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Justin Gehtland; Bruce A. Tate

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6.2 Tools for Extension



When I worked at a startup


called AllMyStuff, we researched
reporting tools. We wanted a tool that our users could drop into our
framework and report on data that we gathered. We wanted to be able
to work the reports into our user interface and we wanted the tool to
use our existing data structures. Sales reps from various companies
all said, "No problem. Our framework is completely
extensible." As you might expect, that
wasn''t the case.


Of course, we wanted to extend the reporting tools in a variety of
ways. Some of the extensions were minor. We needed the product to
support our customers'' databases. Most supported
JDBC and all supported major database vendors. In other cases, we
wanted to extend the reporting packages in ways that the original
designers had not intended. We found that supporting a Java API was
not nearly enough. We needed the user interface to live with ours
without source code changes. We needed to integrate the security of
the reporting package to the security of the application. We wanted
our customers to be able to access extensions through configuration
rather than coding changes.


When you''re planning to build an extensible
framework in the Java environment, you''ve got an
incredibly broad selection of tools to choose from. The continuum of
techniques ranges from requiring massive, invasive change to nothing
more than configuration changes. In Figure 6-3, I
identify four types of extension based on the level of effort it
takes to extend the framework. The hardestand most
usefultype of extension to provide requires no code changes or
installation. The application automatically recognizes the need to
change and does the work. Norton Antivirus auto-update provides this
type of support. The next-most stringent model requires only
scripting or configuration. All other support is included or
retrieved automatically. The next option, the plug-in, requires the
user to provide and configure a compatible module.
That''s a very useful and common design for
enterprise programming, and the one that will get most of our focus
in this chapter. Finally, other modes of extension require coding
changes. I won''t spend as much time with these. The
tools that you use to provide each type of support overlap but are
different.





Figure 6-3. Different models of extension place different burdens on the user



6.2.1 Standards



Your first tool is a clear understanding


of the key standards that
improve extension. Using these and exposing relevant configuration
can give you an important head start. This is the paradox: by
choosing standards, you will limit your own choices, but
you''ll dramatically improve the choices that your
customers can make after deployment time. The most critical standards
for you are external touch points, like user interfaces, databases,
transactions, or communication. Table 6-1 shows
some important standards that you may choose to support. If you need
a service in one of these areas, consider the corresponding standard.




Table 6-1. Java standards


Acronym




Meaning




Purpose




JAAS




Java Authentication and Authorization Service




Security of applications




JCA




Java Cryptography Architecture




Encryption of Java data




XML




eXtensible Markup Language




Structuring and representing data




JMS




Java Messaging Service




Messaging




JTA




Java Transaction API




Transactions




JCA




J2EE Connection Architecture




Connections management




JNDI




Java Naming and Directory Interface




Naming and registration of core Java services and components




JDBC




Java DataBase Connectivity




Relational database API




JDO




Java Data Objects




Transparent persistence for Java




RMI




Remote Method Invocation




Remote procedure calls in Java




IIOP




Internet Inter-Orb Protocol




Java-to-CORBA connectivity




Servlet




Server-side component, invoked via HTTP




JSP




Java Server Pages




A markup language for user interfaces that accepts dynamic content
and compiles to a servlet



Be careful: remember, you are what you eat. If you choose EJB CMP
because it''s the most pervasive standard for
persistence, you''re completely missing the point of
this book. Choose an implementation that works for you and works
well.



6.2.2 Configuration



Configuration has long been the



bane of Java developers everywhere. Most
developers focus on reuse strategies that involve coding changes,
such as introducing an API. If you''re like most
developers, you probably save configuration issues for the end of a
projectbut if you don''t want to force
developers to handle all possible extensions, you must address
configuration early. Many developers choose to write configuration
libraries themselves because so many Java libraries and tools have
been so poor for so long. Configuration problems have plagued J2EE as
well. In the recent past, EJB, JNDI, and J2EE security have all had
vastly different configuration strategies, formats, and models.
Recent Java developers have faced these problems:



Inconsistent strategies




Java frameworks and applications had no consistent API for
configuration. On the server side, that''s still
true. The Preferences API is strongly oriented toward client-side
configuration.




