![What is a Module ?
• Module is a collection of packages designed for reuse.
• Modular JAR is a regular JAR with a module descriptor[a module-
info.class] in its root folder.
• A key motivation of the module system is strong encapsulation](https://image.slidesharecdn.com/javaplatformmodulesystem-180123110304/85/Java-Platform-Module-System-3-320.jpg)
![Why modules?
There is no way to reuse a behavior defined inside a package in another
package unless and until it is public . But public is too open .
[Developers started using JDK internals which was not supposed to be
used ex: sun.misc.Unsafe Using it, you can create an instance of a class
without invoking it’s constructor code, initialization code, various JVM
security checks and all other low level things. Even if class has private
constructor, then also you can use this method to create new
instance.].
JDk8 contains about 218 packages. This weakens encapsulation. This
was one of the main reason why JDK itself was modularized.](https://image.slidesharecdn.com/javaplatformmodulesystem-180123110304/85/Java-Platform-Module-System-5-320.jpg)
![Public is no more an golden switch
• Public no longer means accessible if the package is not exported by
the parent module[module which has that class]
• This is the point of controversy in Java9 due to which many libraries
like Glassfish was failing on JDK-9 because they were using jdk
internals which are no longer exported.
Temporary access : ‘--illegal-access=permit’
Proposal: Allow illegal reflective access by default in JDK 9
http://mail.openjdk.java.net/pipermail/jigsaw-dev/2017-
May/012673.html](https://image.slidesharecdn.com/javaplatformmodulesystem-180123110304/85/Java-Platform-Module-System-10-320.jpg)
![JDEPS : Dependency Analyzer
• JDeps is a "Java [static] class dependency analyzer" that is useful
for quickly identifying "static dependencies of applications and
libraries.
• The input can be a path name to a .class file or a JAR File.
• Example : jdeps dependency list for junit-jar. –s option is for summary](https://image.slidesharecdn.com/javaplatformmodulesystem-180123110304/85/Java-Platform-Module-System-12-320.jpg)
![Module-info.java
Module com.example.myapp{
exports com.example.myapp.utils;
requires java.base;
}
• This module[com.example.myapp] only needs types from the base
module 'java.base’ because every Java module needs 'java.base', it is
not necessary to explicitly require it.
• This module[com.example.myapp] only exports the package
com.example.myapp.utils for public use to other modules. Classes in
other packages cannot be accessed by other modules.](https://image.slidesharecdn.com/javaplatformmodulesystem-180123110304/85/Java-Platform-Module-System-18-320.jpg)
![JLINK : Uses
class App {
public static void main(String[]args) {
System.out.prinltn("Hello World");
}
}
To execute this small “hello world” application, we require the
following .class files:
• App.class
• String.class
• System.class
• Object.class](https://image.slidesharecdn.com/javaplatformmodulesystem-180123110304/85/Java-Platform-Module-System-23-320.jpg)
Java Platform Module System
- 1. EXPLORING JAVA9 MODULES By Vignesh Ramesh
- 2. About Me • I am Vignesh . Insanely passionate full stack developer with high proficiency in Java, Scala, Node.js, JavaScript. Github : https://github.com/vickii Linkedin : https://www.linkedin.com/in/vickyramesh/ StackOverFlow :
- 3. What is a Module ? • Module is a collection of packages designed for reuse. • Modular JAR is a regular JAR with a module descriptor[a module- info.class] in its root folder. • A key motivation of the module system is strong encapsulation
- 4. Why Modules? Whenever a new JVM is started the bootstrap classloader is responsible to load key Java classes (from java.lang package) and other runtime classes to the memory first. rt.jar is about 64mb in jdk-8. Do we need all the classes runtime classes? Depends on the Application right?
- 5. Why modules? There is no way to reuse a behavior defined inside a package in another package unless and until it is public . But public is too open . [Developers started using JDK internals which was not supposed to be used ex: sun.misc.Unsafe Using it, you can create an instance of a class without invoking it’s constructor code, initialization code, various JVM security checks and all other low level things. Even if class has private constructor, then also you can use this method to create new instance.]. JDk8 contains about 218 packages. This weakens encapsulation. This was one of the main reason why JDK itself was modularized.
- 6. Maven modules vs JPMS MAVEN MODULES JPMS Build Time only. Everything is on classpath during runtime where no boundaries are available Compile time, Run Time Sub-project Packages
- 7. Module vs JAR • A module can decide which classes and interfaces to export to other modules that require it. A Jar has no such encapsulation mechanism. • The name is of a JAR can be changed. As long as the JVM classloader finds the needed class on the classpath (which can be composed of a single JAR, multiple JARs, or a mix between directories or JARs), the name of the JAR file can be anything. However, the name of a module can be explicitly referenced in the declaration of other modules, and such the name defines it and cannot be changed freely.
