From Idea to IDE

How Java Features Are Considered, Designed, And Shipped

Nicolai Parlog

nipafx.dev / @nipafx

KA Entwicklertag

@entwicklertag

Developer Advocate

Java Team at Oracle

Text Blocks

A text block is a multi-line string literal that avoids the need for most escape sequences, automatically formats the string in a predictable way, and gives the developer control over the format when desired.

language feature added in Java 15
triggered additions to String:
formatted, indent, stripIndent, translateEscapes

Hallmarks

Java features:

come as syntax, VM capabilities, APIs
(or any combination)
specified in JLS, JVMS, Javadoc, etc. &
implemented in openjdk/jdk
fulfill clearly defined requirements
fit in seamlessly & have good tool support

Why did this take so long?

Evolution

But where does a feature come from?

Long story:

starts with an idea
refinement in explorations
prototype and proposal
preview feature
finalized feature

From Idea to IDE

Features

Previews

Proposals

Explorations

Ideas

Previews in Java 21

foreign function & memory API (3rd)
string templates (1st)
simplified main (1st)
unnamed patterns & variables (1st)
scoped values (1st)
structured concurrency (1st)

Specification

The concept of preview features was:

introduced by JEP 12
refined by a recent JEP draft
specified by JLS (e.g., for 20)

Artifacts

Required artifacts for preview features:

specified in JLS:
- all preview features are mentioned
- JLS changes/diffs are linked
implemented in openjdk/jdk, but:
- disabled by default
- by compiler and runtime

Enabling

Enabling:

for compiler and runtime
with --enable-preview
generates compiler warnings
pins Java version

Example

public class TextBlocks {

	public static void main(String[] args) {
		System.out.println("""
				Hello,
				text blocks!""");
	}

}

Enabling For Compilation

Compiling with Java 14:

$ javac TextBlocks.java
> TextBlocks.java:4: error: text blocks are a preview
> feature and are disabled by default.
>                 System.out.println("""
>                                    ^
>   (use --enable-preview to enable text blocks)
> 1 error

$ javac --enable-preview --release 14 TextBlocks.java
> Note: TextBlocks.java uses preview language features.
> Note: Recompile with -Xlint:preview for details.

Enabling For Execution

Running with Java 14:

$ java TextBlocks
> Error: LinkageError occurred while loading main class
> TextBlocks
>        java.lang.UnsupportedClassVersionError:
>        Preview features are not enabled for
>        TextBlocks (class file version 58.65535).
>        Try running with '--enable-preview'

$ java --enable-preview TextBlocks
> Hello,
> text blocks!

Version Pinning

Compiled with Java 14, launched with 15:

$ java --enable-preview TextBlocks
> Error: LinkageError occurred while loading main class
> TextBlocks
>        java.lang.UnsupportedClassVersionError:
>        TextBlocks (class file version 58.65535)
>        was compiled with preview features that are
>        unsupported. This version of the Java Runtime
>        only recognizes preview features for class
>        file version 59.65535

Goals & Considerations

Goals of preview features:

prevent (accidental) use in production
foster early IDE support
user experimentation and reports
review and refinement

Considerations:

"Is it surfaced correctly?"
"Are there unforeseen edge cases?"

Evolution

Preview features evolve a bit
(particularly on the surface)
and usually finalize.

My guesstimates:

duration: 12-24 months
rate of change: 5%
rate of progression: 95%

From Idea to IDE

Features

Previews

Proposals

Explorations

Ideas

Some Current Proposals

Defined in JDK Enhancement Proposals (JEPs)
(essentally decision records):

asynchronous stack trace VM API (JEP 435)
statements before super() (JEP 447)
compact object headers (JEP 450)

Specification

JEPs were introduced by JEP 1.

It was superseded by JEP 2.0 draft.

JEP Filters

JEPs (as JBS issues) by states:

JEP Progression (All)

drafted: 437
open:
- draft: 51
- candidate & active: 33
- candidate & dormant: 13
closed:
- withdrawn: 18
- rejected: 31
- delivered: 291

JEP Progression (Closed)

drafted: 340
withdrawn: 18
rejected: 31
delivered: 291 (85%)

Artifacts

Proposals are defined by JEPs.

Often also come with implementations in
project forks or feature branches of openjdk/jdk, e.g.:

Goals & Considerations

Goals of JEPs:

expose proposal to wider audience
seek feedback on the proposed solution
motivate, describe, archive proposals
with their contexts and consequences

Proposal considerations:

"Is this the right way to do it?"

Evolution

Proposals evolve noticeably
(particularly until Candidate)
and most are delivered.

My guesstimates:

duration: 6-36 months
rate of change: 40%
rate of progression: 85%

From Idea to IDE

Features

Previews

Proposals

Explorations

Ideas

Some Ongoing Explorations

"withers" for records (design notes)
declared patterns (email thread)
shifting and constraining computation (white paper)

Artifacts

mailing list exchanges
meetings
design notes (e.g. for Amber)
prototypes
conference talks
JBS issues

(Roughly in order.)

