spring-spel

Configuration values and bean wiring often require dynamic expressions — accessing system properties, evaluating conditions, performing string operations, or calling methods. Static property files (@Value("${key}")) can only do basic key lookups; developers need runtime expression evaluation within the Spring container.

SpEL (Spring Expression Language) is a powerful expression language that supports querying and manipulating objects at runtime. It can be used in XML and annotation-based configurations, providing dynamic value resolution, method invocation, collection manipulation, and boolean evaluation.

1. Syntax: Expressions start with #{ and end with } — e.g., #{systemProperties['user.dir']}
2. Property access: #{beanName.property} accesses any Spring bean’s property
3. Method invocation: #{beanName.method(args)} calls methods on beans
4. Operators: Arithmetic (+, -, *, /), relational (==, lt, gt), logical (and, or, not)
5. Collection selection: #{users.?[age > 25]} filters collections; .^ first match, .$ last match
6. Collection projection: #{users.![name]} extracts specific property from each element
7. Safe navigation: #{bean?.property} — returns null if bean is null instead of NPE
8. Ternary: #{bean.score > 50 ? 'Pass' : 'Fail'}

• Dynamic evaluation: Expressions are evaluated at runtime, not compile time
• Bean access: Directly reference Spring beans by name (#{myBean.count})
• Null-safe: ?. operator prevents NullPointerException in navigation chains
• Collection operators: Selection (?[]), projection (![]), sorting according to a property
• Type operators: T(java.lang.Math).PI references static types and methods
• Integration: Works with @Value, XML <property>, security annotations, and more

• Built from: Spring Framework — SpEL is a Spring Core module
• Built from: Spring IoC Container — SpEL evaluates within the container’s context
• Related: Spring Annotations — @Value with SpEL provides dynamic property injection
• Contrasts with: EJB Query Language (EJB-QL) — EJB-QL queries entities; SpEL queries beans and system properties
• Related: Java Operators — SpEL operators extend Java’s with collection-safe operations

• Syntax confusion: #{} (SpEL evaluation) vs ${} (property placeholder) — they can be nested: #{${property}}
• Performance: Complex SpEL expressions are evaluated on every access — avoid in hot paths
• Method invocation: Only public methods on beans can be called; private/static methods require T() type operator
• Security: SpEL can call any bean method and access any property — never use user-provided input in SpEL expressions (security risk)