Learning how JVM works

Java is one of the most successful and widely used languages. Java has been very successful because of its write once, run anywhere design principle. JVM realizes this design principle by sitting between the application code and the machine code and interpreting the application code to machine code.

Traditionally, there two ways of running application code:

• Compilers: Application code is directly compiled to machine code (in C, C++). Compilers go through a build process of converting the application code to machine code. Compilers generate the most optimized code for a specific target architecture. The application code has to be compiled to target architectures. In general, the compiled code always runs faster than interpreted code, and issues with code semantics can be identified during compilation time rather than runtime.
• Interpreters: Application code is interpreted to machine code line by line (JavaScript and so on). Since interpreters run line by line, the code may not be optimized to the target architecture, and run slowly, compared to the compiled code. Interpreters have the flexibility of writing once and running anywhere. A good example is the JavaScript code that is predominantly used for web applications. This runs pretty much on different target browsers with minimal or no changes in the application code. Interpreters are generally slow and are good for running small applications.

JVM has taken the best of both interpreters and compilers. The following diagram illustrates how JVM runs the Java code using both the interpreter and compiler approaches:

Figure 1.1 – Java compiler and interpreter

Let's see how this works:

• Java Compiler (javac) compiles the Java application source code to bytecode (intermediate format).
• JVM interprets the bytecode to machine code line by line at runtime. This helps in translating the optimized bytecode to target machine code, helping in running the same application code on different target machines, without re-programming or re-compiling.
• JVM also has a Just-In-Time (JIT) compiler to further optimize the code at runtime by profiling the code.

In this section, we looked at how Java Compiler and JIT work together to run Java code on JVM at a higher level. In the next section, we will learn about the architecture of JVM.