Java Bytecode

Unlike the typical conversion of a programming language to code that works on one specific type of computer platform, Java bytecode is an intermediary level of code designed to run on JVM software. This bytecode is compact and easy for the JVM to process.

This “two-step design” makes Java programs highly “portable, ” meaning they can run on many platforms without modifications.

Over the years, JVM software has been developed for many different computer platforms, thus enabling Java code to work on them. 

Some call this benefit of Java “write once, run anywhere” or “WORA” for short. 

Origins of Java Bytecode

Java was developed in the mid-1990s by James Gosling and his team at Sun Microsystems to create a platform-independent, object-oriented programming language. Java bytecode was designed as a low-level, portable representation of Java code, allowing it to run on any platform with a compatible Java Virtual Machine (JVM).

This vision has certainly been accomplished, given the wide variety of computer systems that support Java programs more than 20 years later. The concept of the translation step to the intermediary Java bytecode has made this possible.

The Java Virtual Machine (JVM)

Knowing about the JVM is helpful in understanding bytecode. The JVM is like a translator that reads the bytecode and interprets it for your specific device.

It is an abstraction layer between the bytecode and the underlying hardware, allowing Java applications to run on various platforms without modification.

The JVM also manages memory allocation and garbage collection, simplifying development and improving application stability. 

Features for the Developer

Java bytecode is loaded and executed by the JVM as needed during an application’s runtime. The JVM’s class loader reads the .class files, verifies their integrity, and loads them into memory for execution.

This dynamic class-loading process enables features like reflection and allows Java applications to be easily extended or updated.

Since Java bytecode is closer to the actual instructions executed by the JVM, debugging tools like the Java Debugger (jdb) allow developers to inspect the execution of their code at the bytecode level, providing greater insight into potential issues. 

Security and Verification

Java bytecode is secure and robust. Before executing any bytecode, the JVM checks its integrity and makes sure it follows specific rules.

This verification process helps prevent malicious code from compromising a system or application. 

Under the Hood 

The JVM and Bytecode are cleverly engineered for speed and flexibility. These aspects are pretty technical but useful to be aware of:

1. Optimizations: Since bytecode is a lower-level representation of Java code, the JVM can perform optimizations to improve performance. For example, the Just-In-Time (JIT) compiler can convert frequently executed bytecode sequences into native machine code, speeding up the execution.
2. Instruction Set: Java bytecode consists of a set of instructions called opcodes, similar to assembly language for a hypothetical stack-based machine. Java bytecode includes about 200 of these opcodes for operations ranging from essential arithmetic functions to object manipulation. Each opcode is a one-byte instruction, making bytecode compact and easy for the JVM to process.
3. Stack-based Architecture: Unlike most real-world processors, such as actual computer CPUs, which use registers for temporary storage, the JVM uses a stack-based architecture for performing computations. This design choice simplifies the bytecode and makes creating a platform-independent representation of the code easier. 

Decompilation to Go Back 

While bytecode isn’t as readable as your original Java code, it can still be decompiled back into source code using tools like JD-GUI or JADX. Decompiling helps developers understand how libraries work or recover lost source code.

However, it also highlights the importance of protecting intellectual property with obfuscation techniques or licensing.

Why Java is Platform Independent | All about Java Byte Code – 7 mins