can proxy some classes but not others

Question

UPDATE solution is Java.lang.reflect.Proxy returning another proxy from invocation results in ClassCastException on assignment

My test code proxies java.sql.Connection.

I create my proxy like so:

log.info("connection is "+connection.getClass().getName()+", "+
    (connection instanceof Connection));
Object proxy = java.lang.reflect.Proxy.newProxyInstance(
    connection.getClass().getClassLoader(),
    connection.getClass().getInterfaces(),
    new MockFailureWrapper(connection));
log.info("proxy is "+proxy.getClass().getName()+", "+
    (proxy instanceof Connection));
return (Connection)proxy;

When I wrap an H2 DB connection, this works perfectly.

When I try and wrap a MySQL connection, the cast of the proxy to Connection in the return fails, even though the connection I'm wrapping is of type Connection. The exception is:

java.lang.ClassCastException: $Proxy11 cannot be cast to java.sql.Connection

The log line for an H2 connection is:

connection is org.h2.jdbc.JdbcConnection, true
proxy is $Proxy9, true

And for the MySQL connection:

connection is com.mysql.jdbc.JDBC4Connection, true
proxy is $Proxy11, false

What's going on, and why can't I wrap MySQL DB connections?

Answer 1

The problem is this line:

connection.getClass().getInterfaces()

It just gives you the interfaces that are directly implemented by the class you would like to proxy. If the Connection-interface is implemented f.eg. by a superclass of the MySql-Connection class (lets say AbstractConnection), your proxy will not implement the Connection interface.

To fix this, add the Connection-Interface to the array of interfaces your proxy should implement.