I'm currently wondering what the actual overhead, in the JVM, is for loading extra classes which are never used.
We have code which iterates all the classes in the class path to find classes which implement a certain interface, we then load them.
This allows custom classes to be simply dropped in a directory and they get loaded and registered.
The side affect is that we hit every class in the class path, causing the classes to load. What would be the affect on the JVMs memory?
Does simply loading classes affect the memory much at all?
As usual, I would advise measuring this for your particular scenario.
Having said that, I'm not sure I'd advise scanning the whole classpath. If you don't control the classpath (it's your customer's, or similar), potentially they could add anything to it, and your process is going to scan anything they drop into their classpath (possibly unrelated to your app).
I'd suggest that you nominate only certain directories/repositories that classes can be uploaded to, and that way you'll restrict the classpath scanning and reduce the chances of inadvertently picking up stuff you don't intend to.
If you use a separate ClassLoader to load those classes and are very careful not to create any references to those classes or instances of them, then when the ClassLoader becomes eligible for garbage collection, so do the classes.
Thus, you could avoid unnecessarily clogging your PermGen space by doing 2 passes with separate ClassLoaders: one to load all the classes and identify those you want to keep, and another to actually use them.
Won't using ClassLoaders in this way have unintended side-effects? Like running static initialisers and so on.
You could use the ServiceLoader mechanism, but if that doesn't suit, you can inspect classes without using ClassLoaders - byte manipulation libraries like BCEL and ASM can be used to just inspect classes.
Yes, this forces the VM to load the class file and examine it (which can be a performance problem). Moreover, if you're using a Sun VM, then these classes will stay in memory forever. the Sun VM puts classes in the so called "PermGen" space which is never garbage collected unless you specify a special option.
So this is generally a bad idea but there are two simple workarounds:
Check the name of the class (the filename). Repeat the name of the interface in the name, so you can easily notice what you have to load and what not.
Use two directories. One contains normal classes, the other the one all those which you want to always load.
A "far out there" suggestion:
could you do the same thing but without actually using the VM? the class file spec is documented, couldn't you write your own app to just read the class files and figure out if they implement your interface/whatever without actually loading them?
That would get you the ability to scan any directories but without any worry of loading classes or static intializers or anything like that.
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 have built in my application the ability to reload classes. However, I need to clarification on the behaviour of the class loader.
Let me explain what I know and then ask the questions...
What I do is provide a special jar that is loaded by a custom classloader. Then during the bootstrap of the jar a bunch of spring beans are created and the ones which implement certain interfaces get registered for use by the central app.
Now I kick off a process in the application which uses these new classes. I can successfully unload the "jar", change classes in the jar and reload and I get the changes. Unloading in class loader parlance means making the class loader that loaded the classes unreachable - this causes any classes loaded by that class loader to be unreachable and thus effectively unloaded.
However, from what I know so far, once the vm has loaded the class it stores it in some shared space so does not have to load it again.
The issue that I have is that sometimes the unload and reload of a new class does not work. The old class stays around. It stands to reason that if I unload a class loader (i.e. make the class loader unreachable) and one of the classes in that class loader is currently in use (an object of that type exists) then the class cannot be unloaded.
Is this true? It seems to be true in practice.
If that is so, how do I successfully unload classes that are in use at the time the class loader goes unreachable. Could I for example, put a weak reference on each class so I can detect when the class goes unreachable and act when the class goes unreachable? (not sure what action I could take though).
Update in response to #Kayaman
My use case is that I have the core application and then based on the customers requirements I can load different classes that implement known interfaces in the core application (so they can be accessed). Then the core application kicks off various processes which use these classes. The big strength of this is that I can update these plugin classes without doing a big redeploy and every customer does not need every one of these. The problem comes in when I want to load a new version of one of these and the current version is in use. Kinda stands to reason that this is not possible.
Conclusion
#Kayamam many thanks for your consult. It has been very helpful. This has codified my thinking somewhat. The conclusion is that no matter what technique I use you cannot ever, with the VM the way it is, reload a class for which there is currently a strongly reachable object. For some of my reloads I have control over these objects as I can make them unreachable before I unload and reload but for other classes I cannot do this... that is where my problem lies. What I need to do is to ring fence the objects of the classes that I wish to reload so that I can the process that is using them to pause while I reload the classes for those objects.
As you said, in order to unload a class, you need to get rid of the classloader. For example URLClassLoader can be used to load classes, and then null out the reference to make it eligible for GC and therefore unload the classes it loaded.
However, all classes know which classloader loaded them. That means that if you've got instances of your classes in use, they have a reference to the Class which has a reference to the ClassLoader and will prevent it from being collected and classes being unloaded. This is understandable, since having an object without a class would be quite an interesting situation.
So for a full reload you need to get rid of old instances and get rid of the classloader. That also avoids situations where you get mysterious exceptions about MyClass != MyClass.
