Friday Jun 24, 2011

Project Coin: JSR 334 has a Proposed Final Draft

Reaching nearly the last phase of the JCP process, JSR 334 now has a proposed final draft. There have been only a few refinements to the specification since public review:

  • Incorporated language changes into JLS proper.

  • Forbid combining diamond and explicit type arguments to a generic constructor.

  • Removed unusual protocol around Throwable.addSuppressed(null) and added a new constructor to Throwable to allow suppression to be disabled. Added disclaimers that OutOfMemoryError, NullPointerException, and ArithmeticException objects created by the JVM may have suppression disabled.

  • Added thread safely requirements to Throwable.addSuppressed and Throwable.getSuppressed.

Next up is the final approval ballot; almost there!

Tuesday May 24, 2011

Project Coin: Documenation at Rampdown

As a follow-up to the Project Coin documentation posted for the JDK 7 developer preview and with work on JDK 7 officially ramping down, I've posted documentation on the semantics of the Project Coin features as implemented in JDK 7 b143 at:
http://cr.openjdk.java.net/~darcy/ProjectCoin/ProjectCoin-Documentation-v0.9375.html

If you have comments or questions about a feature, after reviewing the relevant discussion sections, feedback can be sent to the coin-dev mailing list.

Friday May 06, 2011

Project Coin: Diamond and Generic Constructors

To address an issue in the specification, the JSR 334 expert group has decided to ban the combination of diamond together with explicit constructor type arguments. For background, a constructor can have two sets of type arguments. In a parameterized class, constructors can have type arguments corresponding to the class's type parameters. This is the common case and the type arguments appear to the right of the class name and "new" as in "new ArrayList<String>()". In addition, if a constructor declares its very own type parameters (which can be done whether or not the enclosing class is parameterized), then those type arguments appear between the "new" token and the class name as in "new <Integer> ConstructorWithTypeParameter(Integer.valueOf(42))".

As with generic methods, the type arguments of generic constructors are usually inferred and not explicitly passed. However, generic constructors are much less common than generic methods.

After the specification change, using diamond to infer the class type parameters but explicitly passing the constructor type parameters is forbidden. However, the other three combinations of explicitly passing type arguments versus inferring them are supported, as shown in the table below.

class Foo<T extends Number> {
      <S extends T> Foo(S s) {super();}
}
Class Constructor Supported Example
Explicit Explicit Yes new <Integer> Foo<Number>(null);
Explicit Inferred Yes new Foo<Integer>(null);
Inferred Explicit No new <Integer> Foo<>(null); // compile error
Inferred Inferred Yes new Foo<>(null);

The substitution that was used to formally describe the semantics of diamond did not fully propagate constraints if there was a typing relationship between the type parameters of the constructor and the type parameters of the class. Lifting the imposed restriction on the banned combination of explicit and inferred type arguments can be revisited in future work in the context of broader type inference getting added to the Java language.

Since generic constructors are so uncommon, adding this restriction about combining them with diamond should have little effect to most programs. For example, none of the uses of diamond in the JDK had to be updated as a result of this change. A changeset implementing this specification change has been pushed and will appear in subsequent promoted builds of JDK 7.

Monday Apr 25, 2011

Project Coin: JSR 334 Public Review Approved

After due consideration by the JCP SE/EE Executive Committee, the public review of JSR 334 has been approved.

Onward to the proposed final draft!

Thursday Apr 21, 2011

Project Coin: IDE support

All three of the NetBeans, IntelliJ, and Eclipse IDEs have some level of support in progress for Project Coin's language features.

The recently released NetBeans 7.0 has support for all the Coin features. As explained in its release notes, beyond just recognizing the features in the editor NetBeans 7.0 provides auto-completion to diamond and gives hints to apply the new features to existing code.

As previously announced for IntelliJ IDEA, the builds of IDEA 10.5 EAP (early access program) provide similar quickfix support.

Finally, an Eclipse branch is hosting their efforts to support Project Coin.

As JSR 334 is not quite done yet, I'd expect the IDEs to be updated after JSR 334 is completed to account for any specification changes made in the interim.

