Java RMI Enhancements

Enhancements in JDK 6

java.rmi.MarshalledObject now generic: The class MarshalledObject now has a type parameter representing the type of the contained serialized object.
Bug fix: Explicit TCP ports freed after remote objects unexported (4457683): In previous releases, after a remote object had been exported on an explicit (non-anonymous) TCP port, the RMI implementation would keep a server socket bound to that port open for the lifetime of the virtual machine, regardless of the life cycle of remote objects exported on that port. This bug has been fixed so that after all remote objects that had been exported on a given non-anonymous port have become unexported (either explicitly or through distributed garbage collection) then the associated server socket will be closed. (There is an equivalent bug for anonymous TCP ports: 6279965.)
Bug fix: Garbage collection of client socket factories (4486732): In previous releases, after a remote invocation was made on a remote stub containing a non-null RMIClientSocketFactory in a given virtual machine, then either that factory object or an equivalent factory object would remain forever reachable in the virtual machine, preventing it (and its defining class loader) from ever being garbage collected; this bug has been fixed. (There is an equivalent bug for socket factories used to export a remote object: 6332603.)
Default GC interval lengthed to one hour (6200091): In previous releases, the maximum interval between garbage collections of the local heap enforced by the RMI implementation (while there are live remote references or exported remote objects), which is controlled by the system properties sun.rmi.dgc.client.gcInterval and sun.rmi.dgc.server.gcInterval, was one minute by default. The default interval is now one hour, to better accommodate applications with large heap sizes without special configuration.

Enhancements and Changes in Previous Releases

Dynamic Generation of Stub Classes (since Java SE 5.0)

This release adds support for the dynamic generation of stub classes at runtime, obviating the need to use the Java Remote Method Invocation (Java RMI) stub compiler, rmic, to pregenerate stub classes for remote objects. Note that rmic must still be used to pregenerate stub classes for remote objects that need to support clients running on earlier versions.

When an application exports a remote object (using the constructors or static exportObject methods1 of the classes java.rmi.server.UnicastRemoteObject or java.rmi.activation.Activatable) and a pregenerated stub class for the remote object's class cannot be loaded, the remote object's stub will be a java.lang.reflect.Proxy instance (whose class is dynamically generated) with a java.rmi.server.RemoteObjectInvocationHandler as its invocation handler.

An existing application can be deployed to use dynamically generated stub classes unconditionally (that is, whether or not pregenerated stub classes exist) by setting the system property java.rmi.server.ignoreStubClasses to "true". If this property is set to "true", pregenerated stub classes are never used.

Notes:

If a remote object has pre-5.0 clients, that remote object should use a stub class pregenerated with rmic or the client will get an ClassNotFoundException deserializing the remote object's stub. Pre-5.0 clients will not be able to load an instance of a dynamically generated stub class, because such a class contains an instance of RemoteObjectInvocationHandler, which was not available prior to this release.
Prior to Java SE 5.0, exporting a remote object would throw a java.rmi.StubNotFoundException if the pregenerated stub class for the remote object's class could not be loaded. With the added support for dynamically generated stub classes, exporting a remote object that has no pregenerated stub class will silently succeed instead. A user deploying a server application to support pre-5.0 clients must still make sure to pregenerate stub classes for the server's remote object classes, even though missing stub classes are no longer reported at export time. Such errors will instead be reported to a pre-5.0 client when it deserializes a dynamically generated stub class.

1 The static method UnicastRemoteObject.exportObject(Remote) is declared to return java.rmi.server.RemoteStub and therefore cannot be used to export a remote object to use a dynamically generated stub class for its stub. An instance of a dynamically generated stub class is a java.lang.reflect.Proxy instance which is not assignable to RemoteStub.

Standard SSL/TLS Socket Factory Classes (since Java SE 5.0)

This release adds standard Java RMI socket factory classes, javax.rmi.ssl.SslRMIClientSocketFactory and javax.rmi.ssl.SslRMIServerSocketFactory, which communicate over the Secure Sockets Layer (SSL) or Transport Layer Security (TLS) protocols using the Java Secure Socket Extension (JSSE). These socket factory classes provide a simple way to use JSSE with Java RMI, enabling enforcement of integrity, confidentiality (through encryption), server authentication, and (optionally) client authentication for remote method invocations. For more information on how to use custom socket factories with Java RMI, see the tutorial "Using Custom Socket Factories with Java RMI". For more information on JSSE (including how to configure it), see the JSSE Reference Guide.

Launching rmid or a Java RMI Server from inetd/xinetd (since Java SE 5.0)

A new feature, provided by the System.inheritedChannel method, allows an application to obtain a channel (java.nio.channels.SocketChannel or java.nio.channels.ServerSocketChannel, for example) inherited from a process that launched the virtual machine (VM). Such an inherited channel can be used to either service a single incoming connection (in the case of a SocketChannel) or accept multiple incoming connections (in the case of ServerSocketChannel). Therefore, Java networking applications launched by inetd (Solaris Operating System) or xinetd (Linux) can now obtain the SocketChannel or ServerSocketChannel inherited from inetd/xinetd.

With the addition of this feature, the Java RMI activation daemon rmid has been enhanced to support being launched from inetd/xinetd so that rmid can be started up only when it receives an incoming connection. For details on the enhancements to rmid to support this feature, see the tools page for rmid (for Solaris, Linux, or Mac OS X operating systems). For details on how to configure inetd to launch rmid, see the tutorial "Configuring inetd to Launch rmid".

An application using Java RMI can also be designed to be launched from inetd or xinetd. For an example on how to implement this technique, see the tutorial "Designing Services to be Launched from inetd."

rmic default stub protocol version option now -v1.2 (since Java SE 5.0)

When rmic is run without an option to specify the JRMP stub protocol version to be used by the generated classes, it now behaves as if the -v1.2 option were specified, instead of the -vcompat option as in previous releases.

