TargetSource.getTarget() is nullable again (for compatibility with MethodInvocation.getThis)

Issue: SPR-15651

Author: jhoeller

spring-aop/src/main/java/org/springframework/aop/target/AbstractPrototypeBasedTargetSource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -97,7 +97,7 @@ else if (target instanceof DisposableBean) {
 
 	private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
 		throw new NotSerializableException("A prototype-based TargetSource itself is not deserializable - " +
-				"just a disconnected SingletonTargetSource is");
+				"just a disconnected SingletonTargetSource or EmptyTargetSource is");
 	}
 
 	/**
@@ -113,13 +113,15 @@ protected Object writeReplace() throws ObjectStreamException {
 			logger.debug("Disconnecting TargetSource [" + this + "]");
 		}
 		try {
-			// Create disconnected SingletonTargetSource.
-			return new SingletonTargetSource(getTarget());
+			// Create disconnected SingletonTargetSource/EmptyTargetSource.
+			Object target = getTarget();
+			return (target != null ? new SingletonTargetSource(target) :
+					EmptyTargetSource.forClass(getTargetClass()));
 		}
 		catch (Exception ex) {
-			logger.error("Cannot get target for disconnecting TargetSource [" + this + "]", ex);
-			throw new NotSerializableException(
-					"Cannot get target for disconnecting TargetSource [" + this + "]: " + ex);
+			String msg = "Cannot get target for disconnecting TargetSource [" + this + "]";
+			logger.error(msg, ex);
+			throw new NotSerializableException(msg + ": " + ex);
 		}
 	}