The ultimate issue is visibility of classes. From most perspectives, there is an execution graph that implies moments when new classes are touched/needed. Where OSGi introduces bundles, it erases the ability to depend on that graph to “expose” the correct version of classes to the correct threads of execution. This is where the bundle model breaks down. When there are a varied set of services with a varied set of versions of various jars, some of which are in conflict with each other, the context class loader exposes the correct version for code to use at that moment the code is running.
This is the number one detriment to how bundles work and resolve in OSGi. Its nearly impossible to use OSGi unless you are in charge of every bundle and update all versions of all software simultaneously. Without context class loading on a per thread basis, you end up pretty restricted in how software can evolve and how deployment of services can be unrelated to each other. Gregg > On Jan 27, 2017, at 11:39 AM, Bharath Kumar <bharathkuma...@gmail.com> wrote: > > Yes Peter. Usage of thread context class loader is discouraged in OSGi > environment. > > http://njbartlett.name/2012/10/23/dreaded-thread-context-classloader.html > > Some of the problems are hard to solve in OSGi environment. For example, > creating dynamic java proxy from 2 or more interfaces that are located in > different bundles. > > http://blog.osgi.org/2008/08/classy-solutions-to-tricky-proxies.html?m=1 > > This problem can be solved using composite class loader. But it is > difficult to write it correctly. Because OSGi environment is dynamic. > > I believe that it is possible to provide enough abstraction in river code, > so that service developers don't even require to use context class loader > in their services. > > > > Thanks & Regards, > Bharath > > > On 27-Jan-2017 6:25 PM, "Peter" <j...@zeus.net.au> wrote: > >> Thanks Gregg, >> >> Thoughts inline below. >> >> Cheers, >> >> Peter. >> >> >> On 27/01/2017 12:35 AM, Gregg Wonderly wrote: >> >>> Is there any thought here about how a single client might use both an >>> OSGi deployed service and a conventionally deployed service? >>> >> >> Not yet, I'm currently considering how to support OSGi by implementing an >> RMIClassLoaderSPI, similar to how Dennis has for Maven in Rio. >> >> I think once a good understanding of OSGi has developed, we can consider >> how an implementation could support that, possibly by exploiting something >> like Pax URL built into PreferredClassProvider. >> >> >> The ContextClassLoader is a good abstraction mechanism for finding “the” >>> approriate class loader. It allows applications to deploy a composite >>> class loader in some form that would be able to resolve classes from many >>> sources and even provide things like preferred classes. >>> >> >> Yes, it works well for conventional frameworks and is utilised by >> PreferredClassProvider, but it's use in OSGi is discouraged, I'd like to >> consider how it's use can be avoided in an OSGi env. >> >> >>> >>> In a Java desktop application, would a transition from a background >>> thread, interacting with a service to get an object from a service which is >>> not completely resolved to applicable loaders still resolve correctly in an >>> EventDispatch Thread? That event dispatch thread can have the context >>> class loader set on it by the thread which got the object, to be the class >>> loader of the service object, to make sure that the resolution of classes >>> happens with the correct class loader such that there will not be a problem >>> with the service object having one set of definitions and another service >>> or the application classpath having a conflicting class definition by the >>> same name. >>> >>> I’ve had to spend quite a bit of time to make sure that these scenarios >>> work correctly in my Swing applications. >>> >> >> Have you got more information? I'm guessing this relates to delayed >> unmarshalling into the EventDispatch thread. >> >> It's early days yet, I'm still working it out what information is required >> to resolve the correct ClassLoaders & bundles, but this is an important >> question, Bharath mentioned Entry's can be utilised for versioning and this >> seems like a good idea. >> >> What follows are thoughts and observations. >> >> A bundle can be created from a URL, provided the codebase the URL refers >> to has an OSGi bundle manifest, so this could allow any of the existing URL >> formats to deliver a proxy codebase for an OSGi framework. When OSGi loads >> the bundle, the package dependencies will be wired up by the local env. If >> the