The primary requirements are:
Apache SOAP and WSDL4J also require:
If you need to access web services over an SSL/HTTPS connection, you need to have Java Secure Socket Extension (jsse) installed and configured. This should be included with Java 1.4 and higher.
Please note that the Apache SOAP documentation also mentions that if you're using XML4J, your classpath must have xerces.jar first and then xml4j.jar next in that order.