* gh-4801:
  Add mainApplicationClass accessor to SpringApplication

Closes gh-4801

If application context refresh fails, SpringApplication will have a 
null context. Previously, this would result in an NPE when looking for
an ExitCodeExceptionMapper.

This commit updates SpringApplication to gracefully handle the context
being null when looking for an ExitCodeExceptionMapper.

Closes gh-4803

Closes gh-4971

Previously, if an application’s main method threw an exception,
MainMethodRunner would catch the exception and call System.exit(1).
This meant that the JVM would exit, irrespective of whether or not
any non-daemon threads were running. In contrast, when an application’s
main method was invoked directly (in an IDE, for example) the JVM
would not exit if one or more non-daemon threads were running. This
is standard JVM behaviour that we should be consistent with in the
launcher.

This commit updates MainMethodRunner to wrap any exception thrown by an
application’s main method in a RuntimeException and rethrow it. This
alllows the JVM to handle the exception and use its normal rules for
deciding whether or not it should exit.

Closes gh-4984

* pr/4987:
  Update inconsistent documentation

Closes gh-4987

While Flyway's `MigrationVersion` is a String value, a simple "0" is
also a valid number. When such value is not wrapped in YAML, an integer
is injected rather than a String which leads to a failure as we can't
convert it.

This commit updates the `StringToMigrationVersionConverter` to also
supports conversion from a Number. This is a convenience for users
using YAML for configuration

Closes gh-4981

This commit makes sure to also include any `GenericConverter` flagged for
inclusion in the conversion service to use to convert configuration keys.

Previously, those were ignored as `Converter` and `GenericConverter` are
two separate interfaces.

Closes gh-4988

The same initializer will receive multiple ContextRefreshedEvents
when their is an application context hierarchy. This commit updates
the initializer to correctly handle multiple ContextRefreshedEvents
and only act upon the first that it receives.

See gh-4871

This commit is a continuation of the changes made in b85b6082. It
addresses an additional problem when testing applications where two
contexts are refreshed in quick succession. In this scenario, it
was possible, theoretically at least, for the first context’s background preinitialization to still be in progress and creating loggers when the
second is refreshed and resets the logger context.

This commit updates BackgroundPreinitializer so that, upon receipt of
a ContextRefreshedEvent, it waits for preinitialization to have
completed. In the scenario described above, this ensures that
preinitialization has completed before the call to refresh() for the
first context returns, thereby preventing it from running in parallel
with the refresh of the second context.

Closes gh-4871

If Both Hazelcast and Hibernate are available, Spring Boot takes the
opinion that Hazelcast can be used for 2nd level caching and therefore
need to start before Hibernate.

Unfortunately, some users require Hibernate in some of their hazelcast
use case so the link is actually reversed. One way for such user is to
disable the auto-configuration that deals with this link. This class is
now public so that users can locate them and exclude them if necessary.

Closes gh-4960

Closes gh-4964

Previously, BackgroundPreinitializer would kick off preinitialization
on a separate thread in response to an ApplicationStartedEvent. This
work would then race with the logging system being set up in response
to an ApplicationEnvironmentPreparedEvent. When Logback’s being used
this race is problematic. As part of Logback’s setup,
LoggerContext.stop() is called. This calls LoggerContext.reset() which
can fail with a ConcurrentModificationException if another thread tries
to create a Logger at the same time. This is a known bug in Logback [1].

This commit updates BackgroundPreinitializer to respond to
an ApplicationEnvironmentPreparedEvent and to order itself so that it’s
called after LoggingApplicationListener has responded to the same event
by initializing the logging system.

Closes gh-4871

[1] http://jira.qos.ch/browse/LOGBACK-397

Spring Boot’s metrics require all values to be Numbers. A Dropwizard
Gauge can have a non-Number value. Previously, to prevent this causing
a problem, MetricRegistryMetricReader would check the value of a Gauge
when it’s being added and ignore it if it had a non-Number value.
Unfortunately, retrieving the value of a Gauge can take a non-trivial
amount of time (hence CachedGauge) so this approach, while functional,
could be improved.

This commit updates the filtering to happen when a Metric is being
retrieved from MetricRegistryMetricReader (via findOne or findAll)
when its value is required anyway. At this point, any Gauge with a
non-Number value is ignored.

Closes gh-4874

Closes gh-4976

EmbeddedVelocityToolboxView is used with both embedded containers and
when a Spring Boot application is deployed to a servlet container. It
process the ServletContext to intercept calls to getResource so that it
can load Velocity’s toolbox.xml file from within an executable jar.
Previously, when it created the proxy, it didn’t specify the ClassLoader
to use. This resulted in the proxy being created using the ClassLoader
that loaded the Class that is being proxied. This fails when the
application is deployed to a ServletContainer as the ClassLoader that
loaded the ServletContext implementation is the container’s ClassLoader
and it cannot see the classes that are specific to the web application.

This commit updates EmbeddedVelocityToolboxView to specify the
ClassLoader to use when creating the proxy. It specifies its own
ClassLoader thereby ensuring that the proxy creation can load
application-specific classes.

Fixes gh-4967

Closes gh-4969

See gh-4969

Mongo’s 2.x driver allowed the credentials list to be null, however
the 3.x driver requires an empty list instead. The behaviour of the 2.x
driver is the same whether the client is created with a null credential
list or an empty credential list.

This commit aligns with the requirements of the 3.x driver by ensuring
that we never pass in a null credential list when creating the client.

Closes gh-4076

Previously, when Spring Session 1.1.0.M1 and DevTools were declared as
dependencies the application failed to start, because
sessionRedisTemplate could not be resolved.

This commit relaxes the dependency for sessionRedisTemplate in
restartCompatibleRedisSerializerConfigurer from
RedisTemplate<String, ExpiringSession> to RedisTemplate<?, ?> and uses
the bean name in a @Qualifer to ensure that the right RedisTemplate
bean gets injected.

Fixes gh-4895
Closes gh-4896

Prior to Tomcat 8.0.30, its default behaviour was for the mapper to
redirect a request for context-root to context-root/. This redirection
was switched off by default in 8.0.30 by default. This has proven
to be problematic and Tomcat 8.0.31 will reinstate the old default.

While we are waiting for Tomcat 8.0.31 to be released, this commit
explicitly sets the default back to what it was in 8.0.29 and earlier
and what it will be in 8.0.31.

Closes gh-4937

Previously, HalJsonMvcEndpoint used a redirect to go from path/ to path.
When the actuator’s configured to use a custom context path this
redirect was leading to an infinite redirect loop.

This commit removes the redirect in favour of updating the controller
advice to apply the links to requests for path and path/.

Closes gh-4853

Previously, MavenSettings used a FileProfileActivator with no
PathTransformer. If a settings.xml file contains a file-activated
profile this would result in an NPE within Maven. This was made worse
by the NPE not being included in the resulting failure message which
hampered diagnosis of the problem.

This commit updates MavenSettings to configure its FileProfileActivator
with a PathTransformer. It also improves the failure message that’s
created from any problems that are reported by Maven while determining
the active profiles to include a problem’s exception if it has one.

Closes gh-4826

Webjars locator is a good hint that the resource chain should be
enabled. The sole presence of the library now enables the resource chain
unless the configuration states otherwise.

Closes gh-4403

Generalize the `skip` property to start and stop goals so that one
can control if the Spring Boot app is starting via a configuration
property.

Note that this only controls the Spring Boot Maven plugin and the
failsafe maven plugin should be updated accordingly.

Closes gh-4922

* pr/4956:
  Document use of maven wrapper

Closes gh-4956

Closes gh-4947