore, they do not have a setDefaultCloseOperation method • When the HTML document containing the applet is closed, then the applet is automatically closed

  9. APPLET CAPABILITIES • Here are some other things that current browers and other applet viewers let applets do: • Applets can usually make network connections to the host they came from. • Applets running within a Web browser can easily cause HTML documents to be displayed. • Applets can invoke public methods of other applets on the same page. • Applets that are loaded from the local file system (from a directory in the user's CLASSPATH) have none of the restrictions that applets loaded over the network do. • Although most applets stop running once you leave their page, they don't have to.

  10. SECURITY RESTRICTION • Applets cannot load libraries or define native methods. • An applet cannot ordinarily read or write files on the host that is executing it. • An applet cannot make network connections except to the host that it came from. • An applet cannot start any program on the host that is executing it. • An applet cannot read certain system properties. • Windows that an applet brings up look different than windows that an application brings up.

  11. URL URL(Uniform Resource Locator) is a reference (an address) to a resource on the Internet. • "URL address” ==> an Internet address • "URL object” ==> an instance of the URL class in a program.

  12. CREATING URL • URL gamelan = new URL("http://www.gamelan.com/"); • URL(URL baseURL, String relativeURL) URL gamelanGames = new URL(gamelan, "Gamelan.net.html");

  13. new URL("http", "www.gamelan.com", 80, "pages/Gamelan.network.html"); • creates a URL object for the following URL: http://www.gamelan.com:80/pages/Gamelan.network.html • NOTE: URLs are "write-once" objects. Once • you've created a URL object, you cannot • change any of its attributes (protocol, • host name, filename, or port number).

  14. You can call the URL's openStream() method to get a stream from which you can read the contents of the URL. • The openStream() method returns a java.io.InputStreamobject.

  15. CONNECTING TO URL • URL object's openConnection method is used to connect to a URL. • Connecting to a URL means initializing a communication link between the Java program and the URL over the network. try { URL yahoo = new URL("http://www.yahoo.com/"); yahoo.openConnection(); } catch (MalformedURLException e) { // new URL() failed . . . } catch (IOException e) { // openConnection() failed . . . }

  16. WRITING TO A URL CONNECTION • Create a URL. • Open a connection to the URL. • Set output capability on the URLConnection. • Get an output stream from the connection. This output stream is connected to the standard input stream of the cgi-bin script on the server. • Write to the output stream. • Close the output stream.

  17. DATAGRAM A datagram is an independent, self- contained message sent over the network whose arrival, arrival time, and content are not guaranteed.

  18. URL, SOCKET vs. DATAGRAM URL or SOCKET • Connection-based • Reliable • Dedicated point-to-point channel • Same order DATAGRAMS • Not connection-based • No guarantees about arrival • Don’t have a dedicated point-to-point channel • The order is not guaranteed

  19. IP ADDRESS ES AND JAVA • Java has a class java.net.InetAddress which abstracts network addresses • Serves three main purposes: • Encapsulates an address • Performs name lookup (converting a host name into an IP address) • Performs reverse lookup (converting the address into a host name)

  20. java.net.InetAddress (1) • Abstraction of a network address • Currently uses IPv4 (a 32 bit address) • Will support other address formats in future • Allows an address to be obtained from a host name and vice versa • Is immutable (is a read-only object) • Create an InetAddress object with the address you need and throw it away when you have finished

  21. java.net.InetAddress (2) • Static construction using a factory method • InetAddressgetByName(String hostName) • hostName can be “host.domain.com.au”, or • hostName can be “130.95.72.134” • InetAddressgetLocalHost() • Some useful methods: • String getHostName() • Gives you the host name (for example “www.sun.com”) • String getHostAddress() • Gives you the address (for example “192.18.97.241”) • InetAddressgetLocalHost() • InetAddress[] getAllByName(String hostName)

  22. java.net.ServerSocket (1) • Listens on well-known port for incoming connections • Creates a dynamically allocated port for each newly established connection • Provides a Socket connected to the new port • Maintains a queue to ensure that prospective clients are not lost

  23. java.net.ServerSocket (2) • Construction: • ServerSocket(int port, int backlog) • Allows up to backlog requests to queue waiting for the server to deal with them • Some useful methods: • Socket accept() • Blocks waiting for a client to attempt to establish a connection • void close() • Called by the server when it is shutting down to ensure that any resources are deallocated • More details in the Javadoc (as always!)

