{"id":81,"date":"2018-07-10T11:26:30","date_gmt":"2018-07-10T11:26:30","guid":{"rendered":"http:\/\/itp16.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=81"},"modified":"2019-05-15T07:14:26","modified_gmt":"2019-05-15T07:14:26","slug":"applet-life-cycle","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/itp16\/chapter\/applet-life-cycle\/","title":{"rendered":"APPLET LIFE CYCLE"},"content":{"raw":"
\"epgp<\/a>\r\n<\/span><\/div>\r\n
\r\n\r\n \r\n\r\nWhatis an Applet?<\/strong>\r\n\r\n \r\n

Applet is a Panel (inherits from Panel), which can be embedded in a webpage (HTML). What are the additional features of Applet compared to a Panel. It has the capability of interacting with the Browser, and has a life cycle defined by the Browser, when it is viewed in a web page on the browser. The life cycle defined by the help of methods invoked by the browser.<\/p>\r\n \r\n

For applet to working a browser, the browser must have JVM, the JVM needs to be started before the Applet can be used. When is the JVM started? It is normally started with the help of method main(). In the case of Browser the main is already started by the browser, before the web page is loaded. The browser has its own Java application, which it uses to manage the Java objects like Applet.<\/p>\r\n \r\n

Browser as a Container for the Applet<\/strong><\/p>\r\n \r\n

Let\u2019s look at the sequence of events within the browser. Firstly, the JVM is already started within the browser with the help of some main method. When a web page is loaded which contains at least one tag for an Applet, then the java application within the browser would first create an instance of \u201cAppletContext\u201d. ie. one instance of \u201cAppletContext\u201d would be created per html page, which contains one or more Applet tags. The java application would parse the data between the <Applet.....> and <\/Applet> tags and create an instance of \u201cAppletStub\u201d for each such applet tag in the page. This instance has all the information specified between the <Applet...> and the <\/applet> tags. In the applet tag one of the attributes is code, eg. <applet code=\".....\"...> some class name is specified. The class name specified should be the name of a class which extends the Applet class. The browser would now send a request to the server from where the web page was loaded to get the contents of the applet class specified in the code attribute. The applet class is stored on the server and needs to be downloaded to the client machine. After downloading the content of the class file the browser would now load the downloaded class and then create an instance of the class using its default constructor. The public default constructor must be available for the applet class. After creating the instance, the browser would invoke the first method on the instance.<\/p>\r\n\r\n<\/div>\r\n

LIFE-CYCLE OF AN APPLET<\/strong><\/p>\r\n \r\n\r\n\"\"\r\n\r\n \r\n

\r\n

which is the first method invoked on an instance of Applet ?<\/strong><\/p>\r\n \r\n

Is it init()? no. The first method invoked on the instance of Applet is \u201cpublic final void setStub(AppletStub stub)\" This is the first method invoked on the instance of Applet and not init(). this method is a final method in the Applet class. The instance of AppletStub created by the browser corresponding to this applet tag is passed to the Applet instance. Also the AppletStub has a method<\/p>\r\n \r\n

\u201cpublic AppletContext getAppletContext()\u201d<\/p>\r\n \r\n

This method would return the \u201cAppletContext\u201d instance corresponding to the web page of this applet. The \u201cAppletContext\u201d instance can be used for interaction with the browser. The Applet instance can get the information mentioned between the <applet...> and <\/applet> tags with the help of methods of the \u201cAppletStub\u201d instance which is given to it in the \u201csetStub\u201d method. This \u201cappletstub\u201d instance is also used for getting access to the \u201cappletcontext\u201d instance for interacting with the browser. So after invocation of \u201csetstub\u201d the applet instance has the knowledge about the information in the applet tag (mainly the parameters) and the capability to interact with the browser.<\/p>\r\n \r\n

The life-cycle of an Applet<\/strong><\/p>\r\n \r\n

It is after \u201csetStub\u201d that the \u201cinit\u201d method is invoked. The \u201cinit\u201d method in the applet class has a blank implementation and can be overridden by the sub class of applet to do any kind of initialization. The browser maintains the state of the applet instance as being active or inactive. The instance of applet is active when the web-page containing it is currently being shown. The browser then invokes the method \u201cstart()\u201d and puts the applet in the active state. if the applet is becoming inactive because it is not on an active page. then the browser would invoke the \u201cstop()\u201d method on the instance and puts it into inactive state. so start and stop methods may be invoked by the browser any number of times depending on the activity of the user. Finally when the web-page containing the applet is being destroyed, in the browser. Then the browser would call the method \u201cdestory()\u201d on the instance of applet to indicate to the applet that it is now going to be destoryed. so the life cycle of an applet starts with \u201csetStub()\u201d, followed by\u00a0 \u201cinit()\u201d\u00a0 followed\u00a0 by \u201cstart()\u201d\u00a0 and then depending on the\u00a0 user activity may call stop and start any no. of times. and ultimately the last method invoked is \u201cdestroy()\u201d.<\/span><\/p>\r\n\r\n