Inconsistent formats




Java frameworks have had no consistent format for configuration. The
Java properties file used simple name-value pairs and other
frameworks used ad hoc XML configuration strategies.




Inadequate tools




The default configuration tool, the Java properties API, did not meet
the most basic needs of configuration. (For example, it
doesn''t support multipart properties.) JNDI, on the
other hand, is much too heavyweight for many applications.





Recently, developers have learned more, and other options have
surfaced that allow much better control. Although
they''re not perfect, there''s a much
broader set of choices. You can choose from several strategies. There
are too many solutions to count, but at least three look promising:











Java Preferences API for client-side configuration




The Java toolkit provides a good



library for client side
configuration, called the Preferences API. It
doesn''t fully support the most common server-side
configuration format, XML, but it does provide enough power and
flexibility for client-side applications.




Apache Digester subproject for server-side configuration




Apache projects broadly use a


configuration tool called Digester,
which takes XML configuration files and translates them to
lightweight objects for configuration and also provides many services
to assist configuration. This tool is probably the preferred way to
deal with generic server-side XML configuration files. J2EE still
does not have a cohesive strategy or API for configuration, so I
choose a reliable open source alternative.




Spring for framework-driven configuration




Often, you want all



configuration
driven by a central framework. At its core, the Spring framework
provides a set of application configuration and assembly tools.
It''s a unified approach that''s
freely available to components across all layers of the application.
We''ll talk more about Spring configuration in
Chapter 8 and Chapter 10.





Study every solution with the same diligence that
you''d use for choosing any other major framework.
Without effective configuration, your only option for extension is
programming intervention. In this chapter, I look briefly at two
possible solutions: the Java Preferences API and to a lesser extent,
Apache Digester.



6.2.2.1 Client-side configuration with Java Preferences



The standard Java API for configuration


is the Java Preferences
API. As you''ll see, it''s designed
primarily for client-side use, but lightweight server-side
applications may make some use of it as well. It''s
designed independently of the backend data store. It lets you store
system level properties and properties for individual
usersthus, then name Preferences.


Preferences databases are stored in two different trees: one for the
user and one for the system. Your primary window into a preferences
data store is the node. You could decide to read the top node out of
either preferences store, the system, or the user, but if you did so,
different applications would potentially step on each other. The
customary solution is to group preferences together under a package,
as in Figure 6-4. You can get the top-level node
for the user tree, for any given package, like this:


Preferences node = Preferences.userNodeForPackage(getClass( ));





Figure 6-4. The Preference API supports two different trees, for the user and the system


Then you can use the Preference API to load and save preferences. You
can always create additional tree nodes to organize things further
but for simple applications, just store preference properties
directly at the package level. For example, to get the
"BackgroundColor" preference to
"White":


node.putString("BackgroundColor", "White");


Preferences are simply name-value pairs. The first parameter is the
key and the second is the value of the preference. You can use
Strings, other primitive types, or byte arrays. The Preference API
does not support complex objects. You''ll read a
preference in much the same way. As you''d expect,
you look up a preference by its key value. For example, to read the
default high score for a game:


node.getInt("HighScore", 0);


Here, the first parameter is once again the key but the second is a
default, just in case the backend data store is not available. By
default, the data is stored in different places on different
operating systems. The Windows system (Windows 2000 and beyond) uses
the registry. The Linux version uses a file.


You can also import or export a preference file as XML with
exportSubtree and exportNode.
Exporting a subtree includes children; exporting a node only exports
a single level. For example, to dump the current
user''s node for a package:


Preferences prefs = Preferences.userNodeForPackage(getClass( ));
FileOutputStream outputStream = new FileOutputStream("preferences.xml");
prefs.exportSubtree(outputStream);


I haven''t shown the whole API, but you have enough
to get started. You may be wondering why the preferences may be
appropriate for the client, but less so for the server. Here are the
most important reasons:



For the Windows operating system, the Preferences API default storage
is to the registry. That''s not the most appropriate
place for all server-side configuration because administrators need
to carefully guard the registry from corruption.



