Java RMI Essentials Based on Mastering RMI Rickard Oberg

2 Essentials of Remote Invocation What is RMI? –The Principles of RMI –How Does RMI Differ from Ordinary Java RMI/JRMP Architecture –Stubs –Marshalling –RMI Threading & Network Connection Management –Distributed Garbage Collection –Naming Summary

3 What is RMI? RMI is a specification (API) for accessing objects from a remote JVM. What is specified? –How objects are to be coded. –How objects can be located & invoked. –How parameters & returned values are passed. Java Remote Method Protocol (JRMP) is Sun’s implementation of the RMI API.

4 The Principles of RMI Meta-principle Make RMI like MI as much as possible. Objects are invoked by calling methods. Expose interfaces not implementation. Exceptions report errors in the computation. GC determines the lifecycle of objects. Get classes that are not part of system classpath via classloading.

5 How Does RMI Differ from Local MI? Remote exceptions –Remote methods throw java.rmi.RemoteExeption. Pass by value –All arguments are pass-by-value. –Serializing may hurt performance for large objects. Latency Invocations take much longer to complete. Security Arguments/returned value are sent over a network. Is privacy an issue?

6 RMI/JRMP Architecture Stubs Marshalling RMI Threading & Network Connection Management Distributed Garbage Collection Naming

7 Stubs Stub –The client has a proxy for the remote object: the stub. –The stub implements the remote object’s interface[s]. –The RMI compiler (rmic) generates the stub. –Remote methods invoked by the client are delegated to the JRMP engine. –The JRMP forwards the call to the server. –The server executes the method. –The result is returned to the client.

8 MyServer MyServer Stub RemoteRef [host=myhost, port=1234, ObjID=0] JRMP > CLIENT MyServerImpl MyServer ServerSocket [port=1234] End-point > Object table > JRMP exported objects SERVER MyServer instance maps to ObjID=0

9 The Remote Interface It is a set of remotely invoked methods. It has the following characteristics: _______________________________________ public interface MyRemoteInterface extends java.rmi.Remote // possibly indirectly { public myMethod( p1, … ) throws java.rmi.RemoteException // declare other remote methods … } _______________________________________ All parameters & return type are serializable.

10 The Hello Interface package masteringrmi.helloworld.interfaces; import java.rmi.Remote; import java.rmi.RemoteException; public interface HelloWorld extends Remote { public String helloWorld( String name ) throws RemoteException; }

11 The Hello Interface … Style: create a contract package It contains the “contract” between client & server: Remote interfaces of server objects Application exceptions throwable by any remote interface Data container classes for data moved between client & server. –Client & server get contract classes & interfaces. –Server also gets implementation classes.

12 Implementing the Remote Interface public class HelloWorldImpl extends UnicastRemoteObject implements HelloWorld { public HelloWorldImpl() throws RemoteException {} public String helloWorld( String name ) { return “Hello “ + name + “!”; }

13 Implementing the Remote Interface … public class HelloWorldImpl implements HelloWorld { public HelloWorldImpl() throws RemoteException { UnicastRemoteObject.exportObject( this ); } public String helloWorld( String name ) { return “Hello “ + name + “!”; }

14 Implementing the Remote Interface … UnicastRemoteObject constructor exports the object Makes it available for incoming invocations. The exportObject method does this explicitly. Extending UnicastRemoteObject inherits distributed implementations of: –equals, hashCode, & toString.

15 RMI/JRMP Architecture Stubs Marshalling RMI Threading & Network Connection Management Distributed Garbage Collection Naming

16 Marshalling Marshalling creates a byte[] from an object. Unmarshalling does the reverse. To marshal, Java serializes the object. To unmarshal, Java deserializes the byte[]. public class foo implements Serializable Foo Copy of Foo Bytes SerializationDeserialization

17 Dynamic Classloading We defer discussion of this until later.

18 Security To dynamically download stubs, the client: –Sets a security manager: ______________________________________________________ import java.rmi.RMISecurityManager;... if ( System.getSecurityManager() == null ) System.setSecurityManager( new RMISecurityManager() ); ________________________________________________ –Has a policy file that permits downloading ________________________________________________ grant { permission java.security.AllPermission; } ________________________________________________

19 Class Versioning Server changes are propagated to clients that subsequently download the stub. What about running clients that already have the stub? Basically, this is a problem. –serial version UID can be used to detect incompatibilities. –Jini further ameliorates the version problem.

20 RMI/JRMP Architecture Stubs Marshalling RMI Threading & Network Connection Management Distributed Garbage Collection Naming

21 RMI Threading & Network Connection Management “Since RMI on the same remote object may execute concurrently, a remote object implementation needs to make sure its implementation is thread-safe.” The RMI specification, section 3.2

22 Network Connections RMI specifies socket factory interfaces to get sockets on the client & server: –Java.rmi.server.RMIClientSocketFactory –Java.rmi.server.RMIServerSocketFactory Override the default implementation (to provide encryption, authentication, …) Selecting an features at run time is desirable: Some jobs want encryption, some do not. Custom implementations are discussed later.

23 Threading Model The JRMP implementation for the server –Instantiates a thread for each connection. –It listens for its associated client’s calls. –It handles each call to completion. The JRMP implementation for the client –Concurrent calls from a client cause concurrent threads connecting to the server. –This may swamp a server. –If your client makes concurrent calls, you may want a different implementation.

24 RMI/JRMP Architecture Stubs Marshalling RMI Threading & Network Connection Management Distributed Garbage Collection Naming

25 Distributed Garbage Collection RMI has distributed garbage collection (DGC). Server tracks clients who have its stub. Server keeps a count of such clients. Count is decremented when client: –explicitly relinquishes reference OR –doesn’t renew its lease (default 10 min.), e.g., client crashed: “Leasing” is due to Miller & Drexler, in –Incentive Engineering for Computational Resource Management. K. E. Drexler & M. S. Miller, B. A. Huberman (ed.), (Studies in Computer Science & Artificial Intelligence), If local & remote references == 0, it is garbage.

26 The Unreferenced Interface A server can implement Unreferenced It has 1 method: unreferenced. It is called when there are no remote references to the object. Being bound in the RMI registry counts as a remote reference. –It must unbind itself before unreferenced can be invoked.

27 RMI/JRMP Architecture Stubs Marshalling RMI Threading & Network Connection Management Distributed Garbage Collection Naming

28 Naming Rmiregistry allows: –A server to store its (serialized) stub on a web server –A client to download & deserialize the stub. The essential information: the url of the serialized stub file.