Monday, 8 December 2014

What might a Beans v2.0 spec contain?

What features should be in scope and out of scope for a Beans specification v2.0?

Note: I am an independent blogger, not an Oracle employee, and as such this blog is my opinion. Oracle owns the JavaBeans trademark and controls what actually happens to the specification.

JavaBeans v1.0

My last blog examined the contents of the current JavaBeans (TM) specification. The key feature elements can be summarized as follows:

  • Properties - accessed using getter and setter methods
  • Events - change events for properties
  • Methods - all public methods
  • BeanInfo - permits customization of properties, events and methods
  • Beans - a utility class to access the features
  • Property editor - a mechanism to edit properties, including conversion to and from a string format
  • GUI support - allows a bean to be manipulated visually

These features were a good fit for 1997, given that the original goal was to be a component system, interoperating with other similar systems like COM/DCOM. These features are no longer what we need, so any Beans v2.0 is naturally going to look a lot different.

Beans v2.0 - Data abstraction

The key goal of a Beans v2.0 spec is to define an abstraction over the data in an object.

When such an abstraction exists, frameworks and libraries become far simpler to write and interact with. For example, a serialization framework at its most basic level simply needs to iterate over all the beans and properties in an object graph in order to perform its task.

In addition to frameworks, many ad-hoc coding tasks become simpler because it is much easier to walk over the object graph of beans and properties. For example, to find all instances of a particular type within an object graph starting from an arbitrary bean.

The data abstraction needs to be fully round-trip. The spec must allow for data to be read from a bean into another format. But it must also provide the tools to allow the original bean to be recreated from the data previously read.

Beans v2.0 - Mutable vs Immutable

The JavaBean v1.0 specification implicitly defines mutable objects. It requires all beans to have a no-args constructor, thus the only way for the properties to get populated is through the use of setters. This mutability is one of the key complaints that people have about JavaBeans, and often Java itself by association.

Any Beans v2.0 specification must support immutable objects.

Immutable classes are now a common design approach to systems and are increasingly popular. Supporting them is thus vital to encompass the range of data objects in use today.

Any Beans v2.0 specification must also still support mutable objects. While there are those that turn their nose up at any mutability, it is still a perfectly valid choice for an application. The decision between mutable and immutable data objects is a trade-off. The Beans v2.0 spec needs to support both options.

The implications of supporting immutable is that an alternative to a no-args constructor and public setters will be needed to support construction. This is likely to be a "bean builder" of some kind.

Beans v2.0 - Beans and Properties

The term "bean" is often associated with a specific Java class, such as Person. The term "property" is often associated with a single field on the class, such as surname or forename.

Any Beans v2.0 spec must not have a reflection-based view of beans and properties.

Firstly, a property is not the same as a field (instance variable). Fields hold the state of an object, but not necessarily in the form that the object wishes to expose (it might be stored in an optimised format for example). As such, it must be possible for a bean to control the set of properties that it publishes.

Secondly, it can be desirable to a bean with a dynamic set of properties. This might consist of a bean with a fixed set of base properties and an extensible map of additional ones, or a bean which is entirely dynamic, much closer to the data model of many dynamic languages.

The implications of this is that there is not a one-to-one mapping between a bean and a Class, nor is there a one-to-one mapping between a property and a java.lang.reflect.Field.

The JavaBeans v1.0 spec treats indexed properties in a special way. I can see no reason to do that in Beans v2.0, particularly given the range of collection types now in use.

Beans v2.0 - Getters and Setters

The current JavaBeans v1.0 spec requires there to be a physical java.lang.reflect.Method instance for both the getter and setter. What Beans v2.0 needs to provide is an abstraction away from this, as it provides many more options.

Any Beans v2.0 spec must not require data access via a java.lang.reflect.Method.

Moving away from direct exposure of reflection raises the level of abstraction. It permits alternative object designs to be exposed as beans.

For example, consider a bean that has no public getters and setters. With the Beans v2.0 abstraction, this could still expose its data as properties, providing interoperation with frameworks, but encapsulating the data for most programmatic uses. This tackles another big criticism of JavaBeans, which is that they cause state to be exposed rather than hidden.

Another example is the HashMap based bean. In this case a Method does not exist for each property, just a single get by name. Being able to expose this as a bean provides the desired dynamic behaviour.