  24. java.net.Socket(1) • Provides access to TCP/IP streams • Bi-directional communication between sender and receiver • Can be used to connect to a remote address and port by using the constructor: • Socket(String remoteHost, int port) • Also used to accept an incoming connection (see ServerSocket)

  25. java.net.Socket (2) • Can obtain access to input and output streams • Input stream allows reception of data from the other party • InputSteamgetInputStream() • Output stream allows dispatch of data to the other party • OutputStreamgetOutputStream()

  26. java.net.DatagramPacket (1) • DatagramPackets normally used as short lived envelopes for datagram messages: • Used to assemble messages before they are dispatched onto the network, • or dismantle messages after they have been received • Has the following attributes: • Destination/source address • Destination/source port number • Data bytes constituting the message • Length of message data bytes

  27. java.net.DatagramPacket (2) • Construction: • DatagramPacket(byte[] data, int length) • Some useful methods: • void setAddress(InetAddressaddr) • InetAddressgetAddress() • void setPort(int port) • intgetPort() • DatagramPackets are not immutable so, in principle you can reuse then, but . . • Experience has shown that they often misbehave when you do -- create a new one, use it once, throw it away!

  28. java.net.DatagramSocket (1) • Used to represent a socket associated with a specific port on the local host • Used to send or receive datagrams • Note: there is no counterpart to java.net.ServerSocket! Just use a DatagramSocket with a agreed port number so others know which address and port to send their datagrams to

  29. java.net.DatagramSocket (2) • Construction: • DatagramSocket(int port) • Uses a specified port (used for receiving datagrams) • DatagramSocket() • Allocate any available port number (for sending) • Some useful methods: • void send(DatagramPacket fullPacket) • Sends the full datagram out onto the network • void receive(DatagramPacket emptyPacket) • Waits until a datagram and fills in emptyPacket with the message • . . . and a few more in the Javadoc

  30. sea.datagram.DatagramSender • This example sends datagrams to a specific host (anywhere on the Internet) • The steps are as follows: • Create a new DatagramPacket • Put some data which constitutes your message in the new DatagramPacket • Set a destination address and port so that the network knows where to deliver the datagram • Create a socket with a dynamically allocated port number (if you are just sending from it) • Send the packet through the socket onto the network

  31. sea.datagram.DatagramSender byte[] data = “This is the message”.getBytes(); DatagramPacket packet = new DatagramPacket(data, data.length); // Create an address InetAddress destAddress = InetAddress.getByName(“fred.domain.com”); packet.setAddress(destAddress); packet.setPort(9876); DatagramSocket socket = new DatagramSocket(); socket.send(packet);

  32. sea.datagram.DatagramReceiver • The steps are the reserve of sending: • Create an empty DatagramPacket (and allocate a buffer for the incoming data) • Create a DatagramSocket on an agreed socket number to provide access to arrivals • Use the socket to receive the datagram (the thread will block until a new datagram arrrives) • Extract the data bytes which make up the message

  33. sea.datagram.DatagramReceiver // Create an empty packet with some buffer space byte[] data = new byte[1500]; DatagramPacket packet = new DatagramPacket(data, data.length); DatagramSocket socket = new DatagramSocket(9876); // This call will block until a datagram arrives socket.receive(packet); // Convert the bytes back into a String and print String message = new String(packet.getData(), 0, packet.getLength()); System.out.println("message is " + message); System.out.println("from " + packet.getAddress());

  34. java.net.MulticastSocket • Subclass of java.net.DatagramSocket • Constructed the same way • Adds some extra methods: • void joinGroup(InetAddress mcastGroup) • Enter the specifies group so that you can send or receive datagrams • void leaveGroup(InetAddress mcastGroup) • Leave a group that you previously joined • void setTimeToLive(int ttl) • Sets how far your datagrams will travel before routers ignore them • int getTimeToLive()

  35. sea.datagram.MulticastSender • Sending similar to the previous example. . • . . .but must register with the multicast group and decide the longevity • The steps involved are: • Create the MulticastSocket. • Join the multicast group(s) (on startup). • Create the DatagramPacket. • Send the packet through the socket. • Leave the multicast group (on exit).

  36. sea.datagram.MulticastSender InetAddress multicastGroup = InetAddress.getByName(multicastGroupAddr); MulticastSocket socket = new MulticastSocket(); socket.joinGroup(multicastGroup); socket.setTimeToLive(5); byte[] data = “This is the message”.getBytes(); DatagramPacket datagram = new DatagramPacket(data, data.length); datagram.setAddress(multicastGroup); datagram.setPort(9876); socket.send(datagram); socket.leaveGroup(multicastGroup);

  37. sea.datagram.MulticastReceiver • The steps are: • Create a multicast socket on an agreed port. • Join the multicast group (on startup). • Create an empty datagram. • Wait for datagram to be delivered on the socket. • Unpack and use the datagram. • Leave the multicast group (on exit).

  38. sea.datagram.MulticastReceiver InetAddress multicastGroup = InetAddress.getByName(multicastGroupAddr); MulticastSocket socket = new MulticastSocket(9876); socket.joinGroup(multicastGroup); byte[] data = new byte[1000]; DatagramPacket packet = new DatagramPacket(data, data.length); socket.receive(packet); String message = new String( packet.getData(), 0, packet.getLength()); socket.leaveGroup(multicastGroup);

More Related