Modularity in the JavaTM Programming Language: JSR 294 and beyondAlex BuckleySun MicrosystemsComputational Theologist

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Disclaimer

Change is inevitable(Except from a vending machine)

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The modularity landscape in JDK7

JSR 294Language and VM features to support module systemsThe OSGi module system is well represented on the EG

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The modularity landscape in JDK7

JSR 294Language and VM features to support module systemsThe OSGi module system is well represented on the EG

JigsawA module system for JDK7, not part of a JSRUses language and VM features from JSR 294

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The modularity landscape in JDK7

JSR 294Language and VM features to support module systemsThe OSGi module system is well represented on the EG

JigsawA module system for JDK7, not part of a JSRUses language and VM features from JSR 294

OpenJDK's Project JigsawHosts the Reference Implementation of JSR 294Hosts the design and implementation of Jigsaw

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What JSR 294 is notJSR 277

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What JSR 294 is notJSR 277Just the 'module' keyword

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What JSR 294 is notJSR 277Just the 'module' keywordA module system

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Modular code

Is clear about what other code it depends on

Is able to evolve without breaking clients

Is able to hide its internals from other modular code

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Modular code

Is clear about what other code it depends on

By explicitly listing its dependencies on other code

Is able to evolve without breaking clients

Is able to hide its internals from other modular code

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Modular code

Is clear about what other code it depends on

By explicitly listing its dependencies on other code

Is able to evolve without breaking clients

By explicitly versioning itself and its dependencies

Is able to hide its internals from other modular code

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Modular code

Is clear about what other code it depends on

By explicitly listing its dependencies on other code

Is able to evolve without breaking clients

By explicitly versioning itself and its dependencies

Is able to hide its internals from other modular code

By explicitly making those classes 'module-private'

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Modular code is static typing++Explicit dependenciesExplicit versioningExplicit accessibility

} Static type information

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Modular code is predictableExplicit dependenciesExplicit versioningExplicit accessibility

Static type information allows early detection of errors that would otherwise occur at runtime

} Static type information

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Modular code is predictableExplicit dependenciesExplicit versioningExplicit accessibility

Static type information allows early detection of errors that would otherwise occur at runtime

Modular code allows the compile-time environment to reflect the runtime environment,so runtime behavior is more predictable

} Static type information

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Dependencies

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Take some existing code...

planetjdk/src/ org/planetjdk/aggregator/Main.java

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Modularize!

planetjdk/src/ org/planetjdk/aggregator/Main.java module-info.java

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module-info.javamodule org.planetjdk.aggregator {

system jigsaw;requires module jdom;requires module tagsoup;requires module rome;requires module rome-fetcher;requires module joda-time;requires module java-xml;requires module java-base;class org.planetjdk.aggregator.Main;

}

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module-info.javamodule org.planetjdk.aggregator {

system jigsaw;requires module jdom;requires module tagsoup;requires module rome;requires module rome-fetcher;requires module joda-time;requires module java-xml;requires module java-base;class org.planetjdk.aggregator.Main;

}

} The logical classpath for this module

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Compiling modular codeplanetjdk/src/

org/planetjdk/aggregator/Main.javamodule-info.java

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Compiling modular code

javac -sourcepath planetjdk/src-d planetjdk/clsorg/planetjdk/aggregator/*.javamodule-info.java

planetjdk/src/org/planetjdk/aggregator/Main.javamodule-info.java

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Compiling modular code

javac -sourcepath planetjdk/src-d planetjdk/clsorg/planetjdk/aggregator/*.javamodule-info.java

planetjdk/src/org/planetjdk/aggregator/Main.javamodule-info.java

planetjdk/cls/org/planetjdk/aggregator/Main.classmodule-info.class

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Questions, questions...

Q: Do you have to recompile module-info.javawhenever you add or remove a dependency?

A: Yes

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Questions, questions...

Q: Do you have to recompile module-info.javawhenever you add or remove a dependency?

A: Yes

Q: Where did the jdom, tagsoup, rome... modulesactually come from?

A: A module system

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Questions, questions...

Q: Do you have to recompile module-info.javawhenever you add or remove a dependency?

A: Yes

Q: Where did the jdom, tagsoup, rome... modulesactually come from?

A: A module systemA: javac's modulepath

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modulepath?

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Let's compile multiple modules together

ConvenientEfficient

1 compiler invocation may be more efficient than n Necessary for mutual dependencies

Module A requires module B which requires ATo start with, all you have are sources for A and BMust compile A and B at the same time

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Compiling multiple modules togetherplanetjdk/src/

org/planetjdk/aggregator/Main.java ... org/joda/time/DateTime.java ...

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Compiling multiple modules togetherplanetjdk/src/

org/planetjdk/aggregator/Main.java ... org/joda/time/DateTime.java ... module-info-for-which-module?.java

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Put multiple locations on classpath?planetjdk/src/org.planetjdk.aggregator/

module-info.javaorg/...

planetjdk/src/org.planetjdk.aggregator

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The classpath grows...

planetjdk/src/org.joda.time/ module-info.java org/...

planetjdk/src/org.planetjdk.aggregator/module-info.javaorg/...

planetjdk/src/org.planetjdk.aggregator:planetjdk/src/org.joda.time

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And grows...

rssutils/rome/com.sun.syndication/ module-info.java com/

planetjdk/src/org.joda.time/ module-info.java org/...

planetjdk/src/org.planetjdk.aggregator/module-info.javaorg/...

planetjdk/src/org.planetjdk.aggregator:planetjdk/src/org.joda.time:

rssutils/rome/com.sun.syndication

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classpath doesn't scale

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classpath doesn't scaleat compile-time or runtime

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classpath recap

rssutils/rome/com.sun.syndication/ module-info.java com/

planetjdk/src/org.joda.time/ module-info.java org/...

planetjdk/src/org.planetjdk.aggregator/module-info.javaorg/...

planetjdk/src/org.planetjdk.aggregator:planetjdk/src/org.joda.time:

rssutils/rome/com.sun.syndication

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s/classpath/modulepath/g;

planetjdk/src:rssutils/rome

rssutils/rome/com.sun.syndication/ module-info.java com/

planetjdk/src/org.joda.time/ module-info.java org/...

planetjdk/src/org.planetjdk.aggregator/module-info.javaorg/...

