Presentation The Heads and Tails of Project Coin

CORE JAVA PLATFORM 22641
Published on: 2011-10-06T16:53:51.000Z
Channel: JavaOne 2011 (all)
Tags: javase7 ProjectCoin oracle JavaOne11
Speakers:

maurizio cimadamore

$speaker.bio

Joseph Darcy

Joe is currently the lead engineer of Project Coin, the effort to select and implement a set of small Java language changes for JDK 7. He also serves as a member of the JDK 7 release team. Concurrently, Joe continues to be the release manager, lead engineer, and quality lead for OpenJDK 6, an open source implementation of the Java SE 6 platform.A longtime member of the JDK engineering group, Joe was previously specification lead for JSR 269, the Pluggable Annotation Processing API, which delivered a standardized annotation processing API and mirror-based language model into JDK 6 to supersede the earlier apt tool from JDK 5. Joe assisted in implementing the JDK 5 language changes with work spanning core reflection, javac hacking, and general library support. Joe holds a master's degree in computer science from UC Berkeley and a master's degree in applied math from Stanford University.

PDF: slides.pdf

Slides:

The Heads and Tails of Project Coin

#include "std_orcl_disclaimer.h"

#include "std_orcl_disclaimer.h" The following is intended to outline our general product direction. It is intended for information purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or functionality, and should not be relied upon in making purchasing decisions. The development, release, and timing of any features or functionality described for Oracle's products remains at the sole discretion of Oracle. 3 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Outline

Outline · · · · Overview and Demo Retrospective on developing JDK 7 Coin features Possible small language changes in JDK 8 and later Q&A 5 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

JDK 7 Survey

JDK 7 Survey · Downloaded JDK 7 GA? JDK7 Mac OS X Port Developer Preview Release http://jdk7.java.net/macportpreview/ · Downloaded JDK 7 update build? http://jdk7.java.net/preview/ · coin-dev subscriber? http://mail.openjdk.java.net/mailman/listinfo/coin-dev/ · OpenJDK contributor? http://openjdk.java.net/contribute/ 6 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Project Coin Benefits

Project Coin Benefits Enjoy improved free code flow today! · Remove extra text to make programs more readable · Encourage writing programs that are more reliable · Integrate well with past and future changes 7 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Coin Constraints

Coin Constraints · Small language changes Specification Implementation Testing · No JVM changes! · Coordinate with forthcoming larger language changes · Beware the hazards of language interactions! 8 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

The Six Coin Features and How They Help

The Six Coin Features and How They Help · Easier to use generics Diamond Varargs warnings · More concise error handling Multi-catch try-with-resources · Consistency and clarity Strings in switch Literal improvements 9 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Project Coin support in IDEs

Project Coin support in IDEs Pick an IDE, any IDE · IntelliJ IDEA 10.5 and later http://blogs.jetbrains.com/idea/2011/02/announcing-intellij-idea-105-with-full-java-7-support/ · Eclipse 3.7.1 and later http://www.eclipse.org/jdt/ui/r3_8/Java7news/whats-new-java-7.html · NetBeans 7.0 and later http://netbeans.org/kb/docs/java/javase-jdk7.html DEMO 10 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

untitled

Varargs Warnings

Varargs Warnings · Summary: no longer receive uninformative unchecked compiler warnings from calling platform library methods: List Arrays.asList(T... a) boolean Collections.addAll(Collection c, T... elements) > EnumSet EnumSet.of(E first, E... rest) void javax.swing.SwingWorker.publish(V... chunks) · New annotation type SafeVarargs 12 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

How Coins were Minted

How Coins were Minted · Internal compiler desugaring strings in switch try-with-resources http://www.usmint.gov/about_the_mint/coinLibrary/ Easy for the programmer, more work for the compiler! · Coordinated Library Changes java.lang.{AutoCloseable, SafeVarargs} Throwable.addSuppressed Serialized form of Throwable, new c'tors · Systematic application of new library features 14 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

// Original sugared switch(s) { case "a": case "b": return 10;

// Original sugared switch(s) { case "a": case "b": return 10; case "c": case "d": return 20; ... // s mapped to offset switch($t) { case 1: case 2: return 10; case 3: case 4: return 20; ... } } 15 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

// Desugared int $t = -1; switch(s.hashCode()) { case 0x61: // "a".hashCode() if(s.equals("a")) $t = 1; break; case 0x62: if(s.equals("b")) $t = 2; break; ... }

