Spring Boot

The Spring Boot Startup Lifecycle (SpringApplication.run)

sequenceDiagram
    autonumber
    participant Main as Main Method
    participant SA as SpringApplication
    participant SARL as SpringApplicationRunListeners
    participant Env as ConfigurableEnvironment
    participant Ctx as ApplicationContext
    participant BF as BeanFactory
    participant Auto as AutoConfigImportSelector
    participant Embed as EmbeddedWebServer

    Main->>SA: run(primarySources, args)
    activate SA

    Note right of SA: 1. Bootstrap Phase
    SA->>SA: Create BootstrapContext
    SA->>SARL: starting()

    Note right of SA: 2. Prepare Environment
    SA->>SA: configureEnvironment()
    SA->>Env: Load Properties (application.properties)
    SA->>SARL: environmentPrepared(environment)

    Note right of SA: 3. Create Context
    SA->>SA: createApplicationContext()

    Note right of SA: 4. Prepare Context
    SA->>SA: applyInitializers()
    SA->>SARL: contextPrepared(context)
    SA->>SA: load(primarySources)
    SA->>SARL: contextLoaded(context)

    Note right of SA: 5. Refresh Context (The Critical Path)
    SA->>Ctx: refresh()
    activate Ctx

    rect rgb(255, 250, 240)
        note right of Ctx: Phase 5a: Bean Definitions
        Ctx->>BF: invokeBeanFactoryPostProcessors()

        Note over Ctx, Auto: Parsing @EnableAutoConfiguration
        Ctx->>Auto: selectImports()
        Auto-->>Ctx: List of Config Class Names

        Ctx->>BF: registerBeanPostProcessors()
        Ctx->>BF: initMessageSource()
    end

    rect rgb(240, 248, 255)
        note right of Ctx: Phase 5b: Web Server Creation
        Ctx->>Ctx: onRefresh()
        Ctx->>Embed: createWebServer() (Tomcat init)
        Note left of Embed: Server created but NOT started
    end

    rect rgb(255, 240, 245)
        note right of Ctx: Phase 5c: Bean Instantiation
        Ctx->>Ctx: finishBeanFactoryInitialization()
        Note over Ctx: Instantiate Singletons (Non-Lazy)
    end

    rect rgb(240, 255, 240)
        note right of Ctx: Phase 5d: Finalize
        Ctx->>Ctx: finishRefresh()
        Ctx->>Embed: start()
        activate Embed
        Note left of Embed: Port 8080 listening
    end

    deactivate Ctx

    SA->>SARL: started(context)
    SA->>SA: callRunners (CommandLineRunner)
    SA->>SARL: running(context)

    deactivate SA

Auto-Configuration

flowchart TD
    Start([App Startup]) --> Scan["@EnableAutoConfiguration"]

    Scan --> Load["Load candidates from:
META-INF/spring/...AutoConfiguration.imports"]

    Load --> Candidates["List of Candidate Config Class Names"]

    Candidates --> Deduplicate["Deduplicate & Remove Exclusions"]

    Deduplicate --> Sort["Sort Candidates
(@AutoConfigureOrder / Before / After)"]

    Sort --> Iterate

    subgraph FilterProcess [Condition Evaluation Loop]
        direction TB
        Iterate[Iterate Config Class X] --> CondClass{"@ConditionalOnClass?"}

        CondClass -- Class Missing --> Discard[Discard Configuration]
        CondClass -- Class Present --> CondProp{"@ConditionalOnProperty?"}

        CondProp -- Prop=false --> Discard
        CondProp -- Prop=true --> CondWeb{"@ConditionalOnWebApplication?"}

        CondWeb -- Mismatch --> Discard
        CondWeb -- Match --> Register[Register Configuration Class]
    end

    Register --> BeanPhase[Process @Bean Methods]

    subgraph BeanLogic [Inside the Configuration Class]
        BeanPhase --> CondBean{"@ConditionalOnMissingBean?"}
        CondBean -- Bean Already Exists --> SkipBean[Skip creating this specific Bean]
        CondBean -- No Bean Found --> RegBean[Register BeanDefinition]
    end