The implications of this are simply that the Beans v2.0 spec abstraction will simply be long the lines of the Map interface get and put.

Beans v2.0 - Annotations

The current JavaBeans v1.0 spec predates annotations in Java. But annotations are now key to many programming practices in Java.

Any Beans v2.0 spec must provide access to property and bean annotations.

Annotations are represented by implementations of the Annotation interface. It so happens that the Annotation interface is not part of reflection. It is possible to implement an annotation yourself (apart from a compiler warning).

What is needed is for the Beans v2.0 spec abstraction to provide access to the set of annotations on the bean, and the set of annotations on each property without going via reflection. With this step, and the ones above, many direct uses of reflection will completely disappear, with the Beans v2.0 abstraction being good enough.

Beans v2.0 - Events

The current JavaBeans v1.0 spec considers events to be first class parts of the abstraction.

There is no good reason to include events in the Beans v2.0 spec.

The current JavaBeans v1.0 spec is GUI focussed, and as such events were an essential element. But the Beans v2.0 spec outlined here focuses on a different goal, abstracting over the data in the object. Round-tripping data from a bean to another format and back to a bean does not require events or a GUI.

There will be those reaching for the comments box right now, demanding that events must be included. But think carefully and you should realise that you don't need to.

None of what is proposed here for Beans v2.0 takes away what works today with JavaBeans v1.0. GUI developers can still carry on using the same getters, setters and events that they do today, including bound and constrained properties. The java.beans package will continue to exist and continue to be available. In addition, JavaFX is the way forward for Java GUIs, and it has its own approach to properties.

Beans v2.0 - Conversion to/from String

The current JavaBeans v1.0 spec includes PropertyEditor. One of the use cases of the class, notably in Spring framework, is to convert a simple object (typically a VALJO) to and from a String. For example, this includes Integer, Enum, Class and Currency.

Any Beans v2.0 spec must tackle conversion to/from a String for simple types.

The most common use case for the Beans v2.0 spec will be to iterate over the object graph and process it in some way, such as to write out a JSON or XML message. In order for this to be practical, all objects in the object graph need to be either a bean, or a simple type convertible to a string. As such, simple type string conversion is very much needed to round out the spec.

The Joda-Convert project contains a simple set of interfaces and classes to tackle this problem, and I imagine any Beans v2.0 spec solution would be similar.

Beans v2.0 - But what does a bean actually look like?

Part of the goal of Beans v2.0 is to allow for different kinds of bean. Immutable as well as mutable. Private getters and setters as well as public. Dynamic map-based structures as well as field-based ones.

Any Beans v2.0 spec should not overly restrict how beans are implemented.

We know that Oracle has no current plans for properties at the language level. As such, it also makes little sense for Beans v2.0 spec to specify a single approach for how classes should be designed. Instead, the spec aims to allow classes that are designed in different ways to all agree on a common way to interact with frameworks and libraries that does not require getters, setters and a no-args constructor.

The Joda-Beans, Lombok, Immutables and Auto projects (and many others) all provide different ways to create beans without writing manual code. All could be adapted to work with the proposed Beans v2.0 spec, as could frameworks like Jackson, Hibernate, Dozer and Groovy (to name but a few).

The simple answer is that the bean could look just like it does today with getters and setters. The real answer is that the bean may look any way it likes so long as it can expose its data via the Beans v2.0 API.

Summary

The current JavaBeans v1.0 spec is very old and not that useful for the kinds of applications we build today. This blog has outlined my opinion on what direction a Beans v2.0 spec should take:

  • Focussed on data abstraction
  • Support for Mutable and Immutable beans
  • Fully abstracted from reflection
  • No requirement for getters and setters
  • No requirement for a no-args constructor
  • Support for dynamically extensible properties
  • Access to bean and property annotations
  • Conversion of simple types to and from a String
  • No support for events

Feel free to comment on the high-level scope outlined above. In particular, if you develop a project that uses the JavaBeans spec (directly or indirectly), please comment to say whether the proposed spec is likely to be sufficient to cover your use cases.

I've also started work on a prototype of the code necessary to support the spec. So also feel free to raise pull requests or join the mailing list / group.

Friday, 28 November 2014

The JavaBeans specification

Have you ever read the JavaBean (TM) specification? It really is from another era!

