Java 9 模块化详解：Project Jigsaw 和可扩展的 Java 应用程序

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"Java 9 模块化揭秘 - Project Jigsaw 和可扩展 Java 应用程序" Java 9 模块化揭秘是一本关于 Java 9 中的模块化特性的书籍，涵盖了 Java 9 中的新特性、Project Jigsaw 和可扩展 Java 应用程序的相关知识点。 **Java 语言简介** Java 语言是 1995 年由 Sun Microsystems 公司推出的编程语言，自出生以来不断演进，成为当今世界上最流行的编程语言之一。每个新的 Java 版本都添加了新的特性，小的、中等的和大的。 **Java 9 的出现** Java 9终于来了！它计划于 2017 年 9 月发布，这距离 Java 8 的发布已经过去了三年多。Java 9 带来了许多新的特性，包括模块化、Project Jigsaw 和可扩展 Java 应用程序等。 **模块化** 模块化是 Java 9 中的一个重要特性，它允许开发者将应用程序拆分成小的、独立的模块，以便更好地管理和维护。模块化可以提高应用程序的可扩展性、可维护性和可重用性。 **Project Jigsaw** Project Jigsaw 是 Java 9 中的一个重要项目，它的主要目标是将 Java 平台拆分成小的、独立的模块，以便更好地管理和维护 Java 平台。Project Jigsaw 包括了两部分：JDK 模块化和 Java 模块化。JDK 模块化将 JDK 拆分成小的模块，以便更好地管理和维护 JDK。Java 模块化将 Java 应用程序拆分成小的模块，以便更好地管理和维护 Java 应用程序。 **可扩展 Java 应用程序** 可扩展 Java 应用程序是 Java 9 中的一个重要特性，它允许开发者创建可扩展的 Java 应用程序，以便更好地适应不断变化的业务需求。可扩展 Java 应用程序可以提高应用程序的可维护性、可扩展性和可重用性。 **小结** Java 9 模块化揭秘是一本关于 Java 9 中的模块化特性的书籍，涵盖了 Java 9 中的新特性、Project Jigsaw 和可扩展 Java 应用程序的相关知识点。这本书适合 Java 开发者，特别是那些想了解 Java 9 中的新特性和模块化特性的开发者。

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xxi

Introduction

The Java programming language, introduced in 1995, has had a very successful story. It’s evolved constantly

since its birth and became one of the most popular programming languages in the world. Every new release

of Java has added new features—small, medium, and big.

Java 9 is finally here! It is scheduled for release in September 2017, more than three years after the

release of Java 8.

Issues with the Monolithic JDK

The release of Java 8 in March 2014 brought very important features to the Java platform like Lambdas and

the Stream API, which were definitely needed by developers. Nevertheless, some well-known weaknesses

of the platform had still not been addressed in Java 8: the huge monolithic JDK and the class path. These

problems are addressed in Java 9 by Project Jigsaw.

The most important feature of Java 9 is by far the new modular system it introduced. Other new features

have been introduced in Java 9, but they’re not the focus of this book. This book covers the new modular

system introduced in Java 9. The big, monolithic, and indivisible JDK has been problematic for a long time.

It’s difficult to install it on small devices because many don’t have enough memory to hold it. In many cases,

a large number of classes that comprise the JDK aren’t needed because the application may not require

them. CORBA, for instance, is still part of the JDK, but it’s rarely used in real projects today. It makes no

sense to use the entire JDK when only a part or a small part of it is needed. The Compact Profiles introduced

in Java 8 recognized the problems caused by the huge JDK and attempted to solve them, but only to a low

degree. The three Compact Profiles still contain a lot of libraries that a developer may not actually need.

There had to be a better way to split the entire JDK and to create a much smaller custom JDK as a runtime

image that contains only the libraries needed and nothing more. Project Jigsaw is that way, as we’ll see

throughout this book.

Big, monolithic software applications present a series of disadvantages. Maintaining them is tough

and expensive, and performing a small change can result into a great effort. In large projects, modularity is

crucial because it allows easy maintenance of the source code by reducing its complexity due to the loose

coupling mechanism it provides.

