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Is it possible to nest a Spring Boot application inside another Spring Boot application?

I have a Spring Boot RESTful microservice that a customer would like to nest inside their Spring Boot application.
Could someone tell me whether this is possible?
I was hoping this would be as simple as adding a dependency on my application in the customers maven pom file and then excluding the tomcat dependency since the customer already uses the embedded tomcat.
Thanks,
Ben

Since they already use Spring Boot to start their app, you can simply mark all Spring Boot dependencies as provided in your Maven POM, this would exclude it from the JAR as well as embedded Tomcat and all related dependencies. Also make sure you don't build your JAR as a Spring Boot executable (should be the default if you're not using the spring-boot-maven-plugin).
On the customer side, they would need to include your JAR as a dependency, and possibly add a scanBasePackages property to their #SpringBootApplication, to auto-discover your application classes, if they don't reside in a package under the one that #springBootApplication is on. Also, they'll need to be mindful of any URI collisions between your app and theirs, as the two will be sharing the same environment.

External web-application configuration in Tomcat

There's a web application and a number of environments in which it works. In each environment it has different settings like DB connection and SOAP ends-points that in their turn are defined in properties-files and accessed in the following way:
config.load(AppProp.class.getClassLoader().getResourceAsStream(
PROPERTIES_FILE_PATH + PROPERTIES_FILE_NAME));
Thus the WAR-files are different for every environment.
What we need is to build a unified WAR-file that doesn't contain any configuration and works in any environment (for now, Tomcat instance) getting its configuration from outside its WAR-file.
The answer Java Web Application Configuration Patterns, to my mind, gives the full set of common approaches but with just few examples. The most attractive way is configuring JNDI lookup mechanism. As I can guess it allows to separately configure web-applications by their context paths. But couldn't find a simple (step-by-step) instructions in both the Internet and the Tomcat's docs. Unfortunately cannot spend much time on studying this complicated stuff in order to just meet so seemingly simple and natural demand :(
Would appreciate your links at the relevant descriptions or any alternative suggestion on the problem.

If its a case of simply deploying your WAR on different environment (executed by different OS user), then you can put all your config files in the user's home folder and load them as:
config.load(new FileInputStream(System.getProperty("user.home") + PROPERTIES_FILE_NAME));
This gives you the isolation and security and makes your WAR completely portable. Ideally though, you should still provide built-in default configuration if that makes sense in your case.

The approach we've taken is based on our existing deployment method, namely to put the WAR files in the filesystem next to the Tomcat, and deploy a context.xml pointing to the WAR file to Tomcat.
The context descriptor allows for providing init parameters which is easily accessible in a servlet. We've also done some work on making this work with CDI (for Glassfish and TomEE dependency injection).
If you only have a single WAR file deployed to this Tomcat instance, you can also add init parameters to the default global context XML. These will be global and you can then deploy the WAR file directly. This is very useful during development.

Hazelcast cache implementation in my application

Below is my scenario from Application perspective.
We have 2 applications (.war) files will be running in a same instance of Application server (mostly Tomcat 8), In production we may deploy App1 on 100 servers and App2 only on 50 server out of those 100 (The App2 does not need to be distributed so much)
Now this 2 applications (.war) depends on a common custom jar (some utility classes)
I am planning to use Jcache API and hazelcast implementation in our apps. I have added following dependency in my pom.xml
<!-- JSR 107 JCache -->
<dependency>
<groupId>javax.cache</groupId>
<artifactId>cache-api</artifactId>
<version>1.0.0</version>
</dependency>
<!-- Hazelcast dependency -->
<dependency>
<groupId>com.hazelcast</groupId>
<artifactId>hazelcast</artifactId>
<version>3.4</version>
</dependency>
Plan is to write a utility CacheManager in this common custom jar which will be shared by App1 and App2.
I am planning to use only the hazelcast server provider as I am doing in-memory cluster i.e. the caching will be in application memory.
Below is the snippet of my code.
public class PPCacheManager {
// Loads the default CacheProvider (HazelCast) from hazelcast.xml which is
// in classpath
private static CachingProvider defaultCachingProvider = Caching.getCachingProvider(); //
// Loads the default CacheManager from hazelcast.xml which is in classpath
private static CacheManager defaultCacheManager = defaultCachingProvider.getCacheManager();
// Some more code goes here...
My hazelast.xml
<hazelcast xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.hazelcast.com/schema/config
http://www.hazelcast.com/schema/config/hazelcast-config-3.4.xsd"
xmlns="http://www.hazelcast.com/schema/config">
<cache name="commonClientCache">
<key-type class-name="java.lang.String"></key-type>
<value-type class-name="java.lang.Object"></value-type>
<statistics-enabled>true</statistics-enabled>
<management-enabled>true</management-enabled>
<read-through>true</read-through>
</cache>
</hazelcast>
Now I have several question around this approach.
Is this a good way to implement the in memory caching (currently we are not looking for cluster caching), should this code be in the common custom jar or somewhere else?
There is some master data from DB which I am planning to load (both applications need this data) so not sure how and where I should load this data into memory. Note: I do not want to do lazy loading; I want to load this master data very first.
Where should I add the cache shutdown code to avoid memory leak issues, as this cache is shared by both the applications.
Update
Also by implementing this approach will I have 2 copies of cache each for application or a single copy will be shared across both?
I have already implemented this approach in my application and from Hazelcast management console I can see that there is only 1 cache is created but it says GET is executed on this cache twice.

