Java™ Application Development on Linux - Dator

142Chapter 5The Sun Microsystems Java Software <strong>Development</strong> KitNext, we have the server main() method. It first sets a security manager.The security manager controls what the VM is allowed to do. A number ofdefault security managers are provided, and here we use one that is designedspecifically to give safe and reasonable defaults for RMI applications. You can,of course, write your own security manager. Security managers use “policyspecifications” to alter their capabilities. For now, we will explain enough torun a simple example. See Section 5.8.4.2 for more information on policies forour example.Remember that main() is static, so there is no instance of SessionImplyet, and thus also no instance of Session. We declare a variable of typeSession, and set it to a new instance of SessionImpl. (There is no needto typecast here because SessionImpl implements Session, thereforeSessionImpl is, among other things, a Session.) We now have an instanceof the server class.Next, the server must make itself available to the world. It does this byregistering itself with the RMI registry (see Section 5.8.3). This is done througha static method of the java.rmi.Naming class, rebind(). Put simply, thismaps a remote object to a string name in the registry. When clients contact theregistry looking for a name then, if a remote object is mapped to that name,the communication can take place (yes, we are simplifying at the moment).The call to rebind() does not return. The server is up and running.Finally, we have the implementation of our remote method, add().This looks like a lot of hassle to go through, and it is, but consider writingan interface that offers, for example, methods like getDirContents(),chDir(), downloadFile(), uploadFile(). You’ve just written somethinglike an FTP server. No matter how many methods you add to your interface,the complexity of the setup code does not increase. Maybe now it looks a littlemore useful?5.8.1.5 Writing the Client ClassAt this point, Example 5.12 should be fairly obvious. Our class has just a singlestatic method, main(). It, like our server side main(), sets up a security manager.It then contacts a registry on the machine named penfold looking for aninstance of a remote interface named Session (again, lookup() is a staticmethod of the java.rmi.Naming class). We store that reference in a variableof type Session called sess. We can then call the add() on sess. We’ll showthe server and client running shortly.