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Compiling modular codejavac -modulepath planetjdk/src:rssutils/rome

org.planetjdk.aggregator/module-info.javaorg.planetjdk.aggregator/org/...com.sun.syndication/module-info.javacom.sun.syndication/com/...

When compiling org.planetjdk.aggregator/* classes,

javac looks for org.planetjdk.aggregator/module-info.java

and infers that the classes belong to the

org.planetjdk.aggregator module

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Running modular codejava -modulepath planetjdk/cls:rssutils/rome

-m org.planetjdk.aggregator

The java launcher looks for the org.planetjdk.aggregator

module on the modulepath and executes its main class

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modulepath scalesat compile-time and runtime

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requires + provides

A provides clause gives information about what the module exports

In Jigsaw, a module can have aliases:module jdk.core {

system jigsaw;provides module java-base;

}This module satisfies requires module java-base

in another module

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Module-level annotationsimport com.foo.annotations.*;@DefaultNonNullmodule org.planetjdk.aggregator {

...}

module-info.class can hold generated annotationse.g. how dependencies were previously resolved

Annotate requires/provides clauses too?

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Versioning

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Rewind: Binary compatibility"Here is a list of some important binary compatible changes

that the Java programming language supports:Reimplementing existing methods, constructors, and

initializers to improve performance.Adding new fields, methods, or constructors to an existing

class or interface.Deleting private fields, methods, or constructors of a class.Moving a method upward in the class hierarchy...."

- Java Language Specification, Ch. 13

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Binary compatibility

Is a set of properties, not a set of featuresDescribes what can be done

"To preserve binary compatibility, a class or interface should treat its accessible members and constructors,their existence and behavior, as a contract with its users"

Does not help with what cannot be doneRemoving membersChanging signatures and thrown exceptions

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More is needed

Sun is often asked to change or remove APIs

People who want this rarely consult with people who would be affected!

But sometimes, it is desirable to refactor or remove

Versioning lets the vector of a change be described

Magnitude: major or minor?

Direction: change of signature? or of behavior?

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Modular code is versionedmodule org.planetjdk.aggregator @ 1.0 {

system jigsaw;requires module jdom @ 1.0;requires module tagsoup @ 1.2;requires module rome @ 1.0;requires module rome-fetcher @ 1.0;requires module joda-time @ 1.6;requires module java-xml @ 7;requires module java-base @ 7;class org.planetjdk.aggregator.Main;

}

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Versioning in JSR 294

Language + VM know that modules are versionedVersion structure, order, and ranges are

implementation details of a module systemStructure 1.2.3.4.5

I,II,III,IV,V..IX,X,L,C,D,MR11V16.4M3.2T200906031105

Order 1.2.3.4 > 1.2.31.6:u13 < 1.7:b60 < 1.6:u14

Range [1.0, 2.0) 1.* 1.*\1.31.0+ >=1.0 1.0

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Versioning in the Jigsaw module systemmodule org.planetjdk.aggregator @ 1.0 {

system jigsaw;requires module jdom @ 1.*;requires module tagsoup @ 1.2.*;requires module rome @ =1.0;requires module rome-fetcher @ =1.0;requires module joda-time @ [1.6,2.0);requires module java-xml @ 7.*;requires module java-base @ 7.*;class org.planetjdk.aggregator.Main;

}

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Language+VM defer to a module systemSuppose a module says:

system jigsaw;requires module M @ 2.*;

javac asks Jigsaw for M @ 2.*Jigsaw will respond with its lowest 2.* version, e.g. 2.1

Hotspot asks Jigsaw for M @ 2.*Jigsaw will respond with its highest 2.* version, e.g. 2.9

M's runtime behavior is predictable up to Jigsaw's version policy

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Accessibility

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Module-private accessibility

To share types and their members across packages but within a module, make them module-privatepublic class Foo {

module String m() { ... }}

Any class in same module as Foo can access Foo.mModule membership inferred from directory layout

Module-private accessibility stored in the classfile and enforced by the VM

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module is a 'restricted keyword'

Adding keywords that break code is frowned uponYou really can write:

package module;module class module {

module module module = new module();module module() {}

}

('can' and 'should' are two different things)

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'package' modifierpackage foo;package class Bar {

package String name;package String getName() {..}

}

(Not completely compelling)

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More information

Subscribe to the JSR 294 observer listCheck out OpenJDK's Project Jigsaw

http://openjdk.java.net/projects/jigsaw/http://cr.openjdk.java.net/~mr/jigsaw/api/

Attend our Modularity BOF7.45pm tonight, Gateway 102/103http://blogs.sun.com/abuckley/http://blogs.sun.com/mr/

Alex Buckleyalex.buckley@sun.com

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Short answer: Yes, with an if

Long answer: No, with a but

Q&A