Communication customization has been widely used and proved having benefits in mobile computing. However, most systems are designed for specific purposes. They may have weaknesses in compatibility and portability, and incur unnecessary overheads in certain situations. Besides, they may be not flexible as a generic system because of requiring particular software facilities or hardware devices. In the paper, we propose TOP, a generic proxy-based system for communication customization in mobile computing. TOP utilizes API hook and packet redirection technologies to provide objects with the service of packet manipulations. Rather than being limited Java Applet, the objects on TOP are Java Application capable of accessing various Java libraries. Communication customization on TOP can fully change an existing protocol's semantics without modifying application programs. We implement TOP on Windows 2000/XP and the experiments show that the performance of TOP is acceptable. Besides, we use TOP to overcome TCP disconnection hazards over wireless LAN as an example of applications on it. TOP, Java, Packet Redirection, API hook.