Count classpath caching test requests only after the prefetcher settles

src/main/java/rife/bld/BaseProject.java

@@ -1669,6 +1669,9 @@ public class BaseProject extends BuildExecutor {
         return dependencies;
     }
 
+    private final Map<String, List<File>> classpathJarsMemo_ = new HashMap<>();
+    private DependencySet resolvedExtensions_ = null;
+
     /**
      * Creates the version resolution for a dependency scope, applying the
      * version overrides from the properties and the BOMs that are
@@ -1708,16 +1711,39 @@ public class BaseProject extends BuildExecutor {
      * @since 2.4.0
      */
     public List<File> dependencyClasspathJars(Scope scope, String groupId, String artifactId) {
+        var coordinate = groupId + ":" + artifactId;
+        var memo_key = "dependency." + scope + "." + coordinate;
+        var memoized = classpathJarsMemo_.get(memo_key);
+        if (memoized != null) {
+            return memoized;
+        }
+
         var declared = dependencies().scope(scope).get(new Dependency(groupId, artifactId));
         if (declared == null) {
-            throw new IllegalArgumentException("Dependency '" + groupId + ":" + artifactId + "' isn't declared in the " + scope + " scope.");
+            throw new IllegalArgumentException("Dependency '" + coordinate + "' isn't declared in the " + scope + " scope.");
+        }
+
+        var dir = scopeLibDirectory(scope);
+        var cache = new BldCache(libBldDirectory(), new VersionResolution(properties()));
+        cache.cacheDependenciesHash(repositories(), dependencies());
+        if (cache.isDependenciesHashValid()) {
+            var cached = cache.getCachedDependencyClasspath(scope, coordinate);
+            if (cached != null) {
+                var jars = cached.stream().map(name -> new File(dir, name)).toList();
+                classpathJarsMemo_.put(memo_key, jars);
+                return jars;
+            }
         }
 
         var resolver = new DependencyResolver(versionResolution(scope), artifactRetriever(), repositories(), declared);
-        var dir = scopeLibDirectory(scope);
-        return resolver.getAllDependencies(Scope.compile, Scope.runtime).stream()
+        var jars = resolver.getAllDependencies(Scope.compile, Scope.runtime).stream()
             .map(dependency -> new File(dir, dependency.toFileName()))
             .toList();
+
+        cache.cacheDependencyClasspath(scope, coordinate, jars.stream().map(File::getName).toList());
+        cache.writeCache();
+        classpathJarsMemo_.put(memo_key, jars);
+        return jars;
     }
 
     private File scopeLibDirectory(Scope scope) {
@@ -1751,6 +1777,13 @@ public class BaseProject extends BuildExecutor {
      * @since 2.4.0
      */
     public List<File> extensionClasspathJars(String groupId, String artifactId) {
+        var coordinate = groupId + ":" + artifactId;
+        var memo_key = "extension." + coordinate;
+        var memoized = classpathJarsMemo_.get(memo_key);
+        if (memoized != null) {
+            return memoized;
+        }
+
         var wrapper = new Wrapper();
         wrapper.currentDir(workDirectory());
         try {
@@ -1762,25 +1795,45 @@ public class BaseProject extends BuildExecutor {
         var properties = new HierarchicalProperties().parent(BuildExecutor.setupProperties(workDirectory()));
         properties.putAll(wrapper.wrapperProperties());
 
+        var resolution = new VersionResolution(properties);
+        var cache = new BldCache(libBldDirectory(), resolution);
+        cache.cacheExtensionsHash(wrapper.repositories(), wrapper.extensions());
+        if (cache.isExtensionsHashValid()) {
+            var cached = cache.getCachedExtensionClasspath(coordinate);
+            if (cached != null) {
+                var jars = cached.stream().map(name -> new File(libBldDirectory(), name)).toList();
+                classpathJarsMemo_.put(memo_key, jars);
+                return jars;
+            }
+        }
+
         var repositories = new ArrayList<Repository>();
         for (var repository : wrapper.repositories()) {
             repositories.add(Repository.resolveRepository(properties, repository));
         }
 
