Fix weaving of already compiled classes when source code is absent

src/it/WeaveDirectoriesNoSources/invoker.properties

@@ -0,0 +1 @@
+invoker.goals = clean process-classes

src/it/WeaveDirectoriesNoSources/pom.xml

@@ -0,0 +1,60 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  <groupId>org.codehaus.mojo.aspectj.it</groupId>
+  <artifactId>weave-directories-no-sources</artifactId>
+  <version>0.0.1-SNAPSHOT</version>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.aspectj</groupId>
+      <artifactId>aspectjrt</artifactId>
+      <version>@aspectjVersion@</version>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <version>3.8.1</version>
+        <executions>
+          <execution>
+            <id>default-compile</id>
+            <configuration>
+              <source>1.8</source>
+              <target>1.8</target>
+              <!-- Compile to a different directory to simulate pre-compiled classes -->
+              <outputDirectory>${project.build.directory}/unwoven-classes</outputDirectory>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+      <plugin>
+        <groupId>org.codehaus.mojo</groupId>
+        <artifactId>aspectj-maven-plugin</artifactId>
+        <version>@project.version@</version>
+        <executions>
+          <execution>
+            <id>weave-classes</id>
+            <phase>process-classes</phase>
+            <goals>
+              <goal>compile</goal>
+            </goals>
+            <configuration>
+              <complianceLevel>1.8</complianceLevel>
+              <!-- Empty sources - we only want to weave pre-compiled classes with aspects from aspectDirectory -->
+              <sources/>
+              <!-- Weave the classes compiled by maven-compiler-plugin -->
+              <weaveDirectories>
+                <weaveDirectory>${project.build.directory}/unwoven-classes</weaveDirectory>
+              </weaveDirectories>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+</project>

src/it/WeaveDirectoriesNoSources/src/main/aspect/foo/Azpect.java

@@ -0,0 +1,13 @@
+package foo;
+
+import org.aspectj.lang.JoinPoint;
+import org.aspectj.lang.annotation.Aspect;
+import org.aspectj.lang.annotation.Before;
+
+@Aspect
+public class Azpect {
+    @Before("execution(* foo.Clazz.doSomething(..))")
+    public void beforeDoSomething(JoinPoint joinPoint) {
+        System.out.println("Before doSomething");
+    }
+}

src/it/WeaveDirectoriesNoSources/src/main/java/foo/Clazz.java

@@ -0,0 +1,7 @@
+package foo;
+
+public class Clazz {
+    public void doSomething() {
+        System.out.println("Doing something");
+    }
+}

src/it/WeaveDirectoriesNoSources/verify.groovy

@@ -0,0 +1,19 @@
+import java.io.*;
+
+File targetClasses = new File(basedir, "target/classes");
+File clazzFile = new File(targetClasses, "foo/Clazz.class");
+
+// Verify that the weaved classes exist - this proves the plugin ran and didn't skip
+if (!clazzFile.exists()) {
+    throw new RuntimeException("Clazz.class was not found in target/classes - plugin execution was skipped!");
+}
+
+// Check that the build log shows the aspectj plugin ran
+File buildLog = new File(basedir, "build.log");
+String logContent = buildLog.text;
+if (!logContent.contains("aspectj:")) {
+    throw new RuntimeException("AspectJ plugin did not run - it was skipped!");
+}
+
+System.out.println("SUCCESS: AspectJ plugin executed with empty sources and weaveDirectories configured");
+return true;

src/main/java/org/codehaus/mojo/aspectj/AbstractAjcCompiler.java

@@ -771,7 +771,17 @@ private boolean hasArgumentsChanged(File outDir) throws MojoExecutionException {
      * Not entirely safe, assembleArguments() must be run
      */
     private boolean hasSourcesToCompile() {
-        return resolvedIncludes.size() > 0;
+        return resolvedIncludes.size() > 0 || hasWeavableContent();
+    }
+
+    /**
+     * Checks if there are weave directories or dependencies configured for weaving.
+     *
+     * @return true if there are weave directories or dependencies configured, false otherwise
+     */
+    private boolean hasWeavableContent() {
+        return (weaveDirectories != null && weaveDirectories.length > 0)
+                || (weaveDependencies != null && weaveDependencies.length > 0);
     }
 
     private boolean hasSourcesChanged(File outDir) {