Let engine utContext.classLoader fallback to UtBot own class path #2523 (#2556)

* Let engine `utContext.classLoader` fallback to UtBot own class path

* Address minor comments from #2556

Author: IlyaMuravjov

utbot-core/src/main/kotlin/org/utbot/common/FallbackClassLoader.kt

@@ -0,0 +1,99 @@
+package org.utbot.common
+
+import java.io.IOException
+import java.net.URL
+import java.net.URLClassLoader
+import java.util.*
+
+/**
+ * [ClassLoader] implementation, that
+ *   - first, attempts to load class/resource with [commonParent] class loader
+ *   - next, attempts to load class/resource from `urls`
+ *   - finally, attempts to load class/resource with `fallback` class loader
+ *
+ * More details can be found in [this post](https://medium.com/@isuru89/java-a-child-first-class-loader-cbd9c3d0305).
+ */
+class FallbackClassLoader(
+    urls: Array<URL>,
+    fallback: ClassLoader,
+    private val commonParent: ClassLoader = fallback.parent,
+) : URLClassLoader(urls, fallback) {
+
+    @Throws(ClassNotFoundException::class)
+    override fun loadClass(name: String, resolve: Boolean): Class<*>? {
+        // has the class loaded already?
+        var loadedClass = findLoadedClass(name)
+        if (loadedClass == null) {
+            try {
+                loadedClass = commonParent.loadClass(name)
+            } catch (ex: ClassNotFoundException) {
+                // class not found in common parent loader... silently skipping
+            }
+            try {
+                // find the class from given jar urls as in first constructor parameter.
+                if (loadedClass == null) {
+                    loadedClass = findClass(name)
+                }
+            } catch (e: ClassNotFoundException) {
+                // class is not found in the given urls.
+                // Let's try it in fallback classloader.
+                // If class is still not found, then this method will throw class not found ex.
+                loadedClass = super.loadClass(name, resolve)
+            }
+        }
+        if (resolve) {      // marked to resolve
+            resolveClass(loadedClass)
+        }
+        return loadedClass
+    }
+
+    @Throws(IOException::class)
+    override fun getResources(name: String): Enumeration<URL> {
+        val allRes: MutableList<URL> = LinkedList<URL>()
+
+        // load resources from common parent loader
+        val commonParentResources: Enumeration<URL>? = commonParent.getResources(name)
+        if (commonParentResources != null) {
+            while (commonParentResources.hasMoreElements()) {
+                allRes.add(commonParentResources.nextElement())
+            }
+        }
+
+        // load resource from this classloader
+        val thisRes: Enumeration<URL>? = findResources(name)
+        if (thisRes != null) {
+            while (thisRes.hasMoreElements()) {
+                allRes.add(thisRes.nextElement())
+            }
+        }
+
+        // then try finding resources from fallback classloaders
+        val parentRes: Enumeration<URL>? = super.findResources(name)
+        if (parentRes != null) {
+            while (parentRes.hasMoreElements()) {
+                allRes.add(parentRes.nextElement())
+            }
+        }
+        return object : Enumeration<URL> {
+            var it: Iterator<URL> = allRes.iterator()
+            override fun hasMoreElements(): Boolean {
+                return it.hasNext()
+            }
+
+            override fun nextElement(): URL {
+                return it.next()
+            }
+        }
+    }
+
+    override fun getResource(name: String): URL? {
+        var res: URL? = commonParent.getResource(name)
+        if (res === null) {
+            res = findResource(name)
+        }
+        if (res === null) {
+            res = super.getResource(name)
+        }
+        return res
+    }
+}

utbot-framework/src/main/kotlin/org/utbot/framework/process/EngineProcessMain.kt

@@ -43,7 +43,6 @@ import org.utbot.spring.process.SpringAnalyzerProcess
 import org.utbot.summary.summarizeAll
 import org.utbot.taint.TaintConfigurationProviderUserRules
 import java.io.File
-import java.net.URLClassLoader
 import java.nio.file.Paths
 import kotlin.reflect.jvm.kotlinFunction
 import kotlin.time.Duration.Companion.seconds
@@ -75,13 +74,21 @@ private var idCounter: Long = 0
 private fun EngineProcessModel.setup(kryoHelper: KryoHelper, watchdog: IdleWatchdog, realProtocol: IProtocol) {
     val model = this
     watchdog.measureTimeForActiveCall(setupUtContext, "UtContext setup") { params ->
-        // - We use `ClassLoader.getSystemClassLoader().parent` as parent to let
-        //   classes like `javax.sql.DataSource` load from URLs like `jrt:/java.sql`.
-        // - We do not use `ClassLoader.getSystemClassLoader()` itself to avoid utbot dependencies like Jackson
-        //   being used instead of user's dependencies, which is important, since they may have different versions.
-        UtContext.setUtContext(UtContext(URLClassLoader(params.classpathForUrlsClassloader.map {
-            File(it).toURI().toURL()
-        }.toTypedArray(), ClassLoader.getSystemClassLoader().parent)))
+        val urls = params.classpathForUrlsClassloader.map { File(it).toURI().toURL() }.toTypedArray()
+        // - First, we try to load class/resource with platform class loader `ClassLoader.getSystemClassLoader().parent`
+        //     - at this step we load classes like
+        //        - `java.util.ArrayList` (from boostrap classloader)
+        //        - `javax.sql.DataSource` (from platform classloader)
+        // - Next, we try to load class/resource from user class path
+        //     - at this step we load classes like class under test and other classes from user project and its dependencies
+        // - Finally, if all else fails we try to load class/resource from UtBot classpath
+        //     - at this step we load classes from UtBot project and its dependencies (e.g. Mockito if user doesn't have it, see #2545)
+        val classLoader = FallbackClassLoader(
+            urls = urls,
+            fallback = ClassLoader.getSystemClassLoader(),
+            commonParent = ClassLoader.getSystemClassLoader().parent,
+        )
+        UtContext.setUtContext(UtContext(classLoader))
     }
     watchdog.measureTimeForActiveCall(getSpringBeanDefinitions, "Getting Spring bean definitions") { params ->
         try {