Since this book is written for beginners, we will assume that you know almost nothing about Java. But even if you do know something, or even a lot, it is always helpful to review the basics, even if it's just so you can feel good about yourself by appreciating how much you have mastered already. So, we will start by defining the terms Java, JVM, compilation, bytecode, and more.
What is Java?
Basic terms
When talking about Java, people use Java, JVM, JDK, SDK, and Java platform as synonyms. The legal definition treats Java as Sun's trademark for a set of technologies, but we typically do not think about Java as a trademark. Most often, when somebody says Java, they mean a programming language that is used by humans to express sets of instructions (programs) that can be executed by a computer (not directly, but after the program is compiled/transformed into code that a computer understands). The human-readable Java program is called source code, and the computer-readable program (after all transformations) is called binary code, because it is expressed only using 1 and 0.
You can find the complete Java Language Specification (description) at https://docs.oracle.com/javase/specs/. It is much more accessible than one might expect, and it can be helpful even for a novice, especially if one uses it as a reference document. Do not feel discouraged by the formal language of the first few sections. Read what you can and come back later, as your understanding of Java grows and the motivation for deeper and more precise definitions increases.
JVM is a program that translates byte code of Java .class files into binary machine code and sends it to a microprocessor for execution.
Have you noticed that there are two similar terms, bytecode and byte code? In conversation, the difference is hardly noticeable, so people use them interchangeably. But there is a difference. Byte code (or Byte Code, to be precise) is a language that can be executed by a special program called JVM. By contrast, bytecode is the format (each instruction occupies one byte, thus the name) of the instructions generated by the Java compiler (another program) that reads the human-readable source code and transforms it into Byte Code.
Bytecode is a binary code expressed in the format JVM understands. JVM then reads (loads, using a program called class loader) bytecodes, transforms the instructions into binary code (instructions in a format a particular computer microprocessor, where JVM is running, understands), and passes the result to the CPU, a microprocessor that executes it.
A class is a file (with the extension .class) produced by the Java compiler (from the source code in a file with the same name and the extension .java). There are more than a dozen JVM implementations, created by different companies, but we will be focusing on Oracle JVM's implementation, which is called HotSpot. In Chapter 11, JVM Processes and Garbage Collection, we will look more closely at JVM's functionality, architecture, and processes.
On the same page as the Java Language Specification (https://docs.oracle.com/javase/specs), you can find the Java Virtual Machine Specification. We recommend that you use it as a source of references for the terminology and for the understanding of JVM functionality.
JDK is a collection of software tools and supporting libraries that allow for the creation and execution of Java language programs.
Since Java 9, applets (components that can be executed in a browser) are not supported anymore, so we will not talk much about them. An application is a Java program that can be (after compilation) executed on a computer where JVM is installed. So, JDK includes at minimum a compiler, JVM, and Java Class Library (JCL)—a collection of ready-to-use procedures that can be called by an application. But in reality, it has many other tools and utilities that can help you to compile, execute, and monitor a Java application. The subset of JDK that includes JVM, JCL, class loader, and supporting files allows the execution (running) of bytecode. Such a combination is called the Java Runtime Environment (JRE). Each Java application is executed in a separate JVM instance (copy) that has its own allocated computer memory, so two Java applications cannot talk to each other directly, but only via the network (web-services and similar means).
Software Development Kit (SDK) is a collection of software tools and supporting libraries that allow the creation of an application using a certain programming language. SDK for Java is called JDK.
So, when people use SDK in reference to JDK, they are correct, but not precise.
The Java platform is composed of a compiler, JVM, supporting libraries, and other tools from JDK.
The supporting libraries in the preceding definitions are Java standard libraries, also called JCL, and are necessary for executing bytecode. If a program requires some other libraries (not included in JCL), they have to be added at compilation time (see Chapter 3, Your Development Environment Setup, which describes how to do it) and included in the generated bytecode. Java platform can be one of the four: Java Platform Standard Edition (Java SE), Java Platform Enterprise Edition (Java EE), Java Platform Micro Edition (Java ME), or Java Card. There used to be the JavaFX Platform, too, but it has been merged into Java SE since Java 8. We will talk about the differences in the next section.
Open JDK is a free and open source implementation of Java SE.
These are the most basic terms. Other terms will be introduced as needed throughout the book, in the corresponding contexts.
History and popularity
Java was first released in 1995 by Sun Microsystems. It was derived from C and C++, but did not allow users to manipulate computer memory on a very low level, which is the source of many difficulties, including memory leak related issues, that C and C++ programmers experience if they are not very careful about it. Java stood out due to its simplicity, portability, interoperability, and safety net, which allowed it to become one of the most popular programming languages. It is estimated that as of 2017, there are close to 20 million programmers in the world (close to 4 million of them are in the US), and approximately half of them use Java. And there are good reasons to believe that the need for software developers, including Java developers, will only grow in the future. So, studying Java looks like a step towards a stable career. And learning Java is not actually very difficult. We will show you how to do it; just continue reading, thinking, and trying all the suggestions in practice on your computer.
Java was conceived as a facility that allows users to write once, run anywhere – that is another term to explain and understand. It means that compiled Java code can run on all computers with operating systems that support Java, without the need for recompilation. As you understand already, support Java means that for each operating system, an interpreter exists that can transform bytecode into binary code. That's how run anywhere is implemented: anywhere where a Java interpreter is available.
After the concept proved to be popular and Java was firmly established as one of the major players among other object-oriented languages, Sun Microsystems made much of its JVM free and open source software, governed by the GNU General Public License (GPL). In 2007, Sun Microsystems made all of its JVM's core code available under free and open source distribution terms, except for a small portion of code to which Sun did not have the copyright. In 2010, Oracle acquired Sun Microsystems and declared itself a steward of Java technology with a relentless commitment to fostering a community of participation and transparency.
Today, Java is used in many areas, most prominently in Android programming and other mobile applications, in various embedded systems (various chips and specialized computers), desktop Graphical User Interface (GUI) development, and a huge variety of web applications, including network applications and web services. Java is also widely used for scientific applications, including the rapidly expanding areas of machine learning and artificial intelligence.
Principles
There were five primary goals in the creation of the Java language, according to Design Goals of the Java TM Programming Language (http://www.oracle.com/technetwork/java/intro-141325.html). The Java language had to be:
- Object-oriented and familiar: This meant that it had to look like C++, but without unnecessary complexities (we will discuss the term object-oriented in Chapter 2, Java Language Basics)
- Architecture-neutral and portable: This meant the ability to use JVM as the environment that isolates the language (source code) from the knowledge of each particular operating system (often called the platform)
- High performance: It should work on par with the leading programming languages of the time
- Interpreted: It can be moved to an executing phase without linking (creating a single executable file from multiple .class files), thus allowing a quicker write-compile-execute cycle (modern JVMs, though, are optimized to keep the binary version of the often used .class files, to avoid repeating interpretation)
- Multithreaded: It should allow several concurrent execution jobs (threads), such as downloading an image and processing other user commands and data at the same time
- Dynamic: Linking should happen during execution
- Secure: It had to be well protected from an unauthorized modification at runtime
The result proved these goals to be well-defined and fruitful, because Java became one of the main languages of the internet era.
