SpringBoot startup principle

This is the 15th day of my participation in the August More text Challenge. For details, see: August More Text Challenge

Start the class

The SpringBoot application has a main entry, the main method, which calls the SpringApplication.run method to launch the program
@ SpringBootApplication comments:
- EnableAutoConfiguration: Automatically configures the Spring framework based on the declared dependencies of the application
- SpringBootConfiguration: Configuration class for the Spring IOC container in the form of JavaConfig
- @componentscan: ComponentScan that automatically discovers and assembles beans. By default, it scans packages in the path where the startup class is located
```
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(...)
public @interfaceSpringBootApplication {... }Copy the code
```

The basic flow

The first part is to initialize the module and configure some basic environment variables, resources, constructors, and listeners
The second part implements the specific startup scheme of the application, including starting the monitoring module, loading the configuration environment module, and creating the context environment module

The third part is the automatic configuration module, which serves as the core of SpringBoot automatic configuration

1) to (2) collecting all kinds of conditions and callback interface, such as ApplictionContextInitializer, ApplictionListener - > notice started () 3. Create and prepare Environment -> Notify environmentPrepared() 4. Create and initialize ApplicationContext, ContextPrepared (); contextLoaded(); refresh ApplictionContext (); The end of theCopy the code

Implementation process

Start the static run method of the class

public static ConfigurableApplicationContext run(Class
         [] primarySources, String[] args) {
    return (new SpringApplication(primarySources)).run(args);
}
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The SpringApplication constructor

public SpringApplication(ResourceLoader resourceLoader, Class
         ... primarySources) {
    // ...
    // Determine the application type, which is divided into responsive Web application, Servlet Web application, and non-Web application
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    // ...
    // Set the initializer
    this.setInitializers(
        this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
    // Set the listener
    this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));
    // Set the entry method
    this.mainApplicationClass = this.deduceMainApplicationClass();
}
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Set the initializer

this.setInitializers(
        this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
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private <T> Collection<T> getSpringFactoriesInstances(Class
         
           type, Class
          [] parameterTypes, Object... args)
          {
    ClassLoader classLoader = this.getClassLoader();
    / /, depending on the type of type ApplicationContextInitializer. Class
    // Read the spring.factories from the meta-INF of the classpath and iterate over the key-value pairs
    Set<String> names = new LinkedHashSet(
        SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    // Instantiate objects according to names
    List<T> instances = this.createSpringFactoriesInstances(
        type, parameterTypes, classLoader, args, names);
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}
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The spring.factories configuration file

org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener
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Setting the listener

this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));
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org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer
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Run method

public ConfigurableApplicationContext run(String... args) {
    / / timer
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    DefaultBootstrapContext bootstrapContext = this.createBootstrapContext();
    // The application context to return
    ConfigurableApplicationContext context = null;
    // Set the java.awt.headless system property to true
    this.configureHeadlessProperty();
    // Load the runtime listener
    SpringApplicationRunListeners listeners = this.getRunListeners(args);
    // Publish the start execution event
    listeners.starting(bootstrapContext, this.mainApplicationClass);

    try {
        // Handle the parameters in the launcher
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        // Prepare the environment by passing in parameters based on the scanned listener objects and functions
        ConfigurableEnvironment environment = this.prepareEnvironment(listeners, bootstrapContext, applicationArguments);
        this.configureIgnoreBeanInfo(environment);
        / / set the Banner
        Banner printedBanner = this.printBanner(environment);
        // Create the Spring container
        context = this.createApplicationContext();
        context.setApplicationStartup(this.applicationStartup);
        // Spring container preprocessing
        this.prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
        / / - > trigger SpringApplicationRunListener contextPrepared execution
        this.refreshContext(context);
        // Spring container post processing
        this.afterRefresh(context, applicationArguments);
        stopWatch.stop();
        // Logs are printed
        if (this.logStartupInfo) {
            (new StartupInfoLogger(this.mainApplicationClass))
            .logStarted(this.getApplicationLog(), stopWatch);
        }
        // Issue the end of startup event
        listeners.started(context);
        // Call the registered Runners, ApplicationRunner, and CommandLineRunner in turn
        this.callRunners(context, applicationArguments);
    } catch (Throwable var10) {
        this.handleRunFailure(context, var10, listeners);
        throw new IllegalStateException(var10);
    }
    try {
     // Publish the application context ready event
        listeners.running(context);
        return context;
    } catch (Throwable var9) {
        this.handleRunFailure(context, var9, (SpringApplicationRunListeners)null);
        throw newIllegalStateException(var9); }}Copy the code

