Processorpublic interface Processor | The interface for an annotation processor.
Annotation processing happens in a sequence of {@linkplain
javax.annotation.processing.RoundEnvironment rounds}. On each
round, a processor may be asked to {@linkplain #process process} a
subset of the annotations found on the source and class files
produced by a prior round. The inputs to the first round of
processing are the initial inputs to a run of the tool; these
initial inputs can be regarded as the output of a virtual zeroth
round of processing. If a processor was asked to process on a
given round, it will be asked to process on subsequent rounds,
including the last round, even if there are no annotations for it
to process. The tool infrastructure may also ask a processor to
process files generated implicitly by the tool's operation.
Each implementation of a {@code Processor} must provide a
public no-argument constructor to be used by tools to instantiate
the processor. The tool infrastructure will interact with classes
implementing this interface as follows:
- If an existing {@code Processor} object is not being used, to
create an instance of a processor the tool calls the no-arg
constructor of the processor class.
- Next, the tool calls the {@link #init init} method with
an appropriate {@code ProcessingEnvironment}.
- Afterwards, the tool calls {@link #getSupportedAnnotationTypes
getSupportedAnnotationTypes}, {@link #getSupportedOptions
getSupportedOptions}, and {@link #getSupportedSourceVersion
getSupportedSourceVersion}. These methods are only called once per
run, not on each round.
- As appropriate, the tool calls the {@link #process process}
method on the {@code Processor} object; a new {@code Processor}
object is not created for each round.
If a processor object is created and used without the above
protocol being followed, then the processor's behavior is not
defined by this interface specification.
The tool uses a discovery process to find annotation
processors and decide whether or not they should be run. By
configuring the tool, the set of potential processors can be
controlled. For example, for a {@link javax.tools.JavaCompiler
JavaCompiler} the list of candidate processors to run can be
{@linkplain javax.tools.JavaCompiler.CompilationTask#setProcessors
set directly} or controlled by a {@linkplain
javax.tools.StandardLocation#ANNOTATION_PROCESSOR_PATH search path}
used for a {@linkplain java.util.ServiceLoader service-style}
lookup. Other tool implementations may have different
configuration mechanisms, such as command line options; for
details, refer to the particular tool's documentation. Which
processors the tool asks to {@linkplain #process run} is a function
of what annotations are present on the {@linkplain
RoundEnvironment#getRootElements root elements}, what {@linkplain
#getSupportedAnnotationTypes annotation types a processor
processes}, and whether or not a processor {@linkplain #process
claims the annotations it processes}. A processor will be asked to
process a subset of the annotation types it supports, possibly an
empty set.
For a given round, the tool computes the set of annotation types on
the root elements. If there is at least one annotation type
present, as processors claim annotation types, they are removed
from the set of unmatched annotations. When the set is empty or no
more processors are available, the round has run to completion. If
there are no annotation types present, annotation processing still
occurs but only universal processors which support
processing {@code "*"} can claim the (empty) set of annotation
types.
Note that if a processor supports {@code "*"} and returns {@code
true}, all annotations are claimed. Therefore, a universal
processor being used to, for example, implement additional validity
checks should return {@code false} so as to not prevent other such
checkers from being able to run.
If a processor throws an uncaught exception, the tool may cease
other active annotation processors. If a processor raises an
error, the current round will run to completion and the subsequent
round will indicate an {@linkplain RoundEnvironment#errorRaised
error was raised}. Since annotation processors are run in a
cooperative environment, a processor should throw an uncaught
exception only in situations where no error recovery or reporting
is feasible.
The tool environment is not required to support annotation
processors that access environmental resources, either {@linkplain
RoundEnvironment per round} or {@linkplain ProcessingEnvironment
cross-round}, in a multi-threaded fashion.
If the methods that return configuration information about the
annotation processor return {@code null}, return other invalid
input, or throw an exception, the tool infrastructure must treat
this as an error condition.
To be robust when running in different tool implementations, an
annotation processor should have the following properties:
- The result of processing a given input is not a function of the presence or absence
of other inputs (orthogonality).
- Processing the same input produces the same output (consistency).
- Processing input A followed by processing input B
is equivalent to processing B then A
(commutativity)
- Processing an input does not rely on the presence of the output
of other annotation processors (independence)
The {@link Filer} interface discusses restrictions on how
processors can operate on files.
