Safe Varargs (original) (raw)

Joe Darcy joe.darcy at oracle.com
Mon Jan 24 22:59:45 PST 2011

FYI, I've pushed a changset [1] to annotate four core library methods with @SafeVarargs:

 * 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> java.util.EnumSet EnumSet.of(E first, E... rest) * protected final void javax.swing.SwingWorker.publish(V... chunks)

I've also posted a description of the methodology used to find candidate methods to be so annotated [2].

-Joe

[1] http://hg.openjdk.java.net/jdk7/tl/jdk/rev/dbfc1e419cb2

[2] "Project Coin: Safe Varargs in JDK Libraries" http://blogs.sun.com/darcy/entry/project_coin_safe_vararg_libraries

On 12/13/2010 3:17 PM, Joe Darcy wrote:

Greetings.

Following up on earlier work, the javac team has pushed a new implementation of Project Coin's simplified varargs method invocation feature. [1] 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. -Joe [1] http://hg.openjdk.java.net/jdk7/tl/jdk/rev/78885e69c42c http://hg.openjdk.java.net/jdk7/tl/langtools/rev/7b99f98b3035 -=-=-=-=-=-=- 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.

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