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Tuesday, December 26, 2017

Java 9 Migration Tips

Java 9 was released this year and it is a much awaited release of Java with a lot of interesting and exciting features. But Upgrading is a concern for any CTO or Infrastructure Team as it may break the existing functionality or create unforeseen issues.

But in a recent presentation, Bernard Traversat who is the Vice President of Development for the Java SE Platform at Oracle provided some tips on successful migration to Java 9. Bernard manages the development team responsible for the Java Language, JVM, Core libraries, Java UI toolkits, and Java deployment stack for Oracle JDK product. So who else is fit enough provide advise on Java 9 Migration than him.

Why Upgrading is required;

  • Upgrades include Security fixes    
  • Upgrades include Regressions fixes    
  • New features with releases
  • More performance    
  • More robustness    
  • Because the top or bottom tier was upgraded (OS’s, Hardware)    
  • Reduce cost of operation via uniformity

Why Upgrading is delayed;

  • Fear of something is going to go wrong.
  • It will cost money if something goes wrong. According InformationWeek IT Downtime Costs $26.5 Billion in Revenue.
  • It requires extra work which costs time and money.
Out of Programming Languages Java, Golang, Python, C# .NET, As per Bernard Only Java is Backward compatible and upgradable.

There are binary, source and runtime compability explicit managed in the Java platform.
Most of the APIs (java.*, java.*) undergo millions of tests to ensure compatibility and have specifications managed under the JCP (Java Community Process).
In order to update a specification of an API, rigorous JCP process is required.


The following things remain the same from Java x to Java 9

  • The class path 
  • Class loading 
  • Not forced to migrate to modules
  • sun.misc.Unsafe works
  • Most existing code should work
  • The IDEs, Maven, etc. all have support for JDK 9 already

Things to look out for when migrating from Java x to Java 9

  • You will have to upgrade the libraries and tools that you use    
  • Build or deployment needs to be adjusted If you use any of the components that are shared between Java SE and Java EE 
  • A small number of supported APIs have been removed    
  • A number of non-API features and tools have been removed        
  • Libraries that you use may show some warnings

Incompatibility issues you will face during or after migration;

No 1 issue will be a Third Party library you use in your older Java code is not compatible with Java 9 anymore. This is addressed as many open source projects and vendors are already changing their code base to Java 9 but in case one hasn't done already until they do your migration will have issues.

Apart from that following can also affect you and you must be aware;

1. JEP-260 Encapsulate most internal APIs - This means Internal APIs will no longer be accessible to developers. If your code uses an internal API then it needs to change.

2. JEP-223 New Version String Scheme - JDK 1.9.0_25 will turn into JDK 9.1.3. If your code contains any version string checking then it has to change.

3. JEP-220 Modular Runtime Images - No more rt.jar and JDK directory structure will change. If your code has any JDK directory checking then it needs to change.

4. Most sun.misc.* and sun.reflect.* are removed - If your code uses any of these packages then it needs to change. Ex:- you can no longer use sun.misc.Base64 class.

5. JEP-214 Remove Java 8 Deprecated GC Combinations - Following GC Combinations will now produce errors. You need to change your Java start up scripts if you use any.

DefNew + CMS       : -XX:-UseParNewGC -XX:+UseConcMarkSweepGC
ParNew + SerialOld : -XX:+UseParNewGC
ParNew + iCMS      : -Xincgc
ParNew + iCMS      : -XX:+CMSIncrementalMode -XX:+UseConcMarkSweepGC
DefNew + iCMS      : -XX:+CMSIncrementalMode -XX:+UseConcMarkSweepGC -XX:-UseParNewGC
CMS foreground     : -XX:+UseCMSCompactAtFullCollection
CMS foreground     : -XX:+CMSFullGCsBeforeCompaction
CMS foreground     : -XX:+UseCMSCollectionPassing

6. Launching JVM with Unrecognized VM Options will result in failure to start

7. java.awt.peer and java.awt.dnd.peer packages will be hidden - If your code uses these it needs to be changed to use supporting APIs

8. Calling Thread.stop(Throwable) method will throw an Exception - This was deprecated for a long time now.

9. JEP-271 Unified GC Logging - Re-implemented GC Logging with the new JVM Logging framework JEP-158, If your Java start up scripts use any of the following they need to change.

