Inter-procedural Null analysis using annotations in Eclipse JDT

The JDT team is extremely excited with the release of the new annotation based null analysis. Before writing anything about it I want to thank Stephan Herrmann (JDT committer from Germany) for all the hard work he has put in on this feature! Feel free to drop him a word of appreciation on stephan AT cs.tu-berlin.de

What is JDT's annotation based null analysis?
To give a bit of a background, you must've noticed that JDT java compiler does some static analysis and based on that warns if you deference a variable that has been analysed to be (potentially) null at runtime, or compare a variable against null when its nullness at runtime can be determined at compile time.

[See line numbers to locate error messages]

However, the warnings are only generated for local variables inside a method. There's no way to find out the change of nullness due to a method call, or for parameters of a method. Consider the example on the left. o1 is a parameter and since we don't know how foo(..) will be called and what value will be passed to o1, we can't do any analysis on it unless its state definitely changes inside foo(..). Also note that even though o1 is assigned return value of method  bar(), which returns null, the compiler cannot figure that out during static analysis and assumes the return value of bar() to be an unknown value.

Null annotations offer a solution here. They are a way of enforcing null contracts in your code to make sure you catch such (potential) NPE's before they occur at runtime and become a stop-ship bug for your product.


How do I use null annotations?
JDT now comes bundled with a JAR (org.eclipse.jdt.annotation) containing null annotations. The types of annotations offered are:
NonNull annotation
Default is @org.eclipse.jdt.annotation.NonNull -

• when this annotation occurs on a return type, it enforces a non null return value for a method.
• when this annotation occurs on a method parameter, it enforces a non null value to be passed to the method call and that only a non null value can be assigned to this parameter inside the method.

Nullable annotation
Default is @org.eclipse.jdt.annotation.Nullable-

• when this annotation occurs on a method return type or parameter, it says that the returned value can be null, and so can be the argument.

NonNullByDefault annotation
Default is @org.eclipse.jdt.annotation.NonNullByDefault-

• This can be applied to a package (in package-info.java), class or method to enforce nullability for all enclosed elements. The annotation alone enforces non nullability of enclosed method return value and parameter values.
• If applied with a an argument i.e. NonNullBeDefault (false), it will cancel out any global default for the particular element.

To use these annotations in your project, right click Project>Build Path>Add External Archives and browse to org.eclipse.jdt.annotation JAR in the "plugins" directory of your eclipse install location. (We are working on the UI for this.)

One can also use their own annotation types as null annotations. Just make sure they are on the project's buildpath.

To enable null annotation based analysis and specify the annotation names, navigate to Preferences>Java>Compiler>Errors/Warning>Null analysis.

(Note also the "use non-null as default" option, which you can use to set a universal (workspace/project level) default to be applied to all method return types and parameters.)







What new errors/warnings should I expect to see with null annotations enabled?
As seen in the above dialog, there are two kind of diagnostics of primary importance - violation of null specification and potential violation of null specification. In addition to these the existing null analysis gets enhanced, and now for annotated method parameters a null state is available inside a method.

Violation of null spec is issued in cases when:
1. You pass a null value as an argument to a method whose corresponding parameter is annotated with @NonNull
2. You assign a @NonNull annotated parameter a null value.
3. You return a null value in a method that is annotated with @NonNull
4. You inherit a method annotated with @NonNull but now loosen the contract by annotating child method as @Nullable.
5. You inherit a method with a parameter annotated with @Null but now tighten the contract by annotating child method's corresponding parameter as @NonNull.

[See line numbers to locate error messages]

Potential violation of null spec is issued when:

1. You pass a value evaluated to be null on some execution path as an argument to a method whose corresponding parameter is annotated with @NonNull
2. You assign a @NonNull annotated parameter a value ascertained to be null on some execution path.
3. You return a possibly null value in a method that is annotated with @NonNull.

[See line numbers to locate error messages]

Insufficient null info is issued when there's not sufficient information to evaluate a given value to be (possibly) null and that value is passed as an argument, assigned to a parameter or returned from a method.

[See line numbers to locate error messages]

Redundant null annotation is issued when an annotation has the same effect as one applied already in an enclosing element.