The run method – loads the runtime listener

SpringApplicationRunListeners listeners = this.getRunListeners(args);
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// Get the listener that is running the listener, and send the corresponding event to the listener at the corresponding stage
private SpringApplicationRunListeners getRunListeners(String[] args) { Class<? >[] types =new Class[]{SpringApplication.class, String[].class};
    return new SpringApplicationRunListeners(
        logger,
        this.getSpringFactoriesInstances(
            SpringApplicationRunListener.class, types, this, args),
        this.applicationStartup);
}
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SpringApplicationRunListener class:

public interface SpringApplicationRunListener {

	// This is called immediately after the run method is called and can be used for very early initialization
    default void starting(ConfigurableBootstrapContext bootstrapContext) {
        starting();
    }

    // When the environment is ready
    default void environmentPrepared(ConfigurableBootstrapContext bootstrapContext, ConfigurableEnvironment environment) {
        environmentPrepared(environment);
    }

    // Call ApplicationContext when it is ready before the resource is loaded
    default void contextPrepared(ConfigurableApplicationContext context) {}// called when the application context is loaded but before it is refreshed

    default void contextLoaded(ConfigurableApplicationContext context) {}// The context has been refreshed and the application started and all CommandLineRunner and ApplicationRunner have not been called before
    default void started(ConfigurableApplicationContext context) {}// The context has been refreshed and the application started and all CommandLineRunner and ApplicationRunner have been called
    default void running(ConfigurableApplicationContext context) {}// called when the startup process fails
    default void failed(ConfigurableApplicationContext context, Throwable exception) {}}Copy the code

Run method – Environment preparation

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners, DefaultBootstrapContext bootstrapContext, ApplicationArguments applicationArguments) {
    // Create Environment objects with different implementations based on different Web types
    ConfigurableEnvironment environment = getOrCreateEnvironment();
    // Configure the environment
    this.configureEnvironment(environment, applicationArguments.getSourceArgs());
    ConfigurationPropertySources.attach(environment);
    // The sending environment is ready to complete the event
    listeners.environmentPrepared(bootstrapContext, environment);
    DefaultPropertiesPropertySource.moveToEnd(environment);
    // Configure the activeProfile in the Environment object based on the spring.profiles. Active property in the command line parameters
    this.configureAdditionalProfiles(environment);
    // Bind the spring.main property of the environment to the SpringApplication object
    this.bindToSpringApplication(environment);
    // If the user manually sets webApplicationType using the spring.main.web-application-type property
    if (!this.isCustomEnvironment) {
        // Convert the environment object to the user set webApplicationType related type, because inherit the same parent class, directly strong
        environment = new EnvironmentConverter(getClassLoader())
            .convertEnvironmentIfNecessary(environment,                             
                                           deduceEnvironmentClass());
    }
    ConfigurationPropertySources.attach(environment);
    return environment;
}
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The run method – prepareContext

private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context, ConfigurableEnvironment environment, SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
    // Set the context
    context.setEnvironment(environment);
    this.postProcessApplicationContext(context);
    / / execution spring. Factories ApplicationContextInitializer object initialize method
    this.applyInitializers(context);
	// The publishing context prepares the completion event to all listeners
    listeners.contextPrepared(context);
    bootstrapContext.close(context);
    if (this.logStartupInfo) {
        this.logStartupInfo(context.getParent() == null);
        this.logStartupProfileInfo(context);
    }

    ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
    beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
    if(printedBanner ! =null) {
        beanFactory.registerSingleton("springBootBanner", printedBanner);
    }

    if (beanFactory instanceof DefaultListableBeanFactory) {
        ((DefaultListableBeanFactory)beanFactory).setAllowBeanDefinitionOverriding(
            this.allowBeanDefinitionOverriding);
    }

    if (this.lazyInitialization) {
        context.addBeanFactoryPostProcessor(
            new LazyInitializationBeanFactoryPostProcessor());
    }
    // Load the resource
    Set<Object> sources = this.getAllSources();
    Assert.notEmpty(sources, "Sources must not be empty");
    // Load the bean into the context object
    this.load(context, sources.toArray(new Object[0]));
    // Send the context loading completion event
    listeners.contextLoaded(context);
}
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Run method – refreshContext

private void refreshContext(ConfigurableApplicationContext context) {
    // Register hooks when the JVM is stopped
    if (this.registerShutdownHook) {
        try {
            context.registerShutdownHook();
        } catch (AccessControlException var3) {
        }
    }

    this.refresh((ApplicationContext)context);
}

protected void refresh(ConfigurableApplicationContext applicationContext) {
    applicationContext.refresh();
}
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Call the ConfigurableApplicationContext refresh method

public interface ConfigurableApplicationContext extends ApplicationContext.Lifecycle.Closeable {
	void refresh(a) throws BeansException, IllegalStateException;
}
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There are three: ConfigurableApplicationContext implementation class

AbstractApplictionContext, ServletWebServerApplicationContext, ReactiveWebServerApplicationContext

AbstractApplictionContext is an abstract class, the other two class inherited AbstractApplictionContext

public void refresh(a) throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        StartupStep contextRefresh = 
            this.applicationStartup.start("spring.context.refresh");

