Running the Java Client and Server
==================================

We now have all of the pieces needed for a simple working RPC example. The instructions on this page assume you have already followed the directives for building and installing the HelloRPC service contract in Java as outlined on the :doc:`Compiling and Installing the Library page <javaBuild>` earlier in the tutorial. If you have not done this, please do so now before trying the steps below.

Java Client and Local Java Server
---------------------------------

To run the HelloRPC example with a Java client and Java Server do the following:

Step 1: Open a new terminal window

Step 2: Move to the HelloRPC directory within your local {{{product}}} git repository if you are not already there. If you use the default location, do the following: ``cd ~/{{{github_directory}}}/examples/helloRPC``.

Note: You must edit the prebuilt client file to point to the local helloRPC server instead of the helloRPC server in the Datawire cloud. To do this, do the following after Step 2:

Step 2.1: Move to the Java source directory (``cd src/main/java/helloRPC``)

Step 2.2: Open HelloRPCClient.java in the text editor of your choice

Step 2.3: Change the URL passed when instantiating the client object from ``http://hello.datawire.io`` to ``http://127.0.0.1:8910/hello``.

[[JMK the current expectation is that everyone will use the prebuilt Java client if they use Java at all as we are not telling them how to write their own yet. However, I am presenting the instructions this way in preparation for having a populated Java section of the walkthrough - this entire page is more-or-less forward looking.]]

Step 3: Run the following command to build the client and the server: ``mvn compile``

Step 4: Run the following command to start the Java server: ``mvn exec:java -Dexec.mainClass=helloRPC.HelloRPCServer``

Step 5: Open a new terminal window

Step 6: Move to the HelloRPC directory within your local {{{product}}} git repository if you are not already there. If you use the default location, do the following: ``cd ~/{{{github_directory}}}/examples/helloRPC``.

Step 7: Run the following command to start the Java client: ``mvn exec:java -Dexec.mainClass=helloRPC.HelloRPCClient``

After some log lines outlining the processing, you should see the following in the terminal window (stdout):

.. code-block:: none

   Request says: Hello from Java!
   Response says: Responding to [Hello from Java!] from Java

The content of the second line is the server regurgitating what the client sent it then identifying its own language. In this case, the Java client sent "Hello from Java!" to the Java server so both language identifiers are Java.

That's it! You should now understand how to define clients and servers to use the basic RPC interaction pattern defined in {{{product}}} and have seen those clients and servers in action in Java. 

Using the Java Client Against the Cloud
---------------------------------------

One of the first things we did in this walkthrough was use the example Python client file against the Datawire cloud and see it respond to the client requests. You can do exactly the same thing with the Java client you wrote by changing the server URL supplied when instantiating the client object. Just swap out http://hello.datawire.io for http://127.0.0.1:8910/hello and run the client again. This time it should tell you that the response is coming from the Datawire cloud.

[[JMK As noted above, the current expectation is that everyone will use the prebuilt Java client if they use Java (at all as we are not telling them how to write their own yet) and this section is not applicable to that case. However, I am including this section to keep the Java page parallel to the Python page and in preparation for having a populated Java section of the walkthrough - this entire page is more-or-less forward looking anyway.]]

Next Steps
----------

You can see the clients and servers running in multiple languages :doc:`here <try>` - you may want to revert any edits you made to run your client against the server in the Datawire cloud first - or just experiment some more on your own with Java!