This interface supports the creation of new files by an annotation
processor. Files created in this way will be known to the
annotation processing tool implementing this interface, better
enabling the tool to manage them. Source and class files so
created will be considered for processing by the tool after the
close
method has been called on the
Writer
or
OutputStream
used to write the contents of the file.
Three kinds of files are distinguished: source files, class files,
and auxiliary resource files.
There are two distinguished supported locations (subtrees
within the logical file system) where newly created files are
placed: one for new source files, and
one for new
class files. (These might be specified on a tool's command line,
for example, using flags such as -s
and -d
.) The
actual locations for new source files and new class files may or
may not be distinct on a particular run of the tool. Resource
files may be created in either location. The methods for reading
and writing resources take a relative name argument. A relative
name is a non-null, non-empty sequence of path segments separated
by '/'
; '.'
and '..'
are invalid path
segments. A valid relative name must match the
"path-rootless" rule of RFC 3986, section
3.3.
The file creation methods take a variable number of arguments to
allow the originating elements to be provided as hints to
the tool infrastructure to better manage dependencies. The
originating elements are the types or packages (representing package-info
files) which caused an annotation processor to
attempt to create a new file. For example, if an annotation
processor tries to create a source file, GeneratedFromUserSource
, in response to processing
@Generate
public class UserSource {}
the type element for
UserSource
should be passed as part of
the creation method call as in:
filer.createSourceFile("GeneratedFromUserSource",
eltUtils.getTypeElement("UserSource"));
If there are no originating elements, none need to be passed. This
information may be used in an incremental environment to determine
the need to rerun processors or remove generated files.
Non-incremental environments may ignore the originating element
information.
During each run of an annotation processing tool, a file with a
given pathname may be created only once. If that file already
exists before the first attempt to create it, the old contents will
be deleted. Any subsequent attempt to create the same file during
a run will throw a FilerException
, as will attempting to
create both a class file and source file for the same type name or
same package name. The initial inputs to
the tool are considered to be created by the zeroth round;
therefore, attempting to create a source or class file
corresponding to one of those inputs will result in a FilerException
.
In general, processors must not knowingly attempt to overwrite
existing files that were not generated by some processor. A Filer
may reject attempts to open a file corresponding to an
existing type, like java.lang.Object
. Likewise, the
invoker of the annotation processing tool must not knowingly
configure the tool such that the discovered processors will attempt
to overwrite existing files that were not generated.
Processors can indicate a source or class file is generated by
including an @Generated
annotation.
Note that some of the effect of overwriting a file can be
achieved by using a decorator-style pattern. Instead of
modifying a class directly, the class is designed so that either
its superclass is generated by annotation processing or subclasses
of the class are generated by annotation processing. If the
subclasses are generated, the parent class may be designed to use
factories instead of public constructors so that only subclass
instances would be presented to clients of the parent class.