Java Program Development and Execution Steps
Java program normally go through five phases. These
are
- Edit,
- Compile,
- Load,
- Verify and
- Execute
We look over all the above mentioned phases in a bit detail. First consider the following
figure that summarizes the all phases of a java program
Edit
Phase 1 consists of editing a file. This is accomplished with an editor program. The
programmer types a java program using the editor like notepad, and make corrections if
necessary.
When the programmer specifies that the file in the editor should be saved, the program is
stored on a secondary storage device such as a disk. Java program file name ends with a
.java extension.
On Windows platform, notepad is a simple and commonly used editor for the beginners.
However java integrated development environments (IDEs) such as NetBeans, Borland
JBuilder, JCreator and IBM’s Ecllipse hava built-in editors that are smoothly integrated
into the programming environment.
Compile
In Phase 2, the programmer gives the command javac to compile the program. The java
compiler translates the java program into bytecodes, which is the language understood by
the java interpreter.
To compile a program called Welcome.java, type
javac Welcome.java
at the command window of your system. If the program compiles correctly, a file called
Welcome.class is produced. This is the file containing the bytecodes that will be
interpreted during the execution phase.
Loading
the program must first be placed in memory before it can be executed. This is
done by the class loader, which takes the .class file (or files) containing the bytecodes
and transfers it to memory. The .class file can be loaded from a disk on your system or
over a network (such as your local university or company network or even the internet).
Applications (Programs) are loaded into memory and executed using the java interpreter
via the command java. When executing a Java application called Welcome, the command
Java Welcome
Invokes the interpreter for the Welcome application and causes the class loader to load
information used in the Welcome program.
Verify
Before the bytecodes in an application are executed by the java interpreter, they are
verified by the bytecode verifier in Phase 4. This ensures that the bytecodes for class that
are loaded form the internet (referred to as downloaded classes) are valid and that they do
not violate Java’s security restrictions.
Java enforces strong security because java programs arriving over the network should not
be able to cause damage to your files and your system (as computer viruses might).
Execute
Finally in phase 5, the computer, under the control of its CPU, interprets the program one
bytecode at a time. Thus performing the actions specified by the program.
Programs may not work on the first try. Each of the preceding phases can fail because of
various errors. This would cause the java program to print an error message. The
programmer would return to the edit phase, make the necessary corrections and proceed
through the remaining phases again to determine id the corrections work properly.