This is one of those strange questions where people would ask - why?
So I will start with why I would like to do this and then go into the issue. I would like to have more control over how the spring context is loaded. For example, I do not want to load the domain and web-api at the same time. That would make the resources available before their dependencies are ready.
There might also be that I need to check the state of something before I can continue this process. So to say, there will be sequential order between modules/contexts. Maybe not just booting but also in shutdown.
So the issue is that I can't find any information on how to load the domain-context, then when that is finished I would check the state and lastly load the api-context. I would like to do all of this from java-code as I need to control the flow of the start up. I have basics working with SpringServlet loading the web-context. What I have not found any information on is if it is possible to load a context, wait and load another context that refers to the first one.
It might be good to know that I am not using JavaEE nor a container. I am only using embeddded Jetty with servlet and spring. So is there a way this can be done?
I'd suggest to consider following:
Read SmartLifeCycle and Phased for extension points on the order of application context life cycle management. The idea is that you have your top-level important beans implement the interfaces such that the standard application context initialization will be also handled to those beans in the order that you customize.
Break your application context XML files into smaller pieces. Use <import /> in the ones that depend on a higher/lower context.
Use depends-on attribute on your mission critical beans to ensure the dependencies.
Use InitializingBean on the ones that you need to make sure a dependency is satisfied for the current bean after it's initialized.
Consider lazy loaded beans and Lazy Proxy.
So the bean will be created only on first usage ...
Related
I am developing a code analysis plugin and a user project may contain Spring definitions.
Analyzing a source class, I need to get information about beans (more specially, which class actually implements an injected interface). Of course, I can take this information from Context. However, starting a context may cause to some side effects (like some db configuring or actions in post processors) in users code that is unacceptable.
How can I get the required information without initializing Context?
How can I start Context in "safe" mode to avoid or at least minimize side effects?
How can I execute the action, before Spring context created? I found only one solution that more or less can satisfy me, it is listening ApplicationStartingEvent but I don't know how correctly do it because Spring doesn't see listener bean because context not created yet. So maybe someone knows ways how to catch ApplicationStartingEvent or maybe another better solution to do it.
Interface ApplicationContextInitializer could suffice for your requirement.
Read ApplicationContextInitializer documentation
Implementing ApplicationContextInitializer allows you to do additional tasks/initializations before persistent bean definition is loaded (i.e. your application-context.xml). One such use is, when you want to select profiles before you will load definitions.
I'm reading through Spring Integration documentation and I still can't get into one thing: does Spring resolve all dependencies and make that automagic IoC dependency injection at compile time or at runtime?
I believed that it is runtime job to wire available components together in a data route from gateway to some data endpoint (e.g. DB). But since most of examples are made using DSL syntax in java, it seems that it's a compile time job.
So, glueing together beans in a data highway can be made only at compile time?
Summing up my comments here :
1) Spring IOC container manages beans from its creation till destruction. What this means is the beans are ready in sort of a bucket, which is a ready to use application. Thus, it is necessary to create the contents of the bucket at compile time, rather then runtime. This does not include hot-swapping of beans.. I hope this is what you were looking for.
2) You can create as many routes as you want, those all beans will be put in the container.... And as far as I understand, you cannot just change your source code and synchronize it with already running one, you have to atleast do a graceful restart. There is a bottom line to this, Spring must see if all beans are properly autowired, no circular dependencies, and there is no expectation of source code during run time. Sure you can get your beans via RMI, but that doesn't count as you have declared it already. So yes, compile time it is
The java DSL syntax is simply a different way of defining a flow definition (a series a bean definitions). The beans are still created and wired together during application initialization (runtime).
My application can't start up, if the PostgreSQL database it's using has not started up yet (the delay between the two is ~2 minutes). It's a fairly complex inherited project and I can't fiddle with the configuration too much.