// Desugared int $t = -1; switch(s.hashCode()) { case 0x61: // "a".hashCode() if(s.equals("a")) $t = 1; break; case 0x62: if(s.equals("b")) $t = 2; break; ... } // Original sugared switch(s) { case "a": case "b": return 10; case "c": case "d": return 20; ... // s mapped to offset switch($t) { case 1: case 2: return 10; case 3: case 4: return 20; ... } } 16 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

try-with-resources desugaring

try-with-resources desugaring try ResourceSpecification Block { final VariableModifiers_minus_final R #resource = Expression; Throwable #primaryException = null; try ResourceSpecificationtail Block catch (Throwable #t) { #primaryException = t; throw #t; } finally { if (#resource != null) { if (#primaryException != null) { try { #resource.close(); } catch(Throwable #suppressedException) { #primaryException.addSuppressed(#suppressedException); } } else { #resource.close(); } } } } 17 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Library support for try-with-resources

Library support for try-with-resources Not just a language feature! · New superinterface java.lang.AutoCloseable All AutoCloseable and by extension java.io.Closeable types usable with try-with-resources · Retrofit Closeable/AutoCloseable to Java SE API JDBC 4.1 retrofitted as AutoCloseable too · Suppressed exceptions are recorded for posterity using a new facility Throwable.addSuppressed Suppressed exception info included in stack trace, etc. 18 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Throwable

Throwable · New field added to store suppressed exception data · HotSpot specially creates certain exception objects Allow stack traces for OutOfMemoryError Need to document and follow a VM libraries protocol · Serial compatibility must be kept with older releases IIOP serialization matters too! · Exception objects should be able to be immutable New constructors added to Throwable Preserve immutability on serialize deserialize cycle 19 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

A Systematic Update

A Systematic Update · Ran annotation processors over the JDK Types to be retrofitted as Closeable/AutoCloseable: Project Coin: Bringing it to a Close(able), http://blogs.sun.com/darcy/entry/project_coin_bring_close/ Methods and constructors to be annotated with @SafeVarargs Project Coin: Safe Varargs in JDK Libraries, http://blogs.sun.com/darcy/entry/project_coin_safe_vararg_libraries/ · Can run the same processors over your own code 20 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Retrospective

Retrospective In the beginning... · A small language change needs to be small in all of: http://www.usmint.gov/about_the_mint/coinLibrary/ Specification Implementation Testing · Not the only relevant dimensions 21 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

http://hubblesite.org/gallery/album/the_universe/pr2009002a/xlarge_web/

for (Coin coin : jdk7Coins){...}

for (Coin coin : jdk7Coins){...} http://www.flickr.com/photos/museumofworldwonders/5446037241/ · · · · · Project Coin specification as a whole Design issues and alternatives, surprises Size of implementation Specification and implementation evolution Bug tail 23 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

24 Cross references in Java SE 7 JLS

JLS Sections Changed by Project Coin

JLS Sections Changed by Project Coin · Improved Literals §3.10.1 "Integer Literals" §3.10.2 "Floating-Point Literals" · Strings in Switch §14.11 "The switch Statement" · Safe varargs §4.8 "Raw Types" §4.12.2 "Variables of Reference Type" §8.4.1 "Formal Parameters" §9.6.3.7 "SafeVarargs" (New!) §15.12.4.2 "Evaluate Arguments" 26 Copyright © 2011, Oracle and/or its affiliates. All rights reserved. · Diamond §15.9 "Class Instance Creation Expressions" §15.9.1 "Determining the Class being Instantiated" §15.9.3 "Choosing the Constructor and its Arguments" §18 "Syntax"

More JLS sections changed by Project Coin

More JLS sections changed by Project Coin · Multi-catch §4.12.4 "final Variables" §11.2.2 "Exception Analysis of Statements" §11.2.3 "Exception Checking" §13.4.4 "Superclasses and Superinterfaces" §14.20 "The try statement" §18 "Syntax" · try-with-resources §4.12.4 "final Variables" §6.3 "Scope of a Declaration" §6.4 "Shadowing and Obscuring" §11.2.2 "Exception Analysis of Statements" §14.20 "The try statement" §14.20.3 "try-with-resources" (New!) §18 "Syntax" 27 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Coin Cross Reference subgraph

