Change default attachedOutputs to include classes and test-classes

src/main/java/org/apache/maven/buildcache/CacheControllerImpl.java

@@ -930,7 +930,7 @@ public void attachGeneratedSources(MavenProject project) throws IOException {
     }
 
     private void attachOutputs(MavenProject project) throws IOException {
-        final List<DirName> attachedDirs = cacheConfig.getAttachedOutputs();
+        final List<DirName> attachedDirs = cacheConfig.getAttachedOutputs(project);
         for (DirName dir : attachedDirs) {
             final Path targetDir = Paths.get(project.getBuild().getDirectory());
             final Path outputDir = targetDir.resolve(dir.getValue());

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

@@ -35,6 +35,7 @@
 import org.apache.maven.model.Plugin;
 import org.apache.maven.model.PluginExecution;
 import org.apache.maven.plugin.MojoExecution;
+import org.apache.maven.project.MavenProject;
 
 /**
  * A java interface to the information configured in the maven-build-cache-config.xml file
@@ -106,7 +107,7 @@ public interface CacheConfig {
 
     String getLocalRepositoryLocation();
 
-    List<DirName> getAttachedOutputs();
+    List<DirName> getAttachedOutputs(MavenProject project);
 
     boolean adjustMetaInfVersion();
 

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

@@ -67,6 +67,7 @@
 import org.apache.maven.model.Plugin;
 import org.apache.maven.model.PluginExecution;
 import org.apache.maven.plugin.MojoExecution;
+import org.apache.maven.project.MavenProject;
 import org.apache.maven.rtinfo.RuntimeInformation;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -95,6 +96,7 @@ public class CacheConfigImpl implements org.apache.maven.buildcache.xml.CacheCon
     public static final String LAZY_RESTORE_PROPERTY_NAME = "maven.build.cache.lazyRestore";
     public static final String RESTORE_ON_DISK_ARTIFACTS_PROPERTY_NAME = "maven.build.cache.restoreOnDiskArtifacts";
     public static final String RESTORE_GENERATED_SOURCES_PROPERTY_NAME = "maven.build.cache.restoreGeneratedSources";
+    public static final String ATTACHED_OUTPUTS_ENABLED_PROPERTY_NAME = "maven.build.cache.attachedOutputs.enabled";
     public static final String ALWAYS_RUN_PLUGINS = "maven.build.cache.alwaysRunPlugins";
     public static final String MANDATORY_CLEAN = "maven.build.cache.mandatoryClean";
 
@@ -572,10 +574,49 @@ public String getLocalRepositoryLocation() {
     }
 
     @Override
-    public List<DirName> getAttachedOutputs() {
+    public List<DirName> getAttachedOutputs(MavenProject project) {
         checkInitializedState();
         final AttachedOutputs attachedOutputs = getConfiguration().getAttachedOutputs();
-        return attachedOutputs == null ? Collections.emptyList() : attachedOutputs.getDirNames();
+        if (attachedOutputs == null) {
+            return getDefaultAttachedOutputs(project);
+        }
+        return attachedOutputs.getDirNames();
+    }
+
+    private List<DirName> getDefaultAttachedOutputs(MavenProject project) {
+        boolean enabled = getProperty(ATTACHED_OUTPUTS_ENABLED_PROPERTY_NAME, true);
+        if (!enabled) {
+            return Collections.emptyList();
+        }
+
+        List<DirName> defaults = new ArrayList<>();
+
+        // Get output directories from project build configuration
+        String buildDirectory = project.getBuild().getDirectory();
+        String outputDirectory = project.getBuild().getOutputDirectory();
+        String testOutputDirectory = project.getBuild().getTestOutputDirectory();
+
+        // Compute relative paths from build directory
+        String classesRelative = getRelativePath(buildDirectory, outputDirectory);
+        String testClassesRelative = getRelativePath(buildDirectory, testOutputDirectory);
+
+        DirName classes = new DirName();
+        classes.setValue(classesRelative);
+        defaults.add(classes);
+
+        DirName testClasses = new DirName();
+        testClasses.setValue(testClassesRelative);
+        defaults.add(testClasses);
+
+        return defaults;
+    }
+
+    private String getRelativePath(String basePath, String fullPath) {
+        // Convert to Path objects and compute relative path
+        Path base = Paths.get(basePath);
+        Path full = Paths.get(fullPath);
+        Path relative = base.relativize(full);
+        return relative.toString();
     }
 