The JavaBean specification

The JavaBean specification, version 1.01, was the last specification that defined the approach to the common bean pattern used in many Java applications. Most developers have never read it, but if you want a history lesson, its a good read.

The spec dates from 1997, JDK 1.1 and Sun Microsystems, a scary 17 years ago. Serialization and Reflection are new technologies, and CORBA is a big deal.

The introduction starts with:

The goal of the JavaBeans APIs is to define a software component model for Java, so that third party ISVs can create and ship Java components that can be composed together into applications by end users.

It goes on to describe the key goal of being able to interact with the OpenDoc, COM/DCOM/ActiveX and LiveConnect object models, so JavaBeans can be embedded inside Word documents and Excel spreadsheets (via a "transparant bridge"). An early example is the desire to support "menubar merging", combining the menu from a JavaBean with that of the host application.

So, what is a JavaBean?:

A Java Bean is a reusable software component that can be manipulated visually in a builder tool.

The explanation goes on to consider builder tools as web page builders, visual application builders, GUI layout builders, or document editors (like a Word document). Beans may have a GUI, or be invisible, however GUI is clearly the main focus. Note that GUI beans are required to subclass java.awt.Component.

The unifying features of a bean are:

  • Introspection - so tools can analyse a bean
  • Customization - so tools can alter a bean
  • Events - AWT style event handing
  • Methods - the set of methods the bean declares as being callable
  • Properties - the set of get/set properties
  • Persistence - long term storage, primarily by serialization

Beans are expected to run in a container, and that container is expected to provide certain behaviour around persistence and gluing beans together. Notably, a connection from one bean to another can either be composition (where the bean is expected to serialize the composed bean) or a "pointer to an external bean", where it is expected that the container will recreate it.

The multi-threading section is very simplistic:

It is the responsibility of each java bean developer to make sure that their bean behaves properly under multi-threaded access. For simple beans this can generally be handled by simply making all the methods “synchronized”.

Another interesting section indicates a future plan to add a "view" layer, where multiple beans can be composed together into a view. To enable this, two methods Beans.instantiate() and Beans.getInstanceOf() are supposed to be used. Casts and instanceof checks are forbidden.

The most classic part of the spec is the scenarios section:

Buying the tools.
1.a) The user buys and installs the WombatBuilder Java application builder.
1.b) The user buys some JavaBeans components from Al’s Discount Component Store. They get a win32 floppy containing the beans in a JAR file. This includes a “Button” component and a “DatabaseViewer” component.
1.c) The user inserts the floppy into the machine and uses the WombatBuilder...

Floppy disks! Well this is 1997...

The section does highlight that bean customization is expected to make available a mechanism of serializing beans as source code, via the PropertyEditor.getJavaInitializationString() method. Standard Java object serialization is a mandatory technique, while the source code route is optional.

Event handling is familiar enough, using java.util.EventListener and java.util.EventObject (think mouse listeners for example). Beans can define their own event hierarchies and publish the events that the bean defines. The listeners are based on a naming convention:

The standard design pattern for EventListener registration is:
public void add<ListenerType>(<ListenerType> listener);
public void remove<ListenerType>(<ListenerType> listener);

And of course these methods are expected to access a Vector to store the listeners. A special case is that of limiting the number of listeners using java.util.TooManyListenersException, not something I've ever seen. Oh, and you are supposed to declare and throw checked exceptions in listeners, not unchecked.

Section 7 covers properties, described as "discrete, named attributes of a JavaBean". They are defined by a naming convention we all know well:

void setFoo(PropertyType value); // simple setter
PropertyType getFoo(); // simple getter

There is also direct support for indexed properties, but only on arrays:

void setter(int index, PropertyType value); // indexed setter
PropertyType getter(int index); // indexed getter
void setter(PropertyType values[]); // array setter
PropertyType[] getter(); // array getter

Note that the spec is slightly unclear here, using "setter" and "getter" as a placeholder for a "setFoo" type name. All getters and setters are allowed to throw checked exceptions.

Bound and constrained properties get a mention, along with PropertyChangeListener, PropertyVetoException and VetoableChangeListener. Vetoing gets complicated when there are multiple event listeners and some may have been notified of a success before the veto occurs. The spec has a suprising amount of detail on these two concepts, including a variety of ways of registering listeners.