-        var resolution = new VersionResolution(properties);
-        var extensions = wrapper.extensions().stream().map(Dependency::parse).toList();
-        var resolved = new ParallelDependencyResolver(resolution, artifactRetriever(), repositories)
-            .resolveAllDependencies(extensions, Scope.compile, Scope.runtime);
+        // the resolved set of all the extensions is reused across
+        // lookups of different dependencies in the same build
+        if (resolvedExtensions_ == null) {
+            var extensions = wrapper.extensions().stream().map(Dependency::parse).toList();
+            resolvedExtensions_ = new ParallelDependencyResolver(resolution, artifactRetriever(), repositories)
+                .resolveAllDependencies(extensions, Scope.compile, Scope.runtime);
+        }
 
-        var dependency = resolved.get(new Dependency(groupId, artifactId));
+        var dependency = resolvedExtensions_.get(new Dependency(groupId, artifactId));
         if (dependency == null) {
-            throw new IllegalArgumentException("Dependency '" + groupId + ":" + artifactId + "' isn't part of the extensions of this project.");
+            throw new IllegalArgumentException("Dependency '" + coordinate + "' isn't part of the extensions of this project.");
         }
 
         var resolver = new DependencyResolver(resolution, artifactRetriever(), repositories, dependency);
-        return resolver.getAllDependencies(Scope.compile, Scope.runtime).stream()
+        var jars = resolver.getAllDependencies(Scope.compile, Scope.runtime).stream()
             .map(d -> new File(libBldDirectory(), d.toFileName()))
             .toList();
+
+        cache.cacheExtensionClasspath(coordinate, jars.stream().map(File::getName).toList());
+        cache.writeCache();
+        classpathJarsMemo_.put(memo_key, jars);
+        return jars;
     }
 
     /**

src/main/java/rife/bld/BldCache.java

@@ -5,6 +5,7 @@
 package rife.bld;
 
 import rife.bld.dependencies.DependencyScopes;
+import rife.bld.dependencies.Scope;
 import rife.bld.dependencies.Repository;
 import rife.bld.dependencies.VersionResolution;
 import rife.bld.wrapper.Wrapper;
@@ -40,12 +41,15 @@ public class BldCache {
     private static final String WRAPPER_PROPERTIES_HASH = Wrapper.WRAPPER_PROPERTIES + PROPERTY_SUFFIX_HASH;
     private static final String BLD_BUILD_HASH = "bld-build" + PROPERTY_SUFFIX_HASH;
 
+    private static final String PROPERTY_SUFFIX_CLASSPATH = ".classpath.";
+
     private static final String PROPERTY_EXTENSIONS_PREFIX = "bld.extensions";
     private static final String PROPERTY_EXTENSIONS_HASH = PROPERTY_EXTENSIONS_PREFIX + PROPERTY_SUFFIX_HASH;
     private static final String PROPERTY_EXTENSIONS_LOCAL = PROPERTY_EXTENSIONS_PREFIX + PROPERTY_SUFFIX_LOCAL;
     private static final String PROPERTY_EXTENSIONS_DOWNLOAD_SOURCES = PROPERTY_EXTENSIONS_PREFIX + PROPERTY_SUFFIX_DOWNLOAD_SOURCES;
     private static final String PROPERTY_EXTENSIONS_DOWNLOAD_JAVADOC = PROPERTY_EXTENSIONS_PREFIX + PROPERTY_SUFFIX_DOWNLOAD_JAVADOC;
     private static final String PROPERTY_EXTENSIONS_DEPENDENCY_TREE = PROPERTY_EXTENSIONS_PREFIX + PROPERTY_SUFFIX_DEPENDENCY_TREE;
+    private static final String PROPERTY_EXTENSIONS_CLASSPATH_PREFIX = PROPERTY_EXTENSIONS_PREFIX + PROPERTY_SUFFIX_CLASSPATH;
 
     private static final String PROPERTY_DEPENDENCIES_PREFIX = "bld.dependencies";
     private static final String PROPERTY_DEPENDENCIES_HASH = PROPERTY_DEPENDENCIES_PREFIX + PROPERTY_SUFFIX_HASH;
@@ -55,6 +59,7 @@ public class BldCache {
     private static final String PROPERTY_DEPENDENCIES_PROVIDED_DEPENDENCY_TREE = PROPERTY_DEPENDENCIES_PREFIX + ".provided" + PROPERTY_SUFFIX_DEPENDENCY_TREE;
     private static final String PROPERTY_DEPENDENCIES_RUNTIME_DEPENDENCY_TREE = PROPERTY_DEPENDENCIES_PREFIX + ".runtime" + PROPERTY_SUFFIX_DEPENDENCY_TREE;
     private static final String PROPERTY_DEPENDENCIES_TEST_DEPENDENCY_TREE = PROPERTY_DEPENDENCIES_PREFIX + ".test" + PROPERTY_SUFFIX_DEPENDENCY_TREE;
+    private static final String PROPERTY_DEPENDENCIES_CLASSPATH_PREFIX = PROPERTY_DEPENDENCIES_PREFIX + PROPERTY_SUFFIX_CLASSPATH;
 