Issues with the Class Path

The problems related to class path have been in Java since the birth of the language. The Java Virtual

Machine (JVM) doesn’t know that a JAR on the class path depends on another JAR. It simply loads a group

of JARs, but it doesn’t check their dependencies. When a JAR is missing, the JVM breaks the execution at

runtime. The JARs can’t define concepts related to accessibility. They don’t define accessibility constraints

like public or private. The entire content of all JARs on the class path is completely visible to all the other

JARs from the class path. There’s no way to declare that some classes in a JAR are private. All classes and

methods are public related to the class path, which leads to a problem sometimes called JAR hell. Conflicts

between versions of JARs can arise, especially when two different versions of the same library are required.

■ IntroduCtIon

xxii

Loading the classes from the class path is a slow process because the JVM doesn’t know where exactly the

class is located and therefore has to check all the existing files from the class path. Jigsaw addresses this

pain point. By taking advantage of reliable configuration, module boundaries are enforced, and the JVM

knows about the dependencies that are needed. This has a positive impact on performance. Java 9 defines

the concept of module path and allows you to have a library as a JAR file on the class path or have the same

library as a module on the module path. This means nobody is forced to turn all their libraries into modules

when they switch to Java 9. The libraries can still be used on the class path, even in Java 9. This is a big

advantage because Java 9 provides a smooth transition of libraries.

The module path introduced in Java 9 tends to solve the problems caused by the class path. It can

replace the class path completely or can interact and work together with the class path.

Modularity is important because it provides a maintainable code base for the future. We should use

modular programming when we want to separate the effort put into design, development, and testing.

Modular programming speeds up development and makes debugging applications easier by reducing

their complexity.

Overview of Chapters

The first chapter of this book describes the concepts that build the foundation of a modular application: high

module cohesion, strong encapsulation, low module coupling and explicit interfaces. It also illustrates some

of the most important principles of modular programming, like: continuity, understandability, reusability,

combinability, and decomposability.

You may wonder why we shouldn’t use OSGi instead of Jigsaw. The reason is that OSGi can’t be used to

modularize the JDK because it’s built on top of the Java Development Kit Platform. Jigsaw isn’t built on top of

the platform but rather directly into the core of it. This allows Jigsaw to completely change the structure of the

JDK. The main differences between Jigsaw and OSGi are described in Chapter 2.

Prior to JDK 9, there was no way to manage modules. That’s where Project Jigsaw comes into action. It

introduces a brand new modular system into the JDK and therefore allows applications to be built on the

skeleton of a modular architecture. It brings flexibility, maintainability, security, and scalability to the Java

platform. It introduces loosely coupled modules by clearly defining the dependencies between the modules.

Project Jigsaw groups the source code of the Java platform into modules and provides a new system to

implement modules as part of the platform. It applies the modular system to the platform and to the JDK

itself and offers programmers the possibility of writing programs using the modular system on top of

the JDK.

The main goal of Project Jigsaw is to modularize the JDK and the runtime. A new component called

module is introduced. Chapter 4 explains what a module is and how to define a module in Java 9. That

chapter also examines how to declare a module’s dependencies, how to encapsulate, and how to hide a

module’s internal implementation. You’ll learn what an unnamed module, a named module, an automatic

module, and an open module are. The chapter introduces the notion of module path and shows how to

build a module graph with no cycles.

Regarding modules declarations, I show practical examples using each of the five clauses introduced in

Jigsaw: the requires clause, the exports clause, the uses clause, the provides clause, and the opens clause.

The objective of the Java modular system is to provide strong encapsulation and reliable configuration.

Chapter 2 explains what these notions mean and how they’re achieved through the modular system in Java 9.

The class path is partially replaced by modules and the module path. The class path–specific well-

known ClassNotFound exceptions are avoided on the module path because the compiler can now test, based

on the modules’ definitions, whether all the modules needed to run the application are available. If they’re

not found, it doesn’t compile the application.

■ IntroduCtIon

xxiii

Accessibility is an important part of the entire ecosystem and is covered throughout this book. The integral

concept regarding the manner in which accessibility is achieved fundamentally changes in Java 9. The public

access identifier that we all know no longer means accessible everywhere and to anyone. Supplementary

accessibility levels have been added in JDK 9 that extend the existing accessibility mechanism by defining

accessibility at the module level. Concepts like direct and implied readability, prerequisites for defining

accessibility, are also outlined in this book. You’ll see how accessibility is imposed by the compiler, by the

virtual machine, or by using core reflection.