Friday Apr 01, 2011

Project Coin: Disabling exception suppression

As part of the library support for the try-with-resources statement, several API changes were made to Throwable including an addSuppressed method to allow suppressed exceptions to be recorded. As discussed on coin-dev, to support VM needs for reusable exception objects, a protocol was devised to disable the suppression mechanism so that a zero-length array would be returned from getSuppressed even if addSuppressed was called with a valid argument. The mechanism was a bit of a kludge, relying on an initial call to addSuppressed with a null argument, and the design was called out as such. I'm happy to report the JSR 334 expert group has devised a more elegant protocol to disable exception suppression: a new constructor is added to Throwable which supports disabling suppression. The existing constructors of Throwable always enable suppression and addSuppressed(null) now always throws a NullPointerException. A few exception and error types in the platform are allowed by behave as if their objects were created with suppression disabled. The fix was recently pushed and will appear in a future JDK 7 build.

Thursday Mar 24, 2011

Project Coin: JSR 334 in Public Review

After successfully going through early draft review, JSR 334 has now entered another phase: public review. Compared to the earlier draft review specification (v0.75), the main changes in the public draft review specification (v0.875) are:

  • The specification for diamond was expanded and clarified. Using diamond on a non-generic class is explicitly forbidden.

  • The specification for multi-catch was expanded and made more precise.

  • Several changes related to the try-with-resources feature:

    • The try-with-resources statement has dropped support for a resource to be given as a general expression without an accompanying explicit variable declaration.

    • An optional trailing semicolon is allowed to terminate the sequence of resources in a resource specification rather than producing a syntax error.

    • The compiler-generated calls to the close method of a resource in a try-with-resources statement only occur if the resource is non-null.

    • Strong warnings were added to the javadoc of AutoCloseable about having the close method throw InterruptedException.

  • The @SafeVarargs annotation was applied to appropriate places in the platform libraries.

  • JLSv3 changes were provided for the simplified varargs method invocation feature.

The public review draft of JSR 334 is the last expected JCP milestone before proposed final draft, so get your comments about the public review in soon!

Tuesday Mar 01, 2011

Project Coin: Developer Preview Documentation

I've posted documentation of the semantics of the Project Coin features as implemented in the JDK 7 developer preview, b130, at:
http://cr.openjdk.java.net/~darcy/ProjectCoin/ProjectCoin-Documentation-v0.83.html

Before sending in comments or questions about a feature to coin-dev, please read the discussion section after a feature. Many design considerations are discussed in those sections. Additionally, some known bugs in the current implementation are noted in the text. In particular, javac in the JDK 7 developer preview erroneously accepts diamond combined with non-generic classes and accepts some uses of diamond with anonymous inner classes. These bugs will be corrected in future builds.

Tuesday Feb 22, 2011

Project Coin: Trying out try-with-resources in the JDK

As part of the "coinification" the JDK libraries, after first forging some diamonds, Stuart has been working on introducing systematic usage of try-with-resources into the JDK code base. Initially, this effort introduced try-with-resources in URLJarFile.java and in javax.sql; more changes are on the way.

Besides directly improving the code base, these efforts also help inform design decisions about how to make the feature more useful in practice.

The try-with-resources statement can also be helpful in new code. In a recent update to the file system API portion of JSR 203, try-with-resources is used throughout the utility methods in java.nio.file.Files and in other portions of the code and tests for JSR 203.

There is a limited amount of time remaining to make adjustments to try-with-resources in JDK 7. Additional reports on experiences using try-with-resources, and other Project Coin features, would be helpful, either to report issues or validate the current design and implementation. For widest discussion, feedback can be sent to the coin-dev alias.

Wednesday Feb 16, 2011

Project Coin:try-with-resources on a null resource

After due consideration the JSR 334 expert group has decided the semantics of the try-with-resources statement on a null resource should be changed as follows: the compiler-generated calls to close a resource will only occur if the resource is non-null.

Concretely, the semantics of the desugaring of the finally block are changed from

finally {
  if (#primaryException != null) {
    try {
      #resource.close();
    } catch(Throwable #suppressedException) {
      #primaryException.addSuppressed(#suppressedException);
    }
  } else {
    #resource.close();
  }
}

to

finally {
  if (#primaryException != null) {
    try {
      if(#resource != null)
        #resource.close();
    } catch(Throwable #suppressedException) {
      #primaryException.addSuppressed(#suppressedException);
    }
  } else {
      if(#resource != null)
        #resource.close();
  }
}

This decision was informed by discussions on coin-dev as well as experiments retrofitting try-with-resources onto the JDK libraries.