     private final File cacheDir_;
     private final VersionResolution resolution_;
@@ -64,6 +69,8 @@ public class BldCache {
     private List<File> extensionsLocalArtifacts_;
     private String extensionsDependencyTree_;
     private String dependenciesHash_;
+    private final java.util.Map<String, String> extensionClasspaths_ = new java.util.LinkedHashMap<>();
+    private final java.util.Map<String, String> dependencyClasspaths_ = new java.util.LinkedHashMap<>();
     private Boolean dependenciesDownloadSources_;
     private Boolean dependenciesDownloadJavadocs_;
     private String dependenciesCompileDependencyTree_;
@@ -477,6 +484,76 @@ public class BldCache {
         return properties;
     }
 
+    /**
+     * Sets the resolved classpath jar names of a single extension
+     * dependency, to store with {@link #writeCache()}.
+     * <p>
+     * The names are only meaningful while the extensions hash is valid,
+     * check {@link #isExtensionsHashValid()} before using them.
+     *
+     * @param coordinate the {@code groupId:artifactId} of the dependency
+     * @param jarNames   the jar file names of its transitive classpath
+     * @see #getCachedExtensionClasspath
+     * @since 2.4.0
+     */
+    public void cacheExtensionClasspath(String coordinate, List<String> jarNames) {
+        extensionClasspaths_.put(coordinate, String.join("\n", jarNames));
+    }
+
+    /**
+     * Retrieves the previously stored classpath jar names of a single
+     * extension dependency.
+     *
+     * @param coordinate the {@code groupId:artifactId} of the dependency
+     * @return the stored jar file names; or {@code null} when nothing was
+     * stored for the coordinate
+     * @since 2.4.0
+     * @see #cacheExtensionClasspath
+     */
+    public List<String> getCachedExtensionClasspath(String coordinate) {
+        var stored = hashProperties().getProperty(PROPERTY_EXTENSIONS_CLASSPATH_PREFIX + coordinate);
+        if (stored == null || stored.isEmpty()) {
+            return null;
+        }
+        return List.of(stored.split("\n"));
+    }
+
+    /**
+     * Sets the resolved classpath jar names of a single dependency in a
+     * scope, to store with {@link #writeCache()}.
+     * <p>
+     * The names are only meaningful while the dependencies hash is valid,
+     * check {@link #isDependenciesHashValid()} before using them.
+     *
+     * @param scope      the scope the dependency is declared in
+     * @param coordinate the {@code groupId:artifactId} of the dependency
+     * @param jarNames   the jar file names of its transitive classpath
+     * @since 2.4.0
+     * @see #getCachedDependencyClasspath
+     */
+    public void cacheDependencyClasspath(Scope scope, String coordinate, List<String> jarNames) {
+        dependencyClasspaths_.put(scope + "." + coordinate, String.join("\n", jarNames));
+    }
+
+    /**
+     * Retrieves the previously stored classpath jar names of a single
+     * dependency in a scope.
+     *
+     * @param scope      the scope the dependency is declared in
+     * @param coordinate the {@code groupId:artifactId} of the dependency
+     * @return the stored jar file names; or {@code null} when nothing was
+     * stored for the coordinate
+     * @since 2.4.0
+     * @see #cacheDependencyClasspath
+     */
+    public List<String> getCachedDependencyClasspath(Scope scope, String coordinate) {
+        var stored = hashProperties().getProperty(PROPERTY_DEPENDENCIES_CLASSPATH_PREFIX + scope + "." + coordinate);
+        if (stored == null || stored.isEmpty()) {
+            return null;
+        }
+        return List.of(stored.split("\n"));
+    }
+
     /**
      * Writes the state of this {@code BldCache} instance to disk.
      *
@@ -490,7 +567,9 @@ public class BldCache {
                 if (!extensionsHash_.equals(properties.get(PROPERTY_EXTENSIONS_HASH))) {
                     properties.put(PROPERTY_EXTENSIONS_HASH, extensionsHash_);
                     properties.remove(PROPERTY_EXTENSIONS_DEPENDENCY_TREE);
+                    properties.keySet().removeIf(key -> key.toString().startsWith(PROPERTY_EXTENSIONS_CLASSPATH_PREFIX));
                 }
+                extensionClasspaths_.forEach((coordinate, jars) -> properties.put(PROPERTY_EXTENSIONS_CLASSPATH_PREFIX + coordinate, jars));
 