[See line numbers to locate error messages]

Note that this is a work in progress and we will continue to polish the same for the Juno release. We're also working on ways to help people use this functionality easily (eg.: Support for nullity profiles for libraries and Quick fixes for null annotations. ) .
For a sneak preview of this feature, drop in at the EclipseCon 2012 tutorial - How To Train the JDT Dragon. If you find any issues while testing this feature, please open a bug. I hope you're as excited as we are about this cool new feature!

Of Compliance and Source settings in Eclipse java compiler

Going through a few stack overflow posts and a few bugs, I realized that there's still a lot of confusion about the  source and compliance level settings available in JDT. Well, frankly, almost everyone I know was confused when they started using JDT, even the current JDT committers themselves!
These settings are available in Preferences>Java>Compiler, or in your project's .settings folder>org.eclipse.jdt.core.prefs file.

So, what do the settings imply and what do they do?

Source compatibility:
This setting should be used to select upto which java version the user intends to use the language features. Eg.: setting it to 1.7 means that the user can use strings in switch, the diamond construct, etc. but setting it to 1.6 will mean that the user can not use such 1.7-only language features.
For the JDT compiler itself, we use a compliance of 1.4. Yes, quite old-fashioned you'd say, but this was a conscious choice to enable people to use the JDT compiler even if they dont have a version of java higher than 1.4 installed on their machines. You can also chose a source setting for your project on such considerations.

Compiler Compliance level:
This setting is a kind of a switch to make the JDT compiler "compliant" to a certain Oracle javac version. This is because not all bugs fixed in the Oracle javac compiler get backported to earlier releases. So, there are many differences between each javac version. There are even cases when only one javac version differs from the others, before and after it. So, we provided the compliance setting for users to replicate that difference in behaviour in the JDT compiler. This is to enable people who compile with ECJ and ship with javac or vice-versa, to see consistent errors (or the lack thereof).

Take, for example, the following case:

package com.example;

import java.util.List;


public class ErasureBug {


    public String output(List<Integer> integers) {


        return "";
    }


    public void output(List<Double> doubles) {


    }
}


This code compiles without error only in javac 6, but fails with a duplicate method erasure in both javac 5 and javac 7. 


So, in Eclipse when the user sets the project "compliance" to 1.6, he/she will be able to compile the code as shown, while in 1.5 and 1.7 modes, the code will fail.

Generated .class files compatibility:
This setting changes the generated bytecode to be compatible to a certain VM version. This is required at runtime when the VM available is lower than the one for which the code is compiled. So, if you set source level at 1.3 , use 1.3 features your code and expect your clients to still be able to use the older 1.2 or lesser VM to execute your code, you should set the .class file compatibility setting to 1.2 or less. This does not work for higher java versions, because suppose you use annotations in your code which only came in 1.5, or instructions such as invokedynamic in 1.7, which introduced a new Bootstrap method attribute, earlier VM's may still not be able to execute the class files. So we cannot guarantee backward compatibility and the UI thus will not allow you to set class file compatibility lower than the source level. However, if you're using features such as generics in 1.5 source level, it's quite safe for a 1.4 VM to execute those. So, you can manually set the org.eclipse.jdt.core.compiler.codegen.targetPlatform property to 1.4 in project>settings folder>org.eclipse.jdt.core.prefs file.


Note that the source level cannot be more than the compliance level.


Hope this gives more clarity on the settings.


EclipseCon update:
I have the following submissions for EclipseCon 2012. Do vote if you find them interesting:

• What's new in JDT
• The holy grail of mobile development
• Agile and collaborative development of mobile apps

Of invalid references to system libraries and execution environments

Picture this:

• You have a plugin that is supposed to be compatible with systems running even on 1.4 JRE i.e. your plugin execution environment is 1.4.
• An unsuspecting developer unintentionally checks out this plugin project into his workspace where he uses a 1.6 JRE by default.
• He starts writing Generics and is even allowed to do so because a 1.6 library is associated with the project in his workspace. The poor guy doesn't yet know what blunder he has done! :)

See an example below where in a plugin with 1.6 compliance, a developer accidentally used Autocloseable (coming from his workspace default 1.7 JRE).

