CompletableFuture / ForkJoinPool Set Class Loader

问题

I tackled down a very specific problem, whose solution seems to be something basic:

My (Spring) application's classloader hierarchy is something like this: SystemClassLoader -> PlatformClassLoader -> AppClassLoader

If I use Java CompleteableFuture to run threads. the ContextClassLoader of the threads is: SystemClassLoader -> PlatformClassLoader -> ThreadClassLoader

Thus, I cannot access any class in AppClassLoader although I have to because all external library classes reside there.

The source base is quite large so I don't want to/can't rewrite all the thread related pieces to something else (e.g. pass a custom executor to each call).

So my question is: How can I make the threads created by e.g. CompleteableFuture.supplyAsync() use the AppClassLoader as a parent? (instead of the PlatformClassloader)

I found out that ForkJoinPool is used to create the threads. But as it seems to me, everything there is static and final. So I doubt that even setting a custom ForkJoinWorkerThreadFactory with a system property will help in this case. Or would it?

Edit to answer the questions from the comments:

where do you deploy to? Is this running within jetty / tomcat / any JEE container?
- I'm using the default Spring Boot setup so an internal tomcat container is used.
What is the exact issue you have?
- The exact issue is: java.lang.IllegalArgumentException: org.keycloak.admin.client.resource.RealmsResource referenced from a method is not visible from class loader
The jobs that you submit to supplyAsync() are created from the AppClassLoader, aren't they?
- The supplyAsync is called from the MainThread which uses the AppClassLoader. But, debugging the applications shows that all such threads have PlatformClassLoader as their parent. As to my understanding, this happens because ForkJoinPool.commonPool() is constructed during the application startup (because it's static) and so uses the default class loader as the parent which is PlatformClassLoader. So, all threads from this pool get PlatformClassLoader as their parent for ContextClassLoader (instead of AppClassLoader).
- When I'm creating my own executor inside the MainThread and pass this executor to supplyAsync everything works - and I can see during debugging that indeed now AppClassLoader is the parent of my ThreadClassLoader. Which seems to affirm my assumption in the first case that the common pool is not created by MainThread at least not when it's using AppClassLoader itself.

Full stacktrace:

java.lang.IllegalArgumentException: org.keycloak.admin.client.resource.RealmsResource referenced from a method is not visible from class loader
    at java.base/java.lang.reflect.Proxy$ProxyBuilder.ensureVisible(Proxy.java:851) ~[na:na]
    at java.base/java.lang.reflect.Proxy$ProxyBuilder.validateProxyInterfaces(Proxy.java:682) ~[na:na]
    at java.base/java.lang.reflect.Proxy$ProxyBuilder.<init>(Proxy.java:628) ~[na:na]
    at java.base/java.lang.reflect.Proxy.lambda$getProxyConstructor$1(Proxy.java:426) ~[na:na]
    at java.base/jdk.internal.loader.AbstractClassLoaderValue$Memoizer.get(AbstractClassLoaderValue.java:327) ~[na:na]
    at java.base/jdk.internal.loader.AbstractClassLoaderValue.computeIfAbsent(AbstractClassLoaderValue.java:203) ~[na:na]
    at java.base/java.lang.reflect.Proxy.getProxyConstructor(Proxy.java:424) ~[na:na]
    at java.base/java.lang.reflect.Proxy.newProxyInstance(Proxy.java:999) ~[na:na]
    at org.jboss.resteasy.client.jaxrs.ProxyBuilder.proxy(ProxyBuilder.java:79) ~[resteasy-client-3.1.4.Final.jar!/:3.1.4.Final]
    at org.jboss.resteasy.client.jaxrs.ProxyBuilder.build(ProxyBuilder.java:131) ~[resteasy-client-3.1.4.Final.jar!/:3.1.4.Final]
    at org.jboss.resteasy.client.jaxrs.internal.ClientWebTarget.proxy(ClientWebTarget.java:93) ~[resteasy-client-3.1.4.Final.jar!/:3.1.4.Final]
    at org.keycloak.admin.client.Keycloak.realms(Keycloak.java:114) ~[keycloak-admin-client-3.4.3.Final.jar!/:3.4.3.Final]
    at org.keycloak.admin.client.Keycloak.realm(Keycloak.java:118) ~[keycloak-admin-client-3.4.3.Final.jar!/:3.4.3.Final]

回答1:

So, here is a very dirty solution of which I'm not proud of and may break things for you if you go along with it:

The problem was that the classloader of the application was not used for ForkJoinPool.commonPool(). Because the setup of commonPool is static and therefor during the application start up there is no easy possibility (at least to my knowledge) to make changes later. So we need to rely on Java reflection API.

create a hook after your application successfully started
- in my case (Spring Boot environment) this will be the ApplicationReadyEvent
- to listen to this event you need a component like the following
```
@Component
class ForkJoinCommonPoolFix : ApplicationListener<ApplicationReadyEvent> {
    override fun onApplicationEvent(event: ApplicationReadyEvent?) {
  }
}
```