                 if (extensionsDependencyTree_ != null) {
                     properties.put(PROPERTY_EXTENSIONS_DEPENDENCY_TREE, extensionsDependencyTree_);
@@ -525,7 +604,9 @@ public class BldCache {
                     properties.remove(PROPERTY_DEPENDENCIES_PROVIDED_DEPENDENCY_TREE);
                     properties.remove(PROPERTY_DEPENDENCIES_RUNTIME_DEPENDENCY_TREE);
                     properties.remove(PROPERTY_DEPENDENCIES_TEST_DEPENDENCY_TREE);
+                    properties.keySet().removeIf(key -> key.toString().startsWith(PROPERTY_DEPENDENCIES_CLASSPATH_PREFIX));
                 }
+                dependencyClasspaths_.forEach((coordinate, jars) -> properties.put(PROPERTY_DEPENDENCIES_CLASSPATH_PREFIX + coordinate, jars));
 
                 if (dependenciesCompileDependencyTree_ != null) {
                     properties.put(PROPERTY_DEPENDENCIES_COMPILE_DEPENDENCY_TREE, dependenciesCompileDependencyTree_);

src/test/java/rife/bld/TestClasspathJars.java

@@ -426,14 +426,324 @@ public class TestClasspathJars {
         FileUtils.writeString(properties, new File(lib_bld, "bld-wrapper.properties"));
     }
 
