How Java is Platform independent
Why Java is known as
platform independent, whats the internal mechanism which enables this.
Most popular Operating Systems are
By definition Operating System is a resource allocator(basic essence of OS ). The real difference comes in the way they allocate resources , the algorithms and optimization each has done to allocate the resources( hard disk, CPU , RAM , network).
Depending on the way of allocating resource the System calls to interact with the Operating Systems are also different for different Operating System. The instructions they need to execute a task is also different.
The executable file that a OS can understand varies across OS.
For a program to become a process, the program needs to be present in the format the OS understands.
Lets Take the case of C language.
- A code written in C compiled in Windows will result in a executable which can never be understood by Ubuntu as the exe file format is different from ELF and vice versa.
- A C library if it needs to be shared across multiple OS platforms , then the code needs to be compiled against the different OS.
- Binary formed on one platform cannot be used on other.
How does JAVA manages to be OS independent.
1. Java code is compiled to create a .class file format.
2. The class file format is neither EXE or ELF its a different file format .
3. The Class file is consumed by the Java Virtual Machine which in turn generates platform dependent instructions.
4. Java Virtual Machine is a abstraction over the actual machine and provides its own instruction set hence its known as Virtual Machine.
5. Java Virtual Machine is a binary and is machine dependent. For windows JVM is a EXE file, for Linux JVM a ELF file.
6. Java Virtual Machine takes .class file as input and generates instruction pertaining to the platform.
7. JVM code has been compiled against the Different OS and on a given platform , the JVM binary specific to the platform needs to be used on that platform.
8. JVM input instruction set is constant across all the platforms , the OS specific instructions generated by the JVM is different for different platform.
9. SUN JVM is written in C while the Java Compiler is written in JAVA.
On Ubuntu JVM file Type is
file /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=8b0e245cdb52ffc478774ec7c54577531ecc9394, stripped