Tuesday Mar 22, 2011

Project Coin EclipseCon 2011

This afternoon at EclipseCon I gave a talk on Project Coin: Small Language Changes in JDK 7; the slides have been posted. An earlier talk at the conference gave a demo of the current Eclipse support for the Project Coin features; my talk included a demo of the "quick fix" hints to introduce Coin features in code provided by NetBeans 7.0 Beta 2.

Monday Mar 14, 2011

JSR 269 Maintenance Review

As a planned part of Java SE 7, JSR 269, which standardized an API for annotation processing, is now undergoing maintenance review. In the JCP, a maintenance review is a process to take comments on small changes so that those small changes can be formally incorporated into an existing specification without running a whole new JSR. The changes being proposed in the JSR 269 maintenance review are the changes already implemented in the JSR 269 APIs in JDK 7. In summary, those proposed changes are:

  • Clarified interaction between the Filer and rounds.

  • Constructors explicitly added to the kinds of elements that can be returned by RoundEnvironment.getElementsAnnotatedWith.

  • New enum constant javax.lang.model.SourceVersion.RELEASE_7.

  • In the package description of javax.lang.model.element, requirements on when a model must be provided are loosened to remove the requirement in case of an "irrecoverable error that could not be removed by the generation of new types," a condition which includes but is not limited to syntax errors.

  • New exception type javax.lang.model.UnknownEntityException added as a common superclass for existing exception types UnknownAnnotationValueException, UnknownElementException, and UnknownTypeException.

  • New enum constant javax.lang.model.element.ElementKind.RESOURCE_VARIABLE.

  • New mixin interfaces Parameterizable and QualifiedNameable added to package javax.lang.model.element. ExecutableElement and TypeElement are retrofitted to extend Parameterizable; PackageElementand TypeElement are retrofitted to extend QualifiedNameable.

  • Behavior of getEnclosingElement method defined to return the generic element of a type parameter instead of null.

  • New interface javax.lang.model.type.DisjunctiveType to model disjunctive types.

  • New enum constant javax.lang.model.type.TypeKind.DISJUNCTIVE to mark disjunctive types.

  • New method visitDisjunctive added to visitor interface javax.lang.model.type.TypeVisitor. Utility visitor implementations updated accordingly.

  • In the package javax.lang.model.type, MirroredTypesException retrofitted to be the superclass of MirroredTypeException.

  • New utility visitors for release 7 in package javax.lang.model.util:

    • AbstractAnnotationValueVisitor7

    • AbstractElementVisitor7

    • AbstractTypeVisitor7

    • ElementKindVisitor7

    • ElementScanner7

    • SimpleAnnotationValueVisitor7

    • SimpleElementVisitor7

    • SimpleTypeVisitor7

    • TypeKindVisitor7

  • The visitors ElementKindVisitor6, ElementScanner6, and SimpleElementVisitor6, are updated to account for new element kind RESOURCE_VARIABLE.

  • The visitor AbstractTypeVisitor6 is updated to account for the possibility of visiting a DisjunctiveType.

  • Definition of documentation comment added to javadoc of javax.lang.model.util.Elements.getDocComment.

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:

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 {
    } catch(Throwable #suppressedException) {
  } else {


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

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,
                3,  // Legal

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

enum PrimaryColor {
    BLUE,  // Legal

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

try Block Catches
try Block Catchesopt Finally
try ResourceSpecification Block Catchesopt Finallyopt
( Resources ;opt )
Resource ; Resources
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();
           try(twrEg1) {
               twrEg1 = new TwrExamples();  // Mutating the variable!
       public void close() {

    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:
    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) {...}
   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.

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

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.

Monday Sep 20, 2010

Project Coin at JavaOne 2010

This morning and early afternoon Maurizio and I gave our JavaOne talk about Project Coin to a packed room; the slides for the talk are now available. The NetBeans demo of Coin support also went smoothly.

As announced earlier at JavaOne, we'll be following the "plan B" option for JDK 7 so the accepted Coin features that are not currently implemented will be reconsidered for JDK 8. In the meantime, we're interested to have more feedback on the Project Coin features

  • Improved numeric literals

  • Strings in switch

  • Reduced varargs warnings

  • Diamond operator

  • Multi-catch with more precise rethrow

  • try-with-resources statement

that are available to test out now in JDK 7 builds.

Friday Sep 10, 2010

Project Coin: JDBC 4.1 and try-with-resources

I'm happy to report that Lance Andersen and the JDBC expert group have decided to support the new try-with-resources statement in JDBC 4.1. Lance just pushed a change set for JDBC 4.1 that retrofits the AutoCloseable interface to the types:

  • java.sql.Connection

  • java.sql.ResultSet

  • java.sql.Statement

The change will be available in JDK 7 promoted builds in due course.

Wednesday Sep 08, 2010

Project Coin possibilities under JDK 7 plan A and plan B

Mark Reinhold has announced in various venues that Oracle is rethinking the plan for JDK 7.