URL doesn't reference a bundle, then we could use Bharath's approach >> and subclass the client's ClassLoader, this does make all the clients >> classes visible to the proxy however, but that would happen anyway in a >> standard Jini / River environment. >> >> OSGi gives preference to already loaded bundles when resolving package >> dependencies, so the client should be careful not to unmarshall any proxy's >> that might require a higher version than the bundle already installed when >> the client bundle resolved its dependencies. >> >> One of the proxy bundle dependencies will be the service api bundle. The >> proxy bundle can limit the service api package / packages to a specific >> version or version range, which it could advertise in an Entry. Boolean >> logic comparison of Entry's would have to be performed locally, after >> matching on the service type (this requires delayed unmarshalling to >> prevent the resolution of unwanted versions). >> >> So, the OSGi bundles installed in remote JVM's communicating with each >> other, may not be exactly the same version, but should be compatible. >> Multiple versions of a package or bundle may be available in a JVM, so we >> need to ensure we've selected one imported by the proxy. >> >> We could define a simple rule, that the first URL in an annotation, must >> be the proxy bundle, with all dependencies to be provisioned by the OSGi >> Framework, which would allow the same codebase annotation to be utilised in >> a non OSGi environment, but this will probably mean the loss of any >> trailing URL annotations if the proxy's remarshalled. >> >> Once the proxy bundle has been loaded, its ClassLoader needs to be the >> default for the remaining stream classes (smart proxy fields), since it >> knows the most about what dependencies are required, these will be visible >> to the proxy's ClassLoader. >> >> If a class doesn't have an existing ClassLoader referenced by annotatation >> and cannot be resolved from the proxy's Bundle, it probably requres a new >> bundle to be resolved. >> >> What's next? Determine what annotations to retrieve from OSGi Bundles. >> >> How PreferredClassProvider currently obtains codebase annotations (Trivia: >> the application/system ClassLoader is a URLClassLoader up to Java 8, but is >> no longer an instance of URLClassLoader in Java 9): >> >> Summary, If ClassLoader: >> >> 1. Is local loader? return java.rmi.server.codebase property or null >> if not defined. >> 2. Is instance of ClassAnnoation, get annotation and return. >> 3. Is instance of URLClassLoader, get URL's as string and return. >> 4. else return java.rmi.server.codebase property or null if not defined. >> >> >> /** >> * Returns the annotation string for the specified class loader >> * (possibly null). If check is true and the annotation would be >> * determined from an invocation of URLClassLoader.getURLs() on >> * the loader, only return the true annotation if the current >> * security context has permission to connect to all of the URLs. >> **/ >> private String getLoaderAnnotation(ClassLoader loader, boolean check) >> { >> >> if (isLocalLoader(loader)) { >> return getClassAnnotation(loader); >> } >> >> /* >> * Get the codebase URL path for the class loader, if it supports >> * such a notion (i.e., if it is a URLClassLoader or subclass). >> */ >> String annotation = null; >> if (loader instanceof ClassAnnotation) { >> /* >> * If the class loader is one of our RMI class loaders, we have >> * already computed the class annotation string, and no >> * permissions are required to know the URLs. >> */ >> annotation = ((ClassAnnotation) loader).getClassAnnotation(); >> >> } else if (loader instanceof java.net.URLClassLoader) { >> try { >> URL[] urls = ((java.net.URLClassLoader) loader).getURLs(); >> if (urls != null) { >> if (check) { >> SecurityManager sm = System.getSecurityManager(); >> if (sm != null) { >> Permissions perms = new Permissions(); >> for (int i = 0; i < urls.length; i++) { >> Permission p = >> urls[i].openConnection().getPermission(); >> if (p != null) { >> if (!perms.implies(p)) { >> sm.checkPermission(p); >> perms.add(p); >> } >> } >> } >> } >> } >> annotation = PreferredClassLoader.urlsToPath(urls); >> } >> } catch (SecurityException e) { >> /* >> * If access was denied to the knowledge of the class >> * loader's URLs, fall back to the default behavior. >> */ >> } catch (IOException e) { >> /* >> * This shouldn't happen, although it is declared to be >> * thrown by openConnection() and getPermission(). If it >> * does happen, forget about this class loader's URLs and >> * fall back to the default