Java Bytecode is a guide to the Java virtual machine. It is like an assembler, an alias for C++ code. After compiling the Java program, there will be a generation of Java bytecode. More precisely, Java bytecode is machine code in the .class files form. Using the Java bytecode, we can achieve platform independence in Java.

While working with the Java virtual machine ecosystem, it is essential to invest some time and understand how it works. Even if you are just starting as a novice, you must understand in simple language and know the explanation of JVM, how it compiles, the bytecode it uses, and how you can read them.

Advantage of Java Bytecode

Platform independence is one of the main reasons James Gosling started creating Java, and bytecode implementation has made it possible. Therefore, bytecode is an essential part of any Java program. The JVM instruction set can vary from system to system; however, they can all be used for bytecode interpretation. Always remember that bytecode is non-executable code and depends on the availability of the interpreter being executed, so comes the importance of JVM.

Bytecode is an important machine-level language that is executed on the JVM – Java Virtual Machine. Anytime you load a class, it receives a bytecode stream for each method of the class. Whenever you call this method in program execution, the bytecode of this method is called. Javac does both programs.

So, it becomes a reality that implementing bytecode makes Java a language that is platform-independent. This enhances the portability of Java, which does not contain languages ​​such as C or C ++. Being portable allows Java to be applicable on different platforms, such as desktops, mobile devices, servers, and more. By interpreting the subcode, Sun Microsystems marks Java as Write once, read anywhere (WORA) in relation to interpreting bytecode.

How does Bytecode Work

When executing a Java program, the compiler will compile this code and generate bytecode for each method in the program in the form of a .class file.

In addition, we can run this bytecode on any other platform. However, bytecode is a non-executable code that needs or depends on an interpreter. The JVM plays an important role in how bytecode works.

Generation of bytecode after the compilation is executed by the JVM – Java virtual machine. The Java virtual machine provides the resources available for execution for seamless execution, which requires the allocation of resources by the processor.

What is the JVM (Java Virtual Machine)?

The JVM is simply a mechanism that reads compiled code in the Java Virtual Machine Specification format and runs on the current computer. The advantage of this method is primarily cross-platform compatibility because the compiled code (known as bytecode) must be platform-independent.

This means that the compiled code on a Linux machine and code compiled on a Windows machine must work in the JVM (Java Virtual Machine) in any way. The compiled .class files can be copied from Linux to Windows and then run without problems and vice versa.

That is, when you install Java on a Windows PC, the platform-specific Java runtime tools use a JIT compiler to execute your code on Windows. On the other hand, Javac compiles your .java files into a common code format.

The bytecode is a format that adheres to the Java virtual machine specification. It has different activation functions based on the current version. These functions are specified per JSR or Java specification requirements and present implementation.

How to read Bytecode?

To make things more practical to increase understanding and how to read bytecode using a simple program.

In this article, we will make use of IntelliJ IDEA, using the ASM Bytecode Outline Plugin; however, you can also utilize the VScode and Javap.

Create a new Java program using the editor dialog.

Create a java file known as DetermineOS.java using the code below:

public class DetermineOS {



public static void main(String[] args) {



String strOSName = System.getProperty("os.name");



System.out.print("Display the current OS name example.. OS is ");



if(strOSName != null)



{



if(strOSName.toLowerCase().contains("linux"))



System.out.println("Linux");



else



System.out.print("not Linux");



}



}



}

The code is written above only retrieves the system property of the os.name, and you can check whether it has the string Linux. Then, your case will determine the strings that will be printed.

Click on View -> Show Bytecode with Jclasslib.

In the panel that will come upon the right side, you will see the information concerning the Bytecode. Some of this information is explained below. They will help you understand how you can read the bytecode easily.

General Information: This particular section shows some information that will help you understand that version of the JVM that compiled this class, the constant available in the constant pool, class access flags, and other counters:

Constant Pool: The JVM loads constant hash maps for each type that appears in the classpath. The mapping essentially consists of literal values, strings, and other types of field references. A unique value references all the values.

Interfaces+Fields: This section shows all interface and field declarations. If you did not specify any, this section would be empty.

Methods: This section contains the center of the bytecode. In your program, you can decide to define the main method. While compiling code using Javac, it creates two inputs, one for the method and another for the main method.

Attributes: Under this section, info about the original source file will appear here. You will see the file name pointing to the Constant Pool map.

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