Table of Contents

Foreword xi

Preface xiii

Part I Introduction to the Java Module System

1 Modularity Matters 3

What Is Modularity? 4

Before Java 9 5

JARs as Modules? 7

Classpath Hell 8

Java 9 Modules 10

2 Modules and the Modular JDK 15

The Modular JDK 16

Module Descriptors 20

Readability 21

Accessibility 22

Implied Readability 23

Qualified Exports 27

Module Resolution and the Module Path 28

Using the Modular JDK Without Modules 30

3 Working with Modules 33

Your First Module 33

Anatomy of a Module 33

Naming Modules 35

Compilation 36

Packaging 37

Running Modules 38

Module Path 40

Linking Modules 41

No Module Is an Island 42

Introducing the Easy Text Example 42

A Tale of Two Modules 44

Working with Platform Modules 48

Finding the Right Platform Module 48

Creating a GUI Module 49

The Limits of Encapsulation 54

Interfaces and Instantiation 55

4 Services 57

Factory Pattern 57

Services for Implementation Hiding 60

Providing Services 60

Consuming Services 62

Service Life Cycle 65

Service Provider Methods 66

Factory Pattern Revisited 68

Default Service Implementations 69

Service Implementation Selection 70

Service Type Inspection and Lazy Instantiation 71

Module Resolution with Service Binding 73

Services and Linking 75

5 Modularity Patterns 79

Determining Module Boundaries 80

Lean Modules 82

API Modules 83

What Should Be in an API Module? 83

Implied Readability 84

API Module with a Default Implementation 88

Aggregator Modules 90

Building a Facade over Modules 90

Safely Splitting a Module 92

Avoiding Cyclic Dependencies 94

Split Packages 94

Breaking Cycles 96

Optional Dependencies 99

Compile-Time Dependencies 100

Implementing Optional Dependencies with Services 104

Versioned Modules 106

Module Resolution and Versioning 107

Resource Encapsulation 109

Loading Resources from a Module 110

Loading Resources Across Modules 111

Working with ResourceBundles 113

6 Advanced Modularity Patterns 115

Strong Encapsulation Revisited 115

Deep Reflection 116

Open Modules and Packages 118

Dependency Injection 121

Reflection on Modules 124

Introspection 124

Modifying Modules 125

Annotations 127

Container Application Patterns 128

Layers and Configurations 129

Classloading in Layers 132

Plug-in Architectures 137

Container Architectures 142

Resolving Platform Modules in Containers 148

Part II Migration

7 Migration Without Modules 153

The Classpath Is Dead, Long Live the Classpath 154

Libraries, Strong Encapsulation, and the JDK 9 Classpath 155

Compilation and Encapsulated APIs 157

Removed Types 160

Using JAXB and Other lava EE APIs 162

The jdk.unsupported Module 165

Other Changes 167

8 Migration to Modules 169

Migration Strategies 170

A Simple Example 170

Mixing Classpath and Module Path 172

Automatic Modules 173

Open Packages 177

Open Modules 179

VM Arguments to Break Encapsulation 180

Automatic Modules and the Classpath 180

Using jdeps 184

Loading Code Dynamically 187

Split Packages 189

9 Migration Case Study: Spring and Hibernate 191

Getting Familiar with the Application 192

Running on the Classpath with Java 9 197

Setting Up for Modules 198

Using Automatic Modules 199

Java Platform Dependencies and Automatic Modules 201

Opening Packages for Reflection 201

Fixing Illegal Access 202

Refactor to Multiple Modules 204

10 Library Migration 207

Before Modularization 208

Choosing a Library Module Name 209

Creating a Module Descriptor 211

Updating a Library with a Module Descriptor 214

Targeting Older Java Versions 215

Library Module Dependencies 216

Internal Dependencies 216

External Dependencies 220

Targeting Multiple Java Versions 221

Multi-Release JARs 221

Modular Multi-Release JARs 224

Part III Modular Development Tooling

11 Build Tools and IDEs 227

Apache Maven 227

Multimodule Projects 229

EasyText with Apache Maven 230

Running a Modular Application with Apache Maven 234

Gradle 235

IDEs 235

12 Testing Modules 237

Blackbox Tests 238

Blackbox Tests with JUnit 240

Whitebox Tests 243

Test Tooling 247

13 Scaling Down with Custom Runtime Images 249

Static Versus Dynamic Linking 250

Using jlink 252

Finding the Right Service Provider Modules 256

Module Resolution During Linking 257

Jlink for Classpath-Based Applications 258

Reducing Size 259

Improving Performance 261

Cross-Targeting Runtime Images 262

14 A Modular Future 263

OSGi 264

Java EE 266

Microservices 267

Next Steps 268

Index 269