     @Override

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

@@ -374,15 +374,17 @@ 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).
+                    When omitted, the cache defaults to saving and restoring the 'classes' and 'test-classes' directories.</description>
                 </field>
             </fields>
         </class>

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.

src/test/java/org/apache/maven/buildcache/xml/CacheConfigImplTest.java

@@ -31,6 +31,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Optional;
 import java.util.Properties;
@@ -39,13 +40,17 @@
 import org.apache.commons.lang3.tuple.Pair;
 import org.apache.maven.buildcache.DefaultPluginScanConfig;
 import org.apache.maven.buildcache.hash.HashFactory;
+import org.apache.maven.buildcache.xml.config.AttachedOutputs;
 import org.apache.maven.buildcache.xml.config.Configuration;
+import org.apache.maven.buildcache.xml.config.DirName;
 import org.apache.maven.buildcache.xml.config.Remote;
 import org.apache.maven.execution.MavenExecutionRequest;
 import org.apache.maven.execution.MavenSession;
+import org.apache.maven.model.Build;
 import org.apache.maven.model.Plugin;
 import org.apache.maven.model.PluginExecution;
 import org.apache.maven.plugin.MojoExecution;
+import org.apache.maven.project.MavenProject;
 import org.apache.maven.rtinfo.RuntimeInformation;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -169,7 +174,7 @@ private void assertDefaults(Map<String, Runnable> overrides) {
         asserts.put("calculateProjectVersionChecksum", () -> assertFalse(testObject.calculateProjectVersionChecksum()));
         asserts.put("canIgnore", () -> assertFalse(testObject.canIgnore(mock(MojoExecution.class))));
         asserts.put("getAlwaysRunPlugins", () -> assertNull(testObject.getAlwaysRunPlugins()));
-        asserts.put("getAttachedOutputs", () -> assertEquals(Collections.emptyList(), testObject.getAttachedOutputs()));
+        // getAttachedOutputs removed - requires MavenProject parameter, tested separately
         asserts.put("getBaselineCacheUrl", () -> assertNull(testObject.getBaselineCacheUrl()));
         asserts.put("getDefaultGlob", () -> assertEquals("*", testObject.getDefaultGlob()));
         asserts.put(
@@ -482,4 +487,111 @@ void testRemoveSaveFinalIgnoredWhenRemoteSaveDisabled() {
                 Pair.of("getUrl", () -> assertEquals("dummy.url.xyz", testObject.getUrl())),
                 Pair.of("isRemoteCacheEnabled", () -> assertTrue(testObject.isRemoteCacheEnabled())));
     }
+
+    @Test
+    void testDefaultAttachedOutputsWhenNotConfigured() {
+        // When attachedOutputs is not configured in XML, should return default list
+        Configuration configuration = new Configuration();
+        // Deliberately not setting attachedOutputs
+        testCacheConfig.setConfiguration(configuration);
+
+        assertEquals(CacheState.INITIALIZED, testObject.initialize());
+
+        // Create mock project with default build configuration
+        MavenProject mockProject = mock(MavenProject.class);
+        Build mockBuild = mock(Build.class);
+        when(mockProject.getBuild()).thenReturn(mockBuild);
+        when(mockBuild.getDirectory()).thenReturn("/project/target");
+        when(mockBuild.getOutputDirectory()).thenReturn("/project/target/classes");
+        when(mockBuild.getTestOutputDirectory()).thenReturn("/project/target/test-classes");
+
+        List<DirName> attachedOutputs = testObject.getAttachedOutputs(mockProject);
+        assertEquals(2, attachedOutputs.size(), "Should have 2 default attached outputs");
+
+        List<String> dirNames = attachedOutputs.stream()
+                .map(DirName::getValue)
+                .collect(Collectors.toList());
+
+        assertTrue(dirNames.contains("classes"), "Should include 'classes' directory by default");
+        assertTrue(dirNames.contains("test-classes"), "Should include 'test-classes' directory by default");
+    }
+
