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java.lang.Objectjavolution.lang.Configurable<T>
public class Configurable<T>
This class facilitates separation of concerns between the configuration logic and the application code.
Does your class need to know or has to assume that the configuration is coming from system properties ??
The response is obviously NO!
Let's compare the following examples:
With the following (using this class):
class Document {
private static final Font DEFAULT_FONT
= Font.decode(System.getProperty("DEFAULT_FONT") != null ? System.getProperty("DEFAULT_FONT") : "Arial-BOLD-18");
...
}
Not only the second example is cleaner, but the actual configuration
data can come from anywhere (even remotely). Low level code does not
need to know.
class Document {
public static final Configurable<Font> DEFAULT_FONT = new Configurable<Font>(new Font("Arial", Font.BOLD, 18));
...
}
Furthermore, with the second example the configurable data is
automatically documented in the JavaDoc (public). Still only instances
of Configurable.Logic
may set this data. There is no chance
for the user to modify the configuration by accident.
Configurable instances have the same textual representation as their
current values. For example:
public static final Configurable<String> AIRPORT_TABLE
= new Configurable<String>("Airports");
...
String sql = "SELECT * FROM " + AIRPORT_TABLE // AIRPORT_TABLE.get() is superfluous
+ " WHERE State = '" + state + "'";
Unlike system properties (or any static mapping), configuration
parameters may not be known until run-time or may change dynamically.
They may depend upon the current run-time platform,
the number of cpus, etc. Configuration parameters may also be retrieved
from external resources such as databases, XML files,
external servers, system properties, etc.
public abstract class FastComparator<T> implements Comparator<T>, Serializable {
public static final Configurable<Boolean> REHASH_SYSTEM_HASHCODE
= new Configurable<Boolean>(isPoorSystemHash()); // Test system hashcode.
...
public abstract class ConcurrentContext extends Context {
public static final Configurable<Integer> MAXIMUM_CONCURRENCY
= new Configurable<Integer>(Runtime.getRuntime().availableProcessors() - 1);
// No algorithm parallelization on single-processor machines.
...
public abstract class XMLInputFactory {
public static final Configurable<Class<? extends XMLInputFactory>> DEFAULT
= new Configurable<Class<? extends XMLInputFactory>>(XMLInputFactory.Default.class);
// Default class implementation is a private class.
...
Reconfiguration is allowed at run-time as configurable can be
notified
of changes in their
configuration values. Unlike system properties, configurable can be
used in applets or unsigned webstart applications.
Here is an example of configuration of a web application from
a property file:
This listener is registered in the
public class Configuration extends Configurable.Logic implements ServletContextListener {
public void contextInitialized(ServletContextEvent sce) {
try {
ServletContext ctx = sce.getServletContext();
// Loads properties.
Properties properties = new Properties();
properties.load(ctx.getResourceAsStream("WEB-INF/config/configuration.properties"));
// Reads properties superceeding default values.
Configurable.read(properties);
} catch (Exception ex) {
LogContext.error(ex);
}
}
}
web.xml
file:
The property file contains the full names of the configurables static
fields and the textual representation of their new values:
<web-app>
<listener>
<listener-class>mypackage.Configuration</listener-class>
</listener>
</web-app>
# File configuration.properties
javolution.util.FastComparator#REHASH_SYSTEM_HASHCODE = true
javolution.context.ConcurrentContext#MAXIMUM_CONCURRENCY = 0
javolution.xml.stream.XMLInputFactory#DEFAULT = com.foo.bar.XMLInputFactoryImpl
Configuration settings are global (affect all threads). For thread-local
environment settings LocalContext.Reference
instances are recommended.
Note: Any type for which a text format is
known
can be configured from
String
properties.
Nested Class Summary | |
---|---|
static class |
Configurable.Logic
This class represents a configuration logic capable of setting Configurable values. |
Constructor Summary | |
---|---|
Configurable(T defaultValue)
Default constructor. |
Method Summary | |
---|---|
T |
get()
Returns the current value for this configurable. |
protected void |
notifyChange()
Notifies this configurable that its runtime value has been changed. |
static void |
read(java.util.Map properties)
Convenience method to read the specified properties (key/value mapping) and reconfigures accordingly. |
java.lang.String |
toString()
Returns the string representation of the value of this configurable. |
Methods inherited from class java.lang.Object |
---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait |
Constructor Detail |
---|
public Configurable(T defaultValue)
Method Detail |
---|
public final T get()
public java.lang.String toString()
toString
in class java.lang.Object
String.valueOf(this.get())
public static void read(java.util.Map properties)
"javolution.context.ConcurrentContext#MAXIMUM_CONCURRENCY"
).
Conversion of String
values is performed
using TextFormat.getInstance(Class)
.
protected void notifyChange()
|
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