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

 - Update copyright headers
 - Use kebab-case for the property name
 - Set the header unconditionally when the filter's registered

Closes gh-4320

See gh-4320
Closes gh-4454

This commit replaces some more occurrances of keySet() and get(key)
with more efficient usage of the map's entry set.

See gh-4813

Closes gh-4813

Closes gh-4885

Closes gh-4935

Previously, the documentation stated that jackson-dataformat-yaml was
the only required dependency. This is incorrect. jackson-databind is
also required.

Closes gh-4924

Previously, MustacheViewResolver would create an InputStreamReader
that wraps the template Resource's InputStream but would fail to close
the Reader. When the InputStream was a FileInputStream, this caused
the resolver to leak file handles.

This commit updates the resolver to close the Reader once the Template
has been compiled, thereby allowing any underlying resources to be
cleaned up immediately, rather than having to wait for the JVM to exit.

Closes gh-4921

PropertiesLauncher creates a ClassLoader that is used by the Launcher
to load an application’s classes. During the creation of this
ClassLoader URLs from its ClassLoader. This resulted resulting in Java
agents that are added to the system class loader via the -javaagent
launch option being available on both the system class loader and the
created class loader. Java agents are intended to always be loaded by
the system class loader. Making them available on another class loader
breaks this model.

This is the same problem that was in ExecutableArchiveLauncher and
that was fixed in ee08667e (see gh-863).

This commit updates PropertiesLauncher so that it skips the URLs of
any Java agents (found by examining the JVM’s input arguments) when
copying URLs over to the new ClassLoader, thereby ensuring that Java
agents are only ever loaded by the system class loader.

Closes gh-4911