Author: limpbizkit
Date: Tue May 19 02:04:30 2009
New Revision: 969
Added:
wiki/CustomInjections.wiki
Log:
Created wiki page through web user interface.
Added: wiki/CustomInjections.wiki
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+++ wiki/CustomInjections.wiki Tue May 19 02:04:30 2009
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+#summary Performing custom injections with type and injection listeners
+=Custom Injections=
+In addition to the standard `...@inject`-driven injections, guice includes
hooks for custom injections. This enables Guice to host other frameworks
that have their own injection semantics or annotations. Most developers
won't use custom injections directly; but they may see their use in
extensions and third-party libraries. Each custom injection requires a type
listener, an injection listener, and registration of each.
+
+[http://google-guice.googlecode.com/svn/trunk/javadoc/com/google/inject/spi/TypeListener.html
TypeListeners]
get notified of the types that Guice injects. Since type
listeners are only notified once-per-type, they are the best place to run
potentially slow operations, such as enumerating a type's members. With
their inspection complete, type listeners may register instance listeners
for values that get injected.
+
+[http://google-guice.googlecode.com/svn/trunk/javadoc/com/google/inject/MembersInjector.html
MembersInjectors]
and
[http://google-guice.googlecode.com/svn/trunk/javadoc/com/google/inject/spi/InjectionListener.html
InjectionListeners] can be used to receive a callback after Guice has
injected an instance. The instance is injected by Guice, then the members
injectors are notified, and finally the injection listeners are notified.
Since they're notified once-per-instance, these should execute as quickly
as possible.
+
+==Example: Injecting a Log4J Logger==
+Guice includes built-in support for injecting `java.util.Logger` instances
that are named using the type of the injected instance. With the type
listener API, you can inject a `org.apache.log4j.Logger` with the same
high-fidelity naming. We'll be injecting fields of this format:
+{{{
+import org.apache.log4j.Logger;
+import com.publicobject.log4j.InjectLogger;
+
+public class PaymentService {
+ @InjectLogger Logger logger;
+
+ ...
+}
+}}}
+We start by registering our custom type listener `Log4JTypeListener` in a
module:
+{{{
+ @Override protected void configure() {
+ bindListener(Matchers.any(), new Log4JTypeListener());
+ }
+}}}
+You can implement the
[http://google-guice.googlecode.com/svn/trunk/javadoc/com/google/inject/spi/TypeListener.html
TypeListener] to scan through a type's fields, looking for Log4J loggers.
For each logger field that's encountered, we register a
`Log4JMembersInjector` on the passed-in
[http://google-guice.googlecode.com/svn/trunk/javadoc/com/google/inject/spi/TypeEncounter.html
TypeEncounter]:
+{{{
+ class Log4JTypeListener implements TypeListener {
+ public <T> void hear(TypeLiteral<T> typeLiteral, TypeEncounter<T>
typeEncounter) {
+ for (Field field : typeLiteral.getRawType().getDeclaredFields()) {
+ if (field.getType() == Logger.class
+ && field.isAnnotationPresent(InjectLogger.class)) {
+ typeEncounter.register(new Log4JMembersInjector<T>(field));
+ }
+ }
+ }
+ }
+}}}
+Finally, implement the `Log4JMembersInjector` to set the logger. We always
set the field to the same instance here. In your application you may need
to compute a value or request one from a Guice provider.
+{{{
+ class Log4JMembersInjector<T> implements MembersInjector<T> {
+ private final Field field;
+ private final Logger logger;
+
+ Log4JMembersInjector(Field field) {
+ this.field = field;
+ this.logger = Logger.getLogger(field.getDeclaringClass());
+ field.setAccessible(true);
+ }
+
+ public void injectMembers(T t) {
+ try {
+ field.set(t, logger);
+ } catch (IllegalAccessException e) {
+ throw new RuntimeException(e);
+ }
+ }
+ }
+}}}
+
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