2.6 Parameter Passing in Remote Method Invocation

An argument to, or a return value from, a remote object can be any Java object that is serializable. This includes Java primitive types, remote Java objects, and non-remote Java objects that implement the java.io.Serializable interface. For more details on how to make classes serializable, see the Java "Object Serialization Specification." Classes, for parameters or return values, that are not available locally are downloaded dynamically by the RMI system. See the section on "Dynamic Class Loading" for more information on how RMI downloads parameter and return value classes when reading parameters, return values and exceptions.

2.6.1 Passing Non-remote Objects

A non-remote object, that is passed as a parameter of a remote method invocation or returned as a result of a remote method invocation, is passed by copy; that is, the object is serialized using the Java Object Serialization mechanism.

So, when a non-remote object is passed as an argument or return value in a remote method invocation, the content of the non-remote object is copied before invoking the call on the remote object.

When a non-remote object is returned from a remote method invocation, a new object is created in the calling virtual machine.

2.6.2 Passing Remote Objects

When passing a remote object as a parameter or return value in a remote method call, the stub for the remote object is passed. A remote object passed as a parameter can only implement remote interfaces.

2.6.3 Referential Integrity

If two references to an object are passed from one VM to another VM in parameters (or in the return value) in a single remote method call and those references refer to the same object in the sending VM, those references will refer to a single copy of the object in the receiving VM. More generally stated: within a single remote method call, the RMI system maintains referential integrity among the objects passed as parameters or as a return value in the call.

2.6.4 Class Annotation

When an object is sent from one VM to another in a remote method call, the RMI system annotates the class descriptor in the call stream with information (the URL) of the class so that the class can be loaded at the receiver. It is a requirement that classes be downloaded on demand during remote method invocation.

2.6.5 Parameter Transmission

Parameters in an RMI call are written to a stream that is a subclass of the class java.io.ObjectOutputStream in order to serialize the parameters to the destination of the remote call. The ObjectOutputStream subclass overrides the replaceObject method to replace each remote object with its corresponding stub class. Parameters that are objects are written to the stream using the ObjectOutputStream's writeObject method. The ObjectOutputStream calls the replaceObject method for each object written to the stream via the writeObject method (that includes objects referenced by those objects that are written). The replaceObject method of RMI's subclass of ObjectOutputStream returns the following:

• if the object passed to replaceObject is an instance of java.rmi.Remote, then it returns the stub for the remote object. A stub for a remote object is obtained via a call to the method java.rmi.server.RemoteObject.toStub.
• if the object passed to replaceObject is not an instance of java.rmi.Remote, then the object is simply returned.

RMI's subclass of ObjectOutputStream also implements the annotateClass method that annotates the call stream with the location of the class so that it can be downloaded at the receiver. See the section "Dynamic Class Loading" for more information on how annotateClass is used.

Since parameters are written to a single ObjectOutputStream, references that refer to the same object at the caller will refer to the same copy of the object at the receiver. At the receiver, parameters are read by a single ObjectInputStream.

Any other default behavior of ObjectOutputStream for writing objects (and similarly ObjectInputStream for reading objects) is maintained in parameter passing. For example, the calling of writeReplace when writing objects and readResolve when reading objects is honored by RMI's parameter marshal and unmarshal streams.

In a similar manner to parameter passing in RMI as described above, a return value (or exception) is written to a subclass of ObjectOutputStream and has the same replacement behavior as parameter transmission.