- 9. Accessibility Accessibility prior to JAVA 9 Accessibility on java9 public but only to specific modules public only within a modules public public to everyone protected protected <package> private <package> private
- 10. Public is no more an golden switch • Public no longer means accessible if the package is not exported by the parent module[module which has that class] • This is the point of controversy in Java9 due to which many libraries like Glassfish was failing on JDK-9 because they were using jdk internals which are no longer exported. Temporary access : ‘--illegal-access=permit’ Proposal: Allow illegal reflective access by default in JDK 9 http://mail.openjdk.java.net/pipermail/jigsaw-dev/2017- May/012673.html
- 12. JDEPS : Dependency Analyzer • JDeps is a "Java [static] class dependency analyzer" that is useful for quickly identifying "static dependencies of applications and libraries. • The input can be a path name to a .class file or a JAR File. • Example : jdeps dependency list for junit-jar. –s option is for summary
- 13. Types of Modules In Java • Named Modules • Unnamed Module • Automatic Modules
- 14. Named Module • Contain a module-info.java • Loaded from the module path. • Only their exported packages are accessible and they can only require and thus read other named modules (which excludes the unnamed module).
- 15. Automatic Modules • Does not contain a module-info.java. • Loaded from the module path. • There name is derived from the jar file name by default. • Explicit naming is recommended by setting the Automatic-Module- Name variable in MANIFEST.MF. • They export all packages and requires all modules, including automatic one’s. • Automatic, or automatically named, modules are JARs without a module-info.java that were loaded from the module path. Their name is derived from the JAR file name, they export every package and read any module, including other automatic ones.
- 16. Un-Named Modules • Does not contain a module-info.java • Loaded from classpath. • Requires all named modules • All classes within the unnamed module can read all other module (named or unnamed) without any explicit declaration of any kind. That also means that older classes (written prior to Java 9) can read all modules defined by the new Module system • Exports all packages • The packages exported by unnamed module can only be read by another unnamed module. It is not possible that a named module can read (requires) the unnamed module
- 17. Creating an Module • In Java 9, you need to have a specific file name by convention in order to define modules with a specific filename. That filename should be called module-info.java. Example: module modulename{} • Don’t worry module is not a keyword in java • If your module name is com.example.myapp then the module.info should be placed at src/com.example.myapp/module-info.java path
- 18. Module-info.java Module com.example.myapp{ exports com.example.myapp.utils; requires java.base; } • This module[com.example.myapp] only needs types from the base module 'java.base’ because every Java module needs 'java.base', it is not necessary to explicitly require it. • This module[com.example.myapp] only exports the package com.example.myapp.utils for public use to other modules. Classes in other packages cannot be accessed by other modules.
- 19. Module-info.java syntax module ${module-name} { requires ${module-name}; exports ${package-name}; exports ${package-name} to ${module-name} , ${module-name}; } Add screenshot of location
- 21. Implied Readability • Let’s look at the java.sql module. It exposes the interface Driver, which returns a Logger via its public method getParentLogger(). Logger belongs to java.logging. Because of that, java.sql requires transitive java.logging, so any module using Java’s SQL features can also access the logging API. JDK Example: module java.sql { requires transitive java.logging; requires java.xml; }
- 22. JLINK : The Java Linker • Jlink is Java’s new command line tool through which we can create our own customized JRE. It is an best illustration of the concept just enough runtime. • Usually, we run our program using the default JRE, but in case if you want to create your own small customized JRE, then you can go with the jlink concept. • This feature is very useful in the era of microservices and containerization • jlink takes into account transitive dependencies from the top modules and generates a custom Java image.
- 23. JLINK : Uses class App { public static void main(String[]args) { System.out.prinltn("Hello World"); } } To execute this small “hello world” application, we require the following .class files: • App.class • String.class • System.class • Object.class
- 24. JLINK : USES • The default JRE contains 4000+ predefined Java .class files. • If I execute my “hello world” application with the default JRE, then all the predefined .class files will be executed. But if I only need 3-4 .class files to execute my “hello world” application, then why I need to maintain the outer .class files? • The default JRE's size in jdk-8 is 203 MB. For executing my simple 1 KB of code, I have to maintain 203 MB of JRE in my machine. It is a complete waste of memory.
- 25. JLINK Commands : jlink --module-path <modulepath> --add-modules <modules> --limit- modules <modules> --output <path> • –module-path specifies where to find the modules for the JDK. These can be jar files, jmod files (a new file format with JDK 9 which is similar to the jar format. • –add-modules adds the modules we need (in this example our app) • –limit-modules limits to just the modules that we know our application needs (sometimes when modules are added, further modules can be added via transitive dependencies. Here we can specify that we only want certain modules) • –output is this directory where the run-time image will be generated.
- 26. OSGI vs JPMS OSGI JPMS Framework Platform Feature. Was used to modularize the java platform itself Complex steep learning curve Simple when compared to OSGI Solves many problems like JAR Hell(Using classloaders),Circular Dependency Not solved/Not supported
- 27. Q & A Links to code : https://github.com/vickii Links to slide : https://www.slideshare.net/VigneshRamesh8 Linked in : https://www.linkedin.com/in/vickyramesh/ Twitter : https://twitter.com/vickiramesh