🍃🎓 SPRING CERTIFICATION QUESTION: How does Spring Boot work? How does it know what to configure?

👩‍🏫 Answer:

Autoconfiguration ⚙️ facility is provided by 𝑠𝑝𝑟𝑖𝑛𝑔-𝑏𝑜𝑜𝑡-𝑎𝑢𝑡𝑜𝑐𝑜𝑛𝑓𝑖𝑔𝑢𝑟𝑒 jar.

This jar lists autoconfiguration classes under 𝐌𝐄𝐓𝐀-𝐈𝐍𝐅/𝐬𝐩𝐫𝐢𝐧𝐠.𝐟𝐚𝐜𝐭𝐨𝐫𝐢𝐞𝐬 file, the presence of which is checked ✅ by Spring Boot.

It's important to emphasize that autoconfiguration classes should be registered 💾 this way only, and should never be the target 🎯 of component scanning.

Usually, autoconfiguration classes are annotated with 𝐂𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐚𝐥 annotation which specifies a condition that should match 🤝 in order for a component to be registered.

Those conditional annotations can be divided 🪓 into the following groups:

• 1. Class conditions (𝐂𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐚𝐥𝐎𝐧𝐂𝐥𝐚𝐬𝐬, 𝐂𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐚𝐥𝐎𝐧𝐌𝐢𝐬𝐬𝐢𝐧𝐠𝐂𝐥𝐚𝐬𝐬}) - which check the presence or absence of a class on the classpath.
• 2. Bean conditions (𝐂𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐚𝐥𝐎𝐧𝐁𝐞𝐚𝐧, 𝐂𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐚𝐥𝐎𝐧𝐌𝐢𝐬𝐬𝐢𝐧𝐠𝐁𝐞𝐚𝐧) - which check the presence or absence of a bean in the 𝐁𝐞𝐚𝐧𝐅𝐚𝐜𝐭𝐨𝐫𝐲.
• 3. Property conditions (𝐂𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐚𝐥𝐎𝐧𝐏𝐫𝐨𝐩𝐞𝐫𝐭𝐲) - which check presence or absence of Spring 𝐄𝐧𝐯𝐢𝐫𝐨𝐧𝐦𝐞𝐧𝐭 property.
• 4. Resource conditions (𝐂𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐚𝐥𝐎𝐧𝐑𝐞𝐬𝐨𝐮𝐫𝐜𝐞) - which check the presence or absence of a resource.
• 5. Application context type conditions (𝐂𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐚𝐥𝐎𝐧𝐖𝐞𝐛𝐀𝐩𝐩𝐥𝐢𝐜𝐚𝐭𝐢𝐨𝐧, 𝐂𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐚𝐥𝐎𝐧𝐍𝐨𝐭𝐖𝐞𝐛𝐀𝐩𝐩𝐥𝐢𝐜𝐚𝐭𝐢𝐨𝐧) - which check the configured type of 𝐀𝐩𝐩𝐥𝐢𝐜𝐚𝐭𝐢𝐨𝐧𝐂𝐨𝐧𝐭𝐞𝐱𝐭.
• 6. SpEL conditions (𝐂𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐚𝐥𝐎𝐧𝐄𝐱𝐩𝐫𝐞𝐬𝐬𝐢𝐨𝐧) - which check a result of specific SpEL expression.

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Spring Conditional Annotations 👉 https://www.baeldung.com/spring-conditional-annotations