Inside your hook you need to set ForkJoinWorkerThreadFactory of commonPool to a custom implementation (so this custom implementation will use the app classloader)

in Kotlin

val javaClass = ForkJoinPool.commonPool()::class.java
val field = javaClass.getDeclaredField("factory")
field.isAccessible = true
val modifiers = field::class.java.getDeclaredField("modifiers")
modifiers.isAccessible = true
modifiers.setInt(field, field.modifiers and Modifier.FINAL.inv())
field.set(ForkJoinPool.commonPool(), CustomForkJoinWorkerThreadFactory())
field.isAccessible = false

Simple implementation of CustomForkJoinWorkerThreadFactory

in Kotlin

//Custom class
class CustomForkJoinWorkerThreadFactory : ForkJoinPool.ForkJoinWorkerThreadFactory {
  override fun newThread(pool: ForkJoinPool?): ForkJoinWorkerThread {
    return CustomForkJoinWorkerThread(pool)
  }
}
// helper class (probably only needed in kotlin)
class CustomForkJoinWorkerThread(pool: ForkJoinPool?) : ForkJoinWorkerThread(pool)

If you need more information about reflection and why it's not good to change final fields please refer to here and here. Short summary: due to optimizations the updated final field may not be visible to other objects and other unknown side effects may occur.

As stated before: this is a very dirty solution. Unwanted side effects may occur if you use this solution. Using reflections like this is not a good idea. If you can use a solution without reflection (and post it as an answer here!).

Edit: Alternative for single calls

Like stated in the question itself: if you only have this problem in a small number of places (i.e. it's no problem to fix the call itself) you can use your own Executor. A simple example copied from here:

ExecutorService pool = Executors.newFixedThreadPool(10);
final CompletableFuture<String> future = 
    CompletableFuture.supplyAsync(() -> { /* ... */ }, pool);

回答2:

It seems that resteasy lib uses thread context classloader to load some resources: http://grepcode.com/file/repo1.maven.org/maven2/org.jboss.resteasy/resteasy-client/3.0-beta-1/org/jboss/resteasy/client/jaxrs/ProxyBuilder.java#21.

When resteasy try to load the requested class, it will ask the thread classloader to look for it and load it if possible, when the requested class is located in a classpath not visible by the classloader, the operation fails.

And that exactly what happens to your application: ThreadClassLoader tried to load a resource located in the application classpath, since resources from this classpath are accessible only from AppClassLoader and its children, then the ThreadClassLoader has failed to load it (ThreadClassLoader is not a child of AppClassLoader).

A possible solution could be to override the thread context ClassLoader by your app ClassLoader: thread.setContextClassLoader(appClass.class.getClassLoader())

回答3:

I ran into something similar and came up with a solution that does not use reflection and seems to work well with JDK9-JDK11.

Here is what the javadocs say:

The parameters used to construct the common pool may be controlled by setting the following system properties:

java.util.concurrent.ForkJoinPool.common.threadFactory - the class name of a ForkJoinPool.ForkJoinWorkerThreadFactory. The system class loader is used to load this class.

So if you rollout your own version of the ForkJoinWorkerThreadFactory and set that instead to use the correct ClassLoader using the system property, this should work.

Here is my custom ForkJoinWorkerThreadFactory:

package foo;

public class MyForkJoinWorkerThreadFactory implements ForkJoinWorkerThreadFactory {

    @Override
    public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
        return new MyForkJoinWorkerThread(pool);
    }

    private static class MyForkJoinWorkerThread extends ForkJoinWorkerThread {

        private MyForkJoinWorkerThread(final ForkJoinPool pool) {
            super(pool);
            // set the correct classloader here
            setContextClassLoader(Thread.currentThread().getContextClassLoader());
        }
    }
}

and then set the system property in your app startup script

-Djava.util.concurrent.ForkJoinPool.common.threadFactory=foo.MyForkJoinWorkerThreadFactory

The above solution works assuming that when the ForkJoinPool class is referenced the first time and it initializes the commonPool, the context ClassLoader for this Thread is the correct one that you need (and is not the System class loader).

Here is some background that might help:

Fork/Join common pool threads return the system class loader as their thread context class loader.

In Java SE 9, threads that are part of the fork/join common pool will always return the system class loader as their thread context class loader. In previous releases, the thread context class loader may have been inherited from whatever thread causes the creation of the fork/join common pool thread, e.g. by submitting a task. An application cannot reliably depend on when, or how, threads are created by the fork/join common pool, and as such cannot reliably depend on a custom defined class loader to be set as the thread context class loader.

As a result of the above backward incompatibility change, things that uses the ForkJoinPool that used to worked in JDK8 may not work in JDK9+ .

来源：https://stackoverflow.com/questions/49113207/completablefuture-forkjoinpool-set-class-loader

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java

Spring

classloader

completable-future