Web application servers (WASs) provide a web computing infrastructure for distributed components. The component structure of statically configured distribution prevents web applications from being adaptive to the changing environmental conditions at runtime. To meet the requirement of dynamic redistribution, WASs should provide the capability to support component migration. The most challenging problem is to maintain component consistency during such a component migration. To resolve this inconsistency problem, some kinds of component migration constrains (CMC) are defined in this paper. A component migration model for J2EE (Java 2 platform enterprise edition) application servers is proposed, and SLB_Copy, SFB_Copy and EB_Copy component migration algorithms are presented. It is proved that SLB_Copy, SFB_Copy and EB_Copy migration algorithms all satisfy the CMC constrains. At present, the migration model is implemented in a J2EE application server, referred to as WebFrame 2.0, and these algorithms are applied to provide numerous services such as the adaptive load balancing service and the failover service.