+    @Test
+    void testExplicitAttachedOutputsOverridesDefaults() {
+        // When attachedOutputs is explicitly configured, should use those values instead of defaults
+        Configuration configuration = new Configuration();
+        AttachedOutputs attachedOutputs = new AttachedOutputs();
+
+        DirName customDir = new DirName();
+        customDir.setValue("custom-output");
+        attachedOutputs.addDirName(customDir);
+
+        configuration.setAttachedOutputs(attachedOutputs);
+        testCacheConfig.setConfiguration(configuration);
+
+        assertEquals(CacheState.INITIALIZED, testObject.initialize());
+
+        // Create mock project (not used when explicit config is set, but required by interface)
+        MavenProject mockProject = mock(MavenProject.class);
+        Build mockBuild = mock(Build.class);
+        when(mockProject.getBuild()).thenReturn(mockBuild);
+        when(mockBuild.getDirectory()).thenReturn("/project/target");
+        when(mockBuild.getOutputDirectory()).thenReturn("/project/target/classes");
+        when(mockBuild.getTestOutputDirectory()).thenReturn("/project/target/test-classes");
+
+        List<DirName> result = testObject.getAttachedOutputs(mockProject);
+        assertEquals(1, result.size(), "Should have 1 explicitly configured output");
+        assertEquals("custom-output", result.get(0).getValue(),
+                "Should use explicitly configured directory, not defaults");
+    }
+
+    @Test
+    void testDefaultAttachedOutputsDisabledViaProperty() {
+        // When attachedOutputs.enabled property is false, should return empty list
+        Configuration configuration = new Configuration();
+        testCacheConfig.setConfiguration(configuration);
+
+        when(mockProperties.getProperty(CacheConfigImpl.ATTACHED_OUTPUTS_ENABLED_PROPERTY_NAME))
+                .thenReturn("false");
+
+        assertEquals(CacheState.INITIALIZED, testObject.initialize());
+
+        // Create mock project
+        MavenProject mockProject = mock(MavenProject.class);
+        Build mockBuild = mock(Build.class);
+        when(mockProject.getBuild()).thenReturn(mockBuild);
+        when(mockBuild.getDirectory()).thenReturn("/project/target");
+        when(mockBuild.getOutputDirectory()).thenReturn("/project/target/classes");
+        when(mockBuild.getTestOutputDirectory()).thenReturn("/project/target/test-classes");
+
+        List<DirName> attachedOutputs = testObject.getAttachedOutputs(mockProject);
+        assertEquals(0, attachedOutputs.size(), "Should return empty list when disabled via property");
+    }
+
+    @Test
+    void testDefaultAttachedOutputsWithCustomDirectories() {
+        // When project has custom output directories, should use those
+        Configuration configuration = new Configuration();
+        testCacheConfig.setConfiguration(configuration);
+
+        assertEquals(CacheState.INITIALIZED, testObject.initialize());
+
+        // Create mock project with custom output directories
+        MavenProject mockProject = mock(MavenProject.class);
+        Build mockBuild = mock(Build.class);
+        when(mockProject.getBuild()).thenReturn(mockBuild);
+        when(mockBuild.getDirectory()).thenReturn("/project/build");
+        when(mockBuild.getOutputDirectory()).thenReturn("/project/build/custom-classes");
+        when(mockBuild.getTestOutputDirectory()).thenReturn("/project/build/custom-test-classes");
+
+        List<DirName> attachedOutputs = testObject.getAttachedOutputs(mockProject);
+        assertEquals(2, attachedOutputs.size(), "Should have 2 default attached outputs");
+
+        List<String> dirNames = attachedOutputs.stream()
+                .map(DirName::getValue)
+                .collect(Collectors.toList());
+
+        assertTrue(dirNames.contains("custom-classes"), "Should include custom output directory");
+        assertTrue(dirNames.contains("custom-test-classes"), "Should include custom test output directory");
+    }
 }