+
+    @Test
+    void testDependencyClasspathJarsCaching() throws Exception {
+        var requests = new java.util.concurrent.atomic.AtomicInteger();
+        var server = createArtifactServer(Map.of(
+            "tool:1.0.0", pom("tool", "1.0.0", dependency("liba", "1.1.0")),
+            "liba:1.1.0", pom("liba", "1.1.0", ""),
+            "tool:2.0.0", pom("tool", "2.0.0", "")), requests);
+        server.start();
+        var tmp = Files.createTempDirectory("classpathjars").toFile();
+        try {
+            var project = new JarsProject(tmp, serverRepository(server));
+            project.dependencies().scope(provided)
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+
+            var jars = project.dependencyClasspathJars(provided, "com.example", "tool");
+            var resolution_requests = settledRequests(requests);
+            assertTrue(resolution_requests > 0);
+
+            // a repeated call in the same build resolves nothing
+            assertEquals(jars, project.dependencyClasspathJars(provided, "com.example", "tool"));
+            assertEquals(resolution_requests, requests.get());
+
+            // a fresh project instance reads the persisted cache instead
+            // of resolving, like a new build invocation would
+            var fresh = new JarsProject(tmp, serverRepository(server));
+            fresh.dependencies().scope(provided)
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            assertEquals(jars, fresh.dependencyClasspathJars(provided, "com.example", "tool"));
+            assertEquals(resolution_requests, requests.get());
+
+            // changing the declared version invalidates the cache
+            var changed = new JarsProject(tmp, serverRepository(server));
+            changed.dependencies().scope(provided)
+                .include(new Dependency("com.example", "tool", new VersionNumber(2, 0, 0)));
+            assertEquals(List.of("tool-2.0.0.jar"),
+                changed.dependencyClasspathJars(provided, "com.example", "tool").stream().map(File::getName).toList());
+            assertTrue(requests.get() > resolution_requests);
+        } finally {
+            server.stop(0);
+            FileUtils.deleteDirectory(tmp);
+        }
+    }
+
+    @Test
+    void testExtensionClasspathJarsCaching() throws Exception {
+        var requests = new java.util.concurrent.atomic.AtomicInteger();
+        var server = createArtifactServer(Map.of(
+            "ext:1.0.0", pom("ext", "1.0.0", dependency("tool", "1.0.0")),
+            "tool:1.0.0", pom("tool", "1.0.0", dependency("liba", "1.1.0")),
+            "liba:1.1.0", pom("liba", "1.1.0", "")), requests);
+        server.start();
+        var tmp = Files.createTempDirectory("classpathjars").toFile();
+        try {
+            var project = new JarsProject(tmp, serverRepository(server));
+            writeWrapperProperties(project, server, "com.example:ext:1.0.0");
+
+            var jars = project.extensionClasspathJars("com.example", "tool");
+            var resolution_requests = settledRequests(requests);
+            assertTrue(resolution_requests > 0);
+
+            // repeated lookups in the same build resolve nothing, also
+            // for another dependency out of the same extension universe
+            assertEquals(jars, project.extensionClasspathJars("com.example", "tool"));
+            assertEquals(resolution_requests, requests.get());
+            var liba_jars = project.extensionClasspathJars("com.example", "liba");
+            assertEquals(List.of("liba-1.1.0.jar"), liba_jars.stream().map(File::getName).toList());
+            var universe_requests = settledRequests(requests);
+            assertEquals(resolution_requests, universe_requests);
+
+            // a fresh project instance reads the persisted cache
+            var fresh = new JarsProject(tmp, serverRepository(server));
+            assertEquals(jars, fresh.extensionClasspathJars("com.example", "tool"));
+            assertEquals(universe_requests, requests.get());
+
+            // changing the declared extensions invalidates the cache
+            writeWrapperProperties(project, server, "com.example:tool:1.0.0");
+            var changed = new JarsProject(tmp, serverRepository(server));
+            assertEquals(List.of("tool-1.0.0.jar", "liba-1.1.0.jar"),
+                changed.extensionClasspathJars("com.example", "tool").stream().map(File::getName).toList());
+            assertTrue(requests.get() > universe_requests);
+        } finally {
+            server.stop(0);
+            FileUtils.deleteDirectory(tmp);
+        }
+    }
+
+
+    @Test
+    void testDependencyClasspathJarsBomChangeInvalidates() throws Exception {
+        var requests = new java.util.concurrent.atomic.AtomicInteger();
+        var server = createArtifactServer(Map.of(
+            "bom1:1.0.0", bomPom("bom1", "1.0.0", dependency("libb", "1.5.0")),
+            "bom1:2.0.0", bomPom("bom1", "2.0.0", dependency("libb", "2.0.0")),
+            "tool:1.0.0", pom("tool", "1.0.0", dependency("libb", "3.0.0")),
+            "libb:1.5.0", pom("libb", "1.5.0", ""),
+            "libb:2.0.0", pom("libb", "2.0.0", ""),
+            "libb:3.0.0", pom("libb", "3.0.0", "")), requests);
+        server.start();
+        var tmp = Files.createTempDirectory("classpathjars").toFile();
+        try {
+            var project = new JarsProject(tmp, serverRepository(server));
+            project.dependencies().scope(test)
+                .include(new Bom("com.example", "bom1", new VersionNumber(1, 0, 0)))
