#expression-language

Posts

• Digging into Tomcat's Source 13: Jasper — How a JSP Becomes a Servlet

  Tomcat · 2026-10-24

  What happens inside Tomcat the first time you request hello.jsp? We follow the Jasper source as JspServlet takes the request, reads the JSP file into a node tree, writes it out as Java source, compiles it with ecj, leaves a .class in the work directory, and then calls that class like any servlet. We open the generated .java file, and use examples to see when an edited JSP gets recompiled (development, modificationTestInterval, checkInterval).

  #source-reading #jsp #servlet #code-generation #expression-language

• Digging into Spring's Source 15: SpEL — How an Expression Is Parsed and Evaluated

  Spring · 2026-10-10

  How a string like "#{T(java.lang.Math).max(a, 3) + 1}" becomes tokens, then a tree, then a value, followed through the source. How operator precedence is expressed as the call order of parser methods, why there are two evaluation contexts (StandardEvaluationContext and SimpleEvaluationContext), the compiler mode that turns an expression into bytecode, and the path a @Value("#{…}") takes inside the container — explained for someone seeing Spring for the first time.

  #source-reading #spel #expression-language #spring-expression