We have now seen enough of the server-side Java mapping to implement a server for the file system we developed in
Chapter 5. (You may find it useful to review the Slice definition for our file system in
Section 5.4 before studying the source code.)
Our server main program, in the file Server.java, uses the
Ice.Application class we discussed in
Section 12.3.1. The
run method installs a shutdown hook, creates an object adapter, instantiates a few servants for the directories and files in the file system, and then activates the adapter. This leads to a
main program as follows:
import Filesystem.*;
public class Server extends Ice.Application {
public int
run(String[] args)
{
// Create an object adapter (stored in the _adapter
// static members)
//
Ice.ObjectAdapter adapter
= communicator().createObjectAdapterWithEndpoints(
"SimpleFilesystem", "default ‑p 10000");
DirectoryI._adapter = adapter;
FileI._adapter = adapter;
// Create the root directory (with name "/" and no parent)
//
DirectoryI root = new DirectoryI("/", null);
// Create a file "README" in the root directory
//
File file = new FileI("README", root);
String[] text;
text = new String[] {
"This file system contains a collection of poetry."
};
try {
file.write(text, null);
} catch (GenericError e) {
System.err.println(e.reason);
}
// Create a directory "Coleridge" in the root directory
//
DirectoryI coleridge
= new DirectoryI("Coleridge", root);
// Create a file "Kubla_Khan" in the Coleridge directory
//
file = new FileI("Kubla_Khan", coleridge);
text = new String[]{ "In Xanadu did Kubla Khan",
"A stately pleasure‑dome decree:",
"Where Alph, the sacred river, ran",
"Through caverns measureless to man",
"Down to a sunless sea." };
try {
file.write(text, null);
} catch (GenericError e) {
System.err.println(e.reason);
}
// All objects are created, allow client requests now
//
adapter.activate();
// Wait until we are done
//
communicator().waitForShutdown();
return 0;
}
public static void
main(String[] args)
{
Server app = new Server();
System.exit(app.main("Server", args));
}
}
The code imports the contents of the Filesystem package. This avoids having to continuously use fully-qualified identifiers with a
Filesystem. prefix.
The next part of the source code is the definition of the Server class, which derives from
Ice.Application and contains the main application logic in its
run method. Much of this code is boiler plate that we saw previously: we create an object adapter, and, towards the end, activate the object adapter and call
waitForShutdown.
As we will see shortly, the servants for our directories and files are of type DirectoryI and
FileI, respectively. The constructor for either type of servant accepts two parameters, the name of the directory or file to be created and a reference to the servant for the parent directory. (For the root directory, which has no parent, we pass a null parent.) Thus, the statement
// Create the root directory (with name "/" and no parent)
//
DirectoryI root = new DirectoryI("/", null);
// Create a file "README" in the root directory
//
File file = new FileI("README", root);
String[] text;
text = new String[] {
"This file system contains a collection of poetry."
};
try {
file.write(text, null);
} catch (GenericError e) {
System.err.println(e.reason);
}
// Create a directory "Coleridge" in the root directory
//
DirectoryI coleridge
= new DirectoryI("Coleridge", root);
// Create a file "Kubla_Khan" in the Coleridge directory
//
file = new FileI("Kubla_Khan", coleridge);
text = new String[]{ "In Xanadu did Kubla Khan",
"A stately pleasure‑dome decree:",
"Where Alph, the sacred river, ran",
"Through caverns measureless to man",
"Down to a sunless sea." };
try {
file.write(text, null);
} catch (GenericError e) {
System.err.println(e.reason);
}
We first create the root directory and a file README within the root directory. (Note that we pass a reference to the root directory as the parent when we create the new node of type
FileI.)
String[] text;
text = new String[] {
"This file system contains a collection of poetry."
};
try {
file.write(text, null);
} catch (GenericError e) {
System.err.println(e.reason);
}
Recall from Section 10.7.3 that Slice sequences by default map to Java arrays. The Slice type
Lines is simply an array of strings; we add a line of text to our
README file by initializing the
text array to contain one element.
Finally, we call the Slice write operation on our
FileI servant by simply writing:
This statement is interesting: the server code invokes an operation on one of its own servants. Because the call happens via a reference to the servant (of type
FileI) and
not via a proxy (of type
FilePrx), the Ice run time does not know that this call is even taking place—such a direct call into a servant is not mediated by the Ice run time in any way and is dispatched as an ordinary Java function call.