Unspecified

Unlike the steps so far, this one is:

mostly unspecified
mostly unstructured
without barrier for contribution

Goals

assess feasibility
shape a general idea
to a specific proposal
estimate complexity and effort

Considerations

"Does it fit into Java / the ecosystem?"
"Can it be implemented?"
"Can it be introduced?"
"What priority does it have?"

Evolution

During exploration, features evolve
fundamentally and many are dropped.

My guesstimates:

duration: 6-60 months
rate of change: 50-500%
rate of progression: 25-75%

From Idea to IDE

Features

Previews

Proposals

Explorations

Ideas

Existing Ideas

If only Java had $FEATURE, this would be so much more elegant.
(Stares procrastinatingly into distance…)

— Every Java Programmer

Existing Ideas

$FEATURE works great in $LANG.
Java needs $FEATURE!

— Every Java/$LANG Programmer

Existing Ideas

Even Brian Goetz has so many,
he needs a bunch of notebooks
to keep track of them.

Free For All

Ideas are cheap…

no specifications
no process
no barriers
no attention

…execution is everything.

Goals & Considerations

Goal is to make Java / the JVM better.

Considerations:

"Is this a good idea?"
"… for Java?"
"… now and/or in the future?"

Evolution

My guesstimates:

duration: 🤷🏾‍♂️
rate of change: 🤷🏾‍♂️
rate of progression: 🤷🏾‍♂️

From Idea to IDE

Features

Previews

Proposals

Explorations

Ideas

Feature Evolution

Feature_final2.docx

Even final features aren’t final!

Evolution

Considerations for feature evolution/removal:

Does it hurt?
Is it used?
Can it be extended/improved?
Would a change/removal hurt?

Some Removals

Single underscore as variable name ⛔⑨:

rarely used
prevents very useful language feature
easy to fix

Concurrent mark sweep GC ⛔⑭:

maintenance cost
lots of alternative GCs
easy to fix

RMI Activation ⛔⑰:

obsolete and disused

Some Terminal Deprecations

Primitive wrapper constructors ⚠️⑯:

prevent primitive types
easy to fix

Applet API ⚠️⑰:

unused and unusable

Finalization ⚠️⑱:

easy to get wrong
peformance & maintenance cost
better mechanisms exist

Prepare Your Code Bases For Future Java
Inside Java Newscast #41

Evolution

Features are extremely stable
but not immutable.

My guesstimates:

rate of change: 0.5%
survival rate: 99.95%

The Full Journey

Got everything?

Let’s rewind.

The Full Journey

	Considerations
Idea	"Can it be good for Java?"
Exploration	"How can it be implemented/introduced?"
Proposal	"Is this the best way to implement it?"
Preview	"Is it surfaced correctly?"
Feature	"Does it hurt? Can it be changed/removed?"

Considerations

Idea

"Can it be good for Java?"

Exploration

"How can it be implemented/introduced?"

Proposal

"Is this the best way to implement it?"

Preview

"Is it surfaced correctly?"

Feature

"Does it hurt? Can it be changed/removed?"

The Full Journey

	Spec	Artifacts	📆	🧬	🚀
Idea	—	🌬, 📔	🤷🏾‍♂️	🤷🏾‍♂️	🤷🏾‍♂️
Explorat.	—	🎟, 📬, …	6-60	50-500%	25-75%
Proposal	JEP 1	JEP / jdk 🌳	6-36	40%	85%
Preview	JLS	JLS diff / jdk	12-24	5%	95%
Feature	JLS	JLS / jdk	∞	0.5%

Nota Bene

Incubating APIs:

belong into Proposal (not Preview)
come with alpha implementations

JEPs:

not all proposals need one
smaller changes rely on JBS issues

Why So Serious?

Every change impacts:

millions of developers
billions of lines of code
industry, research, games, hobbies
billions of Euros of annual revenue

Better safe than sorry.

So long…

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code fccparlog

bit.ly/the-jms

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From Idea to IDE

How Java Features Are Considered, Designed, And Shipped

Text Blocks

Hallmarks

Evolution

From Idea to IDE

Previews in Java 21

Specification

Artifacts

Enabling

Example

Enabling For Compilation

Enabling For Execution

Version Pinning

Goals & Considerations

Evolution

From Idea to IDE

Some Current Proposals

Specification

JEP Filters

JEP Progression (All)

JEP Progression (Closed)

Artifacts

Goals & Considerations

Evolution

From Idea to IDE

Some Ongoing Explorations

Artifacts

Unspecified

Goals

Considerations

Evolution

From Idea to IDE

Existing Ideas

Existing Ideas

Existing Ideas

Free For All

Goals & Considerations

Evolution

From Idea to IDE

Feature_final2.docx

Evolution

Some Removals

Some Terminal Deprecations

Evolution

The Full Journey

The Full Journey

The Full Journey

Nota Bene

Why So Serious?

So long…​

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Image Credits

So long…