'java' - The Java Launcher

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This chapter describes:

• "java" Command
• Hello.java - My First Application
• "-classpath" - Specifying Class Path
• "-jar" - Executable JAR Files
• "-X" Options to Control Memory Size
• "javaw" - Launching Java Applications without Console

"java" Command

"java": A command line tool that launches Java applications. It starts a Java virtual machine, loads the specified class, and invokes that class's main method. "java" has the following syntax:

java [options] class [arguments]

where "options" is a list of options; "class" is the full name of a Java class to be launched; "arguments" is a list of arguments to be passed to the main method of the class to be launched.

Another way of launching a Java class is to use the "-jar" option:

java [options] -jar file [arguments]

where "file" is a JAR file, which should contain a "Main-Class" attribute in the manifest file. The "Main-Class" attibute defines the Java class to be launched.

Commonly used options are:

• "-help" - Displays a short help text.
• "-verbose" - Generates verbose output to standard output.
• "-version" - Prints the version information of the launcher.
• "-classpath classpath" - Specifies a list of path names where the launcher will search for compiled type definitions. If "-classpath" is not specified, the current directory will be used as the class path.
• "-Dproperty=value" - Defines a new system property, which can be accessed by the application.

Hello.java - My First Application

To test the Java launcher, I wrote the following Java file, Hello.java:

class Hello {
   public static void main(String[] a) {
      System.out.println("Hello world!"); 	
   }
}

Here is what I did in a command window to compile Hello.java into Hello.class:

>javac Hello.java

>dir Hello.*
   416 Hello.class
   116 Hello.java

>java Hello
Hello world!

As you can see, launching a Java application is easy.

(Continued on next part...)

Part:   1   2  3  4