【springboot源码分析】1.SpringApplication实例化

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一般springboot(版本2.0.6)项目启动代码

package com.xll;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class RpcClientApplication {

    public static void main(String[] args) {
        SpringApplication.run(RpcClientApplication.class, args);
    }

}

@SpringBootApplication+SpringApplication.run(RpcClientApplication.class, args)这两句，就可以启动一个springboot项目了，需要注意的是，如果注解SpringBootApplication没有配置scanBasePackages，默认会去扫描启动类（在这里是run方法中的第一个参数RpcClientApplication.class）所在包下的类并进行注入。

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直接跟踪进入SpringApplication.run(RpcClientApplication.class, args) 方法中

public static ConfigurableApplicationContext run(Class<?> primarySource,
			String... args) {
	return run(new Class<?>[] { primarySource }, args);
}

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

run方法有两个，并且都是public，因此支持启动多个springboot应用同时启动。

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跟踪new SpringApplication(primarySources)

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();
}

SpringApplication构造函数主要做了四件事情

1. 推断Web应用类型，这里属于SERVLET
2. 设置初始化器
3. 设置监听器
4. 推断出main函数所在的类

需要设置哪些初始化器和需要设置哪些监听器，这些信息是从哪里获取的呢？答案就是：META-INF/spring.factories，springboot会去读取项目根路径下所有META-INF（包括所有的依赖）文件夹下的spring.factories文件里面的内容，可以看一下spring.factories里面内容都有些什么，以spring-boot-2.0.6.RELEASE.jar为例

# PropertySource Loaders
org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader

# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener

# Error Reporters
org.springframework.boot.SpringBootExceptionReporter=\
org.springframework.boot.diagnostics.FailureAnalyzers

# Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\
org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor

# Failure Analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindValidationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.UnboundConfigurationPropertyFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoSuchMethodFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyNameFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyValueFailureAnalyzer

# FailureAnalysisReporters
org.springframework.boot.diagnostics.FailureAnalysisReporter=\
org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter

可以看到ApplicationContextInitializer.class和ApplicationListener.class的value值有很多，这些就是需要设置的类。META-INF/spring.factories这个文件在springboot自动配置起到非常重要的作用，在之后会分析到它的作用。需要注意的是，之前有的springboot版本，每次有需求都会从磁盘中读取META-INF/spring.factories的内容，但是这个版本只有第一次会去磁盘中读取，然后缓存起来，之后按需进行实例化