Resolve remaining nullability warnings

Issue: SPR-15869

spring-aop/src/main/java/org/springframework/aop/target/CommonsPool2TargetSource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 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.
@@ -24,6 +24,7 @@ import org.apache.commons.pool2.impl.GenericObjectPool;
 import org.apache.commons.pool2.impl.GenericObjectPoolConfig;
 
 import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 
 /**
  * {@link org.springframework.aop.TargetSource} implementation that holds
@@ -230,6 +231,7 @@ public class CommonsPool2TargetSource extends AbstractPoolingTargetSource implem
 	 */
 	@Override
 	public Object getTarget() throws Exception {
+		Assert.state(this.pool != null, "No Commons ObjectPool available");
 		return this.pool.borrowObject();
 	}
 
@@ -238,17 +240,19 @@ public class CommonsPool2TargetSource extends AbstractPoolingTargetSource implem
 	 */
 	@Override
 	public void releaseTarget(Object target) throws Exception {
-		this.pool.returnObject(target);
+		if (this.pool != null) {
+			this.pool.returnObject(target);
+		}
 	}
 
 	@Override
 	public int getActiveCount() throws UnsupportedOperationException {
-		return this.pool.getNumActive();
+		return (this.pool != null ? this.pool.getNumActive() : 0);
 	}
 
 	@Override
 	public int getIdleCount() throws UnsupportedOperationException {
-		return this.pool.getNumIdle();
+		return (this.pool != null ? this.pool.getNumIdle() : 0);
 	}
 
 
@@ -257,8 +261,10 @@ public class CommonsPool2TargetSource extends AbstractPoolingTargetSource implem
 	 */
 	@Override
 	public void destroy() throws Exception {
-		logger.debug("Closing Commons ObjectPool");
-		this.pool.close();
+		if (this.pool != null) {
+			logger.debug("Closing Commons ObjectPool");
+			this.pool.close();
+		}
 	}