SpringBoot spring.factories loading timing source code analysis

The role of spring.factories

This is similar to the SPI function in Java. When SpringBoot starts, it will read the META-INF/spring.factories files under all jar packages;

And map the implementation classes corresponding to the interfaces/abstract classes in the file, and instantiate the corresponding implementation classes when needed

Let’s analyze the source code and take a lookspring.factoriesUsage scenarios

Source code analysis

Start SpringApplication and look at the construction method

	public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) {
		this.resourceLoader = resourceLoader;
		Assert.notNull(primarySources, "PrimarySources must not be null");
		this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
		this.webApplicationType = WebApplicationType.deduceFromClasspath();
		setInitializers((Collection) getSpringFactoriesInstances(
				ApplicationContextInitializer.class));
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		this.mainApplicationClass = deduceMainApplicationClass();
	}

The method getSpringFactoriesInstances(ApplicationContextInitializer.class) is used to obtain the specified class instance in Spring; and when obtaining it, the corresponding instance class is instantiated based on the content in the file path META-INF/spring.factories read in the entire project. ;

For example, the ApplicationContextInitializer here is an interface, so which of its implementation classes should be instantiated? Then look for the META-INF/spring.factories file; then we found this file in the spring-boot:2.1.0jar package

Read that the implementation class that needs to be instantiated is

org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer

and also found this file in spring-boot-autoconfigure-2.1.0.RELEASE.jar

Then the two implementation classes in the file will also be instantiated; plus the above 4, there are 6 in total

org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

You can see that not only the instance classes of org.springframework.context.ApplicationContextInitializer are parsed; but all of them are parsed and saved. Other classes will need it next time When instantiated, it can be taken directly from the memory;

After the above process obtains the instance class, the next step is the instantiation process

	private  Collection getSpringFactoriesInstances(Class type,
			Class[] parameterTypes, Object... args) {
		ClassLoader classLoader = getClassLoader();
		// Use names and ensure unique to protect against duplicates
		Set names = new LinkedHashSet<>(
				SpringFactoriesLoader.loadFactoryNames(type, classLoader));
		List instances = createSpringFactoriesInstances(type, parameterTypes,
				classLoader, args, names);
		AnnotationAwareOrderComparator.sort(instances);
		return instances;
	}

MethodcreateSpringFactoriesInstancesIt is the process of creating an instance; you can see that the corresponding interface class is passed inorg.springframework.context.ApplicationContextInitializer ;Then the corresponding implementation class will be instantiated;

	private  List createSpringFactoriesInstances(Class type,
			Class[] parameterTypes, ClassLoader classLoader, Object[] args,
			Set names) {
		List instances = new ArrayList<>(names.size());
		for (String name : names) {
			try {
				Class instanceClass = ClassUtils.forName(name, classLoader);
				Assert.isAssignable(type, instanceClass);
				Constructor constructor = instanceClass
						.getDeclaredConstructor(parameterTypes);
				T instance = (T) BeanUtils.instantiateClass(constructor, args);
				instances.add(instance);
			}
			catch (Throwable ex) {
				throw new IllegalArgumentException(
						"Cannot instantiate " + type + " : " + name, ex);
			}
		}
		return instances;
	}

If there is nothing special to explain about the instantiation process;

There is a method above AnnotationAwareOrderComparator.sort(instances);is used to sort all instances; the implementation class needs to implement Interface Ordered; The smaller the value returned by getOrder, the higher the priority

Usage

After knowing the usage of spring.factories, then we You can use this feature to achieve your own purposes;

For example, we can also write an implementation class of the interface classApplicationContextInitializer.

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