The introspection section finally talks in detail about using reflection ad design patterns to find getters and setters, plus the ability to use a more specific BeanInfo:

By default we will use a low level reflection mechanism to study the methods supported by a target bean and then apply simple design patterns to deduce from those methods what properties, events, and public methods are supported. However, if a bean implementor chooses to provide a BeanInfo class describing their bean then this BeanInfo class will be used to programmatically discover the beans behaviour.

...within Java Beans the use of method and type names that match design patterns is entirely optional. If a programmer is prepared to explicitly specify their properties, methods, and events using the BeanInfo interface then they can call their methods and types whatever they like. ... Although use of the standard naming patterns is optional, we strongly recommend their use as standard naming conventions are an extremely valuable documentation technique

Bean properties, getters and setters, are matched based on the "get" and "set" prefix, and they may be defined in a superclass. Read-only and write-only properties are permitted. Boolean properties may use the "is" prefix, which may be used instead of, or in addition to, the normal "get" prefixed method.

Bean events are similarly matched by "addFooListener" and "removeFooListener" pattern matching. Both must be present for an event to be recognised.

Bean methods are simply defined as all public methods of the class. The list of methods includes getters, setters and event methods.

The BeanInfo class is used to selectively replace parts of the reflected information. Thus, the properties could be defined, but the events obtained by reflection. The java.beans.Introspector class is used to obtain the combined view, including the BeanInfo and reflected data.

It turns out that there are specific rules around capitalization in property names:

Thus when we extract a property or event name from the middle of an existing Java name, we normally convert the first character to lower case. However to support the occasional use of all upper-case names, we check if the first two characters of the name are both upper case and if so leave it alone. So for example,
“FooBah” becomes “fooBah”
“Z” becomes “z”
“URL” becomes “URL”

When customizing a bean, the default approach is to take the set of properties and expose an editor. This would operate as a key-value table, but where each property can nominate a java.beans.PropertyEditor to provide a GUI for defining that property. The classic example is a colour picker for a property of type Color. Again, any GUI is an AWT Component. And you can register an editor simply by naming it after the property being configured - Foo class is edited by FooEditor in the same package or a set of search packages.

The official tutorial is also instructive.

The simple view is that a bean just needs to be serializable, have a no-args constructor and have getters/setters matching a naming convention. While this is true, the actual spec has a lot more in it, including many rules that most developers do not follow, such as the use of Beans.instantiate() and Beans.getInstanceOf().

Summary

This has been a whistlestop tour of the JavaBeans spec. What hits you between the eyes when you read it is a sense of how old it is. The world has very much moved on.

More significantly however, is that the "beans" that are so common in Java code, consisting of mutable classes with getters and setters are not what the authors of the spec had in mind. As such, we probably shouldn't really call them beans.

But perhaps it is time to write up a JavaBeans v2 spec to rethink the problem being solved in the absence of a properties language feature.

Saturday, 15 November 2014

Devoxx 2014 - Whiteboard votes

As is traditional, the attendees of Devoxx vote on certain topics on whiteboards. Each result must be treated with care - there is no validation process, and only a fraction of the 3500 attendees actually bother to vote. Nevertheless, the results can be interesting.

What version of Java do you use at work right now?

A high level question focussed on Java at the day job. Java 8 hasn't surpassed Java 6 yet, but it is doing pretty well. Remember that Java 7 is End-Of-Life in April 2015!

Java 91
Java 863
Java 7160
Java 681
Java 511
Java 43
Java 31
Dalvik6
Other4


What IDE do you use?

A sense of how the IDE market is looking. The interesting data point is how Eclipse is falling - compare with 2008, where Eclipse led IntelliJ by almost 2 to 1, and NetBeans by almost 4 to 1. Sadly, the absolute number of votes is a lot lower than 6 years ago, so be careful with comparisons.

Eclipse36
NetBeans26
IntelliJ75
Vi/vim8
Other1

Beyond this, I heard various people comment on IBM's apparent withdrawal of funding for Eclipse IDE and what that might mean looking forward.

What JVM language are you using?

