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The Java® programming language is a general-purpose, concurrent, object-oriented language. Its syntax is similar to C and C++, but it omits many of the features that make C and C++ complex, confusing, and unsafe. The Java platform was initially developed to address the problems of building software for networked consumer devices. It was designed to support multiple host architectures and to allow secure delivery of software components. To meet these requirements, compiled code had to survive transport across networks, operate on any client, and assure the client that it was safe to run.
The popularization of the World Wide Web made these attributes much more interesting. Web browsers enabled millions of people to surf the Net and access media-rich content in simple ways. At last there was a medium where what you saw and heard was essentially the same regardless of the machine you were using and whether it was connected to a fast network or a slow modem.
Web enthusiasts soon discovered that the content supported by the Web's HTML document format was too limited. HTML extensions, such as forms, only highlighted those limitations, while making it clear that no browser could include all the features users wanted. Extensibility was the answer.
The HotJava browser first showcased the interesting properties of the Java programming language and platform by making it possible to embed programs inside HTML pages. Programs are transparently downloaded into the browser along with the HTML pages in which they appear. Before being accepted by the browser, programs are carefully checked to make sure they are safe. Like HTML pages, compiled programs are network- and host-independent. The programs behave the same way regardless of where they come from or what kind of machine they are being loaded into and run on.
A Web browser incorporating the Java platform is no longer limited to a predetermined set of capabilities. Visitors to Web pages incorporating dynamic content can be assured that their machines cannot be damaged by that content. Programmers can write a program once, and it will run on any machine supplying a Java run-time environment.
The Java Virtual Machine is the cornerstone of the Java platform. It is the component of the technology responsible for its hardware- and operating system-independence, the small size of its compiled code, and its ability to protect users from malicious programs.
The Java Virtual Machine is an abstract computing machine. Like a real computing machine, it has an instruction set and manipulates various memory areas at run time. It is reasonably common to implement a programming language using a virtual machine; the best-known virtual machine may be the P-Code machine of UCSD Pascal.
The first prototype implementation of the Java Virtual Machine, done at Sun Microsystems, Inc., emulated the Java Virtual Machine instruction set in software hosted by a handheld device that resembled a contemporary Personal Digital Assistant (PDA). Oracle's current implementations emulate the Java Virtual Machine on mobile, desktop and server devices, but the Java Virtual Machine does not assume any particular implementation technology, host hardware, or host operating system. It is not inherently interpreted, but can just as well be implemented by compiling its instruction set to that of a silicon CPU. It may also be implemented in microcode or directly in silicon.
The Java Virtual Machine knows nothing of the
Java programming language, only of a particular binary format, the
class file contains Java Virtual Machine instructions
(or bytecodes) and a symbol table, as well as
other ancillary information.
For the sake of security, the
Java Virtual Machine imposes strong syntactic and structural constraints on the code
class file. However, any language with functionality that can
be expressed in terms of a valid
class file can be hosted by the
Java Virtual Machine. Attracted by a generally available, machine-independent
platform, implementors of other languages can turn to the Java Virtual Machine as a
delivery vehicle for their languages.
The Java Virtual Machine specified here is compatible with the Java SE 12 platform, and supports the Java programming language specified in The Java Language Specification, Java SE 12 Edition.
In the Second Edition of The Java® Virtual Machine Specification, Chapter 2 gave an overview of the Java programming language that was intended to support the specification of the Java Virtual Machine but was not itself a part of the specification. In The Java Virtual Machine Specification, Java SE 12 Edition, the reader is referred to The Java Language Specification, Java SE 12 Edition for information about the Java programming language. References of the form: (JLS §x.y) indicate where this is necessary.
In the Second Edition of The Java® Virtual Machine Specification, Chapter 8 detailed the low-level actions that explained the interaction of Java Virtual Machine threads with a shared main memory. In The Java Virtual Machine Specification, Java SE 12 Edition, the reader is referred to Chapter 17 of The Java Language Specification, Java SE 12 Edition for information about threads and locks. Chapter 17 reflects The Java Memory Model and Thread Specification produced by the JSR 133 Expert Group.
Throughout this specification
we refer to classes and interfaces drawn from the Java SE Platform
API. Whenever we refer to a class or interface (other than those
declared in an example) using a single
identifier N, the intended reference is to the
class or interface named N in the package
java.lang. We use the fully qualified name for classes or interfaces
from packages other than
Whenever we refer to a class
or interface that is declared in the package
or any of its subpackages, the intended reference is to that class or
interface as loaded by the bootstrap class loader
Italic is used for Java Virtual Machine "assembly language", its opcodes and operands, as well as items in the Java Virtual Machine's run-time data areas. It is also used to introduce new terms and simply for emphasis.
This is non-normative information. It provides intuition, rationale, advice, examples, etc.