java 9 modules: the duke yet lives that osgi shall depose

Java 9 Modules. The Duke Yet Lives That OSGi Shall Depose

Nikita Lipsky

Excelsior LLC

Modules in IDE

Modules in Mavencom.foo

app1.0

com.fooparse-api

com.foopersist-api

org.apachecommons

org.apachecommons-io

Modules in Mavencom.foo

app1.0

com.fooparse-api

com.foopersist-api

org.apachecommons

Why Jigsaw is not OSGi?

Mark Reinhold (the Chief Architect of the Java Platform Group):

“…As it (OSGi) stands, moreover, it’s useful for library and application modules but, since it’s built strictly on top of the Java SE Platform, it can’t be used to modularize the Platform itself”

Question: why the presence of j.l.Object in Java Language Specification, implemented in Java in turn, does not lead to a bootstrap problem?

Факт: Существует реализация Java SE, где OSGi поддерживается на уровне JVM (на уровне платформы).

Question: why the presence of j.l.Object in Java Language Specification, implemented in Java in turn, does not lead to a bootstrap problem?

Fact: There is a Java SE implementation that supports OSGi at the JVM level (at the platform level).

Nikita Lipsky• 20+ years in software development

• Excelsior JET project initiator– 16+ years of contributions– compiler engineer – team lead– product lead– etc.

• Twitter: @pjBooms

Excelsior JET?• AOT-centric Java SE implementation

– certified as Java Compatible since 2005

• AOT compiler + Java Runtime– mixed compilation: AOT + JIT– AOT support for custom classloaders (Eclipse RCP, Tomcat)

• Toolkit– Startup Optimizer– Deployment

Agenda

• Why OSGi

• How OSGi does it

• Why NOT OSGi

• Why Jigsaw is not OSGi

• Jigsaw mantra

• Jigsaw problems

Where OSGi?

Standardized by OSGi Alliance (IBM, Adobe, Bosch, Huawei, NTT, Oraсle)

Implementations:

• Equinox – Eclipse IDE, Eclipse RCP, IBM Websphere

• Apache Felix– Oracle WebLogic, Glassfish, Netbeans

Why OSGi?

Why OSGi?• Modularity

– Reduced complexity– Hide internals (encapsulation)– Dependency management and easy deployment

• Dynamic updates• Versioning• Lazy• Simple, fast, small, secure, etc.https://www.osgi.org/developer/benefits-of-using-osgi/

Модульная система OSGi

OSGi Module SystemOSGi module – Bundle:• Jar or directory• Import/export is defined in META-INF/MANIFEST.MF:

Manifest-Version: 1.0 Bundle-ManifestVersion: 2

Bundle-SymbolicName: B

Bundle-Version: 1

Bundle-Name: B Bundle

Export-Package: org.bar

Import-Package: org.foo;version=“[1,2)”

OSGi Module System

• OSGi bundle imports/exports

– packages (Import-Package/Export-Package)

– services (Import-Service/Export-Service).

• May import other bundles directly

– Require-Bundle directive

– However it is not the best practice (less flexible)

OSGi Module System

Bundle A

Class ABundle B

Class B

Bundle C

Class CExport-Package: packageCImport-Package: packageA

Export-Package: packageAI

Import-Package: packageA,packageC

OSGi Runtime

• Resolves Import/Export of OSGi bundles(wiring)

• Defines bundle life cycle

– May start (activate) bundles lazily

– Enables on-the-fly bundle updates without system restart (aka hot redeploy).

Jar/Classpath HellApp1.0

LibraryFoo2.0

LibraryBar3.0

LibraryBaz2.1

LibraryBaz3.1

Versioning in OSGi

• OSGI resolves JAR hell:

– import/export is qualified by version

– If two bundles require a library bundle of two different versions, both versions will be loaded

Why OSGi?So, OSGi promises:• Modularity

– explicit dependencies– encapsulation

• JAR Hell problem resolution– via versioning

• Hot ReDeploy– via ability to update a separate bundle dynamically

How OSGi?

Versioning in OSGi

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.

Versioning in OSGi

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.

Versioning in OSGi

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.

Versioning in OSGi

Thus each OSGi bundle is loaded by its own classloader:

• Subtype of java.lang.ClassLoader

• Unique class namespace

• No conflicts with classes of other bundles

Versioning in OSGi

com.fooApp1.0

com.fooparse-api

com.foopersist-api

org.apachecommons

Different versions of apache commons may simultaneously present in the JVM

Each bundle hasits own classloader

Encapsulation in OSGi

Encapsulation in OSGisrc/com/foo/exported/A.java:

package com.foo.exported;

import com.foo.internal.B;

public class A {B useB;

src/com/foo/internal/B.java:

package com.foo.internal;

public class B {

How to make the class B inaccessible from outside?