The Preferences API specifies two trees, system
and user. That''s not the best
possible organization for server-side configuration, which must also
support other concepts like clusters, and user trees are less
important.



While the Preferences API supports XML import and export, there are
more direct and efficient ways to deal with XML.




If you want to use a configuration service for a more sophisticated
server-side application, I recommend that you look beyond the
properties and Preferences APIs. In the next section,
you''ll see a high-level overview of the solution
used by many Apache projects.



6.2.2.2 Server-side configuration with Apache Digester



The Apache project
has


a set of common Java utilities
grouped into the Commons project. One of the tools,
Digester, pareses XML files, helps map them onto
objects, and fires rules based on patterns. It''s
helpful any time you need to parse an XML tree and especially useful
for processing configuration files. Figure 6-5
shows how it works. You start with a configuration file, then add
patterns and associated rules to Digester. When Digester matches a
pattern, it fires all associated rules. You can use the prepackaged
rules provided by Digester, or you can write your own.





Figure 6-5. Apache''s digester makes it easy to parse configuration files


For the most part, you''re going to want Digester to
create objects that correspond to your configuration tree. You also
might want it to do special things to your objects when it encounters
them, like open a connection or register a data source.
Here''s how to process a configuration file with
Digester:



Create a configuration file. Your configuration file is a simple
hierarchical XML file. For example, this application requires an
output file for a log:


<config>
<logFile>
<fileName>myfile.txt</fileName>
<path>c:\logfiles\</path>
</logFile>
</config>



Create any objects that map to the configuration file In this case,
the logFile XML node maps onto a class called
LogFileConfig. It looks like this:


public class LogFileConfig 
private String fileName;
private String path;
public String getFileName( ) {
return fileName;
}
public void setFileName(String name) {
fileName=name;
}
public String getPath( ) {
return path;
}
public void setPath(String name) {
path=name;
}
}


Notice that you need to use the JavaBeans specification because
Digester uses the JavaBeans API to populate the new classes the
configuration creates. This structure mirrors the structure of the
XML tree.



Create your Digester object. Creating and configuring Digester varies
based on the application you''re configuring. In our
case, it just means configuring a new Digester. To simplify things,
I''ll also set validation to false.


class ConfigManager {
public void configureIt( ) {
Digester digester = new Digester( );
digester.setValidating(false);



Add your patterns and rules. A rule is an action
that you want performed when the parser identifies a pattern. We want
the configuration engine to create a LogFileConfig
object and set the properties according to the values in the XML
input file. Apache has prepackaged rules to create an object and set
the properties. The rules look like this:


    digester.addObjectCreate( "config/logFile", LogFileConfig.class );
digester.addBeanPropertySetter( "config/logFile/path", "path" );
digester.addBeanPropertySetter( "config/logFile/fileName", "fileName" );


The rule to create a new object is first. The method name has two
parts: add means we''re adding a
new rule to Digester and ObjectCreate is the
action the rule performs. The first parameter is the pattern and the
second refers to the class for the new object. In other words, when
the parser encounters an XML <logFile>
underneath <config>, it fires the rule to
create an object of class LogFileConfig.
Similarly, the second and third rules set the properties from the
values specified in the XML file.



Run it. All that remains is to run the configuration file:



myLogConfig = digester.parse( );


I like the Digester framework because it''s simple
and lets you create a transparent model for your
configurationyou can separate the configuration from the
configuration implementation. That means you''re not
lobbing DOM trees all over your application.



6.2.3 Class Loading



Configuration lets administrators



rather than programmers specify what an
application should look like. Often, you''ll want to
allow the administrators to configure classes that you might not have
anticipated when you build your application, or even classes that
don''t exist yet. In fact, that''s
the whole pointgiving the users the ability to extend a system
in ways you might not foresee. It''s called
delayed binding.


The problem with delayed binding is this: if you
don''t know the name of the class, you
can''t create it with a constructor.
You''ll need to load the class yourself or use a tool
such as Spring or Digester that will do it for you. Fortunately,
class loading and reflection give you enough horsepower to get the
job done.


