Java虚拟机规范JDK12详解：必备文档

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本文档是Java虚拟机规范（JVM）的Java SE 12版本，由Oracle America, Inc.等专家编撰，发布日期为2019年3月。这份官方文档对于Java开发者来说至关重要，它详细阐述了Java虚拟机的工作原理、数据类型、内存结构以及运行时数据区域等内容。 1. **历史回顾**：章节1介绍了Java虚拟机的历史背景，概述了其发展和在Java平台中的核心作用。从1997年起，随着JDK12的发布，Java Virtual Machine经历了多次迭代和改进，以适应不断变化的技术需求。 2. **Java虚拟机结构**：在第二部分，重点讲解了Java类文件格式，这是JVM执行的基础。类文件是编译后的Java代码，包含了方法的二进制表示。此外，还介绍了Java的数据类型，包括基本数据类型（如整数、浮点数、布尔值）和引用数据类型。 - **基本数据类型**：包括各种整数类型（如byte、short、int、long）、浮点数（float和double）以及boolean类型。这些类型对应不同的值集和范围。 - **返回地址类型**：用于跟踪方法调用的上下文，确保程序可以正确地返回到调用的位置。 - **引用数据类型**：如对象和数组，它们存储在堆区，而非栈区，是Java面向对象编程的核心概念。 3. **运行时数据区域**：这部分深入解析了JVM中的几个关键内存区域，如程序计数器（PC Register，用于跟踪当前执行的指令）、虚拟机栈（存放局部变量和方法调用帧）、堆（存放所有对象实例和数组），这些都是JVM管理和垃圾回收的主要区域。 4. **反馈机制**：文档鼓励读者提供反馈，表明规范的编写者重视与开发者社区的互动，以便不断优化和完善Java虚拟机的设计。 JVM规范JDK12版文档是Java开发者深入了解Java虚拟机内部工作原理和优化性能的基石，通过阅读和理解这些内容，开发者能够更好地编写高效、可移植的Java应用程序。同时，随着技术的发展，这份文档也会定期更新，以反映最新的语言特性和优化策略。

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

2.3 Primitive Types and Values THE STRUCTURE OF THE JAVA VIRTUAL MACHINE

2.3.2 Floating-Point Types, Value Sets, and Values

The floating-point types are float and double, which are conceptually associated

with the 32-bit single-precision and 64-bit double-precision format IEEE 754

values and operations as specified in IEEE Standard for Binary Floating-Point

Arithmetic (ANSI/IEEE Std. 754-1985, New York).

The IEEE 754 standard includes not only positive and negative sign-magnitude

numbers, but also positive and negative zeros, positive and negative infinities, and

a special Not-a-Number value (hereafter abbreviated as "NaN"). The NaN value

is used to represent the result of certain invalid operations such as dividing zero

by zero.

Every implementation of the Java Virtual Machine is required to support two

standard sets of floating-point values, called the float value set and the double value

set. In addition, an implementation of the Java Virtual Machine may, at its option,

support either or both of two extended-exponent floating-point value sets, called

the float-extended-exponent value set and the double-extended-exponent value set.

These extended-exponent value sets may, under certain circumstances, be used

instead of the standard value sets to represent the values of type float or double.

The finite nonzero values of any floating-point value set can all be expressed in

the form s ⋅ m ⋅ 2

(e - N + 1)

, where s is +1 or -1, m is a positive integer less than

, and e is an integer between E

min

= -(2

K-1

-2) and E

max

= 2

K-1

-1, inclusive, and

where N and K are parameters that depend on the value set. Some values can

be represented in this form in more than one way; for example, supposing that a

value v in a value set might be represented in this form using certain values for

s, m, and e, then if it happened that m were even and e were less than 2

K-1

, one

could halve m and increase e by 1 to produce a second representation for the same

value v. A representation in this form is called normalized if m ≥ 2

N-1

; otherwise

the representation is said to be denormalized. If a value in a value set cannot be

represented in such a way that m ≥ 2

N-1

, then the value is said to be a denormalized

value, because it has no normalized representation.

The constraints on the parameters N and K (and on the derived parameters E

min

and E

max

) for the two required and two optional floating-point value sets are

summarized in Table 2.3.2-A.

