Listnening 1 - Structure of the interface
3.2. Developing a class which implements the interface from 3.1 (Figure 2)
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- Import packages java.rmi and java.rmi.server;
- Extend the class RemoteObject or some subclasses specific to this class (in some real applications you will find a frequently extension of subclass UnicastRemoteObject, because of the fact that the class has point-to-point communication with some streams over TCP);
- Implements the interface from 3.1 IHello. Here will implements the body of the getGreetingMessage() method.
- It is necessary to provide a constructor for the implemented object. If we don’t have any implementation for the object, we will leave it empty. The constructor needs to throw RemoteException. In order for everything to be OK and understandable, we will add (append) Implementation keyword next to the name of our interface with the goal to form the name of the class that is implemented.
The class that will implements IHello will look like in Figure 3 and code listening 2. The keyword @Override shows us that the method getGreetingMessage() is overridden.
Figure 3. Implementing the IHello (see step 3.1) interface
Listening 2 - Implementation of the IHello interface
3.3. Develop the server component (process)
The task of the server component is to create object(s) from the class that we have implemented above. The next step is to register them with the help of a naming service, called registry.
The registration process is done with a rebind() method of class Naming (this class is from java.rmi package) and it has two argument:
- A string that keeps the name of the remote object. The string has a URL form that is preceded by the rmi protocol.
- A reference to the object that is remote (as a parameter of type Remote).
Next we will provide the code (Figure 4 and code listening 3) for the server component. The server contains one single method, main. In order to catch the different types of exception, the method main throws Exception.
Figure 4. The server
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The method will set a connection between the name of the object and his reference. The clients will have the possibility to use the remote object’s name in order to retrieve a specific reference of that object using the registry.
The URL string indicates the name of the remote object that is stored on a host machine. In order to keep it simple, we will use localhost (the default value that RMI is assuming). The default port of RMI is 1099 and it can be changed if you wish.
3.4. Develop the client component (process)
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The client goal is to obtain a reference for the remote object through the registry. In order to accomplish this, we use the lookup method which can be found in Naming class. The lookup method receive as a parameter the URL that the server generated when binding the object reference to the object’s name in the registry. The lookup method returns a Remote reference. This reference need to be typecast into a Hello reference (and not a HelloImplementation reference!). After the Hello reference is obtained, it can be used to invoke (call) the method that we have made it available in the interface.
The code is listed below, in Figure 5 and code listening 4.
Figure 5. The client
Listening 4 - Client Implementation
4. How the running process is done?
In order to run the application, we need to compile all the four files. This is done from the Command Prompt window using javac command. First, we have to navigate to the path where we have the files, in my case it will be on E:ProiecteJavaRMIsrc (see Figure 6). In case that you have created your files with NetBeans (which I recommend for the beginning to do like this), the files are stored in src folder.
Figure 6. The location of the files
Next, we will compile the files (use the same order as I give you) using javac command as it follows:
- javac IHello.java
- javac HelloImplementation.java
- javac HelloServerComponent.java
- javac HelloClientComponent.java
When you try to compile one of the file and if you receive the message from Figure 7, then you have two options: first, you can copy all the files in your Java folder (path in my case is C:Program FilesJavajdk1.8.0_40bin) or second, you can add it as an environment variable (see the annexes of the article to see how to proceed).
Figure 7. Command javac not found
Step 1 - After you have configured your javac command to the environment variables, let’s start to compile all the files. If everything is going OK, your command prompt should look like in Figure 8.
Figure 8. Compiling the files
Step 2 - Next, execute the rmic command for HelloImplementation (see Figure 9). Read the message after the command has been executed.
Figure 9. Running rmi command
Step 3 – Run rmiregistry command (see Figure 10). You will not receive any message, just a blinking pointer on the next line of your window and the title of the window is changed.
Figure 10. Running rmiregistry command