Java虚拟机规范详解：JDK15版

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"JDK15的Java虚拟机规范提供了Java SE 15版的详细技术规格，由Tim Lindholm、Frank Yellin、Gilad Bracha、Alex Buckley和Daniel Smith等人编写。该规范的版本号为15，于2020年9月发布，由Oracle America, Inc.版权所有。此规范的目的是为了定义Java虚拟机（JVM）的工作原理，以确保Java程序的可移植性和兼容性。文档内容涵盖了Java虚拟机的各个方面，包括历史背景、结构、类文件格式、数据类型、运行时数据区域等关键概念。 1. 引言部分介绍了Java虚拟机的历史，以及其在Java平台中的核心作用。它还阐述了规范的组织结构和使用的符号约定，鼓励读者提供反馈以改进规范。 2. 虚拟机的结构是文档的重点，其中详细描述了： - 类文件格式：这是编译后的Java程序的基本单元，包含类和接口的信息。 - 数据类型：分为原始类型（如整型和浮点型）和引用类型（如对象和数组）。原始类型进一步细分为积分类型、浮点类型和布尔类型，而引用类型则涉及对象的表示。 - 运行时数据区域：包括程序计数器、JVM栈、堆、方法区、运行时常量池和本地方法栈，这些都是JVM执行程序时存储不同类型数据的地方。 3. 其中，运行时数据区域的描述详细解释了每个区域的功能： - 程序计数器：用于记录当前线程正在执行的字节码指令的地址。 - JVM栈：每个方法都有一个独立的栈帧，用于存储局部变量、操作数栈和方法返回信息。 - 堆：是所有对象实例和数组的存储区域，实现垃圾回收机制。 - 方法区：存储类和接口的结构信息，如常量、字段和方法数据。 - 运行时常量池：存放各种字面量和符号引用。 - 本地方法栈：为Java Native Interface（JNI）调用的本地方法服务。这个规范不仅对理解JVM的工作原理至关重要，也为开发者提供了编写兼容Java虚拟机的代码的指导。对于想要深入理解Java内存模型、垃圾收集机制、字节码执行过程的开发者来说，这份文档是宝贵的参考资料。"

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 binary32 and 64-bit binary64 floating-point formats for IEEE 754

values and operations, as specified in the IEEE 754 Standard (JLS §1.7).

In Java SE 15 and later, the Java Virtual Machine uses the 2019 version of the IEEE 754

Standard. Prior to Java SE 15, the Java Virtual Machine used the 1985 version of the IEEE

754 Standard, where the binary32 format was known as the single format and the binary64

format was known as the double format.

IEEE 754 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

−

N + 1)

, where s is +1 or

−1,

m is a positive integer less than

, and e is an integer between E

min

−(2

−1

−2)

and E

max

= 2

−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 subnormal. 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 subnormal value, because

its magnitude is below the magnitude of the smallest normalized value.

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 binary32 floating-point format defined by IEEE 754, 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

binary64 floating-point format defined by IEEE 754, 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,

respectively, the binary32 extended and binary64 extended floating-point formats

defined by IEEE 754. 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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