Change default attachedOutputs to include classes and test-classes

Previously, when attachedOutputs was not specified in maven-build-cache-config.xml,
the build cache would return an empty list, meaning no directories were cached
by default. This caused issues where compiled classes were not restored from cache,
leading to build failures or inconsistent behavior.

Changes:
- Modified CacheConfigImpl.getAttachedOutputs() to return a default list containing
  'classes' and 'test-classes' directories when attachedOutputs is not configured
- Added getDefaultAttachedOutputs() helper method to create the default configuration
- Updated usage.md to reflect that classes and test-classes are now restored by default
- Updated build-cache-config.mdo documentation to document the new default behavior

This change addresses user reports in issues apache#259 and apache#340 where builds failed
after cache restoration due to missing compiled classes. It also aligns with
user expectations from PR apache#177 discussion where this default was requested.

Fixes: apache#259, apache#340
Related: apache#177

Author: cowwoc

src/main/java/org/apache/maven/buildcache/xml/CacheConfigImpl.java

@@ -575,7 +575,24 @@ public String getLocalRepositoryLocation() {
     public List<DirName> getAttachedOutputs() {
         checkInitializedState();
         final AttachedOutputs attachedOutputs = getConfiguration().getAttachedOutputs();
-        return attachedOutputs == null ? Collections.emptyList() : attachedOutputs.getDirNames();
+        if (attachedOutputs == null) {
+            return getDefaultAttachedOutputs();
+        }
+        return attachedOutputs.getDirNames();
+    }
+
+    private List<DirName> getDefaultAttachedOutputs() {
+        List<DirName> defaults = new ArrayList<>();
+
+        DirName classes = new DirName();
+        classes.setValue("classes");
+        defaults.add(classes);
+
+        DirName testClasses = new DirName();
+        testClasses.setValue("test-classes");
+        defaults.add(testClasses);
+
+        return defaults;
     }
 
     @Override

src/main/mdo/build-cache-config.mdo

@@ -374,19 +374,20 @@ under the License.
           -->
         <class>
             <name>AttachedOutputs</name>
-            <description>Section relative to outputs which are not artifacts but need to be saved/restored.</description>
+            <description>Section relative to outputs which are not artifacts but need to be saved/restored.
+            If not specified, defaults to caching 'classes' and 'test-classes' directories.</description>
             <fields>
                 <field>
                     <name>dirNames</name>
                     <association>
                         <type>DirName</type>
                         <multiplicity>*</multiplicity>
                     </association>
-                    <description>Path to a directory containing files which needs to be saved/restored (relative to the build directory).</description>
+                    <description>Path to a directory containing files which needs to be saved/restored (relative to the build directory).
+                    Default: classes and test-classes directories are cached if this section is not specified.</description>
                 </field>
             </fields>
-        </class>
-        <class>
+        </class>        <class>
             <name>DirName</name>
             <description><![CDATA[Path to a directory containing files which needs to be saved/restored (relative to the build directory).
             <br>

src/site/markdown/usage.md

@@ -72,8 +72,7 @@ When a configuration is disabled by default in the config, it can be enabled via
 
 Build cache extension is generally compatible with IDEs with one limitation:
 
-* The cache doesn't restore the entire project state. Compiled classes, unpacked artifacts, and similar ones typically
-  will not be restored in the build directory (aka `target`). Configure your IDE to not use Maven
-  build (`target`) directories for compilation and execution. In that case, IDE will provide fast compilation using
-  native caches, and
-  the build cache will supplement that with fast builds.
+* The cache restores `classes` and `test-classes` directories by default, but may not restore other artifacts like
+  unpacked dependencies or generated resources. Configure your IDE to not use Maven build (`target`) directories for
+  compilation and execution if you experience issues. In that case, IDE will provide fast compilation using
+  native caches, and the build cache will supplement that with fast builds.