We’ll look at the new concept of modular JAR files and the great advantage it brings: the possibility

of compiling a library with JDK 9+, using it on the module path for JDK 9+, and compiling it with JDK8 or

earlier and using it on the class path. Because the class path can still be used, the migration of libraries

to JDK 9 is smooth. Even if the libraries contain a module descriptor and are to be treated as “modules,”

they will still work on previous versions of JDK 9 because their module descriptor won’t be taken into

consideration on the class path. By using modular JARs, developers have the freedom to decide whether they

want to switch to the module platform or not.

We’ll highlight the distinctions between regular and modular JARs with some examples and describe

the new format for files introduced in Java 9, called JMOD, which is very similar to the format of JAR files.

We’ll go over the new JMOD tool and describe its use in detail.

The compilation of multiple modules using the --module-source-path option is illustrated with some

explanatory examples. We’ll also describe the enhancements added to the jar tool and show how to use it to

package everything as a modular JAR or as a JMOD file.

We’ll see how to run the compiled classes and the module-info.class using the java launcher. New

options like --module-path and -m, introduced in JDK9, are covered thoroughly. When attempting to run an

application using the -m option, which tells the launcher where the main module is, a resolution is triggered.

We’ll describe in detail all the steps involved when running a Java modular application, including resolution

triggering and generation of the module graph. We’ll also look at special cases like when a module is missing

and the startup fails, and we’ll present some workarounds for this, such as using the newly introduced java

launcher option --show-module-resolution.

We’ll also put modular JARs on the class path and see how to successfully run them. This is very

important: We’ll explain how to mix the class path and the module path when running with the java

launcher. For this, we’ll take advantage of the newly introduced --add-modules command-line option.

Chapter 3 describes the JEP 200, called the Modular JDK. This is the JEP that splits the JDK into a set of

modules. We’ll consider the new structure of the JDK together with its modules. We’ll talk about platform

modules and show the module graph that represents the new modular structure of the JDK. We’ll examine

the graph, show how modules depend on each other, and learn how to list all the modules from the system

using the --list-modules command-line option. We’ll also explain the notions of standard module and

non-standard module.

It’s beyond the scope of this book to go through every module in detail. You can find a comprehensive

list of all the existing modules on the Open JDK website at http://cr.openjdk.java.net/~mr/jigsaw/ea/

module-summary.html.

The JEP 260, Encapsulate most internal APIs, is also covered in this book. In order to manage

incompatibilities, all non-critical internal APIs were encapsulated by default. Besides that, all critical internal

APIs for which supported replacements exist in JDK 8 were encapsulated. Other critical internal APIs weren’t

encapsulated. Since they were deprecated in JDK 9, a workaround via a command-line flag is provided.

A linker and a new phase called link-time were introduced in Java 9. This phase is optional and is

executed after the compile-time but before the runtime. It basically assembles the modules in order to form

a runtime image. Runtime images allow us to create custom JDKs that contain only the minimum number of

modules necessary to run our application. The minimum possible runtime would contain a single module,

the java.base module. Runtime images allow us to scale down the JDK or to scale it up based on our needs.

They’re a replacement of the runtime rt.jar.

■ IntroduCtIon

xxiv

The linker represents a new phase of the development life-cycle. It improves performance by selecting

only the minimum modules that it needs to successfully compile the code and provides many optimization

options for the future.

Jigsaw makes it possible to install separate modules as part of the JDK platform installation. It also

allows us to dynamically include other additional modules in the JDK runtime image. We’ll talk about the

changes related to the binary structure of the JRE and the JDK and about the new structure of the legacy JDK

image. You’ll learn more about all these new concepts in Chapters 5 and 7.

In Chapter 6 you’ll learn what services are, and we’ll describe through examples the notions of service

interface and service providers. We’ll show how to define service providers in modules and how to make

them available to other modules.

In Chapter 8 you’ll see how to migrate applications and libraries to modules in a smooth way. We’ll

describe how to migrate an application to Java 9 using the top-down migration strategy. For this, we’ll look

at a concrete example of how to migrate an application that contains some third-party JARs to modules.

