In order to write and execute a software program you need the following
1) Editor – To type your program into, a notepad could be used for this
2) Compiler – To convert your high language program into native machine code
3) Linker – To combine different program files reference in your main program together.
4) Loader – To load the files from your secondary storage device like Hard Disk, Flash Drive , CD into RAM for execution. The loading is automatically done when your execute your code.
5) Execution – Actual execution of the code which is handled by your OS & processor.
With this background, refer the following video & learn the working and architecture of the Java Virtual Machine.
In this tutorial, you will learn-
To understand the Java compiling process in Java. Let's first take a quick look to compiling and linking process in C.
Suppose in the main, you have called two function f1 and f2. The main function is stored in file a1.c.
Function f1 is stored in a file a2.c
Function f2 is stored in a file a3.c
All these files, i.e., a1.c, a2.c, and a3.c, is fed to the compiler. Whose output is the corresponding object files which is the machine code.
The next step is integrating all these object files into a single .exe file with the help of linker. The linker will club all these files together and produces the .exe file.
During program run a loader program will load a.exe into the RAM for the execution.
Let's look at the process for JAVA. In your main you have two methods f1 and f2.
- main method is stored in file a1.java
- f1 is stored in file as a2.java
- f2 is stored in file as a3.java
The compiler will compile the three files and produces a corresponding .class file which consists of BYTE code. Unlike C, no linking is done.
The Java VM or Java Virtual Machine resides on the RAM. During execution, using the class loader the class files are brought on the RAM. The BYTE code is verified for any security breaches.
Next, the execution engine will convert the Bytecode into Native machine code. This is just in time compiling. It is one of the main reason why Java is comparatively slow.
There is more to the JVM architecture which will be discussed in the later tutorial.
NOTE: JIT or Just-in-time compiler is the part of the Java Virtual Machine (JVM). It interprets part of the Byte Code that has similar functionality at the same time.
JVM stands for Java Virtual Machine. It is the engine that drives the Java Code. It converts Java bytecode into machines language.
- In other programming language, the compiler produces code for a particular system. But Java compiler produces code for a Virtual Machine.
- In JVM, Java code is compiled into bytecode. This bytecode gets interpreted on different machines
- Between host system and Java source, Bytecode is an intermediary language.
- JVM is responsible for allocating a memory space.
- Programming languages are classifies as
- Higher Level Language Ex. C++ , Java
- Middle Level Languages Ex. C
- Low Level Language Ex Assembly
- finally the lowest level as the Machine Language.
A compiler is a program which converts a program from one level of language to another. Example conversion of C++ program into machine code.
The java compiler is a convert's high level java code into bytecode (which is also a type of machine code).
A interpreter is a program which converts a program at one level to another programming language at the same level. Example conversion of Java program into C++
In Java , the Just In Time Code generator converts the bytecode into the native machine code which are at the same programming levels.
Hence java is both compiled as well as interpreted language.
The two main reasons behind the slowness of Java are
Dynamic Linking = Unlike C, linking is done at run-time , every time the program is run in Java.
Run-time Interpreter = The conversion of byte code into native machine code is done at run-time in Java which furthers slows down the speed
However, the latest version of Java have addressed the performance bottlenecks to a great extent.
- JVM or Java Virtual Machine is the engine that drives the Java Code. It converts Java bytecode into machines language.
- In JVM, Java code is compiled to bytecode. This bytecode gets interpreted on different machines
- JIT or Just-in-time compiler is the part of the Java Virtual Machine (JVM). It is used to speed up the execution time
- In comparison to other compiler machine, Java may be slow in execution.