Escaping The Jar Hell With Jigsaw Layers

Jigsaw modules are often criticized for the lack of versioning that exists in alternative module systems for Java, such as OSGi. One of the main purposes of versioning is to solve the so called Jar Hell problem that arises when your application depends on two different versions of the same library. While Jigsaw is able to detect this conflicting situation, it won’t allow you to load both versions that are placed on the module path. However, it is not always possible to eliminate all version conflicts from your application, because other versions of the same libraries can come to your application indirectly via dependencies that you do not control. Fortunately, there is a native solution for the problem in Jigsaw called Layers. In this session, I will show what problems Jigsaw could have introduced if it had explicit versions for modules, and how Jigsaw Layers in conjunction with Jigsaw Services help to solve the Jar Hell problem safely.

Transcript

1. Escaping The Jar Hell With Jigsaw Layers Nikita Lipsky
2. Jigsaw and versioning 2
3. Nikita Lipsky •  20+ years in software development •  Excelsior JET project initiator –  18+ years of contributions –  compiler engineer –  product department lead –  etc. •  Twitter: @pjBooms 3
4. Agenda •  Very short introduction to Jigsaw •  Jar Hell problem •  Versioning as a solution to Jar Hell problem •  Problems with versioning •  Jigsaw Layers •  Jar Hell resolution using Jigsaw Layers •  Demo 4
5. Модульная система OSGi 5
6. Module example // src/java.sql/module-info.java module java.sql { requires transitive java.logging; requires transitive java.xml; exports java.sql; exports javax.sql; exports javax.transaction.xa; uses java.sql.Driver; } 6
7. Java Platform Module System Jigsaw module •  imports: –  modules: requires –  services: uses •  exports: –  packages: exports –  servicies: provides SomeService with ServiceImpl 7
8. Java Platform Module System Module A Class A Module B Class B Module C Class C exports packageC; requires: A exports packageA; I requires A; requires C; 8
9. Jar Hell 9
10. Jar/Classpath Hell App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 10
11. Jar/Classpath Hell App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 11
12. Jar/Classpath Hell App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 12
13. Jar/Classpath Hell App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 13
14. Jar/Classpath Hell App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 14
15. Jar/Classpath Hell App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 15
16. Versioning •  Jigsaw module might have a version in the first drafts of Jigsaw to solve Jar Hell problem – import/export was qualified by version – If two modules require a library module of two different versions, both versions will be loaded 16
17. Versioning However versioning was removed later … 17
18. Versioning However versioning was removed later … Why? 18
19. Versioning Question: How to implement versioning? Task: For given A module importing Lib (v1) library, and B module importing Lib (v2), it is required that both versions of Lib working without conflicts. Solution: load the versions of the library by different classloaders. 19
20. Versioning Question: How to implement versioning? Task: For a given module A importing library Lib (v1), and module B importing Lib (v2), it is required that both versions of Lib work without conflicts. Solution: load the versions of the library by different classloaders. 20
21. Versioning Question: How to implement versioning? Task: For a given module A importing library Lib (v1), and module B importing Lib (v2), it is required that both versions of Lib work without conflicts. Solution: load versions of the library by different classloaders. 21
22. Versioning •  ClassLoader forms a unique class namespace •  If each module is loaded by its own classloader then there will be no conflicts with classes of other modules 22
23. Versioning com.foo App 1.0 com.foo parse-api 2.0 com.foo persist-api 3.0 org.apache commons 2.1 org.apache commons 3.1 CL1 CL2 CL3 CL4 CL5 Different versions of apache commons may simultaneously present in the JVM 23
24. Versioning problems 24
25. Versioning Versioning immediately means: 1 module ßà 1 classloader 25
26. Versioning Versioning immediately means: 1 module ßà 1 classloader It was exactly so in the first Jigsaw drafts! 26
