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The Java Virtual Machine Specification Java SE 14 Edition PDF 下载
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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 systemindependence, 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.
2
INTRODUCTION Organization of the Specification 1.3
The Java Virtual Machine specified here is compatible with the Java SE 14
platform, and supports the Java programming language specified in The Java
Language Specification, Java SE 14 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 systemindependent 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 14 Edition, the
reader is referred to The Java Language Specification, Java SE 14 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 14 Edition, the reader is referred to Chapter 17 of The Java Language
Specification, Java SE 14 Edition for information about threads and locks. Chapter
17 reflects The Java Memory Model and Thread Specification produced by the JSR
133 Expert Group.
3
1.4 Notation INTRODUCTION
1.4 Notation
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 java.lang.
Whenever we refer to a class or interface that is declared in the package java or
any of its subpackages, the intended reference is to that class or interface as loaded
by the bootstrap class loader (§5.3.1).
Whenever we refer to a subpackage of a package named java, the intended
reference is to that subpackage as determined by the bootstrap class loader.
The use of fonts in this specification is as follows:
• A fixed width font is used for Java Virtual Machine data types, exceptions,
errors, class file structures, Prolog code, and Java code fragments.
• 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.
Non-normative information, designed to clarify the specification, is given in
smaller, indented text.
This is non-normative information. It provides intuition, rationale, advice, examples, etc.
1.5 Feedback
Readers are invited to report technical errors and ambiguities in The Java® Virtual
Machine Specification to jls-jvms-spec-comments@openjdk.java.net.
Questions concerning the generation and manipulation of class files by javac (the
reference compiler for the Java programming language) may be sent to compilerdev@openjdk.java.net. 4
CHAPTER 2
The Structure of the Java
Virtual Machine
THIS document specifies an abstract machine. It does not describe any particular
implementation of the Java Virtual Machine.
To implement the Java Virtual Machine correctly, you need only be able to
read the class file format and correctly perform the operations specified therein.
Implementation details that are not part of the Java Virtual Machine's specification
would unnecessarily constrain the creativity of implementors. For example, the
memory layout of run-time data areas, the garbage-collection algorithm used, and
any internal optimization of the Java Virtual Machine instructions (for example,
translating them into machine code) are left to the discretion of the implementor.
All references to Unicode in this specification are given with respect to The
Unicode Standard, Version 12.1.0, available at https://www.unicode.org/.
2.1 The class File Format
Compiled code to be executed by the Java Virtual Machine is represented using
a hardware- and operating system-independent binary format, typically (but not
necessarily) stored in a file, known as the class file format. The class file format
precisely defines the representation of a class or interface, including details such
as byte ordering that might be taken for granted in a platform-specific object file
format.
Chapter 4, "The class File Format", covers the class file format in detail.
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