ch03.htm

corba比较入门级的介绍详细间接了corba访问发布各种细节。

	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">TRANSIENT</TD>
		<TD ALIGN="LEFT">Transient failure--re-issue request.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">FREE_MEM</TD>
		<TD ALIGN="LEFT">Cannot free memory.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">INV_IDENT</TD>
		<TD ALIGN="LEFT">Invalid identifier syntax.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">INV_FLAG</TD>
		<TD ALIGN="LEFT">Invalid flag was specified.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">INTF_REPOS</TD>
		<TD ALIGN="LEFT">Error accessing interface repository.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">BAD_CONTEXT</TD>
		<TD ALIGN="LEFT">Error processing context object.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">OBJ_ADAPTER</TD>
		<TD ALIGN="LEFT">Failure detected by object adapter.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">DATA_CONVERSION</TD>
		<TD ALIGN="LEFT">Data conversion error.</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">OBJECT_NOT_EXIST</TD>
		<TD ALIGN="LEFT">Nonexistent object--delete reference.</TD>
	</TR>
</TABLE>

<H2><A NAME="Heading32"></A><FONT COLOR="#000077">The any Type</FONT></H2>
<P>For those occasional methods that need to accept any sort of CORBA object as a
parameter or take a parameter that could potentially be one of several unrelated
data types (in other words, none of the data types are inherited from any of the
others), IDL provides the any type. When any is used as the type of a parameter or
return value, that value can literally be any IDL type. A method that accepts an
any as an input parameter will usually need to determine precisely what type of object
is being passed before it can manipulate the object; you will see how this is done
when you begin implementing CORBA clients and servers.


<BLOCKQUOTE>
	<P>
<HR>
<B>Note:</B>For a method that must accept one of various types of CORBA objects as
	a parameter, the any is not always the only choice. If the type of the parameter
	can be one of only a few types, and the type of the parameter is known, then the
	union type might be a better choice. However, with the union type, all of the types
	must be known in advance; this is not true for an any type. An any parameter can
	literally hold any type of object, even one that is unknown to the server receiving
	the any parameter. 
<HR>


</BLOCKQUOTE>

<P>For an example of the any type, see Listing 3.12. In this example, the browseObject()
method accepts a single parameter, called object, of type any. A client calling this
method can use an object of any IDL type to fill this parameter. Internally to browseObject(),
the method will attempt to determine what the actual type of object is. If it determines
that object is a type of object it can interact with, it will do so. Otherwise, it
will raise the UnknownObjectType exception, which is returned to the caller.
<H4><FONT COLOR="#000077">Listing 3.12. any example.</FONT></H4>
<PRE><FONT COLOR="#0066FF">1: interface ObjectBrowser {
2:     exception UnknownObjectType {
3:         string message;
4:     };
5:     void browseObject(in any object) raises (UnknownObjectType);
6: };</FONT></PRE>
<H2><A NAME="Heading33"></A><FONT COLOR="#000077">The TypeCode Pseudotype</FONT></H2>
<P>Along with the any type comes the TypeCode pseudotype. TypeCode is not actually
an IDL type; instead, it provides type information to a CORBA application. In most
method calls, the types of the parameters passed to the method are known because
they are specified by the IDL method signatures. However, when a method accepts an
any type as an argument, the actual type of that object is unknown. This is where
TypeCode comes in. Because every CORBA type--both standard types such as long and
string and user-defined types such as Television--has a unique TypeCode associated
with it, a method implementation can determine which type of object was sent through
its any parameter. When the object's type is determined, the method can act on that
object. Think of TypeCodes as a sort of runtime-type information for CORBA applications.
<H2><A NAME="Heading34"></A><FONT COLOR="#000077">Summary</FONT></H2>
<P>This chapter presented the basic data types provided by CORBA's Interface Definition
Language (IDL), including integer and floating point numeric types, Boolean values,
and characters and character strings. Because many of these data types closely resemble
data types of programming languages like C, C++, or Java, readers who are already
familiar with one of these languages will have little difficulty assimilating the
IDL types.</P>
<P>You have expanded your knowledge of IDL to now include higher-level data types--particularly
the user-defined types--and their uses. Most importantly, you are familiar with the
interface construct and how it defines the behavior of CORBA objects. Because the
interface is one of the fundamental IDL data types, a clear understanding of this
construct is essential to the design of CORBA applications. Indeed, you cannot design
or implement a CORBA application without interfaces.</P>
<P>Today you have seen some very useful IDL data types and constructs, in particular,
the sequence and array types for storing multiple values of similar types. You learned
about the exceptions in IDL, including the predefined CORBA standard exceptions.
Finally, you looked at the any type, which can contain a value of any IDL type, and
its counterpart, the TypeCode pseudotype, for determining unknown data types passed
to a method. These constructs--particularly the sequences and exceptions--are essential
for building robust CORBA applications.</P>
<P>You've covered just about everything there is to know about IDL; now it's time
to apply it. In the next chapter--Day 4, &quot;Building a CORBA Application&quot;--you'll
do just that, translating IDL definitions into working CORBA server and client applications.
<H2><A NAME="Heading35"></A><FONT COLOR="#000077">Q&amp;A</FONT></H2>