-XX:+PrintGC will be changed to -Xlog:gc
-XX:+PrintGCDetails will be changed to -Xlog:gc*
-xloggc: will be changed to -Xlog:gc:

10. JEP-248 G1 is the new Garbage Collector - Serial GC is and will remain the default garbage collector for Windows 32 bit.


Migration to Java 9 is not as scary as it looks and will be easy with proper planning. Inorder to harness the full features it is advised to migrate
in small increments so that issues arising can also be mitigated easily.


Friday, December 8, 2017

Writing Reactive Repositories for Spring Data with Mongodb

1. Overview

Reactive Programming has been alive for sometime now. Programming frameworks like Akka, Reactive Streams, Reactor, RxJava etc are good examples. In simple terms reactive programming is about writing non blocking software that are asynchronous and event driven.
Reactive Programming requires a small number of threads to scale vertically (Scale up inside a single JVM) instead of horizontally (Scale out to different nodes by means of clustering).
With Spring 5.0 there is out of the box support for Reactive Programming and now Spring Data project also has Reactive support. Now we will be looking at those latest features here in detail:

2. Setup

In order to use Spring Data Reactive Repositories we need to include spring-boot-starter-data-mongodb-reactive, de.flapdoodle.embed.mongo (for testing), rxjava and rxjava-reactive-streams. Plus reactive mongo-db driver is needed to make full use of the reactive capabilities. The maven dependencies will look like below:

Complete pom.xml file can be found at the Github repository listed at the conclusion section.

3. Enabling Spring Data Reactive Repositories for Mongodb

As the title suggests we will have a look the Spring Data Reactive Repositories with Mongodb. The new @EnableReactiveMongoRepositories is introduced to enable Reactive Repository support for Mongodb. The following configuration enables Spring Data Reactive Repositories for Mongodb:

4. Reactive Repositories

Spring Data project uses the repositories programming model which is the most high-level abstraction to deal with data. They’re comprised of a set of CRUD methods defined in a Spring Data provided interface and domain-specific query methods.
Mainly by using an interface named CrudRepository which exposes methods like findOne, delete, save. With Reactive Programming support Spring Data project has now introduced two more interfaces named ReactiveCrudRepository and RxJava2CrudRepository (for RxJava project support)
A typical Spring Data Reactive Repository would look like below:

And a RxJava2 version of the same Repository would look like below:

Note that Spring 5.0 Reactor Project specific Flux is returned in ReactiveTaxiRepository and RxJava project specific Flowable is returned in RxJava2TaxiRepository.
These repositories are really identical to standard Spring Data Repositories except for the fact that now they can return and/or accept as parameters, reactive elements such as Flux, Mono and Flowable.  By default, reactive repositories use Project Reactor types but other reactive libraries can also be used such as RxJava2 as shown above.

5. Using Spring Data Reactive Repositories for Mongodb

When using the new Spring Data Reactive Repositories, We can use the full features of Reactive Programming provided by the entities Flux, Mono or Flowable (RxJava2).
And now we look at Reactor version would like below:

And now we look at the RxJava2 version:

The above codes will find Taxis by Number CAL-4259 and collect that Flux stream or Flowable stream into a List and will block until the collection is finished.

6. Streaming Data with Tailable Cursor

Spring Data Reactive Repositories provide a way to Stream Data as it arrives into Mongodb with a @Tailable annotation, sort of like an Event Source system. Sticking to our example of Taxis, we can Subscribe to a Tailable Stream and while being subscribed, insert Taxi entities into Mongodb.
This will enable to see newly added Taxi entities coming into system in real time in a streaming manner until the subscription is disposed of. Simulating Taxis entering into a City in real time:

7. Advantages

Compared to a Standard Spring Data Repository, A Reactive Repository provides all the features of Reactive Programming to Data Retrieval. Just by using Reactive Repositories, we can easily filter, process, aggregate data returned declaratively and use asynchronous capabilities provided out of the box in Reactive Programming.

8. Conclusion

Reactive Programming provides a lot of features such Functional, Declarative style of Coding which is being rapidly adopted by developers and enables to write scale-able, easy to understand code.
Now with Spring Data Reactive Repositories these features can be easily incorporated into the existing features of Spring Data project. The complete Source code for the project can be found at GitHub .