Update the documentation to include a section about ServletContext
initialization with embedded servlet containers. This update is to
primarily highlight that `WebApplicationInitializers` and Servlet 3.0+
`ServletContainerInitializers` are not called.

Fixes gh-4643

Closes gh-4789

Prevent potential SpEL injection attacks by ensuring that whitelabel
error view SpEL placeholders are not recursively resolved.

Fixes gh-4763

Previously, ActiveMQ's pooled connection factory was not closed as
part of the application context being closed. This would leave
non-daemon threads running which could cause shutdown to hang unless
the JVM itself was shutting down (in which case a shutdown hook would
stop the pool).

This commit configures each pooled connection factory bean with a
custom destroy method so that the pool is stopped as part of the
application context being closed. To allow the destroy method to only
be declared when the connection factory is pooled, the bean method
has been split into two; one for pooled and one for non-pooled. This
is a partial backport of the changes made in bedf2edf.

Closes gh-4748

Closes gh-4754

Commit 8c140092 removed the endpoints.metrics.filter.enabled property so
that endpoints.metrics.enabled is used for both disabling the endpoint
and the servlet filter that records interactions.

This was an unfortunate decision as it was no longer possible to only
disable the servlet filter. The endpoints.metrics.filter.enabled property
has therefore been restored.

Closes gh-4365

Previously, BasicErrorController would return the response status
set in the javax.servlet.error.status_code request attribute when
serving JSON but would also return a 200 OK response when serving
HTML. This didn’t cause much trouble when a person was browsing, but
proved problematic for machine clients that request text/html and care
about the response status. For example, the success handler would be
driven for an XHR request even though the response was really an error.

This commit updates BasicErrorController to set the response status for
text/html responses to match the status that it would use in an
application/json response.

Closes gh-4694

Closes gh-4510

This reverts commit bb9e1be7.

4.1.7.RELEASE is incompatible with Spring IO Platform 1.1.x due to a
new dependency on kryo-shaded. kryo-shaded was introduced in Kryo
3.0.x but IO Platform 1.1.x uses Kryo 2.

See gh-4719

Closes gh-4738

Tomcat 8.0.30 has changed to using relative URIs in its redirects
by default. To avoid any problems that this behaviour change may
causes we override the default and configure Tomcat to continue to
use absolute URIs.

Closes gh-4715

Closes gh-4721

Closes gh-4720

Closes gh-4719

Closes gh-4718

Closes gh-4717

Closes gh-4716

Closes gh-4714

Closes gh-4713

Previously, LoggingApplicationListener would clean up the logging
system in response to any application context with which it was
registered being closed. This caused problems when a child context was
closed. Specifically, closing the child context would cause any
SLF4J-based logging systems to unregister the JUL bridge handler
preventing an JUL logging being bridged into Logback or Log4J2.

This commit updates LoggingApplicationListener so that the logging
system is only cleaned up when a root application context is
closed.

Closes gh-4651

Closes gh-4606

Some libraries like aspectj are using findResource to see the raw
bytecode of a class. It will even call findResource for every method of
every class of beans that are post processed. This can be significant
performance hit on startup when LaunchedURLClassLoader and there are a
lot of nested jars.

See gh-3640
Fixes gh-4557

Previously, EmbeddedWebApplicationContext used synchronized, but did
not do so consistently. It also synchronized on this so its lock was
exposed outside of the class, creating a risk of deadlock if a caller
synchronized incorrectly. Furthermore, not all fields on the class
were sychronized so the class wasn't truly thread-safe.

This commit attempts to rectify some of the problems above. The use
of synchronized has been dropped in favour of using a volatile field
for the embedded servlet container. Whenever this field is accessed,
a local variable is used to "cache" the value thereby preventing a
change on another thread from causing unwanted behaviour such as an
NPE.

Closes gh-4593

See gh-4401

Closes gh-4397

Closes gh-4588

Closes gh-4520

Fixed initially in #3725

The target attribute is effectively checked for null beforehand so this
additional defensive check can be removed.

Closes gh-4567

Closes gh-4551

Previously, UndertowEmbeddedServletContainerFactory always used
Undertow’s default ServletContainer. This meant that if there were two
UndertowEmbeddedServletContainers created, they would share the same
ServletContainer and the second one that was created would overwrite
the deployment for the first. This resulted in a async request
handling failing as the attempt to look up the deployment for the
first embedded Undertow instance would incorrectly find the deployment
for the second.

This commit fixes the problem by ensuring that each 
UndertowEmbeddedServletContainerFactory uses a separate Undertow
ServletContainer instance.

Closes gh-4329

This reverts commit 331a26a4.

Fixes gh-4464

The CLI application advertises `-cp` support but it appears that only
`--cp` is really supported. The fix for gh-178 forgot to update the
call to `getParser().parse(...)`.

See gh-178

Update EmbeddedWebApplicationContext so that the servlet container is
shutdown after the context is closed. Unfortunately shutting the
container down before the context has been closed causes exceptions if
the `/shutdown` actuator endpoint is used. It can also cause the Tomcat
classloader to throw IllegalStateExceptions if resources are accessed
during shutdown.

As this commit effectively reverts 0069e41c we need to fix the shutdown
deadlock issue reported in gh-4130 in a different way. The deadlock can
be caused when an incoming HTTP connection occurs whilst the context is
closing. The incoming connection triggers the `FrameworkServlet` to
call `initWebApplicationContext` which in turn calls `refresh`. The
`FrameworkServlet` checks `ApplicationContext.isActive()` before
performing an initialization but prior to this commit we would set
active to `false` before stopping the servlet container. We now override
`onClose` rather than `doClose` in `EmbeddedWebApplicationContext` to
ensure that the active flag is only set to `false` once the servlet
container has been stopped.

See gh-4130
Fixes gh-4396

Spring Initalizr now bundles a wrapper script for the build system. While
that wrapper has the necessary execute flag in the zip archive, that flag
is lost as the zip abstraction does not honor those.

The init command now makes sure to restore the execute flag on `mvnw`
and `gradlew` if necessary.

Unfortunately, this can't be tested as the Windows build would fail to
assert that the executable flag has been propertly set.

Closes gh-4392

Add missing documentation

See gh-4220