In summary, the two plans under consideration with their ship dates are:

   Plan A:  JDK 7 (as currently defined)                      Mid 2012

   Plan B:  JDK 7 (minus Lambda, Jigsaw, and part of Coin)    Mid 2011
            JDK 8 (Lambda, Jigsaw, the rest of Coin, ++)     Late 2012

Oracle is leaning heavily toward plan B, but is also soliciting feedback on the plans, as indicated in Mark's announcement.

For Project Coin, the maximum likelihood outcome under plan B is that the currently implemented Coin features:

will be included in JDK 7, subject to future refinements and revisions (and even possible removals) as experience is gained with the features. The remaining accepted Coin features would then be reconsidered in the context of other language changes being planned for JDK 8.

Friday Aug 20, 2010

Project Coin: Try out try-with-resources

As of build 105 of JDK 7, the compiler and libraries have support for Project Coin's try-with-resources statement (ARM blocks).

We're curious to get feedback on how try-with-resources works in practice, so please download the build, use the feature to on your favorite Closeable type, and report back on how it goes.

Thursday Aug 05, 2010

Project Coin: Uniform Suppression in try-with-resources

As previously considered, the specification of the try-with-resources statement has been updated to require uniform suppression of all throwables. That is, the semantics of the code around the implicitly generated calls to close was previously

    try {
    } catch(Exception #suppressedException) {

and has been replaced with

    try {
    } catch(Throwable #suppressedException) {

(The initial implementation actually implemented the latter semantics; a test has been added to verify the behavior.)

Thursday Jul 22, 2010

Writing javac regression and unit tests for new language features

With Java language changes in progress for JDK 7, such as Project Coin's strings in switch, improved exception handling, and try-with-resources statement, writing effective regression and unit tests for javac is an integral component of developing the new language features.

Unit and regression tests differ from conformance tests. Unit and regression tests focus on achieving code coverage on an implementation while conformance tests focus on specification coverage, having tests that probe a high percentage of the testable assertions in the specification. The underlying functionality probed by both styles of testing is shared (source code is accepted or rejected, compiled class files do or do not implement the required semantics), but the different test methodologies differently categorize the space of concerns to be spanned. For more information on the methodology of conformance testing, see these blog entries about finding assertions in the Java language specification and on direct and derived assertions.

While there is overlap in the functionality testable by unit/regression tests and conformance tests, a key difference is that unit/regression tests can be written against implementation-specific functionality while conformance tests must be valid for any implementation (since all implementations are written against same specification). For example, a conformance test suite, like the JCK compiler test suite, can test that a particular source file is properly accepted or rejected by a compiler. Testing that the compiled class file(s) accurately implement the semantics of the source is also in-bounds for a conformance test. (The JCK test suite uses an interview process to configure the test suite to work with a particular compiler so that compiler-specific properties such as how acceptance/rejection of sources is indicated can be accommodated.) The JCK compiler test suite includes both positive tests (testing that programs that are in the language are properly accepted) and negative tests (testing that programs that are outside the language are properly rejected). Besides properties that can be verified in conformance-style testing, the regression and unit tests for a new language feature as implemented in javac also need to verify that various javac-specific properties hold.

For both positive and negative aspects of a language feature, unit and regression tests can cover all properties of interest for a particular compiler. A subset of those properties are also in the domain of conformance tests:

  • Negative Tests

    • Conformance and unit/regression: Invalid source files are rejected. This includes that sources only valid under, say, -source 7, are rejected when javac is run under -source 6 and earlier source settings.

    • Unit/regression only: The expected error messages are provided that reference the right source locations.

  • Positive Tests

    • Conformance and unit/regression: Valid source is compiled.

    • Conformance and/or unit/regression: Proper modeling of the new language construct. Depending on the language feature, the feature may be surfaced in the standard language model API javax.lang.model.\*, which can be tested with a conformance test. However, aspects of a language feature reflected in the javac tree API are only testable with regression or unit tests.

    • Conformance and unit/regressoin: Resulting class files are structurally well-formed. This includes passing verification and other checks done by the JVM upon loading and linking.

    • Unit/regression only: Resulting class files follow compiler-specific idioms. There are many ways a compiler can transform source code into valid class files which have the correct operational semantics.

    • Conformance and unit/regression: Resulting class file(s) when run have correct operational semantics. (Being able to run a class file implies the class file is well-formed.) Tests of operational semantics should be structured so that the code in question must positively run for the test to pass. For example, if a piece of code, like a string switch, was erroneously compiled to no byte codes, the test should fail.