An attempt to see what languages people are using. Whenever this is asked, it is always an opportunity for advocates of a language to push hard, so again take the results with a good pinch of salt:

Java214
Scala64
Groovy62
Clojure15
Rhino/JS6
Ceylon3
Kotlin1
Cobol1


Is merging Scala into Java a good idea?

An odd question with a predictable answer - WTF!

Yes3
No26
WTF?! seriously?52
Depends2


What JavaScript framework/library do you use?

An open question that showed a clear dominance for Angular.

Angular51
Ember4
Reactive2
Backbone6
Knockout7
Angular-Dart3
Dojo5
JQuery9
Meteor2


Is this the end for Angular?

Angular 2.0 - "no migration path, not backwards compatible". Whether this is factually correct or not, this was the question asked.

Yes4
No47
Maybe12
Hate JavaScript24
Follow GWT1


What desktop UI technology have you used in the last 3 years?

A sense of where the desktop UI is today. Given that JavaFX has only been truly useful in Java SE 8, its not a bad showing for FX.

None39
Swing41
JavaFX15
MS .NET10
Chrome apps2
Eclipse RCP14
NetBeans platform2
QT5
VCL1
CLI4
OGLPHI1
Applet2
Broswer/HTML7


How do you create WebApps?

An open question on web technologies. Bear in mind that some options were added after others, notably JAX-RS+JavaScript, which was added long after REST+JavaScript. I think its fair to say that there are an odd mix of answers, but a clear leader in REST + JavaScript.

JSF+JavaEE44
REST+JavaScript101
JAX-RS+JavaScript9
Play23
Swing MVC76
Wicket26
JavaScript library50
Grails8
GWT30
Vaadin20
Apache Sling6
WebSockets+JavaScript6
Spray+AkkaHTTP6
Ruby on Rails2
ZK framework4
OmniFaces4
PrimeFaces10
REST+Dart2
Fluent-HTTP1
AngularJS23
Polymer3
By hand, with love2
Web motion3
Struts v1 or v211
Beehive2
JRuby3
Bootstrap2


What one thing would you add to Java (your top priority)?

A traditional Devoxx question, with a long shopping list of answers, as always. Note however that Brian Goetz indicated strongly that properties as a language feature is not happening.

Properties54
Better handling of null33
Multi-line strings33
Safe null dereference23
Interpolation of strings22
Immutability21
Value types16
Unsigned int11
Too bloated!8
Better logging7
Pattern matching8
Sub-package scope4
Tail recursion3
Solve expression problem2
Algorithmic data types2
Class versioning2
Cluster aware JVM1
Performance1
Free$ embedded support1
Inner functions1
Generators and co-routines1
Operator overloading1 (plus 2 minus 1 !)
Enterprise modularity1


Summary

Lots of interesting, and no doubt slightly dubious, statistics to examine. Use with care, and congratulations to the Devoxx team for another great conference!

Thursday, 13 November 2014

No properties in the Java language

Here at Devoxx 2014, Brian Goetz went a little further than he has before about whether properties will ever be added to Java as a language feature.

No properties for you

Properties as a language feature in Java has been talked about for many years. But ultimately, it is Oracle, the Java stewards, who get to decide whether it should be in or out.

Today, at Devoxx, Brian Goetz, Java Language Architect, went a little further than he has before to indicate that he sees little prospect of properties being added to the language at this point.

Oracle have chosen to prioritize lambdas, modularization and value types as key language features over the JDK 8 to 10 timescale. As such, there was no realistic prospect of properties before JDK 11 anyway. However, the indications today were reasonably clear - that it isn't on the radar at all.

When quizzed, Brian indicated two main reasons. First, that it is perhaps too late for properties to make a difference as a language feature, with compatibility with older getter/setter beans being a concern. Second, that there are many different, competing visions of what properties means, and there is an unwillingness to pick one.

Competing visions include:

  • Just generate getters, setters, equals, hashCode and toString
  • Generate observable properties for GUIs
  • Raising the level of abstraction, treating a property as a first class citizen
  • Immutability support, with builders for beans
  • New shorter syntax for accessing properties

While it is easy to argue that these are competing visions, it is also possible to argue that these are merely different requirements, and that all could be tackled by a comprehensive language enhancement.

(Just to note that "beans and properties" does not imply mutable objects. It is perfectly possible to have immutable beans, with or without a builder, but immutable beans are harder for frameworks to deal with. Value types are intended for small values, not immutable beans.)