        // Step 1: Prepare the preparation for updating the upper and lower
        prepareRefresh();

        // Step 2: Get the BeanFactory inside the context
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Step 3: Prepare the BeanFactory
        prepareBeanFactory(beanFactory);

        try {
            // Step 4: Allow post-processing of bean factories in context subclasses
            postProcessBeanFactory(beanFactory);

            StartupStep beanPostProcess = 
                this.applicationStartup.start("spring.context.beans.post-process");
            // Step 5: Call the factory BeanFactoryPostProcessor registered as a bean in the context
            invokeBeanFactoryPostProcessors(beanFactory);

            // Step 6: Register the interceptor created by the interceptor bean
            registerBeanPostProcessors(beanFactory);
            beanPostProcess.end();

            // Step 7: initialize internationalization related MessageSource
            initMessageSource();

            // Step 8: Initialize the container event announcer for publishing events
            initApplicationEventMulticaster();

            // Step 9: Initialize some special beans
            onRefresh();

            // Step 10: Register all listeners with the event broadcasters created in the first two steps
            registerListeners();

            // Step 11: Finish the bean initialization (instantiate all singletons BeanDefinition)
            finishBeanFactoryInitialization(beanFactory);

            // Step 12: afterRefresh
            finishRefresh();
        } catch (BeansException ex) {
            if (logger.isWarnEnabled()) {
                logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
            }

            // Destroy singletons that have been created to avoid resident resource occupancy
            destroyBeans();

            // Reset 'active' flag.
            cancelRefresh(ex);

            throw ex;
        } finally {
            // Reset the public cache because some of the metadata used to create the bean may no longer be neededresetCommonCaches(); contextRefresh.end(); }}}Copy the code

The realization of the ServletWebServerApplicationContext:

@Override
public final void refresh(a) throws BeansException, IllegalStateException {
    try {
        super.refresh();
    } catch (RuntimeException ex) {
        WebServer webServer = this.webServer;
        if(webServer ! =null) {
            webServer.stop();
        }
        throwex; }}Copy the code

Automatic configuration

@EnableAutoConfiguration

EnableAutoConfiguration is part of the @SpringBootApplication
```
@Import({AutoConfigurationImportSelector.class})
public @interfaceEnableAutoConfiguration {... }Copy the code
```
AutoConfigurationImportSelector SpringFactoriesLoader will all eligible @ Configuration Configuration class is loaded into the IOC container

AutoConfigurationImportSelector

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    // Load the spring.factories configuration information
    List<String> configurations = 
        SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
                                               getBeanClassLoader());
    // ...
    return configurations;
}
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SpringFactoriesLoader.loadFactoryNames

SpringFactoriesLoader is an abstract class with static properties defined in the class that define the path to which it loads resources
```
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
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```
There are also three static methods:
- LoadFactories: Loads and instantiates the specified factoryClass
- LoadFactoryNames: Loads a collection of names for the specified factoryClass
- InstantiateFactory: Instantiates the specified factoryClass
Many xxxAutoConfiguration classes are configured with spring.factories. When loaded into the container, these configuration classes are full of @conditional annotations indicating the valid conditions, so that the corresponding content can be automatically configured under appropriate conditions

The sample

Hello.java

public class Hello {

    private String msg;

    public String getMsg(a) {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg; }}Copy the code

HelloProperties.java

@ConfigurationProperties(prefix = "hello") 
public class HelloProperties {

    private String msg;

    public String getMsg(a) {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg; }}Copy the code

HelloAutoConfiguration.java

/ / configuration class
@Configuration
// Add HelloProperties to the IOC container
@EnableConfigurationProperties(HelloProperties.class)
// Determine whether the Hello class exists in the classpath. If so, the class bean will be instantiated
@ConditionalOnClass(Hello.class)
// The configuration determines whether to instantiate beans of this class
@ConditionalOnProperty(prefix="hello", value="enabled", matchIfMissing = true)
public class HelloAutoConfiguration {

    @Autowired
    private HelloProperties helloProperties;

    @Bean
    // If there is no Hello bean in the container, configure a Hello bean
    @ConditionalOnMissingBean(Hello.class)
    public Hello hello(a) {
        Hello hello = new Hello();
        hello.setMsg(helloProperties.getMsg());
        returnhello; }}Copy the code

Create a new meta-INF /spring.factories in the resource directory

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.example.demo.pojo.HelloAutoConfiguration
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application.properties

hello.enabled=true
hello.msg=abcde
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The configuration takes effect after the test

@SpringBootTest
class DemoApplicationTests {

    @Autowired
    Hello hello;

    @Test
    void contextLoads(a) { System.out.println(hello.getMsg()); }}Copy the code

Start the class

The basic flow

Implementation process

Automatic configuration

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