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精通Java 9至11：Java并发、集合与最新I/O API

Java

Java11

Nutshell

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"Java in a Nutshell 7th Edition 是一本专为Java程序员设计的指南，涵盖了从Java 9到11的最新更新。这本书由Benjamin J. Evans和David Flanagan共同撰写，旨在帮助既有经验的开发者充分利用新版本的Java特性，同时也适合初学者学习。书中包含了大量示例，展示了如何利用现代Java API和最佳实践进行开发。" 在这本书的第一部分，读者将获得Java编程语言和Java平台核心运行时的快速且全面的介绍。这部分内容会涉及Java 9至11的重要变化，帮助读者迅速掌握语言细节。此外，还会讲解面向对象编程的基础，如使用基本的Java语法，以及深入探讨泛型、枚举、注解和lambda表达式等现代Java特性。书的第二部分是一个对核心概念和API的详尽参考，涵盖了在Java环境中进行实际编程工作所需的关键知识。这包括面向对象设计的基本技术，例如类、接口和继承。同时，还深入讲解了并发性和内存管理，解析了它们之间的相互作用，这对于理解和编写高效多线程程序至关重要。书中还涵盖了Java集合框架，指导读者如何有效地操作数据结构，并处理常见的数据格式。此外，还深入研究了Java最新的I/O API，特别是异步通道的使用，这在处理高性能I/O任务时非常有用。值得一提的是，本书还介绍了如何在Java虚拟机（JVM）上利用Nashorn JavaScript引擎执行JavaScript代码，从而实现Java与JavaScript的交互。本书的每个版本都会根据Java的最新进展进行更新，确保读者能够跟上Java技术的步伐。无论是对于Java开发者提升技能，还是新手入门，"Java in a Nutshell 7th Edition"都是一本不可或缺的参考书。通过阅读这本书，读者将能够充分理解并应用Java 9到11的各种新特性，提高编程效率和代码质量。

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Chapter 1. Introduction to the

Java Environment

Welcome to Java 11.

That version number probably surprises you as much as it does us. It seems

like only yesterday that Java 5 was the new thing, and yet here we are, 14

years and 6 major versions later.

You may be coming to the Java ecosystem from another language, or maybe

this is your first programming language. Whatever road you may have

traveled to get here, welcome—we’re glad you’ve arrived.

Java is a powerful, general-purpose programming environment. It is one of

the most widely used programming languages in the world, and has been

exceptionally successful in business and enterprise computing.

In this chapter, we’ll set the scene by describing the Java language (which

programmers write their applications in), the Java Virtual Machine (which

executes those applications), and the Java ecosystem (which provides a lot of

the value of the programming environment to development teams).

We’ll briefly cover the history of the Java language and virtual machine,

before moving on to discuss the lifecycle of a Java program and clear up

some common questions about the differences between Java and other

environments.

At the end of the chapter, we’ll introduce Java security, and discuss some of

the aspects of Java that relate to secure coding.

The Language, the JVM, and the Ecosystem

The Java programming environment has been around since the late 1990s. It

comprises the Java language, and the supporting runtime, otherwise known as

the Java Virtual Machine (JVM).

At the time that Java was initially developed, this split was considered novel,

but recent trends in software development have made it more commonplace.

Notably, Microsoft’s .NET environment, announced a few years after Java,

adopted a very similar approach to platform architecture.

One important difference between Microsoft’s .NET platform and Java is that

Java was always conceived as a relatively open ecosystem of multiple

vendors, albeit led by a steward who owns the technology. Throughout Java’s

history, these vendors have both cooperated and competed on aspects of Java

technology.

One of the main reasons for the success of Java is that this ecosystem is a

standardized environment. This means there are specifications for the

technologies that comprise the environment. These standards give the

developer and consumer confidence that the technology will be compatible

with other components, even if they come from a different technology

vendor.

The current steward of Java is Oracle Corporation (who acquired Sun

Microsystems, the originator of Java). Other corporations, such as Red Hat,

IBM, Amazon, AliBaba, SAP, Azul Systems, and Fujitsu are also heavily

involved in producing implementations of standardized Java technologies.

TIP

As of Java 11, the primary reference implementation of Java is the open source OpenJDK, which

many of these companies collaborate on and base their shipping products upon.