As is well known, I have argued for properties as a language feature for many years. As such, I find the hardening of the line against properties to be intensely disappointing.

Every day, millions of developers interact with beans, getters, setters and the frameworks that rely on them. Imagine if someone today proposed a naming convention mechanism (starts with "get") for properties? It would be laughable in any language, never mind the world's most widely used enterprise development language.

Instead, developers use IDEs to generate getters, setters, equals and hashCode, adding no real value to their code, just boilerplate of the kind Java is notorious for.

Consider that with 9 million Java developers, there could easily be 50,000 new beans created every day, each with perhaps 100 lines of additional boilerplate getters/setters/equals/hashCode/toString - easily 5 million lines of pointless code created, per day.

(And plenty more code in serialization and mapping code that either tries to deduce what the properties are, or results in lots of additional manual mapping code)

Realistically however, we have to accept that in all likelihood Java will never have properties as a language feature.

Summary

The Java language is not getting properties.

Comments welcome, but please no "Foo language has properties" comments!

Wednesday, 12 November 2014

Converting from Joda-Time to java.time

What steps are needed to upgrade from Joda-Time to Java SE 8 date and time (JSR-310)?

From Joda-Time to java.time

Joda-Time has been a very successful date and time library, widely used and making a real difference to many applications over the past 12 years or so. But if you are moving your application to Java SE 8, its time to consider moving on to java.time, formerly known as JSR-310.

The java.time library contains many of the lessons learned from Joda-Time, including stricter null handling and a better approach to multiple calendar systems. I use the phraseology that java.time is "inspired by Joda-Time", rather than an exact derivation, however many concepts will be familiar.

Converting types

Classes not including a time-zone:

Joda-Time java.time Notes
LocalDate LocalDate Same concept - date without time or time-zone
YearMonthDay
LocalTime LocalTime Same concept - time without date or time-zone
TimeOfDay
LocalDateTime LocalDateTime Same concept - date and time without time-zone
MonthDay MonthDay Same concept - month and day without time-zone
YearMonth YearMonth Same concept - year and month without time-zone
- Year New concept - a value type for the year
- Month New concept - an enum for the month-of-year
- DayOfWeek New concept - an enum for the day-of-week
Partial - Not included in java.time

Classes including a time-zone or representing an instant:

Joda-Time java.time Notes
DateTime ZonedDateTime Same concept - Date and time with time-zone
OffsetDateTime New concept - Date and time with offset from Greewich/UTC
MutableDateTime - Not included in java.time, use immutable ZonedDateTime instead
DateMidnight - Deprecated as a bad idea in Joda-Time, Use LocalDate or ZonedDateTime
- OffsetTime New concept - Time with offset from Greewich/UTC
Instant Instant Same concept - Instantaneous point on timeline
DateTimeZone ZoneId Same concept - Identifier for a time-zone, such as Europe/Paris
ZoneOffset New concept - An offeset from Greenwich/UTC, such as +02:00

Amounts of time:

Joda-Time java.time Notes
Duration Duration Same concept - Amount of time, based on fractional seconds
Period Period and/or Duration Similar concept - Amount of time
Joda-Time Period includes years to milliseconds, java.time only year/month/day
MutablePeriod - Not included in java.time, use immutable Period or Duration instead
Years - Not included in java.time, use Period instead (or ThreeTen-Extra)
Months
Weeks
Days
Hours - Not included in java.time, use Duration instead
Minutes
Seconds
Interval - Not included in java.time
MutableInterval - Not included in java.time

Formatting:

Joda-Time java.time Notes
DateTimeFormatter DateTimeFormatter Same concept - an immutable formatter
DateTimeFormatterBuilder DateTimeFormatterBuilder Same concept - a builder of the formatter
DateTimeFormat DateTimeFormatter Concepts exposed as static methods on DateTimeFormatter
ISODateTimeFormat
PeriodFormatter - Not included in java.time
PeriodFormatterBuilder
PeriodFormat
ISOPeriodFormat

Other classes and interfaces:

Joda-Time java.time Notes
- Clock New concept - Standard way to pass around the current time
Chronology Chronology Similar concept - Very different API and implementation
ISOChronology IsoChronology Similar concept - Very different API and implementation
DateTimeFieldType ChronoField Similar concept - Very different API and implementation
DateTimeField
DurationFieldType ChronoUnit Similar concept - Very different API and implementation
DurationField
PeriodType - Not included in java.time
Readable* Temporal* The Readable* interfaces are most closely replaced by the Temporal* interfaces
It is strongly recommended to use the value types, not the temporal interfaces

Odds and Ends

In most cases, the table above will be sufficient to identify the class to convert to. After that, in most cases the method needed will be obvious. Some special cases are noted below.

