Java SE 8虚拟机详解：新特性和class文件深度解析

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1.2 The Java Virtual Machine INTRODUCTION

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.

1.2 The Java Virtual Machine

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 format. A 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 in a 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.

INTRODUCTION Organization of the Specification 1.3

The Java Virtual Machine specified here is compatible with the Java SE 8 platform,

and supports the Java programming language specified in The Java Language

Specification, Java SE 8 Edition.

1.3 Organization of the Specification

Chapter 2 gives an overview of the Java Virtual Machine architecture.

Chapter 3 introduces compilation of code written in the Java programming

language into the instruction set of the Java Virtual Machine.

Chapter 4 specifies the class file format, the hardware- and operating system-

independent binary format used to represent compiled classes and interfaces.

Chapter 5 specifies the start-up of the Java Virtual Machine and the loading,

linking, and initialization of classes and interfaces.

Chapter 6 specifies the instruction set of the Java Virtual Machine, presenting the

instructions in alphabetical order of opcode mnemonics.

Chapter 7 gives a table of Java Virtual Machine opcode mnemonics indexed by

opcode value.

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 8 Edition, the

reader is referred to The Java Language Specification, Java SE 8 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 8 Edition, the reader is referred to Chapter 17 of The Java Language

Specification, Java SE 8 Edition for information about threads and locks. Chapter

17 reflects The Java Memory Model and Thread Specification produced by the JSR

133 Expert Group.

2.2 Data Types THE STRUCTURE OF THE JAVA VIRTUAL MACHINE

2.2 Data Types

Like the Java programming language, the Java Virtual Machine operates on two

kinds of types: primitive types and reference types. There are, correspondingly, two

kinds of values that can be stored in variables, passed as arguments, returned by

methods, and operated upon: primitive values and reference values.

The Java Virtual Machine expects that nearly all type checking is done prior

to run time, typically by a compiler, and does not have to be done by the Java

Virtual Machine itself. Values of primitive types need not be tagged or otherwise

be inspectable to determine their types at run time, or to be distinguished from

values of reference types. Instead, the instruction set of the Java Virtual Machine

distinguishes its operand types using instructions intended to operate on values of

specific types. For instance, iadd, ladd, fadd, and dadd are all Java Virtual Machine

instructions that add two numeric values and produce numeric results, but each is

specialized for its operand type: int, long, float, and double, respectively. For a

summary of type support in the Java Virtual Machine instruction set, see §2.11.1.

The Java Virtual Machine contains explicit support for objects. An object is

either a dynamically allocated class instance or an array. A reference to an object

is considered to have Java Virtual Machine type reference. Values of type

reference can be thought of as pointers to objects. More than one reference to an

object may exist. Objects are always operated on, passed, and tested via values of

type reference.

2.3 Primitive Types and Values

The primitive data types supported by the Java Virtual Machine are the numeric

types, the boolean type (§2.3.4), and the returnAddress type (§2.3.3).

The numeric types consist of the integral types (§2.3.1) and the floating-point types

(§2.3.2).

The integral types are:

• byte, whose values are 8-bit signed two's-complement integers, and whose

default value is zero

• short, whose values are 16-bit signed two's-complement integers, and whose

default value is zero

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