Question: how to make an internal class of a module declared public inaccessible outside?

Answer: classloaders (!) may hide internal classes.

Dynamic Updates

Task: update one (!) changed bundle in a running program without stopping it

Dynamic Updates

Task: update one (!) changed bundle in a running program without stopping it

Solution: CLASSLOADERS!

(unload old bundle, load new bundle by a new classloader)

Bundle life cycle

Lazy start

Start of bundles in OSGi is implemented via the bundle activators feature:• Each bundle may have an activator

– Bundle-Activator manifest directive– Implementation of an interface with methods start(), stop()

– Static class initializer analogue for bundles

Lazy start

Start of bundles in OSGi is implemented via the bundle activators feature:

public interface BundleActivator {void start(BundleContext context) throws Exception;void stop(BundleContext context) throws Exception;

Lazy start

• Bundle start can be defined by OSGi configuration

• Otherwise the bundle starts when it is required by other started bundles

Lazy start

Task: start (load) bundles only when they become needed for program execution

Lazy start

Solution: CLASSLOADERS (again!) in OSGi invoke the start()method of the bundle activator right before loading of the first class of a bundle

Since classloading in the JVM is lazy, bundle activation becomes lazy AUTOMATICALLY

Lazy start

How OSGi?

All solved in one fell swoop!48

Why Not OSGi?

Modularity?Bundle A

Bundle B

Modularity?Bundle A

Bundle B Bundle C

Modularity?Bundle A

Bundle B Bundle C

Bundle D

Modularity?Bundle A

Bundle B Bundle C

Bundle D

Modularity?Bundle A

Bundle B Bundle C

Bundle D

Modularity?Bundle A

Bundle B Bundle C

Bundle D

Modularity?

OSGI allows cycles in

the dependency graph

Exercise: understand, why

that is bad

On the fly updates?

Bundle ABundle B

On the fly updates?

Bundle ABundle B

Let’s update bundle B

On the fly updates?

Bundle ABundle B

On the fly updates?

Bundle A

Bundle B

On the fly updates?

Bundle A

NEWBundle B

On the fly updates?

Bundle ANEWBundle B

On the fly updates?

Bundle ABundle B

Does it work for bundle A?

On the fly updates?

Bundle B

Class B

Bundle A

Class А

On the fly updates?

Bundle B

Class B

Bundle A

Class А

If bundle B imports bundle А then there is a class from B that references a class from A

Symbolic references resolution

B.java:class B {

A useA;int f1 = A.f;int f2 = A.foo();

A.java:class A {

static int f;static int foo(){};

B.class…

CONSTANT_Class: A

CONSTANT_FieldRef: A.f@int

CONSTANT_MethodRef: A.foo()I

A.class…

Field: f@int

Method: foo()I

• A class references other classes and their fields/methods symbolically

• JVM resolves those symbolic references with real values at runtime

• Once a reference is resolved, the value of the reference is never changed!

On the fly updates?

• If a reference from a class B to a class A is resolved then the class А cannot be unloadedfrom the JVM without unloading the class B

On the fly updates?

• If a reference from a class B to a class A is resolved then the class А cannot be unloadedfrom the JVM without unloading the class B

• Hence if a bundle В imports a bundle А then the bundle А cannot be unloaded without also unloading the bundle B

On the fly updates?

Bundle ABundle B

Let’s update bundle A

On the fly updates?

Bundle ABundle B

On the fly updates?

Bundle A

Bundle B

On the fly updates?

Бандл ABundle B

On the fly updates?

Now let’s remember

cyclic dependencies

Exercise: try to update

SWT bundle in running Eclipse

On the fly updates?

• On the fly updates in OSGi work, more or less, only for leaf bundles that are not imported by other bundles – plugins

On the fly updates?

• On the fly updates in OSGi work, more or less, only for leaf bundles that are not imported by other bundles – plugins

• There are much simpler ways than OSGi to implement plugins

On the fly updates?

Even leaf bundles are not so easy to unload from the JVM:

• The classes from leaf bundles can live in the JVM after their unloading

– Known problem: Classloaders Memory Leak

One does not simply

unload a class from the JVM84

Versioning?

Versioning?App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

Versioning?App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

Versioning?App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

Versioning?App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

Versioning?App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

Versioning?App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

Versioning?App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

Versioning?App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

I’m “A”!

Versioning?App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

I’m “A”!No, I’m

“A”!

Versioning?App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

AAAAA!AAAAA!

