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Java RMI, JAX-RPC and JWSDP. B. Ramamurthy. Inside RMI. Basic RMI classes: /usr/java1.1/src/java/rmi java.rmi.registry.* java.rmi.Naming class (static/class methods) java.rmi.Remote interface (marker interface) java.rmi.server.*

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    1. Java RMI, JAX-RPC and JWSDP B. Ramamurthy

    2. Inside RMI • • Basic RMI classes: /usr/java1.1/src/java/rmi • java.rmi.registry.* • java.rmi.Naming class (static/class methods) • java.rmi.Remote interface (marker interface) • java.rmi.server.* • Default RMI port 1099 • Both lookup from local and remote are acceptable.

    3. Implementation of RMI (5.2.5)

    4. The role of proxy and skeleton in remote method invocation server client remote skeleton object B object A proxy for B Request & dispatcher for B’s class Reply servant Communication Communication Remote reference Remote module reference module module module Object A invokes a remote object in Object B for which it holds a remote object reference. “System Model”

    5. RMI Internals: Communication Module • Carries out request-reply protocol; • On the client side {message type, message id, remote reference to object} are gathered and sent out. At most once invocation semantics; • On the server side, it gets local reference for remote reference from remote reference module, invokes a dispatcher with this reference. • See UnicastRemote (implements UnicastRemote)

    6. RMi Internals: Remote Reference module • Responsible for translating between local and remote object references and for creating remote object references. • A remote object table has a mapping between local and remote references. A table at server (entry for object ref for B) and a table at client (entry for object ref for proxy B).

    7. RMI Internals: Remote References • Action of remote reference module: See interface • When a remote object is to be passed as argument or result for the first time, the remote ref is asked to create a remote ref object which is added to the table. • When a remote object reference arrives in a request or reply, the remote ref module is asked for corresponding local object ref, which may either a proxy or remote object. If it is not in the table RMI runtime creates it and asks remote ref module to add it to the table.

    8. RMI Internals: RMI software • Layer of software between application level objects and communication and remote reference modules: “Middleware” • Proxy: provides remote access transparency. One proxy for every remote object in the client. • Dispatcher: A server has one dispatcher and skeleton for each class representing a remote object. • It receives request message from comm. Module • It used MessageId to select appropriate method in skeleton. • Proxy and dispatcher use same MessageId. • Skeleton: A class of remote object has a skeleton that implements of the remote interface. All the access dependencies are hidden in this class. A remote object has a servant that directly implements the methods. Java 5 creates this dynamically. • Proxies, dispatcher and skeleton are automatically generated by interface compiler. • Binder: binds textual names to remote object references. RMiRegistry is a binder; Naming class; see fig.5.13 • Server Threads: one thread per invocation • Distributed garbage collection: See Andrew Birell’s paper [1995].

    9. RMI Internals: Distributed Garbage Collection • Based on reference counts. • Local garbage collectors and a distributed support. • Each server holds the list of processes that hold remote object references: for example, B.Holders • When a client C first receives a remote reference to a particular remote object, say B, it makes a addRef(B) invocation to server of that remote object and then creates proxy; server adds C to B.Holders. • When client C’s garbage collector finds that proxy is no longer reachable (ref count), it makes a removeRef(B) invocation to server and then deletes proxy; the server removes C from B.Holders. • When B.Holders is empty, server’s local garbage collector will reclaim the space occupied B unless there are any local holders. • These extra calls for updates occur during proxy creation and deletion and do not affect normal opertion. • Tolerates communication failures: addRef() and removeRef() are idempotent: effects of N > 0 identical requests is the same as for a single request. • If addRef() fails with an exception, proxy is not created, removeRef() is transmitted; removeRef() failures are dealt with by “leases” (Jini kind).

    10. RMI Internals: Use of Reflection • What is reflection? See Reflection package • Reflection enables Java code to discover information about the fields, methods and constructors of loaded classes, and • To use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions. • Reflection feature allowed for dynamic creation of skeleton and proxy in Java 2 version onwards. • Read more about reflection model of computing.

    11. A Little bit of Reflection • Method class, invoke method • Invoke method requires two parameters: first the object to receive invocation, second an array of Object parameters. • Invoke executes the method on the object and returns result as Object. • Method m; • Object result = M.invoke(String, Args);

    12. Using Reflection in RMI • Proxy has to marshal info. about a method and its arguments into a request message. • For a method it marshals an object of class Method into the request. It then adds an array of objects for the method’s arguments. • The dispatcher unmarshals the Method object and its arguments from request message. • The remote object reference is obtained from remote ref. table. • The dispatcher then calls the “invoke” method on the object reference and array of arguments values. • After the method execution the dispatcher marshals the result or any exceptions into the reply message.