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            assertEquals(List.of("tool-1.0.0.jar", "libb-1.5.0.jar"),
+                project.dependencyClasspathJars(test, "com.example", "tool").stream().map(File::getName).toList());
+            var warm_requests = requests.get();
+
+            // a different BOM version changes the hash and the pinned jar
+            var changed = new JarsProject(tmp, serverRepository(server));
+            changed.dependencies().scope(test)
+                .include(new Bom("com.example", "bom1", new VersionNumber(2, 0, 0)))
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            assertEquals(List.of("tool-1.0.0.jar", "libb-2.0.0.jar"),
+                changed.dependencyClasspathJars(test, "com.example", "tool").stream().map(File::getName).toList());
+            assertTrue(requests.get() > warm_requests);
+        } finally {
+            server.stop(0);
+            FileUtils.deleteDirectory(tmp);
+        }
+    }
+
+    @Test
+    void testDependencyClasspathJarsOverrideChangeInvalidates() throws Exception {
+        var requests = new java.util.concurrent.atomic.AtomicInteger();
+        var server = createArtifactServer(Map.of(
+            "tool:1.0.0", pom("tool", "1.0.0", dependency("libb", "2.0.0")),
+            "libb:2.0.0", pom("libb", "2.0.0", ""),
+            "libb:2.5.0", pom("libb", "2.5.0", "")), requests);
+        server.start();
+        var tmp = Files.createTempDirectory("classpathjars").toFile();
+        try {
+            var project = new JarsProject(tmp, serverRepository(server));
+            project.dependencies().scope(test)
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            assertEquals(List.of("tool-1.0.0.jar", "libb-2.0.0.jar"),
+                project.dependencyClasspathJars(test, "com.example", "tool").stream().map(File::getName).toList());
+            var warm_requests = requests.get();
+
+            // a version override changes the hash and the resolved jar
+            var overridden = new JarsProject(tmp, serverRepository(server));
+            overridden.properties().put(VersionResolution.PROPERTY_OVERRIDE_PREFIX, "com.example:libb:2.5.0");
+            overridden.dependencies().scope(test)
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            assertEquals(List.of("tool-1.0.0.jar", "libb-2.5.0.jar"),
+                overridden.dependencyClasspathJars(test, "com.example", "tool").stream().map(File::getName).toList());
+            assertTrue(requests.get() > warm_requests);
+        } finally {
+            server.stop(0);
+            FileUtils.deleteDirectory(tmp);
+        }
+    }
+
+    @Test
+    void testDependencyClasspathJarsRepositoryChangeInvalidates() throws Exception {
+        var requests1 = new java.util.concurrent.atomic.AtomicInteger();
+        var requests2 = new java.util.concurrent.atomic.AtomicInteger();
+        var poms = Map.of("tool:1.0.0", pom("tool", "1.0.0", ""));
+        var server1 = createArtifactServer(poms, requests1);
+        var server2 = createArtifactServer(poms, requests2);
+        server1.start();
+        server2.start();
+        var tmp = Files.createTempDirectory("classpathjars").toFile();
+        try {
+            var project = new JarsProject(tmp, serverRepository(server1));
+            project.dependencies().scope(test)
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            project.dependencyClasspathJars(test, "com.example", "tool");
+            assertTrue(requests1.get() > 0);
+
+            // a different repository list changes the hash, the second
+            // server sees the resolution instead of the cache answering
+            var moved = new JarsProject(tmp, serverRepository(server2));
+            moved.dependencies().scope(test)
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            moved.dependencyClasspathJars(test, "com.example", "tool");
+            assertTrue(requests2.get() > 0);
+        } finally {
+            server1.stop(0);
+            server2.stop(0);
+            FileUtils.deleteDirectory(tmp);
+        }
+    }
+
+    @Test
+    void testDependencyClasspathJarsScopeKeysIsolated() throws Exception {
+        var requests = new java.util.concurrent.atomic.AtomicInteger();
+        var server = createArtifactServer(Map.of(
+            "tool:1.0.0", pom("tool", "1.0.0", ""),
+            "tool:2.0.0", pom("tool", "2.0.0", "")), requests);
+        server.start();
+        var tmp = Files.createTempDirectory("classpathjars").toFile();
+        try {
+            var project = new JarsProject(tmp, serverRepository(server));
+            project.dependencies().scope(provided)
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            project.dependencies().scope(test)
+                .include(new Dependency("com.example", "tool", new VersionNumber(2, 0, 0)));
+
+            // the same coordinate is cached per scope, with its own version
+            assertEquals(List.of("tool-1.0.0.jar"),
+                project.dependencyClasspathJars(provided, "com.example", "tool").stream().map(File::getName).toList());
+            assertEquals(List.of("tool-2.0.0.jar"),
+                project.dependencyClasspathJars(test, "com.example", "tool").stream().map(File::getName).toList());
+            var warm_requests = settledRequests(requests);
+
+            var fresh = new JarsProject(tmp, serverRepository(server));
+            fresh.dependencies().scope(provided)
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            fresh.dependencies().scope(test)