The change allows idioms like

try(Resource r = methodThatMightReturnNull()) {
    if (r == null)
       return; // nothing to do
}

to complete normally without generating a null pointer exception. Note that the programmer still has responsibility to check for a null resource if the resource is used inside the try block; the generated null check does not occur before the try block is entered.

Implementing the change is being tracked under Oracle bug 7020047 "Project Coin: generate null-check around try-with-resources close call."

Monday Jan 31, 2011

Project Coin: How to Terminate try-with-resources

In addition to mulling over nulling in the try-with-resources statement, the JSR 334 expert group has decided to slightly amend the syntax of try-with-resources statement: an optional trailing semicolon will now be allowed to terminate the list of resources.

Previously, a semicolon could only be used as a separator between two resources as in

try(Resource r0 = new Resource(); Resource r1 = new Resource(); Resource r2 = new Resource()) {...}

or reformatted as

try(Resource r0 = new Resource();
    Resource r1 = new Resource();
    Resource r2 = new Resource()) {...}

However, including an extraneous semicolon at the end of the list would be rejected as a syntax error:

try(Resource r0 = new Resource();
    Resource r1 = new Resource();
    Resource r2 = new Resource();) {...}  // Illegal under JSR 334 EDR!

While requiring a semicolon at the end of a list of resources would be excessive, especially when there is only a single resource being managed, optionally allowing a terminating resource offers several advantages. First, when adding a resource to the list of resources being managed, there are fewer necessary edits when cutting and pasting. More importantly, programmatic generation of code is simplified since the code generation logic does not need to know ahead of time whether or not a particular resource will be the last one when the declaration for that resource is generated. The simple rule "terminate a resource declaration with a semicolon" will result in acceptable code, even if the code is not ideal style-wise. Finally, allowing an optional trailing semicolon is consistent with the handling of commas in array initializers,

int[] values = {1,
                2,
                3,  // Legal
               };

and the handling of commas in the declaration of enum constants.

enum PrimaryColor {
    RED,
    GREEN,
    BLUE,  // Legal
    ;
}

The full amended grammar for try-with-resources which allows the optional trailing semicolon is:

TryStatement:
try Block Catches
try Block Catchesopt Finally
try ResourceSpecification Block Catchesopt Finallyopt
ResourceSpecification:
( Resources ;opt )
Resources:
Resource
Resource ; Resources
Resource:
VariableModifiersopt Type VariableDeclaratorId = Expression

The necessary compiler changes to implement the revised grammar have been pushed into a JDK 7 integration repository and will appear in a promoted build in due course.

Monday Jan 24, 2011

Project Coin: Safe Varargs in JDK Libraries

Back for JDK 7 build 123, the language support for the Project Coin's safe varargs feature was pushed; the time has come to update the libraries to take advantage of this feature.

Following the same general methodology used to systematically flush out types that should be made Closeable or AutoCloseable, I wrote an annotation processor to identify candidate varargs methods and constructors where adding a @SafeVarargs annotation might be appropriate.

Several JDK library methods were known candidates for @SafeVarargs; running the annotation processor found another one. The complete list of methods to be annotated in a java.\* or javax.\* package is:

  • public static <T> List<T> java.util.Arrays.asList(T... a)
  • public static <T> boolean java.util.Collections.addAll(Collection<? super T> c, T... elements)
  • public static <E extends Enum<E>> java.util.EnumSet<E> EnumSet.of(E first, E... rest)
  • protected final void javax.swing.SwingWorker.publish(V... chunks)

After this update, many fewer spurious unchecked warnings will be reported when calling core library classes.

Friday Jan 21, 2011

Project Coin: JSR 334 Expert Group Update

Besides working to address issues identified in the EDR draft, such as refining the diamond specification, the JSR 334 expert group has been considering other matters as well. One change being contemplated is removing the ability to have multiple underscores between digits of a literal; under that possible change, no underscore or a single underscore would be allowed. The primary consideration here is to prevent abuses of the underscores in literal feature that would obscure program meaning; on the other hand, there may be reasonable uses of repeated underscores that are desirable to allow.

The expert group has agreed to one significant change from the EDR draft: as called out as a possibility in the draft, support has been dropped for the general expression form of the try-with-resources statement. That is, support has been removed for passing a resource as a general expression without an accompanying explicit variable declaration.