    13. JAX-RPC • JAX-RPC (The Java API for XML-based RPC) is designed to provide a simple way for developers to create Web services server and Web services client. • Based on remote procedure calls; so the programming model is familiar to Java developers who have used RMI or CORBA. • Major difference between RMI and JAX-RPC is that messages exchanged are encoded in XML based protocol and can be carried over a variety of transport protocols such as HTTP, SMTP etc. • You can use JAX-RPC without having to be an expert in XML, SOAP, or HTTP.

    14. The JAX-RPC Programming Model • Services, ports and bindings • JAX-RPC web service servers and clients • JAX-RPC service creation • JAX-RPC client and server programming environments • Stubs and ties • Client invocation modes • Static and dynamic stubs and invocation

    15. Services, ports and bindings • Service endpoint interface (SEI) or service endpoint that defines one or more operations that the web service offers. • Access to an endpoint is provided by binding it to a protocol stack through a port. • A port has an address that the client can use to communicate with the service and invoke its operations. • An endpoint can be bound to different ports each offering a different suite of protocols for interaction.

    16. Endpoint, Port and binding Web service endpoint Port1 port2 port3 Web services Client SOAP 1.1 over https SOAP1.1 Over http Other. Ex: ebXML over SMTP https 1.1 transport soap1.1 messages

    17. Web Service Clients and Servers • JAX-RPC maps a • web service operation to a java method call. • service endpoint to a Java Interface. • Thus one way to begin implementation of a web service in JAX-RPC is to define a Java interface with a method for each operation of the service along with a class that implements the interface. Of course, following the rules of remote invocation etc. • Now visualize client/server invocation in the same address space and lets compare it with remote invocation.

    18. Local Date Service //server public class DataService { public Data getDate() { return new Date();} //client Public class Appln { public static void main (..) { DateService instance = new DateService(); Date date = instance.getDate(); System.out.println (“ The date is” + date); } • In the case of the remote call a layer of software is used to convey the method call from client to server. This layer of software is provided by JAX-RPC runtime.

    19. JAX-RPC service creation • A service definition describes the operations that it provides and the data types that they require as argument and provide as return values. • This definition can be made available as a document written in WSDL. • From a WSDL document, JAX-RPC can generate the Java code required to connect a client to a server leaving one to write only the logic of the client application itself. • Since WSDL is language independent the server can be in .net, Jax-rpc or any other compatible platform.

    20. JAX-RPC service creation (contd.) • Define the service a Java interface. • Generate WSDL using the tools provided with JAX-RPC package. • Advertise it in a registry for the client to lookup and import it.

    21. Client and Server Programming Environment • JAX-RPC API is distributed over a set of packages: • javax.xml.rpc • javax.xml.rpc.encoding • javax.xml.rpc.handler • javax.xml.rpc.handler.soap • javax.xml.rpc.holders • javax.xml.rpc.server • javac.xml.rpc.soap

    22. Stubs and Ties • Client Side: Stub object has the same methods as the service implementation class. • Client application is linked with the stub. • When it invokes a method stub delegates the call to the JAX-RPC runtime so that appropriate SOAP message can be sent to the server. • On completion the result return back in the reverse path as above. • Server side: • Message received must be converted into a method call on actual service implementation. This functionality is provided by another piece of glue called tie. • Tie extracts method name and parameter from SOAP message. • Tie also converts the result of the method call back into a response message to be returned to client JAX-RPC runtime. JAX-RPC comes with tools to generate these.

    23. Client Invocation Modes • Synchronous request-response mode (tightly coupled). • One-way RPC (loosely coupled): no value returned, no exception thrown, need to bypass stub layer, use Dynamic Invocation Interface (DII).

    24. SEI Invocation Code • Service End Point (SEI) invocation code: Stub stub = (Stub)(new MyHelloService_Impl().getHelloIFPort()); stub._setProperty(javax.xml.rpc.Stub.ENDPOINT_ADDRESS_PROPERTY, "http://localhost:8080/hello-jaxrpc/hello"); HelloIF hello = (HelloIF)stub; resp = hello.sayHello(request.getParameter("username"));