How Does Java Code Run?

How Does Java Code Run?

Java Runtime Process for Beginners

Table of contents


Java is a popular programming language known for its platform independence and robustness. As a software engineer, my name is Kunal Gavhane, and I've come to appreciate the power of Java in my work. When writing Java code, understanding how it runs under the hood can help developers like me optimize their programs and debug errors effectively. In this blog post, we will explore the process of how Java code runs, step by step.

  1. Java Compilation

Before a Java program can be executed, it needs to be compiled into a bytecode format that can be understood by the Java Virtual Machine (JVM). The Java compiler (javac) converts the human-readable Java source code (.java files) into platform-neutral bytecode (.class files).

During compilation, the Java compiler performs syntax and type-checking, ensuring that the code adheres to Java language rules. Any errors or issues discovered during compilation will be reported as compile-time errors.

2.Java Virtual Machine (JVM)

Java code is executed within the Java Virtual Machine (JVM). The JVM acts as an interpreter that reads and executes the bytecode generated during the compilation phase. It abstracts the operating system and hardware, allowing Java programs to run consistently across different platforms.

The JVM consists of different components, including the Class Loader, Just-In-Time (JIT) Compiler, and Garbage Collector. Let's take a closer look at each of these components.

3.**Class Loading**

The Class Loader is responsible for finding, loading, and verifying the Java class files. It locates the bytecode (.class files) required by the program and loads them into memory. The classes are loaded dynamically as they are referenced by the program, allowing for flexibility and efficient memory usage.

4.Bytecode Execution

Once the classes are loaded, the bytecode is interpreted and executed by the JVM. The JVM goes through the bytecode line by line and performs the necessary actions specified in the code. The execution involves utilizing the JVM runtime environment, memory management, and thread scheduling.

5.Just-In-Time (JIT) Compilation

To improve performance, the JVM employs a Just-In-Time (JIT) compiler. The JIT compiler analyzes the bytecode as it executes and identifies frequently executed sections of code (hot spots). It then compiles these hot spots into native machine code specific to the underlying hardware architecture. The native code is executed directly by the CPU, resulting in faster execution.

6.Memory Management

Java features automatic memory management through the Garbage Collector (GC). The GC monitors the memory usage of the program, identifies unreferenced objects, and reclaims memory occupied by those objects. This process helps prevent memory leaks and eliminates the need for manual memory deallocation.

7.Exception Handling

Java provides robust exception handling mechanisms. If an exception occurs during code execution, the JVM catches and handles it appropriately. Exceptions can be thrown explicitly by the programmer or generated by the JVM at runtime. Proper exception handling helps maintain program integrity and prevents unexpected crashes.


The Java program continues executing until it completes its tasks or encounters an exit condition. At termination, any remaining system resources are released, and the program exits gracefully. This includes closing open files, terminating network connections, and freeing memory.

Understanding how Java code runs enables developers to write more efficient, optimized, and reliable programs. From the compilation phase to bytecode execution within the JVM, each step plays a crucial role in the Java code execution process. Moreover, features such as JIT compilation, memory management, and exception handling enhance the performance and stability of Java applications. By grasping these concepts, developers can harness the power of Java to build robust software solutions.

Remember, writing proper Java code leads to efficient execution and fewer runtime errors, making your Java applications more scalable and maintainable.

Note: This blog post provides a simplified overview of the Java code execution process. Advanced topics such as threading, optimization techniques, and JVM internals have not been covered in detail.