1. Improved literals

1. Improved literals · Grammar changes a bit tricky to get right; multiple underscores between digits: Digits: Digit Digit DigitsAndUnderscoresopt Digit DigitsAndUnderscores: DigitOrUnderscore DigitsAndUnderscores DigitOrUnderscore 29 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Implication of multiple underscores

Implication of multiple underscores Do we want this to be allowed? // Courtesy Josh Bloch int bond = 0000_____________0000________0000000000000000__000000000000000000+ 00000000_________00000000______000000000000000__0000000000000000000+ 000____000_______000____000_____000_______0000__00______0+ 000______000_____000______000_____________0000___00______0+ 0000______0000___0000______0000___________0000_____0_____0+ 0000______0000___0000______0000__________0000___________0+ 0000______0000___0000______0000_________000+__0000000000+ 0000______0000___0000______0000________0000+ 000______000_____000______000________0000+ 000____000_______000____000_______00000+ 00000000_________00000000_______0000000+ 0000_____________0000________000000007; 30 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

What about octal?

Implementing literals

2. Strings in switch: specification change

2. Strings in switch: specification change JLS §14.11 The switch Statement "The type of [the switch] Expression must be char, byte, short, int, Character, Byte, Short, Integer, String, or an enum type (§8.9), or a compile-time error occurs." 33 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Strings in switch proposal form

Strings in switch proposal form instead of hashCode. Generally a String.equals() check will be needed to verify the candidate string's identity in addition to the evaluation of the screening PROJECT COIN SMALL LANGUAGE CHANGE PROPOSAL FORM v1.0 AUTHOR(S): Joseph D. Darcy function because multiple string inputs could evaluate to the same result. OVERVIEW A single case label, a single case label with a default, and two case labels can be special-cased to just equality checks without function evaluations. If there are collisions in String.hashCode on the set of case labels in a switch block, a different function without collisions on that set of inputs should be used; for Provide a two sentence or shorter description of these five aspects of the feature: example ((long)s.hashCode<<32 ) + s.length()) is another candidate function. FEATURE SUMMARY: Should be suitable as a summary in a language tutorial. Here are desugarings to currently legal Java source for the two examples above where the default hash code do not collide: Add the ability to switch on string values analogous to the existing ability to switch on values of the primitive types. // Simple Example if (s.equals("foo")) { // cause NPE if s is null processFoo(s); } // Advanced example { // new scope for synthetic variables boolean MAJOR ADVANTAGE: What makes the proposal a favorable change? More regular coding patterns can be used for operations selected on the basis of a set of constant string values; the meaning of the new construct should be $take_default = false; boolean $fallthrough = false; $default_label: { switch(s.hashCode()) { // cause NPE if s is null case 3482567: // "quux".hashCode() if (!s.equals("quux")) { $take_default = true; break $default_label; } processQuux(s); $fallthrough = true; case 101574: // "foo".hashCode() if (!$fallthrough && obvious to Java developers. !s.equals("foo")) { $take_default = true; break $default_label; } $fallthrough = true; case 97299: // "bar".hashCode() if (!$fallthrough && !s.equals("bar")) { MAJOR BENEFIT: Wh

Strings in switch spec. and implementation

Strings in switch spec. and implementation What is there to discuss? · What does switching on a null do? · Can null be a case label? · JDK 7 implementation relies on a particular algorithm be used for String.hashCode · Many other implementation options Secondary hash if collisions on primary hash Perfect hash functions Labeled-break 35 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Project Coin: Taking a Break for Strings in Switch

