java11jvm特性（EN-官方）

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, andany 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 TheUnicode Standard, Version 10.0.0, available at http://www.unicode.org/.

2.1 The class File Format

Compiled code to be executed by the Java Virtual Machine is represented usinga hardware- and operating system-independent binary format, typically (but notnecessarily) stored in a file, known as the class file format. The class file formatprecisely defines the representation of a class or interface, including details suchas byte ordering that might be taken for granted in a platform-specific object fileformat.

Chapter 4, "The class File Format", covers the class file format in detail.

2.2 Data Types

THE STRUCTURE OF THE JAVA VIRTUAL MACHINE

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 zeroTHE STRUCTURE OF THE JAVA VIRTUAL MACHINE

Primitive Types and Values

2.37

• int, whose values are 32-bit signed two's-complement integers, and whose default value is zero

• long, whose values are 64-bit signed two's-complement integers, and whose default value is zero

• char, whose values are 16-bit unsigned integers representing Unicode code points in the Basic Multilingual Plane, encoded with UTF-16, and whose default value is the null code point ('\u0000')

The floating-point types are:

• float, whose values are elements of the float value set or, where supported, the float-extended-exponent value set, and whose default value is positive zero

• double, whose values are elements of the double value set or, where supported, the double-extended-exponent value set, and whose default value is positive zero

The values of the boolean type encode the truth values true and false, and the default value is false.

The First Edition of The Java® Virtual Machine Specification did not consider boolean to be a Java Virtual Machine type. However, boolean values do have limited support in the Java Virtual Machine. The Second Edition of The Java® Virtual Machine Specification clarified the issue by treating boolean as a type.

The values of the returnAddress type are pointers to the opcodes of Java Virtual Machine instructions. Of the primitive types, only the returnAddress type is not directly associated with a Java programming language type.

2.3.1 Integral Types and Values

The values of the integral types of the Java Virtual Machine are:

• For byte, from -128 to 127 (-27 to 27

- 1), inclusive

• For short, from -32768 to 32767 (-215 to 215 - 1), inclusive

• For int, from -2147483648 to 2147483647 (-231 to 231 - 1), inclusive

• For long, from -9223372036854775808 to 9223372036854775807 (-263 to 263

- 1), inclusive

• For char, from 0 to 65535 inclusive2.3

Primitive Types and Values

THE STRUCTURE OF THE JAVA VIRTUAL MACHINE

8

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 than2N, and e is an integer between Emin = -(2K-1-2) and Emax = 2K-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 fors, m, and e, then if it happened that m were even and e were less than 2K-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 ≥ 2N-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 ≥ 2N-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 Emin and Emax) for the two required and two optional floating-point value sets are summarized in Table 2.3.2-A.

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