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Is the Spring Boot CGLIB proxy reliable?

For a multi-tenant system, I decided that using a proxy in bean definitions would be a logical way to go.
At this point I developed a custom scope. I want to use a proxy. I saw that Spring Boot uses a third party library called CGLIB in ScopedProxyMode.TARGET_CLASS method. It is written in the Github repo of this library that there are problems for java 17 and above.
However, as far as I can see, Spring Boot does not directly use this library, it has wrapped it. Is it safe to choose TARGET_CLASS as proxy mode at this point? Is this issue guaranteed by Spring Boot?

Can I look up an EJB via JNDI and invoke methods using reflection?

I'm using EJB 3.1. I need to get a references to one of the EJBs in a servlet and I'd rather not put an EJB interface jar on the class path to get the code to compile.
Is it possible to look an EJB via JNDI and find the method I want to invoke using reflection without ever strongly typing the object to an interface?

Yes, if you're looking up a local EJB interface, then you can look up and invoke a local EJB within the same application using reflection.
This should work if you're using a direct lookup or an EJB ref lookup because the Java EE spec requires the application server to make EJB module classes available to WARs within the same application. The EJB spec doesn't require support for local interfaces across applications, so if that's what you're doing, you'll have to check with your application server vendor.
This will not work in general for remote EJB interfaces because a client proxy needs to be created. If you're using RMI-IIOP (EJB 2.x remote or EJB 3 extending java.rmi.Remote), you might be able to cast the EJB lookup result to javax.rmi.CORBA.Stub and use the _servant_preinvoke or _invoke methods the same as a generated stub method would do.
(Ultimately, this is a lot of caveats just to avoid a compile-time dependency. It's probably not worth the fragility, so I would recommend finding a way to solve that and compile normally.)

Inject a bean from another application context?

Is it possible to inject a bean from a web application that deploy in another server!
I declare a scenario to myself, I have two web application that use spring framework and deploy separately in different application servers (one is TOMCAT and another one is WEBLOGIC),the first application has ServiceA and the second one has ServiceB, now I want to inject ServiceB in ServieA?
I try to do this with RMI once an another one with JMS, now I am wondering that:
Is it possible with another thing?
Is there any active project about this scenario exist?
How can share application context in spring framework, is it possible?
Thanks.

Bean is just an object in JVM. You certainly cannot use an object from one JVM in another JVM straightforward.
But you can do 2 things:
Use proxies - some objects that will have the same interface but invoke somehow to the proper server as implementation.
Use service-oriented architecture (SOA). Each server should have some limited set of beans that are responsible for their functionality. And all beans can interact with each other.
Maybe OSGI is suitable for this.

Web services, JAX-RS is the simplest. But JAX-WS provides you with the tools to automatically generate the client code.

dependency injection in a servlet 3.0 web app?

I'm trying to write a servlet 3.0 web app, just to learn basic servlet handling. Normally i would use spring.
Now I have a servlet which access a DAO which queries the database. Now, what is the best way to instantiate that DAO and use it? My best guess would be to have a private property in the servlet and create an instance of the DAO when the servlet is created.
But, will the servlet be created multiple times?
Is there something similar to springs dependency injection available in servlet 3.0?

EJB 3 dependency injection is extremely simple to use. A single annotation, #EJB, causes the injection of a declared bean. The injection of the SomeDAO bean into a Servlet 3.0 looks like this:
#WebServlet(name="Messenger", urlPatterns={"/Messenger"})
public class Messenger extends HttpServlet {
#EJB
SomeDAO someDAO;
}
The injected SomeDAO bean could be an interface or a no-interface view bean. As long as there is only one implementation of the interface, it will be injected without any ceremony.

javax.servlet API is one of the technologies included in java-ee.
CDI, is the Context and Dependency Injection technology in java-ee
So to answer your question, your use case could be solved by using only CDI and Servlets.
But most of the application servers that supports above (e.g. TomEE, Glassfish webprofiles), will also support EJB (which uses cdi) and JPA. EJB+JPA can be used to implement DAOs.
Arjan Tijms has put together a nice link overview of what is included and happening in the java-ee-7 world

How to use dependency injection in servlet?

How to inject an object to a servlet?
I mean, that I cannot use a constructor DI because servlets are instantiated by a servlets container. And I also don't see a nice way of implementing setter-based DI for a servlet.
Should I use servlet listener? Are there any best-practices?
P.S. I don't have neither Spring nor Guice nor any other DI framework, I'm interested in manual dependency injection.

This is possible under Servlet 3.0. You register a ServletContextListener which programmatically registers Servlet instances with the addServlet(String, Servlet) method of ServletContext just before the app starts. Since you're instantiating the Servlet instances yourself, you can give them proper constructors and inject dependencies.
I created an example a while ago that illustrates the basic technique.

You can consume services which are created/managed by some IOC container (Spring, Guice)
You could create a ContextAware implementation and Pull out the beans as and when needed from Servlet

You could use JNDI, the Java Naming and Directory Interface, and #Resource to inject it.