Java actually comprises several different but related environments and

specifications, such as Java Mobile Edition (Java ME), Java Standard

Edition (Java SE), and Java Enterprise Edition (Java EE). In this book, we’ll

only cover Java SE, version 11, with some historical notes related to when

certain features were introduced into the platform.

We will have more to say about standardization later, so let’s move on to

discuss the Java language and JVM as separate but related concepts.

What Is the Java Language?

Java programs are written as source code in the Java language. This is a

human-readable programming language, which is strictly class based and

object oriented. The language syntax is deliberately modeled on that of C and

C++ and it was explicitly intended to be familiar to programmers coming

from those languages.

NOTE

Although the source code is similar to C++, in practice Java includes features and a managed

runtime that has more in common with more dynamic languages such as Smalltalk.

Java is considered to be relatively easy to read and write (if occasionally a bit

verbose). It has a rigid grammar and simple program structure, and is

intended to be easy to learn and to teach. It builds on industry experience

with languages like C++ and tries to remove complex features as well as

preserving “what works” from previous programming languages.

Overall, Java is intended to provide a stable, solid base for companies to

develop business-critical applications. As a programming language, it has a

relatively conservative design and a slow rate of change. These properties are

a conscious attempt to serve the goal of protecting the investment that

organizations have made in Java technology.

The language has undergone gradual revision (but no complete rewrites)

since its inception in 1996. This does mean that some of Java’s original

design choices, which were expedient in the late 1990s, are still affecting the

language today—see Chapters 2 and 3 for more details.

Java 8 added the most radical changes seen in the language for almost a

decade (some would say since the birth of Java). Features like lambda

expressions and the overhaul of the core Collections code were enormously

popular and changed forever the way that Java developers write code. Since

then, the platform has produced a release (Java 9) that adds a major (and

long-delayed) feature: the platform modules system (JPMS).

With that release, the project has transitioned to a new, much faster release

model where new Java versions are released every six months—bringing us

up to Java 11. The Java language is governed by the Java Language

Specification (JLS), which defines how a conforming implementation must

behave.

What Is the JVM?

The JVM is a program that provides the runtime environment necessary for

Java programs to execute. Java programs cannot run unless there is a JVM

available for the appropriate hardware and OS platform we wish to execute

on.

Fortunately, the JVM has been ported to run on a large number of

environments—anything from a set-top box or Blu-ray player to a huge

mainframe will probably have a JVM available for it.

Java programs are typically started from a command line like this:

java <arguments> <program name>

This brings up the JVM as an operating system process that provides the Java

runtime environment, and then executes our program in the context of the

freshly started (and empty) virtual machine.

It is important to understand that when the JVM takes in a Java program for

execution, the program is not provided as Java language source code. Instead,

the Java language source must have been converted (or compiled) into a form

known as Java bytecode. Java bytecode must be supplied to the JVM in a

format called class files (which always have a .class extension).

The JVM provides an execution environment for the program. It starts an

interpreter for the bytecode form of the program that steps through one

bytecode instruction at a time. However, production JVMs also provide a

runtime compiler that will accelerate the important parts of the program by

replacing them with equivalent compiled machine code.

You should also be aware that both the JVM and the user program are

capable of spawning additional threads of execution, so that a user program

may have many different functions running simultaneously.

The design of the JVM built on many years of experience with earlier

programming environments, notably C and C++, so we can think of it as

having several different goals—which are all intended to make life easier for

the programmer:

Comprise a container for application code to run inside

Provide a secure and reliable execution environment as compared to

C/C++

Take memory management out of the hands of developers

Provide a cross-platform execution environment

These objectives are often mentioned together in discussions of the platform.

We’ve already mentioned the first of these goals, when we discussed the

JVM and its bytecode interpreter—it functions as the container for

application code.

We’ll discuss the second and third goals in Chapter 6, when we talk about

how the Java environment deals with memory management.

The fourth goal, sometimes called “write once, run anywhere” (WORA), is

the property that Java class files can be moved from one execution platform

to another, and they will run unaltered provided a JVM is available.

This means that a Java program can be developed (and converted to class

files) on a machine running macOS, and then the class files can be moved to

Linux or Microsoft Windows (or other platforms) and the Java program will

run without any further work needed.

NOTE

The Java environment has been very widely ported, including to platforms that are very different

from mainstream platforms like Linux, macOS, and Windows. In this book, we use the phrase “most

implementations” to indicate those platforms that the majority of developers are likely to encounter;

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