WeakReference (or PhantomReference would probably be better here) would allow you to notice when your existing objects get collected, you just need to make sure you're tracking them all.
Spring adds to the complexity here, so I strongly recommend spending some time imagining that this approach is impossible, and to see if Spring has something that could be used to fulfil your business requirement. After all it does a lot of classloading itself, so you might be reinventing the wheel in a less clear way.
With quick Googling I found this http://docs.spring.io/spring-boot/docs/current/reference/html/howto-hotswapping.html which mentions among other things Spring Loaded which can apparently do class reloading and then some.
In trying to learn about Java class loaders from Wikipedia, I think I can see why they have the three major class loaders:
1) Bootstrap class loader
2) Extensions class loader
3) System class loader
They go on to say you can define your own classloader. I'm not sure I see the value in defining your own, but the following quote from Wikipedia really makes me wonder:
The most complex JAR hell problems arise in circumstances that take
advantage of the full complexity of the classloading system. A Java
program is not required to use only a single "flat" classloader, but
instead may be composed of several (potentially very many) nested,
cooperating classloaders. Classes loaded by different classloaders may
interact in complex ways not fully comprehended by a developer,
leading to errors or bugs that are difficult to analyze, explain, and
resolve.
If it's so complex, why bother with it? Shouldn't the three already-defined classloaders be enough?
(And yes, for those curious, I did run into a ClassCastException that I didn't think should have happened, much like the graphic labelled Figure 2. Class identity crisis. I'm trying to understand the background is all.)
Certain use cases require custom classloaders.
A few examples:
Dynamically adding new folders/jars to be loadable. (Without restarting the whole application).
Dynamically removing folder/jars from being loadable.
Runtime bytecode generation with javassist.
Multiple (actually used at the same time) versions of the same classes in the same application/jvm
I don't want to use the URL Classloader to load classes.
I want to implement this myself.
I don't want to use a solution like JRebel (although it's great).
I've got prior experience of JavaAssist, bytecode generation, implementing javaagent class transformers etc.
I would like to write a javaagent which hooks into the classloader or defines it's own system classloader.
I'll store the class files in an in memory cache, and for particular files, periodically reload them from disk.
I'd prefer to do this in a way which doesn't involve continuously polling the file system and manually invalidating specific classes. I'd much rather intercept class loading events.
I last messed around with this stuff 4 years ago, and I'm sure, although my memory may deceive me that it was possible to do, but 8 hours of searching google doesn't present an obvious solution beyond building a patched JVM.
Is this actually possible?
I've created a stub implementation at https://github.com/packetops/poc_agent if anyone's interested in a simple example of javaagent use.
update
Just found this post - I may have been using the wrong approach, I'll investigate further.
It depends on what you want to do. If you want to reload your classes and define new ones, then you are fine with implementing your own classloader, as you already found.
If you want to replace existing classes, things become more "envolved". You can do this by implementing your own tiny Java agent. See the Java documentation, how to do this: http://docs.oracle.com/javase/7/docs/api/java/lang/instrument/package-summary.html
With the instrumentation mechanism you can not freely redefine classes, quote from Instrumentation.redefineClass:
The redefinition may change method bodies, the constant pool and attributes. The redefinition must not add, remove or rename fields or methods, change the signatures of methods, or change inheritance. These restrictions maybe be lifted in future versions. The class file bytes are not checked, verified and installed until after the transformations have been applied, if the resultant bytes are in error this method will throw an exception.
If you want to do more, you need to load it again. This can be done under the same name, by using a different classloader. The previous class definition will be unloaded, if no one else is using it any more. So, you need to reload any class that uses your previous class also. Utlimatly, you end up reinventing something like OSGi. Take a look at: Unloading classes in java?
I have a servlet which takes in some parameters. One of the parameters include the class name which should do some operations based on the other parameters sent to the servlet. The class name parameter is dynamic so the servlet wouldn't know which class to load beforehand.
Since the classes change frequently (for maintenance) i need to keep the classes outside of the Application since I cannot restart server in the production environment.
Also I need to keep in mind if the classes change the servlet needs to load up the most recent version of the class.What is the best option that i have.
Use a rule engine.
Use dynamic class loading but I am scared that after some there will be simply too many classes loaded in the memory and permgen space will suffer.
Please advise. Thanks in advance
This sounds like potentially dangerous in terms of security, but I am assuming you have taken steps to mitigate already.
The most straight-forward way, I believe, is to use dynamic class loading. Construct a URL classloader to load off a fixed file system location (make sure people cannot arbitrarily run any class they want!) load the class using that and execute the code (possibly via reflection as the class will not be visible on your web application's classloader) and properly dispose of the classloader and any objects created (such that there are no memory leak) As you mentioned, you might need to increase PermGen, but that goes for any solution that involves loading additional classes (which is part of your requirement)