<DL>
	<DD><B>Q What's the point of having both</B> sequence <B>and array types?</B><BR>
	<B><BR>
	A</B> Array types are useful when the number of elements in an array is fixed and
	known in advance. In many instances, though, this is not the case; arrays often vary
	in size as members are dynamically added and removed. Consequently, a dynamically
	sizable array, such as the IDL sequence, is often much more convenient. For maximum
	flexibility, IDL offers both.<BR>
	<B><BR>
	Q What are all those</B> exception<B>s</B> <B>and when (or how) are they raised?</B><BR>
	<B><BR>
	A</B> First of all, nondistributed methods have little need for many of the CORBA
	standard exceptions, but they make a lot more sense in a distributed environment.
	As mentioned previously, the standard exceptions are generally not raised by any
	code that you will write; rather, the exceptions in this set are raised automatically
	by the ORB when an error condition occurs. Remember that all IDL methods implicitly
	raise these exceptions, even the accessor/mutator methods generated by the IDL compiler
	for each attribute.<BR>
	<B><BR>
	Q Why isn't (insert your favorite primitive data type here) supported?</B><BR>
	<B><BR>
	A </B>Because one primary goal of IDL is to be language-neutral, it isn't practical
	to support all primitive data types contained in all languages. Rather than attempt
	to provide this level of support (nearly impossible to achieve), IDL provides the
	most common and useful primitive data types.<BR>
	<B><BR>
	Q How does an object modify one of its </B>attribute<B>s when that </B>attribute<B>
	is declared to be </B>readonly<B>?</B><BR>
	<B><BR>
	A</B> If you're asking questions like this, you're definitely thinking ahead. When
	you learn about implementation of IDL interfaces on Day 4, it will become clear how
	this is accomplished. For the time being, though, remember that the object implementing
	an interface has full access to its own state. A readonly attribute is mapped to
	a method of that object, and that method can return any value the object wants. Typically,
	the implementing object will define actual attributes of its own that correspond
	to attributes in the IDL interface, although this isn't strictly necessary.<BR>
	<B><BR>
	Q Because Java does not support multiple inheritance, how is multiple inheritance
	of IDL interfaces achieved in Java?<BR>
	<BR>
	A</B> You'll see when you begin implementing IDL interfaces in Java that the IDL
	language mapping for Java maps IDL interfaces to Java interfaces. Although multiple
	inheritance of classes is not supported in Java, multiple inheritance of interfaces
	is.
</DL>

<H2><A NAME="Heading36"></A><FONT COLOR="#000077">Workshop</FONT></H2>
<P>The following section will help you test your comprehension of the material presented
in this chapter and put what you've learned into practice. You'll find the answers
to the quiz in Appendix A.
<H3><A NAME="Heading37"></A><FONT COLOR="#000077">Quiz</FONT></H3>

<DL>
	<DD><B>1</B>. Define a type (using typedef) called temperatureSequence that is a
	sequence of sequences of floats (yes, this is legal).<BR>
	<B><BR>
	2</B>. Why might a type as described in the preceding question be useful?<BR>
	<B><BR>
	3</B>. Why are exceptions useful?<BR>
	<B><BR>
	4</B>. Why is the module construct useful?<BR>
	<B><BR>
	5</B>. Name some practical uses for the octet data type.<BR>
	<B><BR>
	6</B>. Define an enumerated type containing the months of the year.<BR>
	<B><BR>
	7</B>. Why might a nonblocking remote method call be advantageous, compared to a
	blocking method call?<BR>
	<B><BR>
	8</B>. Imagine a compound data type with a large number of data members. This data
	type will frequently be used by a client application, which will generally need to
	access each of the data members. Would it be more efficient to encapsulate this data
	type into a struct or an interface? Why?<BR>
	<B><BR>
	9</B>. Because an IDL method can return a value, what is the purpose of out and inout
	parameter types?<BR>
	<B><BR>
	10</B>. Why is a oneway method unable to return any value to the caller? Can you
	think of a mechanism, using oneway calls, to return a result to the caller?
</DL>

<H3><A NAME="Heading38"></A><FONT COLOR="#000077">Exercises</FONT></H3>

<DL>
	<DD><B>1</B>. Consider the following classes: Conduit, Faucet, FuseBox, Outlet, Pipe,
	WaterHeater, WaterPump, and Wire. How would you partition these classes? What relationships,
	if any, are there between the partitions you have created?<BR>
	<B><BR>
	2</B>. Create an interface that describes a clock/radio (which can set the hours,
	set the minutes, set the alarm time, and so on).<FONT COLOR="#000077"></FONT>
</DL>

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