Project Coin: Taking a Break for Strings in Switch http://blogs.oracle.com/darcy/entry/project_coin_string_switch_break static void f(String s) { // Original sugared code switch (s) { case "azvl": System.out.println("azvl: "+s); // fallthrough case "quux": System.out.println("Quux: "+s); // fallthrough case "foo": int i = 5; //fallthrough case "bar": System.out.println("FooOrBar " + (i = 6) + ": "+s); break; case "bmjrabc": // same hash as "azvl" System.out.println("bmjrabc: "+s); break; case "baz": System.out.println("Baz " + (i = 7) + ": "+s); // fallthrough default: System.out.println("default: "+s); } } 36 Copyright © 2011, Oracle and/or its affiliates. All rights reserved. static void f(String s) { // Desugared code $exit: { int i$foo = 0; $default_label: { $baz: { $bmjrabc: { $bar: { $foo: { $quux: { $azvl: { switch(s.hashCode()) { // cause NPE if s is null case 3010735: // "azvl" and "bmjrabc".hashCode() if (s.equals("azvl")) break $azvl; else if (s.equals("bmjrabc")) break $bmjrabc; else break $default_label; case 3482567: // "quux".hashCode() if (!s.equals("quux")) // inequality compare break $default_label; break $quux; case 101574: // "foo".hashCode() if (s.equals("foo")) // equality compare break $foo; break $default_label; case 97299: // "bar".hashCode() if (!s.equals("bar")) break $default_label; break $bar; case 97307: // "baz".hashCode() if (!s.equals("baz")) break $default_label; break $baz; default: break $default_label; }//switch }//azvl System.out.println("azvl: "+s); // fallthrough } //quux System.out.println("Quux: "+s); // fallthrough } //foo i$foo = 5; }//bar System.out.println("FooOrBar " + (i$foo = 6) + ": "+s); break $exit; }//bmjrabc System.out.println("bmjrabc: " + s); break $exit; } //baz System.out.println("Baz " + (i$foo = 7) + ": "+s); // fallthrough }//default_label System.out.println("default: "+s); }//exit }

Strings in switch implementation history

Strings in switch implementation history · Original changeset quickly followed by small refinement · Bug fix in 7u2: switch(s) { case("a"): // Extra parentheses // would crash javac ... 37 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

3. SafeVarargs

3. SafeVarargs Background · Often preferable to have a sound system of warnings No missed cases (no false negatives) But may have false positives 38 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

"...the [Java] language is designed to guarantee that if your entire application has been compiled without unchecked warnings [...], it is type safe."

"...the [Java] language is designed to guarantee that if your entire application has been compiled without unchecked warnings [...], it is type safe." Generics in the Java Programming Language http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf Gilad Bracha, Computational Theologist (emer.) 39 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Heap Pollution

Heap Pollution "Heap pollution can only occur if the program performed some operation involving a raw type that would give rise to a compile-time unchecked warning [...] or if the program aliases an array variable of non-reifiable element type through an array variable of a supertype which is either raw or non-generic." JLS §4.12.2 Variables of Reference Type 41 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Unchecked Warnings and Soundness

Unchecked Warnings and Soundness · Unchecked warnings are intended to be a sound analysis · From a certain point of view, it would be correct (but unhelpful!) to always emit an unchecked warning · Pre-JDK 7, always got an unchecked warning when calling certain varargs library methods Bad, and complicated interaction between generics and arrays But usually nothing actually dangerous happens! 42 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Limits of SafeVarargs

Limits of SafeVarargs · Impractical to standardize a sufficiently powerful sound analysis in JDK 7 Just "read-only" not sufficient Just "invariant" not sufficient Aliasing complicates analysis (Standardizing definitely assigned/definitely unassigned dataflow analysis tricky!) · Leave such standardization for future work; quality of implementation issue in the meantime 43 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

SafeVarargs history

SafeVarargs history · Prototype based on SuppressWarnings · Second implementation based on SafeVarargs What's in a name? @TrustMe @CleanVarargs ... · Very extensive regression/unit tests using auto-generated source files over 7 variables 44 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Designing the SafeVarargs annotation type

Designing the SafeVarargs annotation type · Annotations on methods are not inherited · @SafeVarargs can therefore only be applied to Static methods Constructors final instance methods · Additional checks on varargs status, etc. · Runtime retention policy 45 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

4. Diamond <>

4. Diamond <> · Diamond uses type inference to figure out types so the programmer doesn't have to write them · Type inference is a constraint satisfaction problem What are the constraints? How can they be satisfied? · Want a unique answer returned by the algorithm 46 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

More than one facet

More than one facet · Two inference schemes proposed, differing in how they gathered constraints · Each sometimes more useful than the other · Use quantitative analysis to help resolve the issue Prototype both schemes Analyze results on millions of files of code 47 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Quantitative Results

Quantitative Results · Both schemes equally effective Each could eliminate a different 90% of the explicit type parameters to constructor calls Verified diamond was a worthwhile feature! · Choose inference scheme with better evolution and maintenance properties · Language designer's notebook: Quantitative language design http://www.ibm.com/developerworks/java/library/j-ldn1/ 48 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Hofstadter's Law: "It always takes longer than you expect, "even when you take into account Hofstadter's Law."