This change means that by default, rmic does not generate any skeleton classes and generates stub classes that only implement the 1.2 stub protocol version. If a remote implementation class needs to be built to support clients running on JDK 1.1, then the -vcompat option must now be specified explicitly. (Also, note that as described above, if a remote implementation class does not need to support clients running on any release prior to Java SE 5.0, then rmic does not need to be run at all for that class.)

See the tools documentation for rmic [Solaris, Linux, or Mac OS X] [Windows] for more information about these command line options.

rmic no longer compiles arbitrary source files in the class path (since Java SE 5.0)

In previous releases, rmic would sometimes, while processing its input class files, attempt to compile arbitrary .java source files that it encountered in the class path. In Java SE 5.0, rmic does not attempt to compile any source files other than those for the stub, skeleton, or tie classes that it generates.

The expectation is that the remote implementation classes passed to rmic, as well as all classes and interfaces that they depend on, have already been successfully compiled with javac before rmic is run. If an existing build procedure depends on the previous behavior of rmic for compiling some of its application source files, then that build procedure will need to be modified to ensure that all necessary classes are correctly compiled with javac before running rmic.

Server-side Stack Traces Now Retained in Remote Exceptions (since Java SE 1.4)

The Java RMI runtime implementation will now preserve the server-side stack trace information of an exception that is thrown from a remote call, in addition to filling in the client-side stack trace as it did in previous releases. Therefore, when such an exception becomes accessible to client code, its stack trace will now contain all of its original server-side trace data followed by the client-side trace.

This feature is made possible by the new "programmatic access to stack trace information" feature of java.lang.Throwable in Java SE 1.4, which included making a Throwable's stack trace data part of its default serialized form. What the client-side Java RMI runtime implementation now does to cooperate with this feature is to append the client-side trace to the unmarshalled server-side trace, rather than simply overwriting with the client-side trace as it did in previous releases.

Certain server applications may wish to prevent any server-side stack trace data from accompanying an exception to be marshalled as the result of a remote call (as part of the exception's default serialized form in Java SE 1.4), perhaps for reasons of performance or confidentiality. In such cases, the implementation-specific system property

sun.rmi.server.suppressStackTraces

can be set to "true" to cause the server-side Java RMI runtime implementation to clear the stack traces of all exceptions thrown from the current virtual machine as the result of remote method invocations.

Service Provider Interface for RMIClassLoader (since Java SE 1.4)

Certain static methods of java.rmi.server.RMIClassLoader now delegate their behavior to an instance of a new service provider interface, java.rmi.server.RMIClassLoaderSpi. The service provider can be configured to augment the Java RMI dynamic class loading behavior for a given application. By default, the service provider implements the standard behavior of all of the static methods in RMIClassLoader. See the class documentation of RMIClassLoader and RMIClassLoaderSpi for more details.

Dynamic Server Host Name (since Java SE 1.4)

The java.rmi.server.hostname property can now be dynamically updated to indicate that future exports should use a new host name. Therefore, the new host name value will be contained in the stub for an object that is exported after the property is updated.

HTTP Fallback Is More Configurable (since Java SE 1.4.1)

The implementation-specific system property sun.rmi.transport.proxy.eagerHttpFallback has been added to allow additional control over when the Java RMI default socket factory will fall back to HTTP tunneling. This property configures the default socket factory so that any SocketException thrown by an initial (direct) connection attempt will trigger HTTP tunneling; this more "eager" fallback strategy can be useful when dealing with firewalls which deny instead of ignore connection attempts to unauthorized ports.

java.rmi.Naming.list Method No Longer Prepends a Scheme to Names Returned (since Java SE 1.4.1)

In previous releases, the Naming.list method prepended the scheme rmi: to each name contained in the returned array of strings. The Naming.list implementation now matches the specification, returning an array of names that are URL-formatted, but without the scheme component.

java.rmi.activation.ActivationGroupDesc (since Java SE 1.3)

The getClassName method, which returns the group's class name, can now return null, indicating the system's default group implementation. Previously, the getClassName method would return the name of the internal implementation class if the default group implementation was chosen when the descriptor was constructed.

Due to this change, if an application running in a Java SE 1.3 virtual machine registers an new activatable object with the ActivationSystem, rmid must also be upgraded to run Java SE 1.3, since a pre-1.3 rmid will not be able to activate the newly registered activatable object.

The Java RMI Stub Compiler, rmic (since Java SE 1.3)

By default, rmic [Solaris, Linux, or Mac OS X] [Windows] now assumes that the destination directory for generated stubs is the package-named subdirectory of the current working directory. If the "-d" option is not specified, the result is the same as though it were specified with the current working directory "." as an argument. The "-d" may still be used to override the default destination directory.

Two new options, "-idl" and "-iiop" have been added to generate IDL and stubs for IIOP, respectively.

The Java RMI Activation Daemon, rmid (since Java SE 1.3)

By default, rmid [Solaris, Linux, or Mac OS X] [Windows] now requires a security policy file.

Serializing remote objects (since Java SE 1.2.2)

Prior to Java SE 1.2.2, an attempt to pass an unexported remote object in a RMI call would result in a java.rmi.StubNotFoundException. This exception was a result of the RMI runtime's failure to locate a stub object during an attempt to replace a remote object implementation with its corresponding stub. In Java SE 1.2.2 and later releases, an unexported remote object passed in an RMI call will no longer result in an exception, but rather the remote object will be serialized instead of its stub. If the remote object implementation is not serializable, an attempt to pass an unexported object in an RMI call will result in a java.rmi.RemoteException with the nested exception java.io.NotSerializableException.