Loading constraints

• Loading constraints prohibit two classes with the same fully qualified name to appear in the namespace of another class

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)

Versioning?

• JVM throws java.lang.LinkageError on loading constraints violation

Versioning?

• JVM throws java.lang.LinkageError on loading constraints violation

• OSGi DOES NOT help developers to avoid loading constraints violation

– Just google for “OSGI” and “LinkageError” to estimate the scale of the problem

Versioning?

There are bundles in the latest Eclipse versions with potential loading constraints violations and nobody cares!

Versioning?

Conclusion: OSGi does not solve JAR Hell but raises it to a new, more sophisticated level.

Versioning?

Conclusion: Do not use two different versions of the same library in one application!

Encapsulation?

Question: well, at least the encapsulation problem is solved by OSGi, right?

Reflection – universal countermeasure

against encapsulation in Java

Encapsulation?

setAccessible(true)

Encapsulation?

Question: well, at least the encapsulation problem is solved by OSGi, right?

Answer: OSGi does not protect from unauthorized access via Reflection

Lazy start?

Lazy start?Bundle A

Bundle B Bundle C

Question: in what order the bundles will be activated?

Lazy start?

Bundle activation order in OSGi depends on classloading order directly

– A bundle is started from loadClass() of the bundle classloader

Lazy start?

Bundle activation order in OSGi depends on classloading order directly

– A bundle is started from loadClass() of the bundle classloader

However, class loading order is not defined by the JVM specification!

Symbolic references resolutionA class may reference other classes and their fields/methods symbolically. A JVM may resolve references:

• Lazily– Each reference is resolved on first access

• Eagerly– All references are resolved at the time of class loading

Lazy start?

The classloading order depends on the class reference resolution scheme in the given JVM implementation: lazy, less lazy, eager.

Lazy start?

Bundle B activator:class B implements BundleActivator {

public void start() {assert A.f!= null;

Bundle A activator:class A implements BundleActivator {

static T f;public void start() {

f = new T();}

Typical case: В thinks that А is already activated, but in fact А can be activated after B, so assertion fails

Lazy start?

• Bundle activation scheme in OSGi is a time bomb:

– If the JVM starts to resolve class references less lazily, practically all OSGi applications will stop working

Why NOT OSGi?

• Modularity with cycles

• Hot Redeploy works for leaf bundles only

• No protection from loading constraints violation

• No protection of implementation details from reflective access

• Bundle activation order depends substantially on class reference resolution scheme

Jigsaw

Jigsaw vs. OSGi

OSGi is dynamic essentially

– modules appear at run time only

Jigsaw vs. OSGi

Jigsaw was thought up as static from the beginning.

Practically all JDK tools know about modules:– javac: respects modules visibility rules

– jdeps: analyses dependencies

– jar, jmod: pack modules

– jlink: prepares final image for deployment

– java: there are modules in runtime as well

Module example// src/java.sql/module-info.javamodule java.sql {

requires transitive java.logging;requires transitive java.xml;

exports java.sql;exports javax.sql;exports javax.transaction.xa;uses java.sql.Driver;

Jigsaw vs. OSGi

Jigsaw prohibits (explicit) cycles

in the dependency graph

Modules import modules

and not packages.

Versioning

There was versioning in the first drafts of Jigsaw (similar to OSGi).

Versioning

However versioning was removed later …

Versioning

However versioning was removed later …

Versioning

Versioning immediately means:

1 module 1 classloader

Versioning

Versioning immediately means:

1 module 1 classloader

It was exactly so in the first Jigsaw drafts!

Jigsaw and classloaders

Jigsaw is not just modules, but also the Java SE platform split into modules.

Backward compatibility problem:

According to the specification

getClassloader() == nullfor core platform classes.то противоречит

That precludes splitting of the platform into modules, with each module loaded by its loader

Backward compatibility problem:

According to the specification

getClassloader() == nullfor core platform classes.то противоречит

That precludes splitting of the platform into modules, with each module loaded by its loader

Problem 2: How to protect developers from loading constraints violation?

Versioning

Another detail: import in early Jigsaw versions (as in OSGi) 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

Versioning

Problem 3: Resolving dependencies (wiring modules) from version ranges is an NP-complete problem!

– Reduced to 3-SAT

Versioning

… after that versioning in JPMS breathed its last.

Versioning

… after that versioning in JPMS breathed its last.

No versioning – no classloaders for modules.

Jigsaw

• No dynamic updates

• No versioning

Jigsaw

• No dynamic updates

• No versioning

So what does it have?

Jigsaw Mantra

Reliable Configuration

Strong Encapsulation

Такая ситуация в Jigsaw просто запрещена!