27. Jigsaw and classloaders Jigsaw is not just modules, but also the Java SE platform split into modules. 27
28. Jigsaw and classloaders Backward compatibility problems: According to the specification getClassloader() == null for core platform classes.то противоречит That precludes splitting of the platform into modules, with each module loaded by its loader 28
29. Jigsaw and classloaders Backward compatibility problems: According to the specification getClassloader() == null for core platform classes.то противоречит It precludes splitting of the platform into modules, with each module loaded by its loader 29
30. Jigsaw and classloaders Backward compatibility problems: Many classloaders for an application may also cause problems: – classloaders delegation Ф 30
31. Jigsaw and classloaders Backward compatibility problems: Even the fact that Application (system) Classloader does not extend URLClassloader anymore causes problems Ф 31
33. Versioning? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 33
34. Versioning? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 A 34
35. Versioning? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 A A 35
36. Versioning? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 A A 36
37. Versioning? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 A A 37
38. Versioning? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 A A 38
39. Versioning? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 A A 39
40. Versioning? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 A A I’m “A”! 40
41. Versioning? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 A No, I’m “A”! A I’m “A”! 41
42. App 1.0 A Versioning? Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 A AAAAA! AAAAA! 42
44. Loading constraints •  Loading constraints prohibit two classes with the same fully qualified name to appear in the namespace of another class 44
45. Loading constraints B.java: class B { T1 f1 = A.f; int f2 = A.m(t2); } A.java: class A { static T1 f; static int m(T2 t) If B is loaded by L1 classloader and A is loaded by L2 then the JVM will check that (T1, L1) = (T1, L2) and (T2, L1) = (T2, L2) == 45
46. Versioning? •  JVM throws java.lang.LinkageError on loading constraints violation 46
47. Versioning? Conclusion: versioning does not solve JAR Hell but raises it to a new, more sophisticated level. 47
48. Versioning Another detail: import in early Jigsaw versions was qualified by not a single version but by а version range: –  A module may declare that it can work with a dependency of “from” version to “to” version 48
49. Versioning // src/module-info.java module my.module@1.0 { //1.2.3.1 <= lib.module version < 4.0 requires lib.module@1.[2.3.1+];2*;3*; } 49
50. Versioning Problem 3: Resolving dependencies (wiring modules) from version ranges is an NP- complete problem! –  Reduced to 3-SAT 50
51. Versioning … after that versioning in JPMS breathed its last. 51
52. Versioning … after that versioning in JPMS breathed its last. No versioning – no classloaders for modules. 52
53. Jar Hell But how to solve the Jar Hell now? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 53
54. Jar Hell Jigsaw simply prohibits this situation! App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 54
55. Jar Hell Stop! Jigsaw does not have versions! App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 55
56. Jar Hell Stop! Jigsaw does not have versions! The most probably two different versions of the same library have same packages. 56
57. Jar Hell Stop! Jigsaw does not have versions! The most probably two different versions of the same library have same packages. Jigsaw prohibits for two modules to have same packages (split packages) 57
58. Jar Hell But how to solve the Jar Hell problem after all? App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 58
59. Jar Hell A possible solution: App 1.0 Foo 2.0 Bar 3.0 Baz 3.1 59
60. Jar Hell A possible solution: App 1.0 Foo 2.5 Bar 3.0 Baz 3.1 60
62. I always dreamed of solving NP-complete problems! 62
63. Jar Hell Another solution: relocate classes of two versions of a library to different Java packages (Maven Shade plugin) 63
64. Jar Hell Another solution: relocate classes of two versions of a library to different Java packages (Maven Shade plugin) It may not work because of reflection. 64
65. Jar Hell Third solution: employ classloaders App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 65 CL1 CL2
