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\name{jserialize}
\alias{.jserialize}
\alias{.junserialize}
\alias{.jcache}
\title{
Java object serialization
}
\description{
\code{.jserialize} serializes a Java object into raw vector using
Java serialization.
\code{.junserialize} re-constructs a Java object from its serialized
(raw-vector) form.
\code{.jcache} updates, retrieves or removes R-side object cache
which can be used for persistent storage of Java objects across
sessions.
}
\usage{
.jserialize(o)
.junserialize(data)
.jcache(o, update=TRUE)
}
\arguments{
\item{o}{Java object}
\item{data}{serialized Java object as a raw vector}
\item{update}{must be \code{TRUE} (cache is updated), \code{FALSE}
(cache is retrieved) or \code{NULL} (cache is deleted).}
}
\value{
\code{.jserialize} returns a raw vector
\code{.junserialize} returns a Java object or \code{NULL} if an error
occurred (currently you may use \code{.jcheck()} to further
investigate the error)
\code{.jcache} returns the current cache (usually a raw vector) or
\code{NULL} if there is no cache.
}
\details{
Not all Java objects support serialization, see Java documentation
for details. Note that Java serialization and serialization of R
objects are two entirely different mechanisms that cannot be
interchanged. \code{.jserialize} and \code{.junserialize} can
be used to access Java serialization facilities.
\code{.jcache} manipulates the R-side Java object cache associated
with a given Java reference:
Java objects do not persist across sessions, because the Java
Virtual Machine (JVM) is destroyed when R is closed. All saved Java
object references will be restored as \code{null} references, since
the corresponding objects no longer exist (see R documentation on
serialization). However, it is possible to serialize a Java object
(if supported by the object) and store its serialized form in
R. This allows for the object to be deserialized when loaded into
another active session (but see notes below!)
R-side cache consists of a serialized form of the object as raw
vector. This cache is attached to the Java object and thus will be
saved when the Java object is saved. rJava provides an automated way
of deserializing Java references if they are \code{null} references
and have a cache attached. This is done on-demand basis whenever a
reference to a Java object is required.
Therefore packages can use \code{.jcache} to provide a way of
creating Java references that persist across sessions. However, they
must be very cautious in doing so. First, make sure the serialized
form is not too big. Storing whole datasets in Java serialized form
will hog immense amounts of memory on the R side and should be
avoided. In addition, be aware that the cache is just a snapshot, it
doesn't change when the referenced Java object is modified. Hence it
is most useful only for references that are not modified outside
R. Finally, internal references to other Java objects accessible
from R are not retained (see below). Most common use of
\code{.jcache} is with Java references that point to definitions of
methods (e.g., models) and other descriptive objects which are then
used by other, active Java classes to act upon. Caching of such
active objects is not a good idea, they should be instantiated by
functions that operate on the descriptive references instead.
\emph{Important note:} the serialization of Java references does NOT
take into account any dependencies on the R side. Therefore if you
hold a reference to a Java object in R that is also referenced by
the serialized Java object on the Java side, then this relationship
cannot be retained upon restore. Instead, two copies of disjoint
objects will be created which can cause confusion and erroneous
behavior.
The cache is attached to the reference external pointer and thus it
is shared with all copies of the same reference (even when changed
via \code{\link{.jcast}} etc.), but it is independent of other
references to the object obtained separately
(e.g., via \code{\link{.jcall}} or \code{\link{.jfield}}).
Also note that deserialization (even automated one) requires a
running virtual machine. Therefore you must make sure that either
\code{\link{.jinit}} or \code{\link{.jpackage}} is used before any
Java references are accessed.
}
%\seealso{
%}
%\examples{
%\dontrun{
%}
%}
\keyword{interface}
|