Several technical problems were identified with allowing a general expression:

  • Syntactic ambiguity: In the parser, it was not always possible to distinguish with one-token look-ahead between the start of an Expression and the start of a Type. Consider code like

      try(i < j // Ternary operator on variables i and j
          ? new Resource1() :
            new Resource2()) {...}
    

    compared to code like

      try(Box < Resource // Simple generic wrapper around a resource
          > resourceBox = Box<>(new Resource1())) {...}
    

    A natural grammatical fallback short of banning Expression would be to only allow a more restricted expression, such as Identifier. However, that restricted grammar would require compiler changes to alert programmers to some surprising legal code, as given in the following examples.

  • Usability issues: Consider a try-with-resources statement being used to manage an existing variable where the variable is mutated inside the try block:

    public class TwrExamples implements AutoCloseable {
       public static void main(String... args) {
           TwrExamples twrEg1 = new TwrExamples();
           System.out.println(twrEg1.hashCode());
    
           try(twrEg1) {
               twrEg1 = new TwrExamples();  // Mutating the variable!
               System.out.println(twrEg1.hashCode());
           }
       }
    
       @Override
       public void close() {
           System.out.println(hashCode());
       }
    }
    

    As try-with-resources was previously specified, this would cause close to be called on the original value, not the value twrEg1 pointed to at the time the try block finishes. In this case, the printed output of the program may be something like:
    1607576787
    1051296202
    1607576787
    which indicates that while close was called on the original value, close was not called on the new TwrExamples object created inside the try-with-resources block. Either policy of calling code on the original value or the value on exit of the block could be problematic. The compiler did not issue any warnings about this situation and warnings should be added if this feature were to be kept. (Mutating a resource variable declared as part of the try-with-resources statement is illegal since such variables are implicitly or explicitly final).

Other complications that stemmed from supporting a general expression as a resource were making sure the specification and implementation accepted both

   try(null) {...}
and
   try(myGenericMethodThatInfersTheTypeOfItsResult()) {}
as valid programs.

The main rationale for supporting general expressions was to allow non-Closeable objects, such as locks, to be easily wrapped in a suitable type to enjoy the call-method-on-block-exit guarantees of try-with-resources. When this is desirable, the same effect can still be achieved with an explicit resource declaration. As experience is gained with try-with-resources, extensions to support other usage patterns will be considered in future releases.

I'm working with the javac team to remove support for this feature in an upcoming JDK 7 build.

Tuesday Jan 11, 2011

Project Coin: JSR 334 EDR now available

The JSR 334 expert group has been hard at work and now the early draft review (EDR) is now available for your reading pleasure, enjoy.

Wednesday Dec 22, 2010

Project Coin at Devoxx 2010

Catching up on blogging, back in November Maurizio and I gave a talk on Project Coin at Devoxx. The video is up on Parleys and I've posted the slides too. These links, along with analogous ones from last year, are available from my new Devoxx talk archive.

JavaPosse episode 330 recorded at Devoxx also covered Project Coin as one of the survey topics (around minute 16 in the podcast). Other than an audience non-reaction for improved integer literals, the other five Coin features received a noticeable amount of applause.

Last year was my first time speaking at Devoxx. This year as before, between sessions a live twitter feed of the conference is projected onto the presentation screens and in the main hallway so there is no waiting for the at times unfiltered feedback from attendees.

Visiting Antwerp again this fall, I has happy to find a few new favorite locations for beer and waffles to enjoy on future visits. Well-aged Trappist beer is a revelation!

Tuesday Dec 21, 2010

New javac warning for setting an older source without bootclasspath

To use javac from JDK N to cross-compiler to an older platform version, the correct practice is to:

  • Use the older -source setting.
  • Set the bootclasspath to compile against the rt.jar (or equivalent) for the older platform.

If the second step is not taken, javac will dutifully use the old language rules combined with new libraries, which can result in class files that do not work on the older platform since references to non-existent methods can get included.

Thanks to work by Jon Gibbons, in JDK 7 build 121 and later javac detects and warns about this suspicious situation; for example:

$ javac -source 6 HelloWorld.java
warning: [options] bootstrap class path not set in conjunction with -source 1.6

One way to address the warning is to set the bootclasspath. If that is inappropriate, the warning can be disabled with a new suboption within the -Xlint family, -Xlint:-options.

With this change, a likely problematic combination of options to javac that can lead to subtle build errors are diagnosed by the compiler and can easily by either directly addressed, or documented as part of the build process via the new -Xlint suboption.