Hazelcast is the perfect solution for what you are trying to do. Definitely no lazy loading. You don't need anything like that if you have shared memory.
As far, as how many instances you'd have inside one (Tomcat) JVM, you'd have two if you instantiate Hazelcast twice. It'd autoincrement the port. However both will belong to the same cluster (you call "cache"), as long as the cluster name is the same. So other than looking a little silly (sharding on a single JVM), you are fine. To avoid it, you can configure one of the wars to instantiate a HazelcastClient. The utility jar can be the same. It should all be in some e.g. Spring config - which every war would have its own copy of. Or you can put that config into two external directories and add to the catalina classpath.
The shutdown code belongs to the same place you instantiated Hazelcast i.e. your two wars will have two shutdown calls. You can do it in Spring's destroy() of any of your high-level config (or autowired) beans or put it in the Web App session listener.

Can a non spring web application use a jar which is built using spring + hibernate

We are building a java application using spring hibernate maven. The jar will be used in a non spring web application that uses ejb and plain jdbc. Ejb will access the jar interface methods which in turn makes hibernate calls.
What changes are needed in the web application to load the spring context during start up? Is it feasible?

If you want to keep your jar decoupled from web application you should do this:
Copy your jar and all dependant jars to webapp WEB-INF\lib directory (or pom if maven)
Your Spring Context should be loaded in a static variable, in a safe thread manner; or if you want to integrate it to the webapp lifecycle you can do it in several ways: web.xml, web-fragment or using a servlet initializer. There are lot of documentation in Internet
If you continue to use hibernate or change to JDBC, the DataSource should be provided generally by the web container, so you must use InitialContext.lookup("") to get the actual DataSource.

Call glassfish ejb from stand-alone application

So I have a simple ejb (#stateless) deployed on a glassfish 3.1 server.
I want to call it from a standalone application.
It's working great if I add the gf-client.jar into my run configuration.
But how can I do if I do not have that file (the server is in another machine) ?
I tried using
<dependency>
<groupId>org.glassfish.common</groupId>
<artifactId>glassfish-naming</artifactId>
<version>LATEST</version>
</dependency>
But I have
Exception in thread "main" javax.naming.NameNotFoundException: java:global
at com.sun.enterprise.naming.impl.TransientContext.resolveContext(TransientContext.java:252)
at com.sun.enterprise.naming.impl.TransientContext.lookup(TransientContext.java:171)
at com.sun.enterprise.naming.impl.SerialContextProviderImpl.lookup(SerialContextProviderImpl.java:58)
at com.sun.enterprise.naming.impl.LocalSerialContextProviderImpl.lookup(LocalSerialContextProviderImpl.java:95)
at com.sun.enterprise.naming.impl.SerialContext.lookup(SerialContext.java:233)
at javax.naming.InitialContext.lookup(Unknown Source)
at be.java.tuto.Application.main(Application.java:17)
Thanks.

EDIT:
I just needed to invoke an EJB deployed on GF from my Tomcat server and resurrected my dependencies. And because I dont want to keep them back for myself :)...
My IDE is Eclipse so I created an User Library containing all the files shown above.
Hope this solves your problem!
I was facing the same problem. For just wanting to invoke a GF session-Bean method I had to add the complete gf-client.jar to my clients classpath.
My problem was that this library is referencing almost the whole GF-libray-folder and even after a clean-up there were >15 referenced jars left which I had to add to my clients classpath.
For me I did't want this overhead so I decided to call the remote method via JAX-WS webservice.
The advantage of using webservises is that it is very easy to add webservice capability to an already existing session-bean by annotating the bean-class with #WebService.
After publishing the bean to the appserver you're able to view your deployed endpoint and getting the WSDL. With this you can generate your webservice-stubs automatically by using the wsimport-tool shipped with your JDK and use this generated files in yor client to invoke the remote method.
See example here.
Once created those files are portable and can be used in any client.
So if your willing to change the way your client calls the remote method this would be a portable, lightweight (except of a bit more http overhead) and easy to implement alternative.
P.S.
You don't lose the ability of invoking your method via EJB-call.
Hope this helped, have Fun!