JAVA COMPILATION PROCESS

Java code does not compile to native code that the operating system executes on the CPU, rather the result of Java program compilation is intermediate bytecode. This bytecode runs in the virtual machine.

Java requires each class to be placed in its own source file, named with the same name as the class name and added suffix .java. This basically forces any medium sized program to be split in several source files. When compiling source code, each class is placed in its own .class file that contains the bytecode. The java compiler differs from gcc/g++ in the fact that if the class you are compiling is dependent on a class that is not compiled or is modified since it was last compiled, it will compile those additional classes. After compiling all source files, the result will be at least as much class files as the sources, which will combine to form Java program. This is where the class loader comes into picture along with the bytecode verifier - two unique steps that distinguish Java from languages like C/C++.

The class loader is responsible for loading each class bytecode. Java provides developers with the opportunity to write their own class loader, which gives developers great flexibility. One can write a loader that fetches the class from everywhere.

STEPS TAKEN BY A LOADER TO LOAD A CLASS:
When a class is needed by the JVM the loadClass(String name, Boolean resolve); method is called passing the class name to be loaded. Once it finds the file that contains the byte code for the class, it is read into memory and passed to the defineClass. If the class is not found by the loader, it can pass the loading to a parent class loader or try to use findSystemClass to load the class from local file system. The Java Virtual Machine Specification is vague on the subject of when and how the Byte Code verifier is invoked, but by a simple test we can infer that the defineClass performs the bytecode verification. The verifier does four passes over the bytecode to make sure it is safe. After the class is successfully verified, its loading is completed and it is available for use by the runtime.

The nature of the Java byte code allows people to easily decompile class files to source. In the case where default compilation is performed, even variable and method names are recovered.