THE STRUCTURE OF THE JAVA VIRTUAL MACHINE Primitive Types and Values 2.3

Table 2.3.2-A. Floating-point value set parameters

Parameter float float-extended-

exponent

double double-extended-

exponent

N 24 24 53 53

K 8

≥ 11

≥ 15

max

+127

≥ +1023

+1023

≥ +16383

min

-126

≤ -1022

-1022

≤ -16382

Where one or both extended-exponent value sets are supported by an

implementation, then for each supported extended-exponent value set there is

a specific implementation-dependent constant K, whose value is constrained by

Table 2.3.2-A; this value K in turn dictates the values for E

min

and E

max

Each of the four value sets includes not only the finite nonzero values that are

ascribed to it above, but also the five values positive zero, negative zero, positive

infinity, negative infinity, and NaN.

Note that the constraints in Table 2.3.2-A are designed so that every element of the

float value set is necessarily also an element of the float-extended-exponent value

set, the double value set, and the double-extended-exponent value set. Likewise,

each element of the double value set is necessarily also an element of the double-

extended-exponent value set. Each extended-exponent value set has a larger range

of exponent values than the corresponding standard value set, but does not have

more precision.

The elements of the float value set are exactly the values that can be represented

using the single floating-point format defined in the IEEE 754 standard, except

that there is only one NaN value (IEEE 754 specifies 2

-2 distinct NaN values).

The elements of the double value set are exactly the values that can be represented

using the double floating-point format defined in the IEEE 754 standard, except

that there is only one NaN value (IEEE 754 specifies 2

-2 distinct NaN values).

Note, however, that the elements of the float-extended-exponent and double-

extended-exponent value sets defined here do not correspond to the values that

can be represented using IEEE 754 single extended and double extended formats,

respectively. This specification does not mandate a specific representation for the

values of the floating-point value sets except where floating-point values must be

represented in the class file format (§4.4.4, §4.4.5).

The float, float-extended-exponent, double, and double-extended-exponent value

sets are not types. It is always correct for an implementation of the Java Virtual

2.3 Primitive Types and Values THE STRUCTURE OF THE JAVA VIRTUAL MACHINE

Machine to use an element of the float value set to represent a value of type float;

however, it may be permissible in certain contexts for an implementation to use

an element of the float-extended-exponent value set instead. Similarly, it is always

correct for an implementation to use an element of the double value set to represent

a value of type double; however, it may be permissible in certain contexts for

an implementation to use an element of the double-extended-exponent value set

instead.

Except for NaNs, values of the floating-point value sets are ordered. When

arranged from smallest to largest, they are negative infinity, negative finite values,

positive and negative zero, positive finite values, and positive infinity.

Floating-point positive zero and floating-point negative zero compare as equal, but

there are other operations that can distinguish them; for example, dividing 1.0 by

0.0 produces positive infinity, but dividing 1.0 by -0.0 produces negative infinity.

NaNs are unordered, so numerical comparisons and tests for numerical equality

have the value false if either or both of their operands are NaN. In particular, a

test for numerical equality of a value against itself has the value false if and only

if the value is NaN. A test for numerical inequality has the value true if either

operand is NaN.

2.3.3 The returnAddress Type and Values

The returnAddress type is used by the Java Virtual Machine's jsr, ret, and jsr_w

instructions (§jsr, §ret, §jsr_w). The values of the returnAddress type are pointers

to the opcodes of Java Virtual Machine instructions. Unlike the numeric primitive

types, the returnAddress type does not correspond to any Java programming

language type and cannot be modified by the running program.

2.3.4 The boolean Type

Although the Java Virtual Machine defines a boolean type, it only provides

very limited support for it. There are no Java Virtual Machine instructions solely

dedicated to operations on boolean values. Instead, expressions in the Java

programming language that operate on boolean values are compiled to use values

of the Java Virtual Machine int data type.

The Java Virtual Machine does directly support boolean arrays. Its newarray

instruction (§newarray) enables creation of boolean arrays. Arrays of type

boolean are accessed and modified using the byte array instructions baload and

bastore (§baload, §bastore).

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