Sharing a ClassLoader with Drools 5.6.0 to allow the in-memory compilation of classes at runtime - classloader

Question

I'm writing a simple utility which reads XML files, converts their nodes into POJOs, loads them into a Drools' WM and finally applies some rules to them. You can find the whole project on my GitHub profile. Unfortunately, despite all my efforts, I couldn't make Drools to "like" any instance of classes that were compiled at runtime. I saw many people also having problems with the ClassLoader so I suspect it might be its fault... I prepared a Minimal Working Example for you to try which is available on GitHub and here below. It requires a few other small files (MemoryFileManager, MemoryJavaClassObject and MemoryJavaFileObject) which are only available on GitHub for brevity. In order to work properly, this example requires that your JVM is a JDK >= 6, and that you have tools.jar or classes.jar on your classpath. The example is the following:
public class Example {
/**
* #param args
*/
public static void main(String[] args) {
// Setting the strings that we are going to use...
String name = "Person";
String content = "public class " + name + " {\n";
content += " private String name;\n";
content += " public Person() {\n";
content += " }\n";
content += " public Person(String name) {\n";
content += " this.name = name;\n";
content += " }\n";
content += " public String getName() {\n";
content += " return name;\n";
content += " }\n";
content += " public void setName(String name) {\n";
content += " this.name = name;\n";
content += " }\n";
content += " #Override\n";
content += " public String toString() {\n";
content += " return \"Hello, \" + name + \"!\";\n";
content += " }\n";
content += "}\n";
String value = "HAL";
String rules = "rule \"Alive\"\n";
rules += "when\n";
rules += "then\n";
rules += " System.out.println(\"I'm alive!\")\n";
rules += "end\n";
rules += "\n";
rules += "rule \"Print\"\n";
rules += "when\n";
rules += " $o: Object()\n";
rules += "then\n";
rules += " System.out.println(\"DRL> \" + $o.toString())\n";
rules += "end\n";
// Compiling the given class in memory
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
JavaFileManager manager = new MemoryFileManager(compiler.getStandardFileManager(null, null, null));
ClassLoader classLoader = manager.getClassLoader(null);
List<JavaFileObject> files = new ArrayList<JavaFileObject>();
files.add(new MemoryJavaFileObject(name, content));
compiler.getTask(null, manager, null, null, null, files).call();
try {
// Instantiate and set the new class
Class<?> person = classLoader.loadClass(name);
Method method = person.getMethod("setName", String.class);
Object instance = person.newInstance();
method.invoke(instance, value);
System.out.println(instance);
System.out.println("We get a salutation, so Person is now a compiled class in memory loaded by the given ClassLoader.");
// Use the same instance in Drools (by means of the shared ClassLoader)
KnowledgeBuilderConfiguration config1 = KnowledgeBuilderFactory.newKnowledgeBuilderConfiguration(null, classLoader);
KnowledgeBuilder builder = KnowledgeBuilderFactory.newKnowledgeBuilder(config1);
builder.add(ResourceFactory.newByteArrayResource(rules.getBytes()), ResourceType.DRL);
if (builder.hasErrors()) {
for (KnowledgeBuilderError error : builder.getErrors())
System.out.println(error.toString());
System.exit(-1);
}
KnowledgeBaseConfiguration config2 = KnowledgeBaseFactory.newKnowledgeBaseConfiguration(null, classLoader);
KnowledgeBase base = KnowledgeBaseFactory.newKnowledgeBase(config2);
base.addKnowledgePackages(builder.getKnowledgePackages());
StatefulKnowledgeSession session = base.newStatefulKnowledgeSession();
session.insert(instance);
session.fireAllRules();
} catch (ClassNotFoundException e) {
System.out.println("Class not found!");
} catch (IllegalAccessException e) {
System.out.println("Illegal access!");
} catch (InstantiationException e) {
System.out.println("Instantiation!");
} catch (NoSuchMethodException e) {
System.out.println("No such method!");
} catch (InvocationTargetException e) {
System.out.println("Invocation target!");
}
System.out.println("Done.");
}
}
If I run the example, I get the following output:
Hello, HAL!
We get a salutation, so Person is now a compiled class in memory loaded by the given ClassLoader.
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
Exception in thread "main" java.lang.NoClassDefFoundError: Object (wrong name: Person)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
at java.lang.ClassLoader.defineClass(ClassLoader.java:642)
at bragaglia.skimmer.core.MemoryFileManager$1.findClass(MemoryFileManager.java:33)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:340)
at org.drools.util.CompositeClassLoader$CachingLoader.load(CompositeClassLoader.java:258)
at org.drools.util.CompositeClassLoader$CachingLoader.load(CompositeClassLoader.java:237)
at org.drools.util.CompositeClassLoader.loadClass(CompositeClassLoader.java:88)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at org.drools.base.ClassTypeResolver.resolveType(ClassTypeResolver.java:155)
at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:174)
at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:135)
at org.drools.rule.builder.GroupElementBuilder.build(GroupElementBuilder.java:67)
at org.drools.rule.builder.RuleBuilder.build(RuleBuilder.java:85)
at org.drools.compiler.PackageBuilder.addRule(PackageBuilder.java:3230)
at org.drools.compiler.PackageBuilder.compileRules(PackageBuilder.java:1038)
at org.drools.compiler.PackageBuilder.compileAllRules(PackageBuilder.java:946)
at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:938)
at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:470)
at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:698)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:51)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:40)
at bragaglia.skimmer.core.Example.main(Example.java:91)
As you can see, the Person class is successfully compiled in memory and instantiated (see the message Hello, HAL! in the output), however if I add it to a WM I get an Exception in thread "main" java.lang.NoClassDefFoundError: Object (wrong name: Person) even if no rule explicitly relies on Persons. Now, I investigated the exception a little bit and I realised that it gets fired when the given class (Person) is not found within the ClassLoader used by Drools. Therefore I changed my code by adding a configuration with a reference to the very same ClassLoader used to compile and instantiate HAL to both the KnowledgeBuilder and the KnowledgeBase, however I might be doing something wrong because I still get the same exception.
Do you have any idea why this happens and how to work around it? Many thanks in advance!