An example of a negative test verifying both conformance and implementation-specific properties is a test included in the changeset for strings in switch:

 * @test  /nodynamiccopyright/
 * @bug 6827009
 * @summary Check for case labels of different types.
 * @compile/fail -source 6 BadlyTypedLabel1.java
 * @compile/fail/ref=BadlyTypedLabel1.out -XDrawDiagnostics BadlyTypedLabel1.java
class BadlyTypedLabel1 {
    String m(String s) {
        switch(s) {
        case "Hello World":
        case 42:
            return ("Don't forget your towel!");

Decoding the initial comments as jtreg directives, @test indicates the file is a jtreg test; jtreg is the test harness used for JDK regression and unit tests. By default, jtreg builds the source file and run its main method; however, for compiler tests that combination of actions is often inappropriate. The directive
@compile/fail -source 6 BadlyTypedLabel1.java
means that jtreg should compile the indicated source and expect a failure, meaning the overall test will fail if the compile succeeds. In this case, the @compile/fail directive tests that a string switch is rejected under -source 6. The next directive
@compile/fail/ref=BadlyTypedLabel1.out -XDrawDiagnostics BadlyTypedLabel1.java
is more specific. Not only must the compilation fail, but as specified in the ref= option to @compile/fail, the reported error messages must match the expected output in file BadlyTypedLabel1.out:

BadlyTypedLabel1.java:13:14: compiler.err.prob.found.req: (compiler.misc.incompatible.types), int, java.lang.String
1 error 

The -XDrawDiagnostics option to javac turns on "raw diagnostics" where the source location and resource keys are output instead of the localized text of the error messages. (For more information about javac diagnostics see Improving javac diagnostics and Playing with formatters.) The effect of the second @compile/fail line is thus to verify the proper error message is generated at "case 42:" where an integer label erroneously occurs inside a string switch. Since this kind of test relies on checking messages reporting at particular code locations, no explicit copyright occurs in such test files so that the golden output files do not have to updated if the length of the copyright notice changes; "/nodynamiccopyright/" indicates the explicit copyright is omitted for this reason.

For positive tests, a @compile directive without /fail declares a compile must succeed for the overall test to pass. Modeling tests can generally be run as annotation processors over selected source code. Since annotation processing is built into javac as of JDK 6, @compile directives are one option for running annotation processing tests. The tree API can also be tested via annotation processors by using the Trees class to bridge between javax.lang.model.element.Element objects and the corresponding abstract syntax trees, a technique also used in some regression tests for ordinary javac bug fixes.

The regression tests for multi-catch include checks for generating idiomatic class files. There are a variety of ways the multi-catch aspect of improved exception handling could be implemented. One way to compile multi-catch would be to duplicate the code blocks for each exception, in source terms treating

try {...}
catch (A | B except) {Statements}


try {...}
catch (A except) {Statements}
catch (B except) {Statements}

However, for javac we do not consider this compilation strategy to result in an acceptable class file. (There are many implicit requirements for generating class files from Java source code, but generally no explicit specification for the compiler's behavior.) Instead, for javac we require catching the multiple exceptions to be represented in the class file as repeated entries in the table of exception ranges stored in the class file to map exceptions to their handling catch blocks. This check is implemented by using javap APIs to introspect on the structure of the exception table.

Testing operational semantics can be challenging due to the difficulty of computing a known-good result all code paths of interest. For strings in switch, testing the operational semantics of the generated code included comparing the control paths taken through a switch on an enum type with the control paths taken through an analogous switch on the names of the enum constants:

private static int 
enumSwitch(MetaSynVar msv) {
    int result = 0;
    switch(msv) {
    case FOO:
        result |= (1<<0);
        // fallthrough:

    case BAR:
    case BAZ:
        result |= (1<<1);

        switch(msv) {
        case QUX:
            result |= (1<<2);

        case QUUX:
            result |= (1<<3);

            result |= (1<<4);
        result |= (1<<5);

    case MUMBLE:
        result |= (1<<6);
        return result;

    case FOOBAR:
        result |= (1<<7);
    result |= (1<<8);
    return result;
private static int 
stringSwitch(String msvName) {
    int result = 0;
    switch(msvName) {
    case "FOO":
        result |= (1<<0);
        // fallthrough:

    case "BAR":
    case "BAZ":
        result |= (1<<1);

        switch(msvName) {
        case "QUX":
            result |= (1<<2);

        case "QUUX":
            result |= (1<<3);

            result |= (1<<4);
        result |= (1<<5);

    case "MUMBLE":
        result |= (1<<6);
        return result;

    case "FOOBAR":
        result |= (1<<7);
    result |= (1<<8);
    return result;

Matching code sections in the two switch statements are identified with the same bit position; executing a code section sets the corresponding bit. The test loops over all enum constants and verifies that the set of code sections run when the enum constant itself is switched on matches the set of code sections run when the name of the enum constant is switched on. Notice that if the string switch statement did nothing, the result would be set to 0, which would not match any proper run of the enum-based switch statement. Inside javac, enum switches and string switches have different implementations so checking the new string switch behavior against the old enum switch behavior is a reasonable approach to validate string switch functionality.

Analogous checks implemented with annotations were used to see if exceptions were caught in the proper catch blocks for improved exception handling.

Analogous approaches were employed to develop the unit and regression tests for try-with-resources.

Thanks to Alex, Maurizio, Jon, and Brian for comments on initial drafts of this entry.




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