Hofstadter's Law: "It always takes longer than you expect, "even when you take into account Hofstadter's Law." Douglas Hofstadter Gödel, Escher, Bach: An Eternal Golden Braid 49 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Law of Evolving the Java Language: "There are always more interactions than you expect, "even when you expect lots of interactions."

Law of Evolving the Java Language: "There are always more interactions than you expect, "even when you expect lots of interactions." CUSTOMER LOGO 50 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Diamond and anonymous classes

Diamond and anonymous classes public class Box { private T value; public Box(T value) { this.value = value; } Object o; List arg = ...; o = new Box<>(arg); T getValue() { return value; } } 51 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Object o; List arg = ...; o = new Box >(arg);

Object o; List arg = ...; o = new Box >(arg){...};

Object o; List arg = ...; o = new a$1(arg);

Object o; List arg = ...; o = new a$1(arg); Anonymous classes translate into a new class file with a full set of attributes. class a$1 extends Box >{...} 54 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Diamond and generic constructors

Diamond and generic constructors class Foo { // The rare generic constructor! Foo(S s) {super();} } Class Explicit Explicit Inferred Constructor Explicit Inferred Explicit Supported Yes Yes No Example new Foo(null); new Foo(null); new Foo<>(null); // compile error Inferred Inferred Yes new Foo<>(null); Project Coin: Diamond and Generic Constructors http://blogs.oracle.com/darcy/entry/project_coin_diamond_generic_constructors 55 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

5. Multi-catch and more precise rethrow

5. Multi-catch and more precise rethrow Does it matter if this code doesn't compile in JDK 7? try { throw new DaughterOfFoo(); } catch (Foo exception) { try { // Pre-JDK 7 throws Foo, now throws DaughterOfFoo throw exception; } catch (SonOfFoo anotherException) { ; // Reachable? } } 56 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Multi-catch evolution

Multi-catch evolution · Reformulated rules for can-throw computation · Used new "effectively final" analysis · What is the meaning of catch(Foo | SonOfFoo e) {...} Explicitly disallowed in JDK 7 Could be added later 57 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

6. try-with-resources

6. try-with-resources Largest of the Coins · Changes to semantics of desugaring Suppress only Exceptions or all Throwables? Null handling · Syntax changes Allow trailing ";" in resource list Forbid expression form without explicit declaration · Throwable changes · AutoCloseable and InterruptedException 58 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Diamond

Diamond Design Complexity try-with-resources Multi-catch and more precise rethrow @SafeVarargs Strings in switch Improved literals Implementation Effort 59 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Project Coin Proposals in JDK 7

Project Coin Proposals in JDK 7 10 70 Submitted Proposals Per Day 8 50 6 40 30 20 4 2 10 0 0 Days after Opening 60 Copyright © 2011, Oracle and/or its affiliates. All rights reserved. Total Submitted Proposals 60

JSR 334 Through the JCP

JSR 334 Through the JCP Expert Group Formation Early Draft Review Jan 11, 2011 Public Review March 24, 2011 Proposed Final June 24, 2011 Draft JSR Approval Ballot Dec 06, 2010 Final July 5, 2011 Approval Ballot JSR Proposal 62 Nov 16, 2010 July 18, 2011 Final Release Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Project Coin Language Changes in JDK 7

Project Coin Language Changes in JDK 7 · Methodical and quantitative approach Decide today what needs to be decided today Consciously leave room for future decisions · Language + library co-evolution (But no VM changes for Coin!) · Smooth transition to new features Widespread tool support Use of new features reads well · What about JDK 8? 63 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

Language/compiler changes expected in JDK 8

Language/compiler changes expected in JDK 8 · Project Lambda: To Multicore and Beyond Alex Buckley and Daniel Smith Bulk Data Handling, Mike Duigou, Thursday, 11:00 am · Project Jigsaw: Putting It All Together, Mark Reinhold Project Jigsaw: Find the Corner Pieces First, Alan Bateman, Mandy Chung, Mark Reinhold · JSR 308 Annotations on Types 64 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.

(Very) Small Language changes

(Very) Small Language changes Under consideration for JDK 8 · Only opportunistic changes given scope of Lambda and Jigsaw · Refinements to JDK 7 Project Coin features try-with-resources on an effectively final variable? Remove some restrictions on diamond? @SafeVarargs on a private method? · Collection literals? · Repeating annotations? · Parameter names available at runtime? 66 Copyright © 2011, Oracle and/or its affiliates. All rights reserved.