My idea would be to have a bean that runs as the first thing in the Spring container. I've tried to configure it with annotations (#PostConstruct) and in xml (<bean id="dbStartupMonitor" class="ee.package.monitoring.DBStartupMonitor" init-method="checkConnection" lazy-init="false" />), but in neither of these cases the checkConnection() seems to be running (no logging appears). In addition, how can I make sure that this bean is created first?
Now, regarding the checkConnection(), the idea would be to check if the database is up. If it's not, sleep for some amount of time and try again. This needs to block the rest of the initialization of the Spring MVC container, but I'm pretty sure it doesn't work that way.
Is it even possible to solve it like this?
The problem was the order of bean initialization. The datasource-dependent beans were initialized first, so after some substantial digging in the bean configuration xml files, I added the depends-on attribute with my DB monitor bean. This ensures that the monitor is initialized first.
Also, the initialization method (init-method) does block, so the "check-sleep-check until database is up" cycle works fine.
I'm about to write a MMO, using HTTP-requests that are responsed with JSON.
I was writing it all in Java EE-style, hoping it won't be hard to port to Java EE than. But then I've found out that my static instance variables for a couple of sinletons weren't created properly - classloader made a bunch of them when calling SingletonClass.getInstance() from servlets.
I was totally desperate and thought adding #Singleton descriptions would help. But things weren't so easy. My classes simply not working while adding them with #EJB ClassName var. Context lookup doesn't work either.
I was trying developing in Eclipse, NetBeans, used Glassfish, tried to set it up, but nothing really helped. I do not know what to do and really desperate now.
All I need is just few classes, that work all the time application is loaded to handle game events and hold logged users data (which is distributed in non-EJB objects that hold user data, monsters and so on), some timed events for each logged user and ability to respond to HTTP POST requests with JSON. I even do not need the ORM, I wrote all queries by myself, but still... Something that had to work simply doesn't work out.
I'm aware that all that sounds messy and non-informative, but I do not know what to do - where is my problem? Maybe, I should fill web.xml, or use different port, or fly to the moon? Or just change programming language? Sorry for your time spent reading this.
UPD. Some application scheme parts. First two from package "server".
#Startup
#Singleton
public class DbWrapper
handles all database connections, DbConnectionPool is non-singleton class, which handles pool of java.sql.Conneciton.
#Startup
#Singleton
#DependsOn("DbWrapper")
public class World
is yet another class to handle all the in-game events, that holds HashMap of logged users. User and Monster classes are from package "entities" (User holds a list of monsters).
Package "servlets" hold HttpServlet descendants, annotated #WebServlet("/pathname"), that try to use
#EJB World world
for example. But such things as world.getUser(id_user) simply won't work.
As for JDBC - postgres jar is included in GlassFish domain's /lib.
As for JSON - I use org.json found here: https://github.com/douglascrockford/JSON-java
I've found out that my static instance variables for a couple of singletons weren't created properly - classloader made a bunch of them when calling SingletonClass.getInstance() from servlets.
First, you should show us the code for one of these singleton classes. You may have made a mistake in the implementation that causes this problem.
It is true that you can get what appear to be multiple instances of a (properly implemented) singleton class in a servlet framework. But in fact they are not what they appear to be. What is actually going on is that you have loaded the class from multiple classloaders, either because you have multiple webapps each loading the class, or because you are redeploying your webapp and the previous deployment is not clearing up properly.
So what can / could you do about this?
You could use a dependency injection framework to configure your webapp, and hence avoid the need for singleton classes.
You could continue using singletons, but track down why you are getting multiple instances, and fix that problem.
You should use singletons really rarely (best would be not to use them). As an alternative use application scoped beans (#Singleton beans should normally work - they should use instance variables though, not static ones).
With Java EE 6 you also can use CDI and thus you don't have to use EJBs if you don't need the additional features they provide (like automatic transaction demarcation, security etc.) or can live with adding those features yourself.
Additionally, you can use CDI in a SE application. Keep in mind though, that you need to define the scope for CDI beans (e.g. #Application, #Request etc.) otherwise the default scope (#Dependant) is used which causes the beans to be copied on every access.