API tooling provides a mechanism to figure out such illegal references to system libraries. In order to set up API tooling for your plugin project, right click on the project > PDE tools > API Tooling setup. Choose your project and click next. When it shows "Add API nature and API analysis builder", click finish. Now make sure your project settings for API tooling (right click > Properties > Plug-in development > API errors/warnings) are set as shown below:

By default, the "Supported Environments" will have no Execution Environments descriptions installed (From 3.8 onwards you will get an error whenever no execution environment description is installed). To install these EE descriptions, click on "install them now" or go to the "Install New Software.." page. Choose the "Eclipse Project update site" if already available or add the URL http://download.eclipse.org/eclipse/updates/<version> , where <version> = 3.6, 3.7, etc.

Uncheck "group by category" option and search for descriptions. Install these descriptions and you're all set to go. (Note: the above steps should be taken if you move to 3.8 Juno and suddenly find the "Missing EE descriptions" error in your project). 

Now with the above "invalid references to system libraries" option set to error/warning, you should get a warning for usage of Autocloseable in the above plugin project, as shown below:

These settings when done once on the project level and released into the repository will make sure that nobody accidentally references any API/class one is not supposed to. Read Olivier's post to know more about project vs. workspace settings

Agile Development of mobile applications using IBM Rational Team Concert and Phonegap for Android, iOS and others

In earlier articles - Developing applications for Android™ using IBM Rational Team Concert in an agile way and Developing applications for Android™ using IBM Rational Team Concert andRational Rhapsody in an agile way we saw how to leverage the power of planning, collaboration, process, source control, build management, etc that IBM Rational Team Concert (RTC) has to offer, to develop and manage Android applications in a team working across geographies and across different platforms. We also saw how a team can model the application, specify UML class diagrams, use case diagrams, and link them with the requirements through Rational Rhapsody. The buck does not stop here. In a world where there are numerous different platforms such as iOS, Windows Mobile, Android, Meego, Blackberry, etc, how does a development team create applications that work seamlessly across different devices supporting different platforms? The brute force approach, ofcourse, is to write fresh code for each platform and then test and manage the application differently for each device or platform configuration. And as it sounds, this approach is not at all scalable and requires immense investment of resources – time, people and money. The other confounding problem that mobile developers can typically come across is how to glue together native and web components into a single mobile application(hybrid application). The way out of this conundrum is offered by new, open source technologies such as PhoneGap.

In this article, we explore what is PhoneGap and how does one use it. We then see, with the help of a demo, how a team can easily develop, test and maintain a cross-platform application working on Rational Team Concert's Eclipse Client and using PhoneGap APIs. This is facilitated by the MDS Applaud PhoneGap Eclipse plugin.

---

About PhoneGap

According to the website, "PhoneGap is an open source solution for building cross-platform mobile apps with modern, standards-based Web technologies. Based on HTML5, PhoneGap leverages web technologies developers already know best... HTML and JavaScript."

The definition itself does not make things very clear though. Let us try understand a bit more. So today, suppose you have a web application that you want to deploy on multiple platform, say Android, iPhone, etc. In this app, you want to use one or more of the platform services to obtain some kind of data or carry out some kind of processing. Lets take, for example, an application that lets you invite friends to a movie as soon as you book a movie ticket. The app lets you select which friends to invite directly from your phone's contacts book. That means, if you're the developer of such an app, you'll most probably have to deal with consuming the platform's contact book using the phone APIs, for doing which, each platform would obviously have its own different ways. So there are 2 problems here:

1) How can you query the platform's contact book from within your web application which is being written in javascript and has no knowledge of the platform's APIs.
2) How can you do so in a platform agnostic way, so that your web app works not just on Android but also on iPhone, notwithstanding the different ways of using the contacts book in both.

PhoneGap aspires to be the answer to both of the above questions, and also allows creation of hybrid applications, consisting of both native and web components. How? It provides APIs which abstract the platform's contact book for you, so that you only deal with the PhoneGap API and let PhoneGap do the rest of the magic for you.

---

Download and installation

1) OS: Windows x86 / Linux x86 .
2) IBM Rational Team Concert: Download/buy from RTC downloads page. Note that if you're using an existing Eclipse installation to install RTC, make sure you have Eclipse 3.5 or lower (Eclipse 3.6 is not yet supported for Android development). RTC can also be installed directly on Eclipse 3.5.x (see http://jazz.net/library/techtip/384).