Many developers never
use dynamic class
loading
, one of the Java language''s most
powerful capabilities. It''s deceptively simple. You
just load a class and use it to instantiate any objects that you
need. You''re free to directly call methods that you
know at compile time through superclasses or interfaces, or methods
that you don''t know until runtime through
reflection.



6.2.3.1 Loading a class with Class.forName



Sometimes, loading

a class is easy. Java provides a simple
method on Class called ForName(String
className)
that loads a class. Calling it invokes a class
loader to load your named class. (Yes, there''s more
than one class loader. I''ll get into that later.)
Then, you can instantiate it using newInstance( ).
For example, to create a new instance of a class called
Dog, use the following lines of code:


Class cls = Class.forName("Dog");



6.2.3.2 Invoking methods



Now you''ve loaded a class; the


next step is to use
it. You''ve got several choices. First, you might
invoke static methods on the class, like main. You
don''t need a class instance to do that. Invoking the
main method looks like this:


myArgs = ...
Method mainMethod = findMain(cls);
mainMethod.invoke(null, myArgs);


Your next option is to create an instance and then call direct
methods of known superclasses or interfaces. For example, say you
know that you''ve loaded an Animal
class or a class that supports the Animal
interface. Further, Animal supports a method
called speak, which takes no arguments. You can
cast your new instance to Animal as you
instantiate it. Understand that your class must have a default
constructor that initializes your object appropriately, because you
can''t pass constructor arguments this way.
Here''s how you would create an instance of
Animal and access its properties:


Animal dog = (Animal)cls.newInstance( );
dog.setName("Rover");
dog.speak( );


When I''m doing configuration of a service, I like to
use the interface approach. An interface best captures the idea of a
service. Other times, you''re instantiating a refined
concept, like a servlet (Tomcat) or component (EJB). For these
purposes, subclasses work fine. Many Java services use a simple class
loading and an interface or abstract class:



The Tomcat servlet engine loads a subclass of a generic servlet.



JDBC applications may know the name of a driver in advance. Still,
they often load a class and invoke static methods on the class object
in order to preserve portability.



EJB containers load a class supporting the interface
EJBObject.




Your next option is to use reflection to access fields or call
methods, as in Chapter 4. Using this technique,
you''ll need to know the name and signature of the
method that you want to call. Remember, Java allows overloading
(methods that have the same name but different signatures).


For the most abstract configuration possible, you may well need
reflection to invoke methods or access properties. For example,
Hibernate and Spring both use configuration and reflection to access
persistent properties. The Digester framework also uses reflection
within many of its rules. All of these techniques depend on dynamic
class loading to do the job. It''s time to dive into
some more details.



6.2.3.3 Which class loader?



Java 2 recently added a lot of flexibility to

the class loader. The changes also
added some uncertainty. Here''s how it works:



Every class has its own class loader.



All loaders are organized in a tree.



The class loader uses a delegating parent model
that works like this:



If a class loader hasn''t seen a class, it delegates
the task of loading the class to its parent before trying to load
itself. That class in turn delegates to its parent and work its way
up the chain.



If the class loader has not loaded the class at the top of the tree,
it tries to load the class. If it fails, it returns control back down
the chain.



A class remains associated with the loader that succeeded in loading
it. You can see the associated class loader with a
Class.getClassLoader( ).




That sounds easy enough. The problem is that Java 2 supports more
than one class loader, as shown in Figure 6-6.
Three different hierarchies load three major types of files. The
system loader loads most applications and uses the class path. The
extension loader loads all extensions, and the bootstrap loader loads
core Java classes in rt.jar.





Figure 6-6. The Java 2 class loader has three major loaders


The Java runtime obviously needs special treatment. Since the Java
runtime is not yet alive when it starts, Sun provides a basic
bootstrap loader to load the Java runtime. It does not load any
application logic at all. Only the most basic Java
classesthose in rt.jarare loaded
by the Bootstrap class loader. It doesn''t have a
parent.


