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.
Related
I have a Java class that gets instantiated by a third-party application as an extension. That is, as per the 3rd party software design, customers like us register our Java class to their application and their application will install it to execute custom logic at the right place and time.
Our custom Java class needs to marshal and unmarshal XML, for which it uses JAXB. It therefore needs a JAXB context.
I naively called JAXBContext.newInstance(MyClass.class) on every call and not-so-quickly discovered that that's a well known recipe for a memory leak. The common prescription is to make one (or at most only a few) JAXB contexts to share among your whole application.
Fine, except the third party application that invokes my class makes each invocation on a new ClassLoader instance that is private to that invocation.
So, even if I put the JAXBContext in a static field or static HashMap<>, it would still be private to the invocation!
QUESTION
How can I, in a class that is instantiated from a private ClassLoader instance, create a singleton to be shared across the JVM?
I am thinking along two possible lines, but I'd like advice on how to make either of them work or any completely different approach anyone has.
The three ideas I had were:
Find somewhere in a JVM class where I could write an object. E.g., if System.setProperty could write instance of Object instead of just String, the idea would be to create the JAXB context and put it in a property, since it is sure that System will already have been instantiated and that the custom classloader instance would inherit it. But System.setProperty does not take an Object value, so I don't know a practical way to do this.
Somehow force a class to load on the parent or root classloader where I could store by JAXB context. I don't know how to do this.
Use a ThreadLocal to store the JAXBContexts. I don't think each invocation is a brand new thread (they're probably reused from a thread pool), so this could maybe be the way to limit my contexts. But how to create the ThreadLocal variable so its shared across the instances? It seems like this leaves me with the same problem.
It sounds like your code is sandboxed with in the 3rd party application. So using static or ThreadLocal won't help since it will only exist within the same classloader, and if that classloader is changed... then the context is lost.
The closest solution is to inject your context into the application code. This is not the best idea, since it can be affected by outsiders and have unexpected consequences. Do note that this means a value you created in your classloader will remain in the application and thus the creating classloader will never be garbage collected. There's also the problem of where JAXB jar comes from. I assume from your code and not the 3rd party. So that jar is loaded each time in a different classloader, so to share an object from there might be a problem and require some proxy.
Honestly there can be so many unforeseen results.
The best idea is to ask the 3rd party to provide you with some API to enable that.
Before continuing, let's see other options:
Is there a replacement for using JAXB? Something that won't present the same memory leak problem.
Is the memory leak that serious? What if there is a temporary memory leak until an API is provided by the 3rd party application.
If you really want to try to inject the object, I'd be happy to help. But, from experience, these kinds of things are messy.
I've tried all the suggested solutions including:
http://snehaprashant.blogspot.com/2009/01/singleton-pattern-in-java.html
How to create a JVM-global Singleton?
But none of these seem to work. They either are not truly Singletons across all ClassLoaders (the first link above) and result in different instances on application reload (I'm using Play Framework 1.3.x which reloads classes upon code change and recompile), or they require the classes to be on the System classpath.
Is there a solution for this? I want the class to be loaded once and even after classes are reloaded with new ClassLoaders, to still have the same one instance of that class.
Is this possible?
I am creating a web front end using an existing back end containing several singleton classes. The DataStore is initialized by passing a user object into it, which is fine in a desktop application environment where the app is launched once on each machine, but will not work in a server side application designed to cater for multiple users.
The database guys are reluctant to change the service layer and remove these singletons to allow an instance per user, or allow a single instance of a service layer object to serve multiple users. This is with good reason, the desktop app has been in use for 10 years and changes could have serious side effects for the desktop app.
I have been asked to investigate using classloaders to create multiple instances of the singletons. I am not comfortable with this idea at all, hacking singletons seems like bad practice, but changing the service layer could take months of work.
I have tested this out already by putting two identical WAR files of my app (with different file names) into Tomcat. Tomcat creates a classloader for each webapp and they worked just fine separately. I only encountered problems when the singletons used System.setProperty/System.getProperty, which is to be expected as the System class comes from a classloader much higher up in the tree.
To get this separation within a single webapp, it starts to get a bit complicated. It seems I would have to create a different classloader for each session, and use the classloader to load either all the classes in the whole service layer or just the ones which are singletons and their dependencies.
The problem comes when I'm thinking about how to use these objects in a session in a servlet. Because each Servlet has a single instance within a Tomcat, getting objects from the session and casting them will not be straightforward. Eg, to get a DataStore object from the session, I would have to cast it to the correct DataStore class loaded by the correct ClassLoader, since a single class loaded by two different ClassLoaders counts as two completely separate classes.