    RegBean --> End([Ready for DI])
    SkipBean --> End

    style FilterProcess fill:#f9f9f9,stroke:#333,stroke-width:2px
    style BeanLogic fill:#f0f8ff,stroke:#333,stroke-dasharray: 5 5
    style Discard fill:#ffcccc,stroke:#333
    style RegBean fill:#ccffcc,stroke:#333

MVC Request Flow

sequenceDiagram
    participant Client
    participant Container as Servlet Container
    participant Filter as Servlet Filters
    participant DS as DispatcherServlet
    participant INT as HandlerInterceptors
    participant HA as HandlerAdapter
    participant RVH as ReturnValueHandler
(RequestResponseBodyMethodProcessor)
    participant MSG as HttpMessageConverter

    Client->>Container: HTTP GET /api/users
    Container->>Filter: doFilter()
    Filter->>DS: doService() -> doDispatch()

    activate DS

    DS->>DS: getHandler(request) (returns Chain)
    DS->>DS: getHandlerAdapter(handler)

    DS->>INT: preHandle()

    alt preHandle returns true
        DS->>HA: handle(request, response, handler)
        activate HA

        HA->>HA: Invoke Controller Method
        note right of HA: Business Logic Execution

        HA->>RVH: handleReturnValue(UserObject)
        activate RVH
        Note right of RVH: Handles @ResponseBody

        RVH->>MSG: write(UserObject, Type, Response)
        activate MSG
        MSG->>MSG: Serialize to JSON
        MSG-->>Container: Write bytes to Response OutputStream
        note left of MSG: Response Committed here!
        deactivate MSG

        RVH-->>HA: return
        deactivate RVH

        HA-->>DS: Return null ModelAndView
        note right of HA: MAV is null because response is committed
        deactivate HA

        DS->>INT: applyPostHandle()
        note right of INT: Called, but cannot change Response
    else preHandle returns false
        DS-->>Client: Request blocked
    end

    DS->>INT: triggerAfterCompletion()

    DS-->>Filter: return
    deactivate DS
    Filter-->>Container: return
    Container-->>Client: HTTP 200 OK (JSON Body)

Bean Lifecycle

flowchart TD
    Start((Start)) --> Instantiate["1. Instantiation
Constructor Call"]

    subgraph Pop [2. Population & Aware]
        direction TB
        Instantiate --> DI["Dependency Injection
Setters / Fields"]
        DI --> BName["BeanNameAware"]
        BName --> BFactory["BeanFactoryAware"]
        BFactory --> AppCtx["ApplicationContextAware
*Applied via BPP*"]
    end

    subgraph Init [3. Initialization]
        direction TB
        AppCtx --> BPPBefore[BeanPostProcessor
.postProcessBeforeInitialization]

        BPPBefore --> PostConstruct[annotation @PostConstruct]
        PostConstruct --> InitBean["InitializingBean
.afterPropertiesSet()"]
        InitBean --> CustomInit[Custom init-method]

        CustomInit --> BPPAfter[BeanPostProcessor
.postProcessAfterInitialization]
    end

    subgraph ProxyPhase [4. The AOP Magic]
        direction TB
        BPPAfter --> IsAOP{Has @Transactional
@Async / @Cacheable?}
        IsAOP -- Yes --> CreateProxy[Wrap Bean in CGLIB/JDK Proxy]
        IsAOP -- No --> ReturnRaw[Keep Raw Bean Instance]
    end

    CreateProxy --> Ready([Bean Ready in Container])
    ReturnRaw --> Ready

    Ready --> Stop((App Shutdown))

    subgraph Destroy [5. Destruction]
        direction TB
        Stop --> PreDestroy[annotation @PreDestroy]
        PreDestroy --> DispBean["DisposableBean
.destroy()"]
        DispBean --> CustomDestroy[Custom destroy-method]
    end

    CustomDestroy --> End((End))

    style ProxyPhase fill:#fff0f5,stroke:#333,stroke-dasharray: 5 5
    style CreateProxy fill:#ffcc00,stroke:#333
    style Ready fill:#ccffcc,stroke:#333