Monday Dec 20, 2010

Project Coin: The Mint is Sprouting

As planned, earlier today Stuart pushed the first set of changes to the JDK libraries to use the new Project Coin features, adding a bit of sparkle with diamonds mounted in java.io, java.lang, java.util, and elsewhere. More diamonds and other shiny coins to follow!

Tuesday Dec 14, 2010

Project Coin: Minty Fresh Libraries

The JDK 7 build has been using -source 7 for some time, but to date use of new language features has been informal and incidental. Supporting Project Coin and JSR 334, Stuart Marks will be leading a "minting" exercise over the JDK code base to systematically update the JDK libraries to take advantage of the Project Coin language features. Efforts will be focused on the src and test portions of the jdk repository of the JDK 7 forest. The first features to be rolled into the code will be diamond and strings in switch.

Review requests for different areas will be sent to the appropriate OpenJDK aliases for review. The proposed changes will consist solely of conversions of existing code to use a new language feature. While use of the new features is encouraged, in areas of the platform where code is kept synchronized with earlier release trains, such code cannot be updated to use the Project Coin features at this time.

For several years, we've been collaborating with the NetBeans team to provide IDE support for the Project Coin features. NetBeans 7 Beta is the latest release to support Project Coin, as detailed in its release notes. We will continue to coordinate with the NetBeans team to update the Project Coin support in NetBeans as refinements are made to the Project Coin features. Additionally, to allow use of NetBeans for JDK development, we will defer updating the JDK to use a Coin feature until there is matching support in a NetBeans build, as there is now for diamond and strings in switch. (The NetBeans 7 Beta supports a slightly older version of multi-catch and try-with-resources.)

Monday Dec 13, 2010

Project Coin: Safe Varargs

Following up on earlier work, the javac team has pushed a new implementation of Project Coin's simplified varargs method invocation feature (jdk, langtools). The changes are scheduled to appear in the promotion of JDK 7 b123.

As envisioned previously, a new @Documented annotation type, java.lang.SafeVararags, can be used to suppress warnings related to unchecked warnings, both the new mandatory warnings at the declaration site of a varargs method/constructor with a non-reifiable element type and the existing unchecked warnings at the call sites of such methods. A systematic application of this annotation to appropriate declarations in the JDK libraries will follow as future work.

Since new unchecked warnings are being introduced, those diligently compiling with options like "-Xlint:unchecked -Werror" will see a build error under JDK 7 if any of the suspicious varargs method declarations are found. To address this, the @SafeVarargs annotation can be applied to the declarations, if appropriate, or the @SuppressWarnings({"unchecked", "varargs"}) annotation can be applied. Unlike @SafeVarargs, the @SuppressWarnings annotation will not squelch unchecked warnings at the call site of the annotated method.

The specification of the new SafeVarargs annotation type is below.


Annotation Type SafeVarargs



@Documented
@Retention(value=RUNTIME)
@Target(value={CONSTRUCTOR,METHOD})
public @interface SafeVarargs
A programmer assertion that the body of the annotated method or constructor does not perform potentially unsafe operations on its varargs parameter. Applying this annotation to a method or constructor suppresses unchecked warnings about a non-reifiable variable-arity (vararg) type and suppresses unchecked warnings about parameterized array creation at call sites.

In addition to the usage restrictions imposed by its @Target meta-annotation, compilers are required to implement additional usage restrictions on this annotation type; it is a compile-time error if a method or constructor declaration is annotated with a @SafeVarargs annotation, and either:

  • the declaration is a fixed-arity method or constructor
  • the declaration is a variable-arity method that is neither static nor final.

Compilers are encouraged to issue warnings when this annotation type is applied to a method or constructor declaration where:

  • The variable-arity parameter has a reifiable element type, which includes primitive types, Object, and String. (The unchecked warnings this annotation type suppresses already do not occur for a reifiable element type.)
  • The body of the method or constructor declaration performs potentially unsafe operations, such as an assignment to an element of the variable-arity parameter's array that generates an unchecked warning.

    Future versions of the platform may mandate compiler errors for such unsafe operations.

Monday Dec 06, 2010

Project Coin: JSR 334 Approved

After due consideration by the JCP SE/EE Executive Committee, JSR 334, Small Enhancements to the Java™ Programming Language has been approved! The work of the expert group now officially gets underway. One of our first tasks will be working on producing a specification of the language changes for early draft review, the next milestone in the JSR process.

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