I have read that using ClassLoaders can cause all sorts of problems with memory leaks if they are not used carefully, and from the sounds of this, if I have 500 users, that is a lot of classloaders and classes loaded by classloaders. Won't I then have also have issues with PermGen?
I suppose from this large explanation, I really have 4 questions:
Hacking a singleton with classloaders in a webapp. Creating potentially hundreds of instances of the same classes designed to be singletons. Is that a terrible idea? So terrible I shouldn't contemplate it?
What is the best way to implement this if I absolutely have to?
How do I cater for casting in a Servlet, if I want to get and set objects into a session?
Will I end up with issues with memory leaks, and PermGen space?
I would really appreciate any suggestions. Thanks :)
It's not ideal, but I don't think it's terrible. In practice, it's not much worse than loading multiple versions of the same class (even without singletons), and that's becoming increasingly common in complex application server environments, particularly those with OSGi.
It's hard to say which approach is best, but to begin with, I would start by creating child class loaders of your web application class loader. Arrange to load implementation classes in the child class loaders (i.e., the ones with the singleton), and it possible, have the implementation class implement an interface that is loaded from the web application class loader.
By loading the interface from the web application class loader. This is basically the same approach that the application server itself is using to invoke your HttpServlet: the interface is loaded by the server, so it can refer to it directly, but the implementation is in a child class loader for your application. You're just creating a secondary layer of interface/impl split for your own convenience.
You'll end up with memory leaks if you store references to the child class loader (or its loaded classes, or instantiated objects from those classes) in a "parent" class loader (e.g., if the singleton registers an MBean, which causes it to get referenced in a JVM-wide object), but that's no different from if you weren't creating child class loaders. If you're dynamically creating/destroying these singletons (thus child class loaders), you'll have to take care that you don't retain references to those child class loaders (or classes/objects) longer than necessary. PermGen is probably more problematic. If you can run with Java 8, that's gone away; otherwise, you might have to increase the default PermGen size depending on how many of these class loaders/singletons you need to create.
I need different classloaders to be able to unload classes. But i need to share objects between them (actually i am getting ClassCastException). So what are the solutions to deal with this?. Thanks
Objects from different classloaders can interact with each other through interfaces and classes loaded by a common classloader.
One of the primary objectives of using separate classloaders is to prevent exactly the kind of thing that you are trying to do. Security and name conflicts are good reasons for keeping this isolation. Here are a few ways that you can circumvent this isolation.
Using the Common Class Loader
Sharing Libraries Across a Cluster
Packaging the Client JAR for One Application in Another
Application
Refer to this link for more details.
Will also mention that if you are using interfaces, you can use java.lang.reflect.Proxy to create an instance of an interface local to your classloader which, under the hood, makes calls with reflection to the "real" (foreign) object from a different classloader. It's ugly, and if parameters or return types are not primitive, you will just be passing the ClassCastException further down the line. While you can rig something up to make this work, in general, it is better to either have a parent classloader with some shared types that you want to be able to use across classloaders, or use a more... serialized format for communication (slower), or only share interfaces that deal in primitives.
In some situation it may be acceptable to 'switch the context' by for using a thread with desired classloader. For example you create SingleThreadPoolExecutor by a context you need and use that singleton instance to execute the code from different context.
Here was my situation: maven multi module web application deployed to tomcat. One web app registered error notification service bean, and DefaultUncaughtExceptionHandler for the whole JVM using the bean to deliver error by an email (with additional logic inside). When exception was thrown by web application which created the bean email was successfully sent, when exception was thrown by different web app, email functionality failed to cope with javax.mail classes (surprisingly not the exception itself). When I executed a method which send email in an executor, both cases start to work.
Im writing an web application using the MVC design pattern.. the application should connect to a RMI server providing the business part (Model)
my problem is, I dont know where i should instantiate the model class and connect to the RMI, providing access to all servlets.. i added a ServletContextListener and shared the reference on the servlet context, but i dont think that is the right way to do it
thanks in advance
You can instantiate your Model/Business classes from your Controller. Better still make a single instance (Just Create One) of each in the start of the application. You can also go lazy-loading about it.
Instantiating Model/Business classes from your JSPs means, instantiating them from View. That, of course, violates the MVC pattern.
Either you can make your Model/Business classes as Singleton, or keep the reference in Servlet Context. Both are fine and the latter is more test friendly. The former can be made test friendly too, if you don't specify the constructor as private but just have a common understanding that it's not there for instantiating it everywhere.