Java: Spring Boot 3: most important changes?

Spring Boot 3 (November 2022) — a major update with many breaking changes.

Simple analogy: Spring Boot 2 → 3 is like moving to a new house. Old furniture (javax) doesn't fit, you need new (jakarta). But the house is better: warmer (Java 17+), more spacious (Virtual Threads), smarter (Observability built-in). It is not a cosmetic update — it is a fundamental modernization of the platform.

Why it matters: Spring Boot 2 reached end of life. Every new project should start with Spring Boot 3. The migration is non-trivial (especially javax → jakarta), but the benefits — native image support, virtual threads, built-in observability, improved security — make it essential.

Key changes:

1. Java 17+ (minimum):

1<java.version>17</java.version>

Java 17 enables sealed classes, records, pattern matching, and text blocks — all used internally by Spring Boot 3.

2. javax → jakarta (the biggest migration effort):

1// Before (Spring Boot 2)
2import javax.persistence.Entity;
3import javax.servlet.http.HttpServletRequest;
4import javax.validation.constraints.NotBlank;
5
6// After (Spring Boot 3)
7import jakarta.persistence.Entity;
8import jakarta.servlet.http.HttpServletRequest;
9import jakarta.validation.constraints.NotBlank;

Every import, every XML namespace, every dependency that references javax must change to jakarta.

3. GraalVM Native Image:

1<dependency>
2    <groupId>org.springframework.boot</groupId>
3    <artifactId>spring-boot-starter-aot</artifactId>
4</dependency>

1./mvnw -Pnative native:compile
2# Result: ./target/myapp (~50MB, startup in 10ms, lower memory usage)

Native images compile Java to native machine code — no JVM required. Startup drops from seconds to milliseconds, memory usage drops from hundreds of MB to tens of MB.

4. Virtual Threads (opt-in, Java 21+):

1spring:
2  threads:
3    virtual:
4      enabled: true  # Spring Boot 3.2+

Virtual threads give you WebFlux-level throughput with standard MVC code. One million concurrent connections without reactive programming.

5. Observability (Micrometer + Tracing):

1<dependency>
2    <groupId>io.micrometer</groupId>
3    <artifactId>micrometer-tracing-bridge-otel</artifactId>
4</dependency>

Built-in distributed tracing — every HTTP call, database query, and message send is automatically traced with trace IDs in logs.

6. Deprecated / Removed:

• spring.factories → AutoConfiguration.imports (new registration mechanism).
• @ConstructorBinding not needed (constructors are default now).
• server.error.include-message = never (security — error messages are hidden by default).
• WebMvcConfigurerAdapter removed — use WebMvcConfigurer directly.

7. Spring Security 6:

• Lambda DSL instead of method chaining:

1// Before (Security 5):
2// http.authorizeRequests().antMatchers("/api/**").authenticated()
3
4// After (Security 6):
5http.authorizeHttpRequests(auth -> auth
6    .requestMatchers("/api/public/**").permitAll()
7    .requestMatchers("/api/**").authenticated()
8);

8. Problem Detail (RFC 7807):

1// Standardized error responses
2@RestControllerAdvice
3public class GlobalExceptionHandler {
4    @ExceptionHandler(UserNotFoundException.class)
5    public ProblemDetail handleNotFound(UserNotFoundException ex) {
6        ProblemDetail problem = ProblemDetail.forStatusAndDetail(
7            HttpStatus.NOT_FOUND, ex.getMessage());
8        problem.setTitle("User Not Found");
9        return problem;
10    }
11}

Migration tips:

• Run the Spring Boot migration tool (spring-boot-migrator) for automated javax → jakarta changes.
• Update all third-party libraries to versions compatible with Jakarta EE.
• Test thoroughly — subtle behavioral changes in Security and auto-configuration may break existing flows.
• Migrate in stages: upgrade Spring Boot first, then enable new features (native, virtual threads) incrementally.