External Documentation

Java
Java 1.7 Javadoc

Spring MVC
Introduction to Spring Web MVC framework
Spring Expression Language (SpEL)
Spring Javadoc
Spring Framework Reference Documentation

Validation
Validation, Data Binding, and Type Conversion
Using Bean Validation
Hibernate Validator

Security
Spring Security 1
Spring Security 2
Expression-Based Access Control

Thymeleaf
Thymeleaf
Thymeleaf - Spring Security 3 integration module

JPA
Introduction to the Java Persistence API
javax.persistence Javadoc
Wikibooks - Java Persistence
Google JPA tutorial

Joda Time
Joda Time - Java date and time API