BlockhoundIntegration SPI plugins ordering

[pderop]

The BlockHound.loadIntegration() method is intended to control ordering of BlockhoundIntegration.applyTo calls with respect to other integrations plugins. So, loadIntegrations() first loads all SPI plugins from META-INF/services, then appends any optional plugins provided in the loadIntegration(...) arguments, and finally sort the list before invoking the plugins applyTo methods.

However, by default, the BlockhoundIntegration.compareTo is implemented like this:

default int compareTo(BlockHoundIntegration o) {
    return 0;
}

So, let's say we have these integration plugins being loaded from META-INF/services/... using the BlockHound.loadIntegrations() method:

    public class I1 implements BlockHoundIntegration {
        public void applyTo(BlockHound.Builder builder) {}
    }

    public class I2 implements BlockHoundIntegration {
        public void applyTo(BlockHound.Builder builder) {}
    }

Now, let's say one want to install two more SPI integration plugin I3,I4; but ensuring the following:

• I3 applyTo method must be called after I1, I2 (or I2, I1).
• I4 applyTo method must be called after I3

then one could write this:

    public abstract class OrderedPlugin implements BlockHoundIntegration {
        final int id;

       public OrderedPlugin(int id) {this.id = id;}

        @Override
        public int compareTo(BlockHoundIntegration o) {
            if (o instanceof OrderedPlugin) {
                return Integer.compare(this.id, ((OrderedPlugin) o).id);
            }
            return 1; // always be placed after default plugins
        }
    }

    public class I3 extends OrderedPlugin {
        public I3() { super(1); }

        public void applyTo(BlockHound.Builder builder) {...}
    }

    public class I4 extends OrderedPlugin {
        public I4() { super(2); }

        public void applyTo(BlockHound.Builder builder) {...}
    }

The problem is that it does not work because CompareTo method is not transitive: when sorting, if I1.compareTo(OrderedPlugin) is called, then 0 is returned, hence we can't really sort the list like we want.

as a work around, you need to programatically install I3, I4, like this: this will ensure that plugins will be applied in desired order: I1, I2, I3, I4, but you then can't declare I3, I4 as SPI plugins.

    BlockHound.install(new I3(), new new I4());

now, @johnrengelman suggests a simple patch from #273: in BlockHoundIntegration, introduce a new getPriority method that is called by default by the compareTo method:

public interface BlockHoundIntegration extends Comparable<BlockHoundIntegration> {
    // ...
    default int getPriority() {
        return 0;
    }

    @Override
    default int compareTo(BlockHoundIntegration o) {
        return Integer.compare(getPriority(), o.getPriority());
    }

This resolves the issue, and plugins can simply refine the getPriority() method is they want to take control on the order, else , by default, plugins applyTo methods will be called in the order on which plugins are loaded (natural ordering).

One thing: in netty, the BlockHound integration is overriding the compareTo method , so, for completeness, a PR should be done on netty asking to not override the compareTo method.

Fixes #273

[simonbasle]

LGTM

[pderop]

@simonbasle , thanks for the review !