[rules-users] how to specify classpath dependencies in .drl files.

[Godmar Back]

PS: it turns out that Drool's class loader in
org/drools/rule/PackageCompilationData.java falls back to getResource
also (that's why I had to delete .class file in my first attempt at
RLoader.)  Installing a class loader that fails getResource() for
.class files makes it even simpler to see that Drools's compilation
machinery relies on it. So, to convince yourselves, use this loader:

// RLoader.java, v2
import java.io.File;
import java.net.URL;

public class RLoader extends ClassLoader {
    public RLoader() { }
    public RLoader(ClassLoader parent) { super(parent); }
    public Class loadClass(String name) throws ClassNotFoundException {
        return getParent().loadClass(name);
    }

    public URL getResource(String name) {
        if (name.endsWith(".class")) {
            System.out.println("Classloader asked for .class file as
resource URL: " + name);
            new Throwable().printStackTrace();
            return null;
        }
        return getParent().getResource(name);
    }
}

and you can observe the same failure - maybe this convinces you.

 - Godmar


On 9/27/07, Godmar Back <godmar at gmail.com> wrote:
> On 9/27/07, Mark Proctor <mproctor at codehaus.org> wrote:
> >
> >  Godmar Back wrote:
> >  I agree that it may not belong into the .drl file, though loading
> > bytecode through the classloader via getResourceAsStream() also seems
> > a rather ad-hoc solution.
> >
> >
> >  I've already told you, that you are mistaken here. You do not need to keep
> > bytecode hanging around, and available via getResourceAsStream() for any
> > compilation to work.  I can generate a class at runtime, in memory, add it
> > to a classloader and have the engine compile against it without
> > getResourceAsStream() being able to search for and return the .class'
> > bytecode.
> >
>
> Ok - here's an experiment you can perform.
>
> Write a class loader RLoader.java as follows:
>
> // RLoader.java
> import java.io.File;
> import java.io.InputStream;
>
> public class RLoader extends ClassLoader {
>     public RLoader() { }
>     public RLoader(ClassLoader parent) { super(parent); }
>     public Class loadClass(String name) throws ClassNotFoundException {
>         return getParent().loadClass(name);
>     }
>     public InputStream getResourceAsStream(String name) {
>         if (name.endsWith(".class")) {
>             System.out.println("Classloader asked for .class file as
> resource: " + name);
>             new Throwable().printStackTrace();
>
>             // delete the .class file to ensure that no other part of
> drools attempts to read it.
>             new File(name).delete();
>             return null;
>         }
>         return getParent().getResourceAsStream(name);
>     }
> }
>
> Now use this class as your system class loader. Specify:
> -Djava.system.class.loader=RLoader
>
> When you then run your application, you will see output such as:
>
> Classloader asked for .class file as resource:
> org/enhancementpatterns/castor/CastorEnhancements/AddVisitor.class
> java.lang.Throwable
>     at RLoader.getResourceAsStream(RLoader.java:13)
>     at org.drools.rule.PackageCompilationData$PackageClassLoader.getResourceAsStream(PackageCompilationData.java:386)
>     at org.drools.commons.jci.compilers.EclipseJavaCompiler$2.isPackage(EclipseJavaCompiler.java:282)
>     at org.drools.commons.jci.compilers.EclipseJavaCompiler$2.isPackage(EclipseJavaCompiler.java:313)
>     at org.eclipse.jdt.internal.compiler.lookup.LookupEnvironment.isPackage(LookupEnvironment.java:1157)
>     at org.eclipse.jdt.internal.compiler.lookup.PackageBinding.findPackage(PackageBinding.java:75)
>     at org.eclipse.jdt.internal.compiler.lookup.PackageBinding.getTypeOrPackage(PackageBinding.java:190)
>     at org.eclipse.jdt.internal.compiler.lookup.CompilationUnitScope.findImport(CompilationUnitScope.java:438)
>     at org.eclipse.jdt.internal.compiler.lookup.CompilationUnitScope.findSingleImport(CompilationUnitScope.java:492)
>     at org.eclipse.jdt.internal.compiler.lookup.CompilationUnitScope.faultInImports(CompilationUnitScope.java:356)
>     at org.eclipse.jdt.internal.compiler.lookup.CompilationUnitScope.faultInTypes(CompilationUnitScope.java:425)
>     at org.eclipse.jdt.internal.compiler.Compiler.process(Compiler.java:589)
>     at org.eclipse.jdt.internal.compiler.Compiler.compile(Compiler.java:411)
>     at org.drools.commons.jci.compilers.EclipseJavaCompiler.compile(EclipseJavaCompiler.java:353)
>     at org.drools.commons.jci.compilers.AbstractJavaCompiler.compile(AbstractJavaCompiler.java:51)
>     at org.drools.rule.builder.dialect.java.JavaDialect.compileAll(JavaDialect.java:332)
>     at org.drools.compiler.DialectRegistry.compileAll(DialectRegistry.java:60)
>     at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:288)
>     at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:158)
>
> Leading to failures such as:
> Exception in thread "main" org.drools.rule.InvalidRulePackage: Rule
> Compilation error : [Rule name=Add Visitors, agendaGroup=MAIN,
> salience=0, no-loop=false]
>     org/enhancementpatterns/castor/test/Rule_Add_Visitors_0.java
> (3:127) : Only a type can be imported.
> org.enhancementpatterns.castor.CastorEnhancements.AddVisitor resolves
> to a package
>
>  - Godmar
>