The sequence that an SRX Series device uses when implementing stateful session security policies using Layer 3 routes is:
An SRX Series device will conduct a longest-match Layer 3 route table lookup before performing a security policy search: When an SRX Series device receives a packet, it first looks up the destination IP address in the routing table and finds the longest matching route to forward the packet. Then, it performs a security policy search based on the source zone, destination zone, source address, destination address, protocol, and application of the packet. If there is a matching policy that allows the packet, it creates or updates a session entry for the packet and applies any security services configured in the policy.
Reference: = [Security Policies Overview], [Security Policy Processing Overview]