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

[Godmar Back]

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