behavior. >> */ >> } >> } >> >> if (annotation != null) { >> return annotation; >> } else { >> return getClassAnnotation(loader); >> } >> } >> >> /** >> * Returns the annotation string for the specified class loader. >> * >> * <p>This method is invoked in order to determine the annotation >> * string for the system class loader, an ancestor of the system >> * class loader, any class loader that is not an instance of >> * {@link ClassAnnotation} or {@link URLClassLoader}, or (for an >> * invocation of {@link #getClassAnnotation(Class) >> * getClassAnnotation(Class)}) a <code>URLClassLoader</code> for >> * which the current security context does not have the >> * permissions necessary to connect to all of its URLs. >> * >> * <p><code>PreferredClassProvider</code> implements this method >> * as follows: >> * >> * <p>This method returns the value of the system property >> * <code>"java.rmi.server.codebase"</code> (or possibly an earlier >> * cached value). >> * >> * @param loader the class loader to obtain the annotation string >> * for >> * >> * @return the annotation string for the class loader, or >> * <code>null</code> >> **/ >> protected String getClassAnnotation(ClassLoader loader) { >> checkInitialized(); >> return codebaseProperty; >> } >> >> >> Gregg >>> >>> On Jan 20, 2017, at 6:08 PM, Peter<j...@zeus.net.au> wrote: >>>> >>>> Looking at your modifications to ServiceDiscoveryManager, I noticed >>>> you've made changes to set the context ClassLoader prior to calling the >>>> lookup service. >>>> >>>> This is eventually utilised by PreferredClassProvider to lookup the >>>> necessary loader to utilise for deserialization of the lookup results. >>>> >>>> I think if we develop a RMIClassLoader provider for OSGi, we can avoid >>>> utilising the context ClassLoader. >>>> >>>> Since all OSGi ClassLoader's are instances of BundleReference, it's easy >>>> to utilise OSGi bundle url anotations (I think this needs to incorporate >>>> bundle versions). I'd also like to utilise Java 9 jrt style URL's. >>>> >>>> Cheers, >>>> >>>> Peter. >>>> >>>> On 20/01/2017 11:09 PM, Bharath Kumar wrote: >>>> >>>>> Thanks Peter for the review. >>>>> >>>>> While creating this POC, I tried to make RIO framework as set of OSGI. >>>>> bundles. Rio project extends LookupDiscoveryManager class in one of the >>>>> class .org.rioproject.impl.client.DiscoveryManagementPool.SharedDi >>>>> scoveryManager. >>>>> That's why I removed the final modifier. >>>>> >>>>> >>>>> Regarding groovy files, >>>>> I have made the org.apache.river as system fragment bundle. So we can't >>>>> import any packages from other bundles. But we can use system bundle's >>>>> packages,. That's why i removed groovy files. If we use these groovy >>>>> files, >>>>> we need to import packages from groovy bundle which is not possible >>>>> here. I >>>>> will check JGDMS to see how it is used. >>>>> >>>>> >>>>> Thanks& Regards, >>>>> Bharath >>>>> >>>>> >>>>> On Fri, Jan 20, 2017 at 6:09 PM, Peter<j...@zeus.net.au> wrote: >>>>> >>>>> Hi Bharath, >>>>>> >>>>>> Re your changes (I've found so far): >>>>>> >>>>>> LookupDiscoveryManager is non final, I'm interested why? >>>>>> >>>>>> BasicInvocationDispatcher, you've set the context class loader around a >>>>>> block of code, to use the ClassLoader passed in during construction. >>>>>> I'm >>>>>> currently investigating addong methods where ClassLoader can be passed >>>>>> in >>>>>> for OSGi. >>>>>> >>>>>> Regarding bundle structure, I've restructured the layout here (so you >>>>>> don't need to delete Groovy config): >>>>>> >>>>>> https://github.com/pfirmstone/JGDMS/tree/Maven_build/modularize/JGDMS >>>>>> >>>>>> The full commit history has been retained, so u can see all changes. >>>>>> >>>>>> Cheers, >>>>>> >>>>>> Peter. >>>>>> >>>>>> Sent from my Samsung device. >>>>>> >>>>>> Include original message >>>>>> ---- Original message ---- >>>>>> From: Bharath Kumar<bharathkuma...@gmail.com> >>>>>> Sent: 20/01/2017 09:42:38 pm >>>>>> To: dev@river.apache.org >>>>>> Subject: Re: OSGi >>>>>> >>>>>> Hello all, >>>>>> >>>>>> I have also added a package in org.apache.river bundle to create the >>>>>> river >>>>>> service in osgi environment ( Here RIver >>>>>> uses NonActivatableServiceDescriptor). >>>>>> >>>>>> package name is org.apache.river.start.ext >>>>>> >>>>>> >>>>>> As river bundle is system fragment, i have to remove >>>>>> the groovy dependency. >>>>>> So i removed groovy files. >>>>>> >>>>>> net.jini.config.Component.groovy >>>>>> >>>>>> net.jini.config.GroovyConfig.groovy >>>>>> >>>>>> >>>>>> >>>>>> Thanks& Regards, >>>>>> >>>>>> Bharath >>>>>> >>>>>> >>>>>> >>>>>> >>
