JAR vs WAR: Key Differences in Java Deployment Explained
JAR (Java ARchive) is a single ZIP file bundling Java classes, resources, and metadata for standalone or micro-service deployment. WAR (Web Application ARchive) is a specialized JAR that contains additional WEB-INF structure, libraries, and JSP/HTML assets specifically for servlet containers like Tomcat.
Devs often toss both files into Docker images or CI pipelines, then panic when Tomcat rejects a plain JAR or “java -jar” chokes on a WAR. The mix-up stems from identical packaging tools and overlapping extensions, hiding the critical deployment target difference until runtime.
Key Differences
JAR targets JVMs directly—think Spring Boot fat JARs with embedded servers. WAR assumes an external servlet container, shipping web.xml, JSPs, and lib folders that the container unpacks and hosts.
Which One Should You Choose?
Pick JAR for micro-services or standalone apps needing fast startup. Choose WAR when you must deploy alongside other web apps in a shared Tomcat, JBoss, or Payara server.
Examples and Daily Life
Upload myapp.jar to AWS Lambda for serverless pay-per-use. Drop myapp.war into /opt/tomcat/webapps/ to coexist with Jenkins and Confluence on the same box.
Can a WAR run with java -jar?
Only if it’s Spring Boot-styled with embedded Tomcat; otherwise you’ll get “no main manifest attribute” errors.
Is a JAR lighter than a WAR?
Usually yes—WARs bundle extra web libraries, JSPs, and container descriptors that JARs skip.