66. Jar Hell Third solution: employ classloaders Be ready to fight with classloaders constraints violation problem 66
67. Jigsaw Layers *
68. Jigsaw Layers * Fourth solution: employ Jigsaw Layers 68
69. Jigsaw Layers * •  Local module system •  Split packages are prohibited inside one layer •  Two modules containing the same package have to belong to different layers 69
70. Jigsaw Layers * * The picture is from Alex Buckley’s presentation: Project Jigsaw Under the hood 70
71. Jigsaw Layers * •  Jigsaw layers are implemented …через загрузчики классов. •  У каждого слоя по крайней мере один свой загрузчик (может быть несколько) •  Но при этом невозможно по построению нарушение classloader constraints! 71
72. Jigsaw Layers * •  Jigsaw layers are implemented via classloaders!ерез загрузчики классов. •  У каждого слоя по крайней мере один свой загрузчик (может быть несколько) •  Но при этом невозможно по построению нарушение classloader constraints! 72
73. Jigsaw Layers * •  Jigsaw layers are implemented via classloaders! •  Each layer has at least one unique classloader (may have several classloaders) 73
74. Jigsaw Layers * •  Jigsaw layers are implemented via classloaders! •  Each layer has at least one unique classloader (may have several classloaders) •  However, classloader constraints are impossible to violate by construction! 74
75. Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 75
76. Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 Boot Layer 76
77. Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 Layer 2 Boot Layer 77
78. Layer 3 Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 Layer 2 Boot Layer 78
79. Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 79
80. Layer 3 Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 Layer 2 Boot Layer 80
81. Layer 3 Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 Layer 2 Boot Layer 81
82. Jigsaw Layers •  Every layer has a parent layer •  Modules of a child layer can import modules of parent layers •  However parent layer modules cannot import child layers modules! •  That’s why it’s impossible to violate classloaders constraints! 82
83. Jigsaw Layers •  Every layer has a parent layer •  Modules of a child layer can import modules of parent layers •  However parent layer modules cannot import child layers modules! •  That’s why it’s impossible to violate classloaders constraints! 83
84. Jigsaw Layers •  Every layer has a parent layer •  Modules of a child layer can import modules of parent layers •  However, parent layer modules cannot import child layers modules! •  That’s why it’s impossible to violate classloaders constraints! 84
85. Jigsaw Layers •  Every layer has a parent layer •  Modules of a child layer can import modules of parent layers •  However, parent layer modules cannot import child layers modules! •  That’s why it’s impossible to violate classloaders constraints! 85
86. Layer 3 Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 Layer 2 Boot Layer A A 86
87. Layer 3 Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 Layer 2 A A Boot Layer 87
88. Layer 3 Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 Layer 2 A A Boot Layer 88
89. Layer 3 Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 Layer 2 A A Boot Layer There is a space between us A place that I can’t go 89
90. Jigsaw Layers Question: if application modules cannot import child layers modules then how they can employ functionality implemented in these child layers? 90
92. Jigsaw Layers Question: if application modules cannot import child layers modules then how they can employ functionality implemented in these child layers? 92
93. Jigsaw Layers Question: if application modules cannot import child layers modules then how they can employ functionality implemented in these child layers? Answer: employ IoC design pattern via Jigsaw Services: child layers can provide services to modules of parent layers 93
94. Jigsaw Layers Question: if application modules cannot import child layers modules then how they can employ functionality implemented in these child layers? Answer: employ IoC design pattern via Jigsaw Services: child layers can provide services to modules of parent layers 94
95. Layer 3 Jigsaw Layers Jar Hell Resolution App 1.0 Library Foo 2.0 Library Bar 3.0 Library Baz 2.1 Library Baz 3.1 Layer 2 Boot Layer 95