3) Android SDK: Download the SDK from Android website - http://developer.android.com/sdk/index.html.

4) Android Development Tools(ADT): This is an eclipse plugin- the equivalent of JDT(Java Development Tools)- for Android Development. ADT can be downloaded and installed using instructions given at http://developer.android.com/sdk/eclipse-adt.html. Please follow the detailed instructions on this page for installing the ADT plugin, and also for setting the location of Android SDK in Eclipse.

5) It is a good idea to read through the following step by step guide to set up RTC and prepare it for Android Development: http://jazzpractices.wordpress.com/2010/08/10/how-to-set-up-rtc-for-android-development/.

Installing PhoneGap

Refer to my earlier blogpost on developing Phonegap applications using Eclipse to read about how to install Phonegap on the RTC Eclipse client.

*Note: You can similarly develop Windows mobile applications using RTC's Visual Studio client.

---

Creating a PhoneGap Android project

Follow the steps below to create a hello world PhoneGap Android application:

1. Create a new PhoneGap project by clicking on the PhoneGap command you see on the coolbar.
   
2. If you don't have the source code for PhoneGap, check "Use Built-in PhoneGap" on the project wizard
   
3. Click Next and create the new Android project in the Android project wizard.
   
4. Launch the new application as an Android application and you will get an application as shown below.
   
   
   
   

(Note: For those who do not have the PhoneGap plugin for Eclipse, you can also simply create an Android project, making sure that the android main activity class extends PhoneGap's DroidGap class instead of Activity class. Make sure you add "phonegap.jar" as an external library to the build path.) 

 

You see that a PhoneGap app is nothing but an android app with the main Activity class extending PhoneGap's DroidGap class and it's onCreate() method loading an HTML page instead of performing any native functionality. You can replace this HTML page by one of your own choice in the assets/www folder. Check out this website for some good examples. Similar applications can also be created on other platforms using the PhoneGap classes specific to that platform. The javascript code can be directly re-used.

---

The Demo

The demo showcases how a distributed team working on a cross-platform, hybrid mobile application can leverage PhoneGap APIs, and also the functionality provided by RTC through the complete lifecycle of the application, right from planning to implementation to design and testing, and even defect tracking.
In the demo, the application to be developed has a native activity to start with, which displays a list of all phone contacts. Clicking a particular contact displays the contact details in another native activity and provides a button to send an email to that contact. Clicking the button launches a web based email client (this is a PhoneGap activity), which has an option to cc more contacts from the phone's contact book. This makes use of the PhoneGap contacts API to fetch a list of the phone contacts from the web based activity. 
 
Check out the video below:

With the recent release of the Rational's Jazz based solution for Application Lifecycle Management, also called Collaborative Lifecycle Management or CLM, it is possible to do much more by integrating multiple tools with RTC. To see Rational Team Concert and CLM live in action, do visit IBM Rational Innovate India 2011, Bangalore. You can catch me there at the RTC solutions center. Also, if you're interested in building JavaME applications, you can also read up my jazz.net article - Developing Java ME applications using Rational Team Concert inan agile way to see how to do so using RTC.

---

For more information

• Android Developer website
  
• Getting started with Rational Team Concert
  
• Developing Android applications using Eclipse
  
• Using PhoneGap in Eclipse to develop Android applications
  
• PhoneGap Android Eclipse quick start 
  
• Rational Solution for Android Mobile Application Development 
      

Android is a trademark of Google Inc. Use of this trademark is subject to Google Permissions.

Java 7: Decoding the new Diamond operator with JDT

Java 7 GA is officially out. Many people have already been trying their hands on the new features, and blogging/commenting about them at various forums. However, I still believe that people do not understand the real meaning of the diamond operator, which brings in "improved type inference for generic instance creation". Being the guy responsible for implementing this feature in Eclipse JDT's BETA java 7 support, I had a tough time understanding the specs at first. But now I have greater clarity. I guess everybody knows by now that diamond operator <> can be used in place of re-specifying type arguments in a class instantiation expression / constructor invocation. This makes the following case quite clear:

Instead of writing the following in java 1.6 and below

List <String> list = new ArrayList<String>();

one can now simply write

List<String> list = new ArrayList<>();

and the compiler will automatically infer that you meant to write ArrayList<String>();

We also know by now, that this is different from

List <String> list = new ArrayList();

since here ArrayList() instantiates a raw type, and the compiler, with -Xlint:unchecked will warn for an unchecked conversion in the above statement.