Many users complained that class paths were getting too long as the
number of Java extensions grew. To simplify Java extensions, Sun
provided an environment variable that pointed to a set of directories
(specified in the environment variable
java.ext.dirs) to hold all Java extensions. That
way, when you want to add a service, like JNDI, you can just drop
your .jar into Java''s
extensions directory.


The system loader is often the loader that your applications will
use. If you don''t specify a loader and you are not a
Java extension, you''re likely going to get the
system loader as your ultimate parent.


The most important thing to keep in mind is that your classes may not
have the same parent. If you''re loading an
extension, it may well use a different loader and check a different
class path. If you''re loading extensions from an
application, for example, your class loader will have a different
ancestor chain. You may have also run across similar problems when
using applications like Tomcat or Ant, which rely on class paths and
environment variables other than the CLASSPATH environment variable.
You don''t have to rely on luck. If you want to be
sure that the new class will use the same class loader chain as your
current thread, specify a class loader when you use
Class.forName. You can get the context class
loader for your current thread like this:


cl=Thread.getContextClassLoader( );


You can even set the current thread''s context class
loader to any class loader you choose. That means all participants in
the thread can use the same class loader to load resources. By
default, you get the app class loader instance.


The short version is that Class.forName(aName)
usually works. If it doesn''t, do a little research
and understand which class loader you''re using. You
can get more details about class loading in an excellent free paper
by Ted Neward at http://www.javageeks.com/Papers/ClassForName/ClassForName.pdf.
Alternatively, some of the resources in the Bibiliography chapter of
this book (Halloway, for instance) are outstanding resources.



6.2.4 What Should You Configure?



After you''ve mastered the basics

for configuration and
class loading, consider what pieces of your applications need
configuration. Several important decisions will guide you.



6.2.4.1 Fundamental concepts



The first step in deciding what to configure is to understand the
fundamental concepts involved. You might just use them as an aid to
your planning or you might use them to help you organize your
configuration implementation. After you''ve decided
on the fundamental concepts of your system, begin to choose the ones
that you''d like to try to configure.


Chapter 3 emphasized the principle
"Do one thing and do it well." For
extensibility, the key is to keep orthogonal concepts
separatenot just in code but in configuration. If
you''re working from a good design and a good
configuration service, it will be easy to organize and expose the
most critical services. Keep the code for individual concepts
independent so you can consider each concept individually. For
example, if you''re considering your data layer, you
may decide to expose your data source, a database name, a schema
name, and your database authentication data. You may decide to
separate your transaction manager, since that may be useful to your
user base should they decide to let your application participate in
an existing transaction.


Keep the Inventor''s Paradox in mind. It will help
you separate important concepts in your configuration. You can often
generalize a configuration option. The Spring framework discussed in
Chapter 8 supports many examples of this.



6.2.4.2 External touch points



All applications access the outside world. You were probably taught
very early in your career to insulate your view from the rest of your
application. Often, you can get great mileage out of insulating other
aspects of your application, as well. For example, you may link to
proprietary applications that manage inventory, handle e-commerce, or
compute business rules. In any of these instances, it pays to be able
to open up the interface to an application. One of the most useful
configuration concepts is to allow your customer to tailor external
touch points. In order to do so, define the way you use your service.
You don''t always need to explicitly configure the
service. You may instead decide to configure a communications
mechanism for an XML message or wire into a standard such as a web
service. The important thing is to allow others to use your
application without requiring major revision. Whether you enable JMS
or some type of RPC, you''ll be able to easily change
the way your clients communicate with your system. Frameworks like
Spring handle this type of requirement very well.



6.2.4.3 External resources



Nearly all enterprise applications need resources.
It''s best to let your clients configure external
services whenever they might need to change them. Data sources
(especially those with connection pools), transaction monitors, log
files, and RPC code all need customization, and it''s
best to do so from a central configuration service.


Sometimes, configuring external touch points requires more than just
specifying and configuring a service. Occasionally, you must build a
custom architecture to plug in specialized services with special
requirements. These types of services require an architecture called
a plug-in.



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