App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

Jigsaw simply prohibits this situation!

App1.0

Foo2.0

Bar3.0

Baz2.1

Baz3.1

Right (reliable) configuration:

App1.0

Foo2.5

Bar3.0

Baz3.1

Reliable configuration:

• All module dependencies are resolved

• No cyclic dependencies

• No modules containing the same packages (split packages)

These properties are checked at startup (wiring)

setAccessible(true)

Java 9 modules are first class citizens

• Define visibility access rules

– via declared export

• There is no access to non-exported functionality outside of the module even via reflection

– Even setAccessible(true) does not work

So Jigsaw does not have problems?

Reliable Configuration?

• Reflective access was prohibited between modules without explicit dependencies in early Jigsaw drafts

• However it had to be relaxed when classloaders had gone from JPMS– because Class.forName() has to work

backward compatible

• Reflective access was prohibited between modules without explicit dependencies in early Jigsaw drafts

• Had to be relaxed when classloaders had gone from JPMS– because Class.forName() had to remain

backward compatible

However if you may have reflective dependencies that are not explicitly declared, where is the guarantee that the resulting configuration is reliable?

Split packages are prohibited, but what about application containers (Tomcat, Java EE)?

Jigsaw Layers *

To solve the application containers problem

Layers feature was introduced in JPMS:

• Local module system for each application in a container

• Two modules containing the same package have to belong to different layers

Jigsaw Layers *

* The picture is from Alex Buckley’s presentation: Project Jigsaw Under the hood154

Strong Encapsulation?

The platform is split into modules:

• Which means that private APIs become really private

The platform is split into modules:

• Which means that private APIs become really private

• But what about

sun.misc.Unsafe ?

Ok, Java community (temporarily)

defended sun.misc.Unsafe (in an unequal fight)!

But what about Dependency Injection?

DI frameworks essentially depend on:

• reflective access to the code in which they inject dependencies

• including non-exported code

module my.module {exports my.module.pack;

module my.module {exports my.module.pack;exports my.module.internal.object.orgy;

module my.moduleO{

exports my.module.pack;}

open module my.module {

exports my.module.pack;}

Open modules were introduced to solve the DI frameworks problem:

• An open module allows reflective access to its non-exported functionality

Open modules provide not-so-strong encapsulation, but it is better than nothing.

Jigsaw

Well, but what benefits me in Jigsaw at last?

Jigsaw benefitsIf all your dependencies are on the classpath now, migrating to the modulepath would improve the architecture of your application by eliminating:• cycles in the dependencies• split packages (jar hell)• unsound access into implementation details of

other modules• dependencies to JDK private API

Jigsaw benefitsIf all your dependencies are on the classpath now, migrating to the modulepath would improve the architecture of your application by eliminating:• cycles in the dependency graph• split packages (jar hell)• unsound access into implementation details of

Jigsaw benefitsIf all your dependencies are on the classpath now, migrating to the modulepath would improve the architecture of your application by eliminating:• cycles in the dependency graph• split packages (jar hell)• unsound reliance upon implementation details of

other modules• dependencies on JDK private APIs

Jigsaw benefits

Jigsaw introduces a migration path to the modulepath:

• Old classpath forms Unnamed Modulе

• Jars from classpath may be temporally moved as is to modulepath as Auto Modules

• Module declaration can be added to auto modules later

Jigsaw benefits

• Old classpath forms Unnamed Module

• Jars from classpath may be temporally moved as is to modulepath as Auto Modules

Jigsaw benefits

• Jars from classpath may be temporarily moved “as-is” to modulepath as Auto Modules

Jigsaw benefits

• Jars from classpath may be temporarily moved “as-is” to modulepath as Auto Modules

Jigsaw benefitsUnfortunately, most Java developers won’t benefit from JPMS immediately:• Java EE standards do not define how they will

interoperate with JPMS yet• Even servlet containers standard knows nothing

about modules so far– Dependencies in war файлах are essentially old plain

classpath!

Jigsaw benefitsUnfortunately, most Java developers won’t benefit from JPMS immediately:• Java EE standards do not define how they will

interoperate with JPMS yet• Even the servlet container standard knows

nothing about modules so far– Dependencies in war files are old plain classpath in

The birth is inevitable!

Conclusion• OSGi is a nice attempt to give modules to Java

developers– but OSGi has many problems unfortunately– including fundamental

• Jigsaw is a carefully designed system without visible fundamental problems– but with a system of checks and balances– there are community acceptance problems

Nikita Lipsky,Excelsior

nlipsky@excelsior-usa.comtwitter: @pjBooms

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