Rounding. Joda-Time has considerable support for rounding, java.time has less, based only on truncation. See the truncatedTo() methods.

End of month. Joda-Time has the withMaximumValue() method on the property object to change a field to the last day of the month. java.time has a more general mechanism, see TemporalAdjusters.lastDayOfMonth().

Nulls. Joda-Time accepts null as an input and handles it with a sensible default value. java.time rejects null on input.

Construction. Joda-Time has a constructor that accepts an Object and performs type conversion. java.time only has factory methods, so conversion is a user problem, although a parse() method is provided for strings.

Time between. Joda-Time provides methods on amount classes such as Days.between(a, b). java.time provides a similar method on the ChronoUnit classes - ChronoUnuit.DAYS.between(a, b).

Additional fields. Joda-Time provides a fixed set of fields in DateTimeFieldType. java.time allows the set of fields to be extended by implementing TemporalUnit, see IsoFields, JulianFields and WeekFields.

Summary

This blog post has outlined the key mappings between Joda-Time and java.time in Java SE 8. If you are writing code in Java SE 8, its time to migrate to java.time, perhaps using ThreeTen-Extra to fill any gaps.

Monday, 10 November 2014

One more library for JDK 9?

On Tuesday night in Belgium at Devoxx I'm asking the question "Is there room for one more library in JDK 9"?

With JDK 9 full of modules, and the long wait until JDK 10 for more interesting things like value types, I pose this question to the community to ask what would you add to JDK 9 if you could?

But, I'm asking more than that. Any additional work for JDK 9 would have to be led by someone outside Oracle capable of drawing together a community. And whatever it is it would need to be tightly focussed - the timescale available is limited.

Think of something that could be expressed in 20 classes or less. Involves only code and no library or JVM changes. Something that the JDK doesn't have, but you think it should. Think about whether you'd put in your time to try and make it happen.

But also think about whether your idea should be in the JDK? Is it better as an external library? What makes it necessary to be in the JDK, especially a modularized JDK? Will Oracle's Java stewards support the idea?

As an example, I'll put forward Joda-Convert, a very small and simple library that provides a mechanism to convert and round-trip a simple object (like a VALJO) to a formatted string, ideal for use in serialization, such as JSON or XML (and noting that JSON is coming to JDK 9...). I'm sure that if you're reading this, you've got some other ideas!

If you've got any thoughts, ideas, or want to suggest something, please add a comment. But remember the rules above - there is no free lunch here. Are you willing to take a lead on your idea?

Thursday, 6 November 2014

Better nulls in Java 10?

Rethinking null when Java has value types.

Null in Java

Was null really the billion dollar mistake? Well, thats an interesting question, but what is certain is that null is widely used in Java, and frequently a very useful concept. But, just because it is useful doesn't mean that there isn't something better.

Optional everywhere!

There are those that would point you to adopt the following strategy - avoid using null everywhere, and use Optional if you need to express the possible absence of something. This has a certain attraction, but I firmly believe the Java is not yet ready for this (in Java SE 8) as per my previous post on Optional.

Specifically, all the additional object boxes that Optional creates will have an effect of memory and garbage collection unless you are very lucky. In addition, its relatively verbose in syntax terms.

Nullable annotations

A second approach to handling null is to use the @Nullable and @NonNull annotations. Except that it is only fair to point out that from an engineering perspective, annotations are a mess.

Firstly, there is no formal specification for nullable annotations. As such, there are multiple competing implementations - one from FindBugs, one from JSR-305, one from IntelliJ, one from Eclipse, one from Lombok and no doubt others. And the semantics of these can differ in subtle ways.