In similar fashion, the remainder of the code creates a subdirectory called Coleridge and, within that directory, a file called
Kubla_Khan to complete the structure in
Figure 13.1.
13.2.2 The FileI Servant Class
Our FileI servant class has the following basic structure:
public class FileI extends _FileDisp
{
// Constructor and operations here...
public static Ice.ObjectAdapter _adapter;
private String _name;
private DirectoryI _parent;
private String[] _lines;
}
The _name and
_parent data members are initialized by the constructor:
public
FileI(String name, DirectoryI parent)
{
_name = name;
_parent = parent;
assert(_parent != null);
// Create an identity
//
Ice.Identity myID = new Ice.Identity();
myID.name = Ice.Util.generateUUID();
// Add the identity to the object adapter
//
_adapter.add(this, myID);
// Create a proxy for the new node and
// add it as a child to the parent
//
NodePrx thisNode
= NodePrxHelper.uncheckedCast(_adapter.createProxy(myID));
_parent.addChild(thisNode);
}
After initializing the _name and
_parent members, the code verifies that the reference to the parent is not null because every file must have a parent directory. The constructor then generates an identity for the file by calling
Ice.Util.generateUUID and adds itself to the servant map by calling
ObjectAdapter.add. Finally, the constructor creates a proxy for this file and calls the
addChild method on its parent directory.
addChild is a helper function that a child directory or file calls to add itself to the list of descendant nodes of its parent directory. We will see the implementation of this function on
page 417.
The remaining methods of the FileI class implement the Slice operations we defined in the
Node and
File Slice interfaces:
// Slice Node::name() operation
public String
name(Ice.Current current)
{
return _name;
}
// Slice File::read() operation
public String[]
read(Ice.Current current)
{
return _lines;
}
// Slice File::write() operation
public void
write(String[] text, Ice.Current current)
throws GenericError
{
_lines = text;
}
The name method is inherited from the generated
Node interface (which is a base interface of the
_FileDisp class from which
FileI is derived). It simply returns the value of the
_name member.
The read and
write methods are inherited from the generated
File interface (which is a base interface of the
_FileDisp class from which
FileI is derived) and simply return and set the
_lines member.
The DirectoryI class has the following basic structure:
package Filesystem;
public final class DirectoryI extends _DirectoryDisp
{
// Constructor and operations here...
public static Ice.ObjectAdapter _adapter;
private String _name;
private DirectoryI _parent;
private java.util.ArrayList _contents
= new java.util.ArrayList();
}
As for the FileI class, we have data members to store the object adapter, the name, and the parent directory. (For the root directory, the
_parent member holds a null reference.) In addition, we have a
_contents data member that stores the list of child directories. These data members are initialized by the constructor:
public
DirectoryI(String name, DirectoryI parent)
{
_name = name;
_parent = parent;
// Create an identity. The parent has the
// fixed identity "RootDir"
//
Ice.Identity myID = new Ice.Identity();
myID.name = _parent != null ? Ice.Util.generateUUID() : "RootDir";
// Add the identity to the object adapter
//
_adapter.add(this, myID);
// Create a proxy for the new node and add it as a
// child to the parent
//
NodePrx thisNode
= NodePrxHelper.uncheckedCast(_adapter.createProxy(myID));
if (_parent != null)
_parent.addChild(thisNode);
}
The constructor creates an identity for the new directory by calling Ice.Util.generateUUID. (For the root directory, we use the fixed identity
"RootDir".) The servant adds itself to the servant map by calling
ObjectAdapter.add and then creates a reference to itself and passes it to the
addChild helper function.
addChild simply adds the passed reference to the
_contents list:
void
addChild(NodePrx child)
{
_contents.add(child);
}
public String
name(Ice.Current current)
{
return _name;
}
// Slice Directory::list() operation
public NodePrx[]
list(Ice.Current current)
{
NodePrx[] result = new NodePrx[_contents.size()];
_contents.toArray(result);
return result;
}
Note that the _contents member is of type
java.util.ArrayList, which is convenient for the implementation of the
addChild method. However, this requires us to convert the list into a Java array in order to return it from the
list operation.
.
Figure 13.1. A small file system.As we will see shortly, the servants for our directories and files are of type DirectoryI and FileI, respectively. The constructor for either type of servant accepts two parameters, the name of the directory or file to be created and a reference to the servant for the parent directory. (For the root directory, which has no parent, we pass a null parent.) Thus, the statementcreates the root directory, with the name "/" and no parent directory.// Create the root directory (with name "/" and no parent)