Note that implementors of this interface may find it convenient
to extend {@link AbstractProcessor} rather than implementing this
interface directly. |
| Methods Summary |
|---|
| public java.lang.Iterable | getCompletions(javax.lang.model.element.Element element, javax.lang.model.element.AnnotationMirror annotation, javax.lang.model.element.ExecutableElement member, java.lang.String userText)Returns to the tool infrastructure an iterable of suggested
completions to an annotation. Since completions are being asked
for, the information provided about the annotation may be
incomplete, as if for a source code fragment. A processor may
return an empty iterable. Annotation processors should focus
their efforts on providing completions for annotation members
with additional validity constraints known to the processor, for
example an {@code int} member whose value should lie between 1
and 10 or a string member that should be recognized by a known
grammar, such as a regular expression or a URL.
Since incomplete programs are being modeled, some of the
parameters may only have partial information or may be {@code
null}. At least one of {@code element} and {@code userText}
must be non-{@code null}. If {@code element} is non-{@code
null}, {@code annotation} and {@code member} may be {@code
null}. Processors may not throw a {@code NullPointerException}
if some parameters are {@code null}; if a processor has no
completions to offer based on the provided information, an
empty iterable can be returned. The processor may also return
a single completion with an empty value string and a message
describing why there are no completions.
Completions are informative and may reflect additional
validity checks performed by annotation processors. For
example, consider the simple annotation:
@MersennePrime {
int value();
}
(A Mersenne prime is prime number of the form
2n - 1.) Given an {@code AnnotationMirror}
for this annotation type, a list of all such primes in the
{@code int} range could be returned without examining any other
arguments to {@code getCompletions}:
import static javax.annotation.processing.Completions.*;
...
return Arrays.asList({@link Completions#of(String) of}("3"),
of("7"),
of("31"),
of("127"),
of("8191"),
of("131071"),
of("524287"),
of("2147483647"));
A more informative set of completions would include the number
of each prime:
return Arrays.asList({@link Completions#of(String, String) of}("3", "M2"),
of("7", "M3"),
of("31", "M5"),
of("127", "M7"),
of("8191", "M13"),
of("131071", "M17"),
of("524287", "M19"),
of("2147483647", "M31"));
However, if the {@code userText} is available, it can be checked
to see if only a subset of the Mersenne primes are valid. For
example, if the user has typed
@MersennePrime(1
the value of {@code userText} will be {@code "1"}; and only
two of the primes are possible completions:
return Arrays.asList(of("127", "M7"),
of("131071", "M17"));
Sometimes no valid completion is possible. For example, there
is no in-range Mersenne prime starting with 9:
@MersennePrime(9
An appropriate response in this case is to either return an
empty list of completions,
return Collections.emptyList();
or a single empty completion with a helpful message
return Arrays.asList(of("", "No in-range Mersenne primes start with 9"));
| | public java.util.Set | getSupportedAnnotationTypes()Returns the names of the annotation types supported by this
processor. An element of the result may be the canonical
(fully qualified) name of a supported annotation type.
Alternately it may be of the form "name.*"
representing the set of all annotation types with canonical
names beginning with "name.". Finally, {@code
"*"} by itself represents the set of all annotation types,
including the empty set. Note that a processor should not
claim {@code "*"} unless it is actually processing all files;
claiming unnecessary annotations may cause a performance
slowdown in some environments.
Each string returned in the set must be accepted by the
following grammar:
- SupportedAnnotationTypeString:
- TypeName DotStaropt
- *
- DotStar:
- . *
where TypeName is as defined in the Java Language Specification.
| | public java.util.Set | getSupportedOptions()Returns the options recognized by this processor. An
implementation of the processing tool must provide a way to
pass processor-specific options distinctly from options passed
to the tool itself, see {@link ProcessingEnvironment#getOptions
getOptions}.
Each string returned in the set must be a period separated
sequence of {@linkplain
javax.lang.model.SourceVersion#isIdentifier identifiers}:
- SupportedOptionString:
- Identifiers
- Identifiers:
- Identifier
- Identifier {@code .} Identifiers
- Identifier:
- Syntactic identifier, including keywords and literals
A tool might use this information to determine if any
options provided by a user are unrecognized by any processor,
in which case it may wish to report a warning.
| | public javax.lang.model.SourceVersion | getSupportedSourceVersion()Returns the latest source version supported by this annotation
processor.
| | public void | init(javax.annotation.processing.ProcessingEnvironment processingEnv)Initializes the processor with the processing environment.
| | public boolean | process(java.util.Set annotations, javax.annotation.processing.RoundEnvironment roundEnv)Processes a set of annotation types on type elements
originating from the prior round and returns whether or not
these annotations are claimed by this processor. If {@code
true} is returned, the annotations are claimed and subsequent
processors will not be asked to process them; if {@code false}
is returned, the annotations are unclaimed and subsequent
processors may be asked to process them. A processor may
always return the same boolean value or may vary the result
based on chosen criteria.
The input set will be empty if the processor supports {@code
"*"} and the root elements have no annotations. A {@code
Processor} must gracefully handle an empty set of annotations.
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