96. Layer 3 Jigsaw Layers Jar Hell Resolution App 1.0 Foo 2.0 Bar 3.0 Baz 2.1 Baz 3.1 Layer 2 App Service Provider 1 App Service Provider 2 Boot Layer 96
98. How to use var finder = ModuleFinder.of(Paths.get(“MyLayer”)); var parent = ModuleLayer.boot(); Configuration cf = parent.configuration().resolve( finder, ModuleFinder.of(), Set.of(“myRootMod")); ModuleLayer myL = parent.defineModulesWithOneLoader( cf, ClassLoader.getSystemClassLoader()); 98
99. How to use var finder = ModuleFinder.of(Paths.get(“MyLayer”)); var parent = ModuleLayer.boot(); Configuration cf = parent.configuration().resolve( finder, ModuleFinder.of(), Set.of(“myRootMod")); ModuleLayer myL = parent.defineModulesWithOneLoader( cf, ClassLoader.getSystemClassLoader()); 99
100. How to use var finder = ModuleFinder.of(Paths.get(“MyLayer”)); var parent = ModuleLayer.boot(); Configuration cf = parent.configuration().resolve( finder, ModuleFinder.of(), Set.of(“myRootMod")); ModuleLayer myL = parent.defineModulesWithOneLoader( cf, ClassLoader.getSystemClassLoader()); 100
101. How to use var finder = ModuleFinder.of(Paths.get(“MyLayer”)); var parent = ModuleLayer.boot(); Configuration cf = parent.configuration().resolve( finder, ModuleFinder.of(), Set.of(“myRootMod")); ModuleLayer myL = parent.defineModulesWithOneLoader( cf, ClassLoader.getSystemClassLoader()); 101
102. How to use var finder = ModuleFinder.of(Paths.get(“MyLayer”)); var parent = ModuleLayer.boot(); Configuration cf = parent.configuration().resolve( finder, ModuleFinder.of(), Set.of(“myRootMod")); ModuleLayer myL = parent.defineModulesWithOneLoader( cf, ClassLoader.getSystemClassLoader()); var services = ServiceLoader.load(myL, MySrv.class); 102
103. Demo https://github.com/pjBooms/Jigsaw-Layers- Example 103
104. Where else we can use layers Problem: the servlet container standard knows nothing about modules so far –  Dependencies in war files are old plain classpath in fact! Solution: it’s possible to fork your favorite app server, adding a Jigsaw layer into its web application loader –  example: 104
105. Jigsaw Mantra Reliable Configuration Strong Encapsulation 105
106. Where else we can use layers Problem: the servlet container standard knows nothing about modules so far –  Dependencies in war files are old plain classpath in fact! Solution: it’s possible to fork your favorite app server, adding a Jigsaw layer into its web application loader –  example: 106
107. Where else we can use layers Problem: the servlet container standard knows nothing about modules so far –  Dependencies in war files are old plain classpath in fact! Solution: it’s possible to fork your favorite app server, adding a Jigsaw layer into its web application loader –  example: https://github.com/pjBooms/tomcat 107
108. Tomcat •  Tomcat classpath contains bootstrap.jar •  Tomcat is loaded by its own classloader (org.apache.catalina.startup.ClassLoaderFactory) •  Each web application is loaded by its own classloader (org.apache.catalina.loader.WebappClassLoaderBase) 108
109. Tomcat •  Any classloader has its own Unnamed module •  Without special efforts from a classloader implementer, classes loaded by a classloader would belong to its Unnamed module – even if сlasses formally belong to a named module (to a jar with module-info.class) 109
110. Modular War Example Example (https://github.com/pjBooms/modular-war-example): –  2 modules: helloworld-provider, helloworld-web-app –  helloworld-web-app outputs module names of loaded classes on a web page: "<H3>HelloServlet module: " + HelloServlet.class.getModule().getName() + "</H3>\n" + "<H3>HelloWorldProvider module: " + HelloWorldProvider.class.getModule().getName() + "</H3>\n" + 110
111. Modular War Example Example (https://github.com/pjBooms/modular-war-example): –  2 modules: helloworld-provider, helloworld-web-app –  helloworld-web-app outputs module names of loaded classes on a web page: "<H3>HelloServlet module: " + HelloServlet.class.getModule().getName() + "</H3>\n" + "<H3>HelloWorldProvider module: " + HelloWorldProvider.class.getModule().getName() + "</H3>\n" + 111
112. Modular War Example If the example is launched on a regular Tomcat: 112
113. Modular War Example On Tomcat fork (https://github.com/pjBooms/tomcat): 113
114. Tomcat fork ClassLoaderFactory.createClassLoader modification: instead of return new URLClassLoader(array); modular layer creation: ModuleFinder finder = ModuleFinder.of(array); … ModuleLayer moduleLayer = parentLayer.defineModulesWithOneLoader(cf, parent); return moduleLayer.findLoader(moduleNames.get(0)); 114
