The 3.1.2 release was performed using JDK 1.5, so it should be 1.5 compatibile. Are you certain that you are using spring-security-web-3.1.2.RELEASE.jar? How did you download it? If you used maven, what does your pom.xml look like?
I downloaded spring-security-web-3.1.2.RELEASE from maven central and FilterChainProxy is marked as major version of 49 which is Java 1.5. See below for the output of javap:
$ javap -verbose org/springframework/security/web/FilterChainProxy | head -n 15
Compiled from "FilterChainProxy.java"
public class org.springframework.security.web.FilterChainProxy extends org.springframework.web.filter.GenericFilterBean
minor version: 0
major version: 49