Exercises in Java

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1:This exercise assumes you have completed the date/time server and client of 7 (all relevant exercises) Run the server with the property javarmidgcleaseValue set to 5 minutes Run the client without this property set and measure the interval between the client exiting and the server exiting Repeat this test, this time setting the javarmidgcleaseValue property to 5 minutes at the client, not the server Does the setting of the javarmidgcleaseValue property take effect at the server or the client 2:Verify or refute the statement in 1823 about distributed garbage collection and callbacks Show your assumptions and experimental method Construct a distributed cycle containing four remote objects and re test, again showing your assumptions and method
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Appendix A Exceptions in RMI
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Class hierarchy Exceptions in servers Exceptions in clients Alphabetic list of exceptions Remarks This appendix discusses some of the issues surrounding exceptions in RMI servers and RMI clients, followed by an alphabetical listing of the exceptions specific to RMI
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A1 Class hierarchy
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The hierarchy diagram in Figure A1 shows the RMI exceptions and their inheritance relationships
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Figure A1 RMI exception class hierarchy
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A2 Exceptions in RMI servers
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An RMI server can encounter an RMI exception when constructing, exporting, binding, unbinding, or unexporting itself An RMI server's implementation of a remote method cannot encounter an RMI exception unless it calls another remote method itself Once exported and bound, while executing remote methods, RMI servers are held robust by the RMI system they will never exit or cease operating because of any uncaught exception whatsoever (They may lose operating threads due to OutOfMemoryError and friends) They are immune to the consequences of runtime exceptions and errors (ie RuntimeException and Error), and to the consequences of RemoteExceptions arising out of any remote method calls they may make themselves If a remote method implementation throws such an exception, it is caught by the RMI system and transmitted to the client If a remote method invocation incurs an exception while entering from or exiting to the RMI framework, the exception is caught by the framework and transmitted to the client The only way an RMI server can exit is by being garbage collected locally This can occur when all remote and local references to the server have been released A binding in the RMI registry constitutes a remote reference, so a server can never exit while bound
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A3 Exceptions in RMI clients
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An RMI client can encounter an RMI exception when executing any remote method, including those of Naming and Registry, as defined by the signature of the remote method RMI exceptions are "checked" exceptions: this means that the Java compiler forces you to handle them
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A4 Alphabetic list of exceptions
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This section lists all the RMI exceptions; explains what they mean; and discusses when they occur and
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why, and what you can do about it We have also included the following frequently encountered non RMI exceptions:
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javalangClassCastException javalangClassNotFoundException javalangNoClassDefFoundError
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A41 javarmiAccessException This indicates that the caller does not have permission to perform the action requested To be specific, it is thrown by the bind, rebind, and unbind methods of Naming and Registry These methods can only be invoked by callers on the same host as the registry concerned It is also thrown by the methods to register and unregister activation groups and activatable objects These methods can only be invoked by callers on the same host as rmid A42 javarmiactivationActivationException ActivationException is the base class for UnknownGroupException and UnknownObjectException A43 javarmiactivationActivateFailedException This is thrown when activation fails during a remote call to an activatable object Activation fails if the object required cannot be activated within its activation group for any reason, including failure to instantiate the object, failure to activate the group itself, or failure to locate the RMI activation system daemon rmid on the target host This exception typically occurs on the first use of a remote activatable stub by an RMI client A44 javarmiAlreadyBoundException This indicates that the name argument to a Namingbind or Registrybind call is already bound If you really want the bind to succeed, it is normally simplest to use the rebind method, which does a forced bind However, if you want to take other actions on detecting an existing binding, you could catch this exception, do an unbind and whatever other processing your application requires, and retry A45 javalangClassCastException This is a frequent "RMI beginners'" error, typically caused by one of the following:
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trying to cast the result of Naminglookup into a remote object implementation class instead of a remote interface declaring a parameter or result of a remote method as a remote object implementation class instead of a remote interface (or an array of implementations instead of an array of interfaces)
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For non beginners, it can also be caused by trying to return a non static inner class of a remote object as the result of a remote method A non static inner class has a hidden reference to the outer class,
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which can't be reconstructed correctly at the receiver Change the inner class to static and the problem will disappear If the inner class needs a reference to the enclosing remote object, you must construct it yourself, and declare its type as the remote interface type, not the remote object type A46 javalangClassNotFoundException In RMI, this exception usually indicates that an RMI code mobility operation has failed It is typically encountered in the following circumstances: 1 When exporting a server: the stub class for the server can't be located at all; possibly it was not generated, or not installed along with the class for the server object 2 When a server is being bound to the registry: the registry can't load the stub for the server from its CLASSP ATH and the server hasn't annotated the stub with a valid javarmiservercodebase setting; the solution is to correct one of these conditions, usually the latter usually the codebase is missing or incorrect, or the specified HTTP codebase server is not running or incorrectly configured 3 When a client is looking up a server in a registry: the client can't load the stub from the server from its CLASSP ATH and the registry didn't transmit any codebase annotation with the stub; the solution is to correct one of these conditions, usually the latter usually it means that the registry was able to load the stub from its CLASSPATH and so didn't transmit any codebase information for the server, and the solution is to fix the registry's CLASSPATH, probably just by running it from a different directory It is important to understand the difference between these situations The first is just an installation error As for the other two, assuming you don't want the registry and clients to load stubs from the CLASSPATH, the second means that the codebase was never correctly set; the third means that the codebase was possibly correct enough originally, but was lost on the way through the registry If you're having trouble sorting all this out, try running the registry with the system property javarmiserverlogCalls set to true and watch the remote calls and exceptions occurring within the registry itself This can be most educational Another difficulty is that prior to JDK 13 this exception is sometimes masked into a RemoteException or ServerException A47 javarmiConnectException This indicates that a connection has been refused to the remote host for a remote method call The detail member contains the original exception, which is likely to be a javanetConnectionException or javanetNoRouteToHostException It is safe to retry an RMI call which gets this exception and still preserve "at most once" semantics A48 javarmiConnectIOException This indicates that an exception has occurred while making a connection to the remote host for a remote method call It differs from ConnectException in that it indicates an I/O problem rather than a real connection problem The exception arises when making a logical RMI connection involves
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I/O on an existing socket connection, rather than creating a new socket connection (which incurs a javarmiConnectException on failure) This happens when re using an existing (cached) connection, or setting up a multiplexed connection The detail member contains the original exception It is safe to retry an RMI call which gets this exception and still preserve "at most once" semantics A49 javarmiserverExportException This occurs when exporting a remote object It indicates that the port requested is already in use, probably by one of the following:
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another process another RMI server in the current JVM with an unequal ServerSocketFactory an explicitly created ServerSocket in the current JVM
RMI servers in the same JVM can share ports with each other if they have null or equal server socket factories, but they can't share ports with other processes of any kind, even other JVMs On Unix like platforms it may also indicate an unprivileged attempt to use a port number below 1024 A410 javarmiMarshalException This is thrown if a javaioIOException occurs when:
marshalling the remote call header or arguments at the client marshalling the remote reply header or result at the server the receiver does not understand the protocol version of the sender Marshalling of arguments occurs at the client; marshalling of the return value occurs at the server Under the covers there is also a remote call header containing among other things a protocol version, which is marshalled and transmitted by both client and server