We’ll see which kind of applications present the risk of breaking when switching to JDK 9. We’ll will give

useful solutions to avoid this, such as searching the code for dependencies, avoiding splitting packages,

and checking the usage of the internal APIs. If you’ve already switched to JDK 9, we recommend trying first

to run your applications with the new JDK to see if it breaks your code. We’ll cover the JDeps tool and how

to use it to audit your code and search for JDK-internal APIs. We’ll discuss the Maven JDeps plugin, which

represents the integration of the JDeps tool with the build tool Maven. We’ll also talk about the impact and

consequences of the removal of rt.jar and tools.jar from the JRE.

Chapter 9 covers the new API introduced in JDK 9 for working with modules. We’ll see how to perform

basic operations on modules.

Chapter 10 gets into some advanced topics like layers, class loading mechanism in JDK 9, multi-release

JAR files, the JMOD tool, and upgradeable modules. We’ll describe the concept of layers, which are a group

of class loaders used to load classes from a module graph. We’ll look at the boot layer and the correlation to

the so-called well-formed graphs.

Chapter 11 talks about how to handle different scenarios for testing modular applications. Three

scenarios are covered: Junit test classes and objects under test residing in different modules, Junit test

classes and objects under test residing in the same module, and only objects under test residing inside a

module. We’ll show how to patch a module and how to use Maven for easing testing.

In Chapter 12 you’ll learn how Jigsaw interacts with build tools like Maven and what kind of support the

most popular IDEs offer for Project Jigsaw.

As you can see, this book is structured into 12 chapters. Chapter 1 covers modular programming

concepts. Chapters 2–9 provide you with a very strong foundation on Project Jigsaw. Chapter 10 describes

some advanced features that will help you understand some complex topics on Jigsaw. Chapter 11 shows

how to test modular applications using Junit. Chapter 12 teaches using Jigsaw together with build tools and

integrated development environments (IDEs).

Each topic should be easy to find. We recommend reading the chapters sequentially in order to

understand all the topics. Some examples build on concepts that were explained in previous chapters.

Who Should Read This Book

This book is intended for anyone who wants to get familiar with the new modularity system introduced in

Java 9. It gives a strong foundation for anyone who wants to understand the core concepts as well as the

advanced concepts of Java 9 modularity. The examples are designed to help you deeply understand all the

notions introduced in Project Jigsaw. We’ve tried, as much as possible, to give a plenty of examples for most

of the theoretical concepts discussed throughout the book.

■ IntroduCtIon

xxv

Whether you already have experience with modular systems or not, this book is for you.

The book can’t cover everything on Java 9 modularity. Java 9 modularity is a very large and complex

subject, and covering every corner of it wouldn’t be possible in a book of this size. However, the book goes

through all the core parts of Java 9 modularity and touches some advanced topics. By reading this book,

you’ll not only understand the concepts behind Java 9 modularity, you’ll also be able to apply them on your

day-by-day projects.

We advise you to try the examples from this book by yourself in order to get familiar with Project Jigsaw.

What You Will Learn

This book aims to provide comprehensive information on the new modular system introduced in Java 9. A

well-structured tutorial is conducted throughout the book by combining theoretical concepts with practical

examples.

Learning to use modularity with Java 9 will help you enhance your technology career and give you very

precious technical skills.

Once you read this book, you’ll be able to develop scalable and modular Java 9 applications and migrate

existing Java applications to Java 9.

Here are some of the most important things you’ll learn in this book:

• What modularity is in general and what advantages it brings

• What Java 9 modularity is and what its goals are

• How the new layout for JDK and JRE looks

• What strong encapsulation and reliable configuration are and how to apply and take

advantage of them

• Which JDK-internal APIs have been encapsulated in Java 9 and which have remained

accessible

• How the JDK was divided into a group of modules

• What the new accessibility rules in JDK 9 are

• How to define a module together with its dependencies

• How to create a modular JAR file and a JMOD file

• How to compile, package, and run a modular Java application using Jigsaw

• How to use the JDeps tool to audit the code, search for dependencies between the

libraries, or generate module descriptors

• How to migrate an application to the modular system

• How to solve migration issues like encapsulated JDK internal APIs, not resolved

modules, split packages, cyclic dependencies, and more

• How to perform top-down migration

• How to define, configure, and use services for modules

• How to combine the module path and the class path to provide backward

compatibility

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