Secondly, they don't actually do anything:

  @javax.annotation.ParametersAreNonnullByDefault
  public class Foo {
    public StringBuilder format(String foo) {
      return new StringBuilder().append(foo);
    }
    public StringBuilder formatAllowNull(@Nullable String foo) {
      return new StringBuilder().append(foo);
    }
  }

Here, I've use @javax.annotation.ParametersAreNonnullByDefault to declare that all parameters to methods in the class are non-null unless otherwise annotated, that is they only accept non-null values. Except that there is nothing to enforce this.

If you run an additional static checker, like FindBugs or your IDE, then the annotation can warn you if you call the "format" method with a null. But there is nothing in the language or compiler that really enforces it.

What I want is something in the compiler or JVM that prevents the method from being called with null, or at least throws an exception if it is. And that cannot be switched off or ignored!

The nice part about the annotations is that they can flip the default approach to null in Java, making non-null the default and nullability the unusual special case.

Null with value types in JDK 10

The ongoing work on value types (target JDK 10 !!) may provide a new avenue to explore. Thats because Optional may be changed in JDK 10 to be a value type. What this might mean is that an Optional property of a Java-Bean would no longer be an object in its own right, instead it would be a value, with its data embedded in the parent bean. This would take away the memory/gc reasons preventing use in beans, but there is still the syntactic overhead.

Now for the hand-waving part.

What if we added a new psuedo keyword "nonnull" that could be used as a modifier to a class. It would be similar to @javax.annotation.ParametersAreNonnullByDefault but effectively make all parameters, return types and variables in the class non-null unless modified.

Then, what if we introduce the use of "?" as a type suffix to indicate nullability. This is a common syntax, adopted by Fantom, Ceylon and Kotlin, and very easy to understand. Basically, wherever you see String you know the variable is non-null and wherever you see String? you know that it might be "null".

Then, what if we took advantage of the low overhead value type nature of Optional (or perhaps something similar but different) and used it to represent data of type String?. Given this, a "nonnull" class would never be able to see a null at all. This step is the most controversial, and perhaps not necessary - it may be possible for the compiler and JVM to track String and String? without the Optional box.

But the box might come in useful when dealing with older code not written with the "nonnull" keyword. Specifically, it might be possible to teach the JVM to be able to auto box and un-box the Optional wrapper. It might even be possible to release a standard annotation jar that projects could use on codebases that need to remain compatible with JDK 8 or 9, where the annotation could be interpreted by a JDK 10 JVM to provide similar nullability information.

  public nonnull class Foo {
    public StringBuilder format(String foo) {
      return new StringBuilder().append(foo);
    }
    public StringBuilder formatAllowNull(String? foo) {
      return new StringBuilder().append(foo);
    }
  }

The example above has been rewritten from annotations to the new style. It is clearly a shorter syntax, but it would also benefit from proper integration into the compiler and JVM resulting in it being very difficult to call "format" with a null value. Behind the scenes, the second method would be compiled in bytecode to take a parameter of Optional<String>, not String.

The hard part is the question of what methods can be called on an instance of String?. The simple encoding suggests that you can only call the methods of Optional<String>, but this might be a little surprising given the syntax. The alternative is to retain the use of null, but with an enforced check before use.

The even harder part is what to do with things like Map.get(key) where null currently has two meanings - not found and found null.

This concept also appears to dovetail well into the value type work. This is because value types are likely to behave like primitives, and thus cannot be null (the value type has to be boxed to be able to be null). By providing proper support for null in the language and JVM, this aspect of value types, which might otherwise be a negative, becomes properly handled.

It should also be pointed out that one of the difficulties that current tools have in this space is that the JDK itself is not annotated in any way to indicate whether null is or is not allowed as an input or return type. Using this approach, the JDK would be updated with nullability, solving that big hurdle.

(As a side note, I wrote up some related ideas in 2007 - null-safe types and null-safe invocation.)

Just a reminder. This was a hand-wavy thought experiment, not a detailed proposal. But since I've not seen it before, I thought it was worth writing.

Just be careful not to overuse Optional in the meantime. Its not what it was intended for, despite what the loud (and occasionally zealot-like) fans of using it everywhere would have you believe.

Summary

A quick walk through null in Java, concluding with some hand-waving of a possible approach to swapping the default for types wrt null.

Feel free to comment on the concept or the detail!