Uses of code mobility in Java

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94 Uses of code mobility
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Code mobility in RMI has a number of important uses, of which the following are merely the most obvious 941 True polymorphism Polymorphism is Greek for "many forms" In object oriented terminology, it means that a derived class can be used wherever any of its base classes can be used This is a fundamental object oriented design technique, and it is also the basic mechanism for exploiting code mobility Just specify your remote interfaces polymorphically This means specifying the result or parameter types (or both) of your remote interfaces in terms of base classes (or Java interfaces), but passing or returning objects of derived classes If the derived class is unknown to the receiver, normally this technique fails with a ClassNotFoundException at the receiver However, if the object is being received by RMI and the class has been annotated with a valid codebase, the RMI runtime in the receiver downloads the class dynamically on demand from the codebase, and execution continues By this means you can truly exploit object oriented polymorphism in RMI applications Consider the following example:
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public interface InterfaceKnownToClient { } public interface RemoteService extends javarmiRemote { InterfaceKnownToClient method() throws RemoteException; }
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A server which implements RemoteService can return any implementation of InterfaceKnownToClient it likes from RemoteServicemethod If the class of the result is not known to the client, and if the correct codebase conditions exist at the server, the implementation class will be downloaded to the client This technique allows the server to vary the implementation class unbeknownst to the client, for application specific reasons It also allows the implementation class itself to evolve without having to be redeployed to all clients, and without being constrained by the versioning rules of serialization The same technique can be used in the reverse direction, allowing the client to supply the implementation class to the server:
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public interface InterfaceKnownToServer { } public interface RemoteService extends javarmiRemote { void method(InterfaceKnownToServer is) throws RemoteException; }
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942 Thinner clients You can use code mobility to reduce the amount of code installed at the client 943 Application rollout You can use code mobility to avoid the "rollout" problem client reinstallations for classes which are likely to evolve over the life of an application By ensuring that such classes are always obtained dynamically, you never have to redistribute them statically You only have to update a single copy of the class at the codebase server: RMI propagates the class as required 944 Application integrity By ensuring that sensitive classes are not installed at the client but are obtained dynamically, you can reduce the possibility of external tampering with class files
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95 Security considerations
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Obviously the "codebase" feature raises questions about security We shouldn't necessarily trust the other end of an RMI communication to provide us with classes willy nilly For this reason, RMI class loading via the codebase is completely disabled unless a Java security manager is running The Java security model provides the ability to tailor completely the security environment for RMI applications generally, and downloaded classes in particular By default, Java's security manager provides complete security if it is installed Even if a security manager is running, every permission the application requires must be explicitly configured The default security configuration doesn't permit any external interactions to occur with the local file system, the network, or the GUI When configuring the application's permissions, classes from different codebases can be assigned different sets of permissions, including the null set no permissions RMI only attempts to download code from the codebase if a Java security manager is installed in the JVM Any RMI server or client which needs to download code must install a security manager Typically the standard javasecuritySecurityManager or its close relation the javarmiRMISecurityManager is used In JDK 11, javalangSecurityManager was an abstract class, and RMISecurityManager was a concrete extension of it In Java 2 (JDK 12 and later), the security architecture was extensively revised: javalangSecurityManager became a concrete class, and RMISecurityManager became a simple derivation from it, with identical behaviour To allow RMI clients or servers to download code, they must run under a security manager with an appropriate security policy file If you specifically want to disable this feature, either don't install a security manager, or install a security policy file which prevents it[1]
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For example, you could disallow network access other than to the RMI server host(s), assuming you have enough control over them to prevent HTTP services being run on
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them A number of possible settings for class downloading suggest themselves:
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disabled throughout (clients and servers) enabled throughout (clients and servers) enabled for all clients and disabled for all servers disabled throughout except from trusted codebases enabled for all clients, and disabled for all servers except from trusted codebases
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With Java's fine grained access control, it is possible to configure security in even more detail You can define a security policy which accepts classes from a number of trusted codebases, but allows classes from each codebase a different set of permissions a different security profile Security management for RMI is discussed in 8
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