AttachmentsClassLoader fails when calling setURLStreamHandlerFactory (#4512)

core/src/main/kotlin/net/corda/core/serialization/internal/AttachmentsClassLoader.kt

@@ -19,6 +19,8 @@ import java.io.ByteArrayOutputStream
 import java.io.IOException
 import java.io.InputStream
 import java.net.*
+import java.lang.reflect.AccessibleObject.setAccessible
+import java.net.URLStreamHandlerFactory
 
 /**
  * A custom ClassLoader that knows how to load classes from a set of attachments. The attachments themselves only
@@ -41,10 +43,11 @@ class AttachmentsClassLoader(attachments: List<Attachment>, parent: ClassLoader
         private val log = contextLogger()
 
         init {
-            // This is required to register the AttachmentURLStreamHandlerFactory.
+            // Apply our own URLStreamHandlerFactory to resolve attachments
-            URL.setURLStreamHandlerFactory(AttachmentURLStreamHandlerFactory)
+            setOrDecorateURLStreamHandlerFactory()
         }
 
+
         // Jolokia and Json-simple are dependencies that were bundled by mistake within contract jars.
         // In the AttachmentsClassLoader we just ignore any class in those 2 packages.
         private val ignoreDirectories = listOf("org/jolokia/", "org/json/simple/")
@@ -115,6 +118,50 @@ class AttachmentsClassLoader(attachments: List<Attachment>, parent: ClassLoader
                 return it.toByteArray()
             }
         }
+
+        /**
+         * Apply our custom factory either directly, if `URL.setURLStreamHandlerFactory` has not been called yet,
+         * or use a decorator and reflection to bypass the single-call-per-JVM restriction otherwise.
+         */
+        private fun setOrDecorateURLStreamHandlerFactory() {
+            // Retrieve the `URL.factory` field
+            val factoryField = URL::class.java.getDeclaredField("factory")
+            // Make it accessible
+            factoryField.isAccessible = true
+
+            // Check for preset factory, set directly if missing
+            val existingFactory: URLStreamHandlerFactory? = factoryField.get(null) as URLStreamHandlerFactory?
+            if (existingFactory == null) {
+                URL.setURLStreamHandlerFactory(AttachmentURLStreamHandlerFactory)
+            }
+            // Otherwise, decorate the existing and replace via reflection
+            // as calling `URL.setURLStreamHandlerFactory` again will throw an error
+            else {
+                log.warn("The URLStreamHandlerFactory was already set in the JVM. Please be aware that this is not recommended.")
+                // Retrieve the field "streamHandlerLock" of the class URL that
+                // is the lock used to synchronize access to the protocol handlers
+                val lockField = URL::class.java.getDeclaredField("streamHandlerLock")
+                // It is a private field so we need to make it accessible
+                // Note: this will only work as-is in JDK8.
+                lockField.isAccessible = true
+                // Use the same lock to reset the factory
+                synchronized(lockField.get(null)) {
+                    // Reset the value to prevent Error due to a factory already defined
+                    factoryField.set(null, null)
+                    // Set our custom factory and wrap the current one into it
+                    URL.setURLStreamHandlerFactory(
+                            // Set the factory to a decorator
+                            object : URLStreamHandlerFactory {
+                                // route between our own and the pre-existing factory
+                                override fun createURLStreamHandler(protocol: String): URLStreamHandler? {
+                                    return AttachmentURLStreamHandlerFactory.createURLStreamHandler(protocol)
+                                            ?: existingFactory.createURLStreamHandler(protocol)
+                                }
+                            }
+                    )
+                }
+            }
+        }
     }
 
     /**
@@ -200,3 +247,4 @@ object AttachmentURLStreamHandlerFactory : URLStreamHandlerFactory {
         }
     }
 }
+