115. Tomcat fork ClassLoaderFactory.createClassLoader modification: instead of return new URLClassLoader(array); modular layer creation: ModuleFinder finder = ModuleFinder.of(array); … ModuleLayer moduleLayer = parentLayer.defineModulesWithOneLoader(cf, parent); return moduleLayer.findLoader(moduleNames.get(0)); 115
116. Tomcat fork ClassLoaderFactory.createClassLoader modification: instead of return new URLClassLoader(array); modular layer creation: ModuleFinder finder = ModuleFinder.of(array); … ModuleLayer moduleLayer = parentLayer.defineModulesWithOneLoader(cf, parent); return moduleLayer.findLoader(moduleNames.get(0)); 116
117. Tomcat fork WebAppClassloaderBase.start() modification modular layer creation: WebResource modules = resources.getResource("/WEB-INF/modules"); if (modules.exists()) { ModuleFinder finder = ModuleFinder.of(Paths.get(modules)); ModuleLayer parent = Catalina.class.getModule().getLayer(); … ModuleLayer moduleLayer = parent.defineModulesWithOneLoader(cf, getParent()); 117
118. Tomcat fork WebAppClassloaderBase.start() modification modular layer creation: WebResource modules = resources.getResource("/WEB-INF/modules"); if (modules.exists()) { ModuleFinder finder = ModuleFinder.of(Paths.get(modules)); ModuleLayer parent = Catalina.class.getModule().getLayer(); … ModuleLayer moduleLayer = parent.defineModulesWithOneLoader(cf, getParent()); 118
119. Tomcat fork WebAppClassloaderBase.start() modification modular layer creation: WebResource modules = resources.getResource("/WEB-INF/modules"); if (modules.exists()) { ModuleFinder finder = ModuleFinder.of(Paths.get(modules)); ModuleLayer parent = Catalina.class.getModule().getLayer(); … ModuleLayer moduleLayer = parent.defineModulesWithOneLoader(cf, getParent()); 119
120. Tomcat fork WebAppClassloaderBase.loadClass() modification The layer is used to load web application classes: if (moduleLayerLoader != null) { try { return moduleLayerLoader.loadClass(name); } catch (ClassNotFoundException e) { // Ignore } } 120
121. Modular War Example Tomcat fork (https://github.com/pjBooms/tomcat): 121
122. Jigsaw Mantra Reliable Configuration Strong Encapsulation 122
123. What about Spring Boot? •  Classes of Spring Boot executable Jar are loaded by a Spring Boot classloader •  The classloader knows nothing about modules •  Hence there are no modules at runtime for Spring Boot applications 123
124. What about Spring Boot? •  Classes of Spring Boot executable Jar are loaded by a Spring Boot classloader •  The classloader knows nothing about modules •  Hence there are no modules at runtime for Spring Boot applications 124
125. What about Spring Boot? •  Classes of Spring Boot executable Jar are loaded by a Spring Boot classloader •  The classloader knows nothing about modules •  Hence there are no modules at runtime for Spring Boot applications 125
126. What about Spring Boot? •  Classes of Spring Boot executable Jar are loaded by a Spring Boot classloader •  The classloader knows nothing about modules •  Hence there are no modules at runtime for Spring Boot applications 126
127. What about Spring Boot? Good news: Spring Boot applications can run without Spring Boot classloader – with all their dependencies put on classpath It is possible to modularize Spring Boot applications using standard migration path! 127
128. What about Spring Boot? Good news: Spring Boot applications can run without Spring Boot classloader – with all their dependencies put on classpath It is possible to modularize Spring Boot applications using standard migration path! 128
129. What about Spring Boot? Exercise: add a modular layer to the Spring Boot classloader. 129
130. Conclusion Versions for modules look like a good idea but there are many problems with them: – backward compatibility – unsafety (classloaders constraints) – NP-completeness on dependencies resolution 130
131. Conclusion Jigsaw Layers allow to solve the Jar Hell problem safely: – no classloaders constraints violations by construction – Jigsaw services help employing functionality implemented in a child layer from a parent layer 131
132. Conclusion Knowledge about the layers can help in modularizing of Java applications including server side applications 132
133. Nikita Lipsky nikita.v.lipsky@gmail.com twitter: @pjBooms 133