ExternalizableConverter is unable to instantiate package-private classes

Sergei Ivanov added a comment - 09/Aug/10 11:37 AM Please see the example below: package test4.data; import java.io.Serializable; public class A implements Serializable { private B something; public A() { } public A( final int number) { this .something = new B(number); } } package test4.data; import java.io.Externalizable; import java.io.IOException; import java.io.ObjectInput; import java.io.ObjectOutput; class B implements Externalizable { private int number; public B() { } B( final int number) { this .number = number; } public void writeExternal( final ObjectOutput out) throws IOException { out.writeInt(number); } public void readExternal( final ObjectInput in) throws IOException, ClassNotFoundException { number = in.readInt(); } } package test4.xstream; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.StringReader; import java.io.StringWriter; import com.thoughtworks.xstream.XStream; import com.thoughtworks.xstream.io.xml.StaxDriver; import test4.data.A; public class TestXStreamRoundtrip { public static void main( String [] args) throws Exception { XStream xstream; xstream = new XStream( new StaxDriver()); xstream.alias( "a" , A.class); final StringWriter writer = new StringWriter(256); ObjectOutputStream out = xstream.createObjectOutputStream(writer, "journal" ); final A sent = new A(4); out.writeObject(sent); out.close(); final StringReader reader = new StringReader(writer.toString()); ObjectInputStream in = xstream.createObjectInputStream(reader); final A received = (A) in.readObject(); in.close(); } } When run, it produces the following exception: Exception in thread "main" com.thoughtworks.xstream.converters.ConversionException: Cannot construct class java.lang.Class : Class com.thoughtworks.xstream.converters.reflection.ExternalizableConverter can not access a member of class test4.data.B with modifiers "public" ---- Debugging information ---- message : Cannot construct class java.lang.Class cause-exception : java.lang.IllegalAccessException cause-message : Class com.thoughtworks.xstream.converters.reflection.ExternalizableConverter can not access a member of class test4.data.B with modifiers "public" class : test4.data.A required-type : test4.data.B path : /a/something line number : 1 ------------------------------- at com.thoughtworks.xstream.converters.reflection.ExternalizableConverter.unmarshal(ExternalizableConverter.java:125) at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:82) at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:63) at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:76) at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.unmarshallField(AbstractReflectionConverter.java:246) at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doUnmarshal(AbstractReflectionConverter.java:218) at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.unmarshal(AbstractReflectionConverter.java:162) at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:82) at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:63) at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:76) at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:60) at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:137) at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:33) at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:923) at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:899) at com.thoughtworks.xstream.XStream$3.readFromStream(XStream.java:1503) at com.thoughtworks.xstream.core.util.CustomObjectInputStream.readObjectOverride(CustomObjectInputStream.java:86) at java.io.ObjectInputStream.readObject(ObjectInputStream.java:345) at test4.xstream.TestXStreamRoundtrip.main(TestXStreamRoundtrip.java:30) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at com.intellij.rt.execution.application.AppMain.main(AppMain.java:110) Caused by: java.lang.IllegalAccessException: Class com.thoughtworks.xstream.converters.reflection.ExternalizableConverter can not access a member of class test4.data.B with modifiers "public" at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65) at java.lang.Class.newInstance0(Class.java:349) at java.lang.Class.newInstance(Class.java:308) at com.thoughtworks.xstream.converters.reflection.ExternalizableConverter.unmarshal(ExternalizableConverter.java:92) ... 23 more

Sergei Ivanov added a comment - 09/Aug/10 11:50 AM Plain Java serialisation writes out and reads back the above test classes without any problems. Digging around XStream code, the culprit turned out to be this line in the ExternalizableConverter.unmarshal(): final Externalizable externalizable = (Externalizable) type.newInstance(); Digging further into Java serialisation code, I found out that they instantiate externalizable objects slightly differently. They obtain an instance of the default Constructor, then they remove access protection from it, and only then they call newInstance() on the constructor object. For reference, please see java.io.ObjectStreamClass.getExternalizableConstructor() private method. I have created a modified version of an ExternalizableConverter, that follows the Java serialisation way of constructing objects, and it worked flawlessly in the end. Please see the attachment.

Jörg Schaible added a comment - 09/Aug/10 4:39 PM Thanks for spotting this. Basically any Externalizable with a non-accessible default constructor were affected. Fixed in HEAD.