Answer 1

Thanks to a discussion with a fiends of mine (#dsotty, if you read this, I'm referring to you), I have found a solution.
The problem was in my MemoryFileManager which was only saving the bytecode of the last compiled class. Anytime I was trying to retrieve a class in a later time, I could only find the result of the last compilation. Drools needs to access the byte code of each class that enters its WM, therefore the MemoryFileManager was raising the ClassNotFoundException that was blocking the execution.
Now, the solution is as simple as replacing the private MemoryJavaClassObject object; inside it with a private Map<String, MemoryJavaClassObject> objects; where the bytecodes of all the classes can be successfully stored. So, anytime I try to compile something, I also store the bytecode in objects and when I try to find a class, I first have to look for an entry with the given name within objects. That's it!
The code for MemoryFileManager follows, for your convenience and ease of understanding:
public class MemoryFileManager extends ForwardingJavaFileManager<StandardJavaFileManager> {
private Map<String, MemoryJavaClassObject> objects;
public MemoryFileManager(StandardJavaFileManager manager) {
super(manager);
this.objects = new HashMap<>();
}
#Override
public ClassLoader getClassLoader(Location location) {
return new SecureClassLoader() {
#Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
MemoryJavaClassObject object = objects.get(name);
if (null == object)
throw new ClassNotFoundException("Class '" + name + "' not found.");
byte[] b = object.getBytes();
return super.defineClass(name, b, 0, b.length);
}
};
}
#Override
public JavaFileObject getJavaFileForOutput(Location location, String name, Kind kind, FileObject sibling) throws IOException {
MemoryJavaClassObject object = new MemoryJavaClassObject(name, kind);
objects.put(name, object);
return object;
}
}
Further improvements are possible but are not included for brevity.