[COLLECTIONS-580] Add javadoc, improve error message and apply review comments.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/branches/COLLECTIONS_3_2_X@1713537 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/commons/collections/functors/InvokerTransformer.java

@@ -18,6 +18,7 @@ package org.apache.commons.collections.functors;
 
 import java.io.IOException;
 import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
 import java.io.Serializable;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
@@ -29,6 +30,17 @@ import org.apache.commons.collections.Transformer;
 
 /**
  * Transformer implementation that creates a new object instance by reflection.
+ * <p>
+ * <b>WARNING:</b> from v3.2.2 onwards this class will throw an
+ * {@link UnsupportedOperationException} when trying to de-serialize an
+ * instance from a {@link ObjectOutputStream} to prevent potential
+ * remote code execution exploits.
+ * <p>
+ * In order to re-enable de-serialization of {@code InvokerTransformer}
+ * instances, the following system property can be used (via -Dproperty=true):
+ * <pre>
+ * org.apache.commons.collections.invokertransformer.enableDeserialization
+ * </pre>
  * 
  * @since Commons Collections 3.0
  * @version $Revision$ $Date$
@@ -160,8 +172,10 @@ public class InvokerTransformer implements Transformer, Serializable {
             deserializeProperty = null;
         }
 
-        if (deserializeProperty == null || !deserializeProperty.equalsIgnoreCase("true")) {
+        if (!"true".equalsIgnoreCase(deserializeProperty)) {
-            throw new UnsupportedOperationException("Deserialization of InvokerTransformer is disabled, ");
+            throw new UnsupportedOperationException(
+                    "Deserialization of InvokerTransformer is disabled for security reasons. " +
+                    "To re-enable it set system property '" + DESERIALIZE + "' to 'true'");
         }
         
         is.defaultReadObject();

src/test/org/apache/commons/collections/functors/TestInvokerTransformer.java

@@ -44,6 +44,7 @@ public class TestInvokerTransformer extends BulkTest {
         Assert.assertNull(System.getProperty(InvokerTransformer.DESERIALIZE));
         System.setProperty(InvokerTransformer.DESERIALIZE, "true");
 
+        try {
             InvokerTransformer transformer = new InvokerTransformer("toString", new Class[0], new Object[0]);
             byte[] data = serialize(transformer);
             Assert.assertNotNull(data);
@@ -53,9 +54,10 @@ public class TestInvokerTransformer extends BulkTest {
             } catch (UnsupportedOperationException ex) {
                 fail("de-serialization of InvokerTransformer should be enabled");
             }
-        
+        } finally {
             System.clearProperty(InvokerTransformer.DESERIALIZE);
         }
+    }
     
     private byte[] serialize(InvokerTransformer transformer) throws IOException {
         ByteArrayOutputStream baos = new ByteArrayOutputStream();