polishing

Reorganized class structure to match our code style (setter for
properties at the top of the class, public method before private
implementation).

Removed DisposableBean as it the lifecycle is already taking care
of removing MBeans on stop.

Cleaned test suite

Issue: SPR-8045

src/asciidoc/index.adoc

@@ -41809,6 +41809,13 @@ With this configuration the `testBean` bean is exposed as an MBean under the
 are exposed as attributes and all __public__ methods (bar those inherited from the
 `Object` class) are exposed as operations.
 
+[NOTE]
+====
+`MBeanExporter` is a `Lifecycle` bean (see <<beans-factory-lifecycle-processor>>)
+and MBeans are exported as late as possible during the application lifecycle by default. It
+is possible to configure the `phase` at which the export happens or disable automatic
+registration by setting the `autoStartup` flag.
+====
 
 
 [[jmx-exporting-mbeanserver]]