Java Buzzwords
While portability and security were the primary motivations behind Java's development, several other key principles influenced its design. The Java design team summarized these foundational ideas as a set of buzzwords that define the language:
Simple: Java has a concise, cohesive set of features that make it easy to learn and use.
Secure: Java provides a secure means of creating and executing Internet applications.
Portable: Java programs can run on any system equipped with a Java Runtime Environment (JRE), making them platform-independent.
Object-Oriented: Java is built on the principles of object-oriented programming.
Robust: Java emphasizes early error checking, strong type checking, and run-time checking to create reliable programs.
Multithreaded: Java has built-in support for multithreaded (concurrent) programming.
Architecture-Neutral: Java is not tied to any specific hardware or operating system.
Interpreted: Java bytecode is executed by the Java Virtual Machine (JVM), allowing for platform independence.
High Performance: Java bytecode is optimized and can be compiled on-the-fly to native machine code using Just-In-Time (JIT) compilation.
Distributed: Java was designed with the distributed nature of the Internet in mind, with features supporting remote method invocation and networking.
Dynamic: Java carries extensive run-time type information to facilitate dynamic linking and execution.
Security in Java
When a program is downloaded and executed on a client computer, it must not pose any harm to the system. Java was specifically designed to address this concern:
Java applications run within a confined execution environment—the Java Virtual Machine (JVM)—which restricts access to critical system resources.
The sandbox model prevents Java programs from performing potentially unsafe operations, like file system or network access, unless explicitly permitted.
Java’s language structure itself enforces safety, for example by eliminating pointers and requiring strong type checking.
Portability
In order to create a truly platform-independent programming language, Java had to overcome the limitations of machine-specific code. Java achieves portability through:
The Java compiler, which outputs bytecode, an intermediate and highly portable representation of a program.
Bytecode is not tied to any specific CPU or operating system, making it universally executable on any platform with a JRE.
This approach ensures that "write once, run anywhere" is not just a slogan, but a functional capability.
The Bytecode: Java’s Magic
At the heart of Java's portability and security lies its use of bytecode:
Bytecode is an intermediate, platform-neutral format generated by the Java compiler.
It is executed by the Java Virtual Machine (JVM), which is part of the Java Runtime Environment (JRE).
The JVM interprets or compiles bytecode into native machine code at runtime, depending on the implementation.
Benefits of Bytecode Execution:
Portability: The same bytecode runs on any platform with a JRE.
Security: Because code is executed within the JVM, it can be monitored and controlled. The JVM enforces runtime constraints and maintains the sandbox environment.
Consistency: All JVMs understand the same bytecode format, ensuring consistent behavior across different systems.
Just-In-Time (JIT) Compilation
A Just-In-Time (JIT) compiler is an advanced feature of the JVM that improves performance.
During program execution, the JIT compiler converts frequently used portions of bytecode into native machine code on the fly.
This real-time compilation allows Java programs to benefit from increased execution speed, approaching that of natively compiled languages.