Package hudson

Class ExtensionFinder

    • Constructor Detail

      • ExtensionFinder

        public ExtensionFinder()
    • Method Detail

      • findExtensions

        @Restricted(org.kohsuke.accmod.restrictions.NoExternalUse.class)
        @Deprecated
        public <T> Collection<T> findExtensions​(Class<T> type,
                                                Hudson hudson)
        Deprecated.
        as of 1.356 Use and implement find(Class,Hudson) that allows us to put some metadata.
      • isRefreshable

        public boolean isRefreshable()
        Returns true if this extension finder supports the refresh() operation.
      • find

        public abstract <T> Collection<ExtensionComponent<T>> find​(Class<T> type,
                                                                   Hudson jenkins)
        Discover extensions of the given type.

        This method is called only once per the given type after all the plugins are loaded, so implementations need not worry about caching.

        This method should return all the known components at the time of the call, including those that are discovered later via refresh(), even though those components are separately returned in ExtensionComponentSet.

        Type Parameters:
        T - The type of the extension points. This is not bound to ExtensionPoint because of Descriptor, which by itself doesn't implement ExtensionPoint for a historical reason.
        Parameters:
        jenkins - Jenkins whose behalf this extension finder is performing lookup.
        Returns:
        Can be empty but never null.
        Since:
        1.356 Older implementations provide findExtensions(Class,Hudson)
      • scout

        public void scout​(Class extensionType,
                          Hudson hudson)
        Performs class initializations without creating instances. If two threads try to initialize classes in the opposite order, a dead lock will ensue, and we can get into a similar situation with ExtensionFinders.

        That is, one thread can try to list extensions, which results in ExtensionFinder loading and initializing classes. This happens inside a context of a lock, so that another thread that tries to list the same extensions don't end up creating different extension instances. So this activity locks extension list first, then class initialization next.

        In the mean time, another thread can load and initialize a class, and that initialization can eventually results in listing up extensions, for example through static initializer. Such activity locks class initialization first, then locks extension list.

        This inconsistent locking order results in a dead lock, you see.

        So to reduce the likelihood, this method is called in prior to find(Class,Hudson) invocation, but from outside the lock. The implementation is expected to perform all the class initialization activities from here.

        See https://bugs.openjdk.java.net/browse/JDK-4993813 for how to force a class initialization. Also see http://kohsuke.org/2010/09/01/deadlock-that-you-cant-avoid/ for how class initialization can results in a dead lock.