+                .include(new Dependency("com.example", "tool", new VersionNumber(2, 0, 0)));
+            assertEquals(List.of("tool-1.0.0.jar"),
+                fresh.dependencyClasspathJars(provided, "com.example", "tool").stream().map(File::getName).toList());
+            assertEquals(List.of("tool-2.0.0.jar"),
+                fresh.dependencyClasspathJars(test, "com.example", "tool").stream().map(File::getName).toList());
+            assertEquals(warm_requests, requests.get());
+        } finally {
+            server.stop(0);
+            FileUtils.deleteDirectory(tmp);
+        }
+    }
+
+    @Test
+    void testClasspathCacheCoexistsWithDependencyTrees() throws Exception {
+        var requests = new java.util.concurrent.atomic.AtomicInteger();
+        var server = createArtifactServer(Map.of(
+            "tool:1.0.0", pom("tool", "1.0.0", "")), requests);
+        server.start();
+        var tmp = Files.createTempDirectory("classpathjars").toFile();
+        try {
+            var project = new JarsProject(tmp, serverRepository(server));
+            project.dependencies().scope(test)
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            project.dependencyClasspathJars(test, "com.example", "tool");
+            var warm_requests = settledRequests(requests);
+
+            // another BldCache user writing to the same file with the same
+            // hash preserves the classpath entries
+            var cache = new BldCache(project.libBldDirectory(), new VersionResolution(project.properties()));
+            cache.cacheDependenciesHash(project.repositories(), project.dependencies());
+            assertTrue(cache.isDependenciesHashValid());
+            cache.cacheDependenciesTestDependencyTree("tree placeholder");
+            cache.writeCache();
+
+            assertEquals("tree placeholder", cache.getCachedDependenciesTestDependencyTree());
+            var fresh = new JarsProject(tmp, serverRepository(server));
+            fresh.dependencies().scope(test)
+                .include(new Dependency("com.example", "tool", new VersionNumber(1, 0, 0)));
+            fresh.dependencyClasspathJars(test, "com.example", "tool");
+            assertEquals(warm_requests, requests.get());
+
+            // and a classpath write with the same hash preserves the tree
+            fresh.dependencyClasspathJars(test, "com.example", "tool");
+            assertEquals("tree placeholder", cache.getCachedDependenciesTestDependencyTree());
+        } finally {
+            server.stop(0);
+            FileUtils.deleteDirectory(tmp);
+        }
+    }
+
+    @Test
+    void testExtensionClasspathJarsUndeclaredAfterCached() throws Exception {
+        var server = createArtifactServer(Map.of(
+            "ext:1.0.0", pom("ext", "1.0.0", dependency("tool", "1.0.0")),
+            "tool:1.0.0", pom("tool", "1.0.0", ""),
+            "other:1.0.0", pom("other", "1.0.0", "")));
+        server.start();
+        var tmp = Files.createTempDirectory("classpathjars").toFile();
+        try {
+            var project = new JarsProject(tmp, serverRepository(server));
+            writeWrapperProperties(project, server, "com.example:ext:1.0.0");
+            assertEquals(1, project.extensionClasspathJars("com.example", "tool").size());
+
+            // the extension that brought the tool in is replaced, a stale
+            // cache entry must not keep answering for it
+            writeWrapperProperties(project, server, "com.example:other:1.0.0");
+            var changed = new JarsProject(tmp, serverRepository(server));
+            var exception = assertThrows(IllegalArgumentException.class,
+                () -> changed.extensionClasspathJars("com.example", "tool"));
+            assertTrue(exception.getMessage().contains("isn't part of the extensions"));
+        } finally {
+            server.stop(0);
+            FileUtils.deleteDirectory(tmp);
+        }
+    }
+
+    // the POM prefetcher can still have a speculative request in flight
+    // when resolution returns, count requests only after they settle
+    private static int settledRequests(java.util.concurrent.atomic.AtomicInteger requests)
+    throws InterruptedException {
+        var last = requests.get();
+        var stable_since = System.currentTimeMillis();
+        while (System.currentTimeMillis() - stable_since < 300) {
+            Thread.sleep(50);
+            var current = requests.get();
+            if (current != last) {
+                last = current;
+                stable_since = System.currentTimeMillis();
+            }
+        }
+        return last;
+    }
+
     private static Repository serverRepository(HttpServer server) {
         return new Repository("http://localhost:" + server.getAddress().getPort() + "/");
     }
 
     private static HttpServer createArtifactServer(Map<String, String> poms)
+    throws IOException {
+        return createArtifactServer(poms, new java.util.concurrent.atomic.AtomicInteger());
+    }
+
+    private static HttpServer createArtifactServer(Map<String, String> poms, java.util.concurrent.atomic.AtomicInteger requests)
     throws IOException {
         var server = HttpServer.create(new InetSocketAddress("localhost", 0), 0);
         server.createContext("/", exchange -> {
+            requests.incrementAndGet();
             var segments = exchange.getRequestURI().getPath().split("/");
             var filename = segments[segments.length - 1];
             byte[] body = null;