Phases of a Java Program

 

  Phase 1: Creating/editing a program

•        The editing phase is the stage where the programmer writes the Java source code with editor.

•        The following are some Java Integrated Development Environments (IDEs).

–       Jbuilder, NetBeans , Sun ONE Studio, Eclipse, jEdit, Jcreator, BlueJ, jGRASP, etc.

–       Most of this editors  can be downloaded for free.

–       Any text editor can also be used for creating Java programs.

–       The filename must match the public class name.

–       Programs written on IDEs  or any other editors are known as Source Code.

–       Java source code file names end with .java extension

Phase 2: Compiling Java Programs into Bytecodes

•        Since a computer cannot understand a source program, program called a compiler is used to translate the source program into a machine language program called an object program.

•        With Java, you write the program once, and compile the source program into a special type of object code, known as bytecode.

o   The Java compiler (javac) checks the code.

o   It detects syntax errors.

o   It converts .java file into .class file.

•        To compile Java Program

o   Javac welcome.java

•        To execute java programs

o   Java welcome

• This .class file contains bytecode, not machine code.
• If there are syntax errors, compilation fails.

 Phase 3: Loading a program

•        A program must be placed in memory before it can execute : a process known as loading.

•        The class loader takes/locates  the .class file (produced during compilation process) containing  the program’s bytecodes and  transform them to primary memory.

•        The class loader also loads any of the .class files provided by java that your program uses.

•        It prepares them for execution.;

•        The .class files can be loaded from a disk on your system or over a network.

Phase 4 : Bytecode Verification

•        Involves examining bytecodes to ensure that they are valid and do not violate Java’s security restriction.

•        Java enforces strong security, to make sure that Java programs arriving over the network do not damage your files or your system (as computer viruses and worms might).

Phase 5: Execution: the program actually runs.

•        The JVM executes the program’s bytecodes, thus performing the actions specified by the program.

•        .class file is not final machine code yet. It is virtual machine code. JVM need a second compilation to convert virtual machine code to real machine code. We call it just-in-time compiler (JIT).

•        This greatly affects the speed of the execution.

•        Traditionally, our C/C++ program can finish all the compilation before execution. The generated machine code could run directly on OS or hardware.