JVM
The JVM is an example of a neat idea called a virtual machine, which is an abstraction. The JVM can be pictured like so:
The JVM is an example of a neat idea called a virtual machine, which is an abstraction. The JVM can be pictured like so:
The JVM executes "bytecode", which are compiled instructions meant for the VM:
Time to get our feet wet (or hands dirty)! Make sure you have (installed these):
After you download and bring up drjava, be sure to go to Edit->Preferences, and set the 'Tools.jar Location' to your OpenJDK 8's lib/tools.jar, like so.
We're going to be using drjava, or an online interface such as repl.it, to compile and run our Java code.
Here is the classic HelloWorld program, Java version - it is written as a class called HelloWorldApp. Save it to a file (THAT MUST BE) called HelloWorldApp.java, inside a HelloWorldApp folder [always make sure that the classname, filename and foldername match]:
class HelloWorldApp {
// say "p s v m" over and over till you like the sound of it :)
public static void main(String[] args) {
System.out.println("Hello, Java World!"); // output
}
}// HelloWorldApp
Compile and run it in the IDE (drjava):
NOTE: running a Java program is done via 'java nameOfClassWithAMain' ('java' invokes the JVM) - this is what the 'run' command in the IDE actually does; compiling a program is done using 'javac'. We compile, then run [just like with C++]. Compilation produces .class files (from .java sources), those are our 'executables' - we don't/shouldn't/can't directly run them, our JVM runs them for us.
We can create a .jar (compressed, .zip equivalent) file out of our HelloWorldApp.class file (plus a 'manifest' file), and execute the .jar file just by double-clicking on it. Real-world Java executables are packaged and distributed as jar files (eg. our drjava IDE).
To create a .jar file:
The above steps should generate a .jar file, which contains our .class file(s) plus a 'manifest'. This standalone file is our 'executable'. We can double-click on it to execute it, as if it is any other app on our machine [double-clicking will simply cause the jar file's 'manifest' to be examined to discover the class that has psvm, and the JVM to be launched, with our psvm class].
When we double-click on the .jar, we get nothing! That's because the program has no UI, just command-line output. We need to run it from a shell, via
java HelloWorldApp [or java -jar HelloWorldApp.jar]
Instead of the command line version we had, here is a GUI-based one. No need to understand the internals right now - let us compile and run:
In drjava, create a .jar as before (create a new project, then do this), and double-click on it, that [too] puts up the same little UI as shown above. In Java, due to the inclusion of the 'Swing' library in the core language, UI is an integral part!
Now let's look at an '**applet**' version of HelloWorld. An applet is bytecode that runs entire inside a browser (whereas the first three HelloWorld versions we ran were **app**s - these run outside the browser, ie. on a JVM atop the underlying OS).
Here is the source - compile it and run [using drjava - click on 'Run']. You will see the following result (FYI, the applet gets run by drjava's 'Applet Viewer' program):
Here is a page that contains an applet version of the .jar file we just created and ran.
Doh!! Error:
Applets are subject to STRICT security (since Java 7), so we need to explicitly add the containing URL to our 'exception' ("OK to run") sites on our computer, via the Java Control Panel [on your PC/Mac]:
After adding the URL (that contains the html page that contains our applet) to the exception list, and reloading:
Success!
NOTE that most browsers - Chrome, Brave, Firefox... simply will NOT run applets, no matter what we do (enable them in the browser, add URLs to the OS exception list). The ONLY browser on the PC that executes applets, is IE:
Moral of the story: if you want interactive graphics etc. in the browser, use JavaScript!!