Head First Java Chapter 17 Notes

Head First Java Chapter 17

This is the Seventeenth Chapter learning of Head First Java by Kathy Sierra and Bert Bates,
This markdown file is my notes for the Seventeenth chapter, not sure if any copyright issues are there.

Package, jars and deployment

JAR

  • Organize your project so that your source code and class files are not in the same directory.
  • A standard organization structure is to create a project directory, and then put a source directory and a classes directory inside the project directory.
  • Organizing your classes into packages prevents naming collisions with other classes, if you prepend your reverse domain name on to the front of a class name.
  • To put a class in a package, put a package statement at the top of the source code file, before import statements.
package com.wickedlysmart;
  • To be in a package, a class must be in a directory structure that exactly matches the package structure. For a class, com.wickedlysmart.Foo, the Foo Class must be in a directory name wickedlysmart, which is in a directory name com.
  • To make your compiled class land in the correct package directory structure under the classes directory, use the -d compiler flag:
% cd sources
% javac -d ../classes com/wickedlysmart/Foo.java
  • To run your code, cd to the classes directory, and give the fully-qualified name of your class.
% cd classes
% java com.wickedlysmart.Foo
  • You can bundle your classes into JAR(Java ARchive) files. JAR is based on the pkzip format.
  • You can make an executable JAR file by putting a manifest into the JAR that states which class has the main() method. To create a manifest file, make a text file with an entry like the following (for example):
Main-Class: com.wickedlysmart.Foo
  • Be sure to hit the return key after typing the Main-Class line, or your manifest may not work.
  • To create a JAR file, types:
jar -cvfm manifest.txt MrJar.jar com
  • The entire package directory structure (and only the directories matching the package) must be immediately inside the JAR file.
  • To run an executable JAR file, type:
java -jar MyJar.jar

Java Web Start

  • Java Web Start technology lets you deploy a stand-alone client application for the web.
  • Java Web Start includes a ‘helper app’ that must be installed on the client (along with java).
  • A Java Web Start (JWS) app has two pieces: and executable JAR and a .jnlp file.
  • A .jnlp file is a simple XML document that describes your JWS application. It includes tags for specifying the name and location of the JAR, and the name of the class with the main() method.
  • When a browser gets a .jnlp file from the server (because the user clicked on a link to the .jnlp file), the browser start up the JWS helper app.
  • The JWS helper app reads the .jnlp file and request the executable JAR from the Web server.
  • When the JWS gets the JAR, it invokes the main() method (specified in the .jnpl file).
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