So far, so good. However, it is worth noting that there's more to it than meets the eye, and people often take a simplistic view of the diamond usage. It is worth remembering that the diamond inference works just like type inference for generic methods, where instead of using <>, it is enough to just forget about specifying anything at all, and leave the inference to the compiler. This is unfortunately or fortunately (depending on whether you prefer to lose your hair or not) a consequence of allowing the diamond operator in ANY class instance creation expression.

Let's take a closer look, and try to understand the various scenarios of diamond usage with the help of Eclipse snippets. The easier part first - let us investigate a couple of cases(i.e. instantiation expressions) where the diamond operator CANNOT be used.

1. Anonymous class declarations                                        
2. Explicit type arguments for the constructor: Lets try to understand this further. A constructor can have two sets of type arguments. A constructor in a parameterized type can have type arguments of the class as its type arguments + it may declare its own. In the example shown below, constructor of Diamond uses T, which is the class type argument and also declares another type argument U. In case the constructor also declares its own type arguments, type parameters will be explicitly specified at the constructor invocation. If this happens, and yet the constructor invocation uses the diamond operator to let the compiler infer the first set of types args (i.e. those coming from the class), then it is a compile time error.                                                                             

Now comes the trickier part. Lets see the valid use cases of the diamond operator. Consider the following (simplest) case and let us see how the inference of type arguments work.


This case is straightforward. The constructors are pretty much chosen by the standard overloading mechanism.  However, this might create the simplistic view that the compiler simply substitutes the type from the RHS in place of the <> operator i.e. in the above case, <> is substituted as String and hence T = String. So all constructors are valid. Let us try understand how this works with a tweaked example:

Oops! This gives a compile error, even though going by the above simplistic view, at line 15, <> should be inferred as Number and hence, T= Number, which means this should be valid. However, even though simple, this is not the way the compiler infers type for the diamond. On the contrary, it tries to :

1. find the matching constructor call with 1 argument, which in the above case, means DiamondTest(T t)
2. It then substitutes the formal parameters of this constructor with the parameters that have been passed at the call site. So, T becomes Integer. (In case of default constructor, or where no type argument is substituted in the matching constructor, by default T gets inferred as Object).
3. Using the inferred type arguments from 1 and 2, the compiler determines the type of the instantiation expression and the exceptions thrown. Hence in this case, new DiamondTest<>(1) gets translated as new DiamondTest<Integer>(1).

Hence, the compile error.  Now let us see some more use cases of the diamond operator:

• Inner classes

Note how the statement at line 21 is not allowed since the new DiamondTest<>() is inferred as new DiamondTest<Object>(). Also, note that as of today, even statement 20 does not work with Oracle Javac because of a bug in their compiler. However, Eclipse JDT compiler correctly compiles it! :)

• Non-variable declaration statements

 

• Return statements

 

• Conditional operators

However, note how the following results in an error

 

We can construct many more scenarios, but in the interest of time (yours and mine ;) ), I'll  leave it here. Go ahead, experiment with the diamond with Eclipse. The Eclipse work for JAVA 7 that was being done in the BETA_JAVA 7 branch has now been merged into the HEAD and R_3_7_maintenance streams and so, you can try next week's builds in either 3.8, 3.7.x or 4.2 streams to get your hands dirty on Java7. Some new JDT features with respect to the diamond operator are:

• Content assist is diamond aware. Wherever legal and non-ambiguous, content assist will insert <> and not the type arguments. Ex: List<String> l = new Arra|  is completed to List<String> l = new ArrayList<>();
• Configurable compiler warning on redundantly specified type arguments.
• Quick assist (CTRL+1) to insert type arguments 
   

For a preview of more new Java 7 features in Eclipse JDT, check out Deepak's blog. If you find any bugs, please open a bug with [1.7] in the summary. Most of the Eclipse JDT team will be at IBM Rational Innovate India 2011, Bangalore on August 10 and 11, 2011. Catch us there!