通过上一章的源码分析，我们知道了spring boot里面的listeners到底是什么（META-INF/spring.factories定义的资源的实例），以及它是创建和启动的，今天我们继续深入分析一下SpringApplication实例变量中的run函数中的其他内容。还是先把run函数的代码贴出来：
/** * Run the Spring application, creating and refreshing a new * {@link ApplicationContext}. * @param args the application arguments (usually passed from a Java main method) * @return a running {@link ApplicationContext} */
public ConfigurableApplicationContext run(String... args) {
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
configureHeadlessProperty();
SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
configureIgnoreBeanInfo(environment);
Banner printedBanner = printBanner(environment);
context = createApplicationContext();
exceptionReporters = getSpringFactoriesInstances(
SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
prepareContext(context, environment, listeners, applicationArguments,
printedBanner);
refreshContext(context);
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
listeners.started(context);
callRunners(context, applicationArguments);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, listeners);
throw new IllegalStateException(ex);
}
try {
listeners.running(context);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, null);
throw new IllegalStateException(ex);
}
return context;
}在listeners启动了以后，我们来看一下ApplicationArguments applicationArguments
= new DefaultApplicationArguments(args); 在DefaultApplicationArguments的构造函数里，我们跟踪过去发现其最终调用的SimpleCommandLineArgsParser.parse函数：
public CommandLineArgs parse(String... args) {
CommandLineArgs commandLineArgs = new CommandLineArgs();
String[] var3 = args;
int var4 = args.length;
for(int var5 = 0; var5 < var4; ++var5) {
String arg = var3[var5];
if(arg.startsWith("--")) {
String optionText = arg.substring(2, arg.length());
String optionValue = null;
String optionName;
if(optionText.contains("=")) {
optionName = optionText.substring(0, optionText.indexOf(61));
optionValue = optionText.substring(optionText.indexOf(61) + 1,
optionText.length());
} else {
optionName = optionText;
}
if(optionName.isEmpty() || optionValue != null && optionValue.isEmpty()) {
throw new IllegalArgumentException("Invalid argument syntax: " + arg);
}
commandLineArgs.addOptionArg(optionName, optionValue);
} else {
commandLineArgs.addNonOptionArg(arg);
}
}
return commandLineArgs;
}从这段代码中我们看到DefaultApplicationArguments其实是读取了命令行的参数。
小发现：通过分析这个函数的定义，你是不是想起了spring boot启动的时候，用命令行参数自定义端口号的情景？
java -jar MySpringBoot.jar –server.port=8000
接着往下看：ConfigurableEnvironment environment = this.prepareEnvironment(listeners, ex);
通过这行代码我们可以看到spring boot把前面创建出来的listeners和命令行参数，传递到prepareEnvironment函数中来准备运行环境。来看一下prepareEnvironment函数的真面目：
private ConfigurableEnvironment prepareEnvironment( SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments) { // Create and configure the environment ConfigurableEnvironment environment = getOrCreateEnvironment(); configureEnvironment(environment, applicationArguments.getSourceArgs()); listeners.environmentPrepared(environment); bindToSpringApplication(environment); if (this.webApplicationType == WebApplicationType.NONE) { environment = new EnvironmentConverter(getClassLoader()) .convertToStandardEnvironmentIfNecessary(environment); } ConfigurationPropertySources.attach(environment); return environment; }在这里我们看到了环境是通过getOrCreateEnvironment创建出来的，再深挖一下getOrCreateEnvironment的源码：
private ConfigurableEnvironment getOrCreateEnvironment() {
if (this.environment != null) {
return this.environment;
}
if (this.webApplicationType == WebApplicationType.SERVLET) {
return new StandardServletEnvironment();
}
return new StandardEnvironment();
}通过这段代码我们看到了如果environment 已经存在，则直接返回当前的环境。
小思考：在什么情况下会出现environment 已经存在的情况？提示：我们前面讲过，可以自己初始化SpringApplication，然后调用run函数，在初始化SpringApplication和调用run函数之间，是不是可以发生点什么？
下面的代码判断了webApplicationType是不是SERVLET，如果是，则创建Servlet的环境，否则创建基本环境。我们来挖一挖webApplicationType是在哪里初始化的：
private static final String REACTIVE_WEB_ENVIRONMENT_CLASS = "org.springframework."
+ "web.reactive.DispatcherHandler";
private static final String MVC_WEB_ENVIRONMENT_CLASS = "org.springframework."
+ "web.servlet.DispatcherServlet";
/** * Create a new {@link SpringApplication} instance. The application context will load * beans from the specified primary sources (see {@link SpringApplication class-level} * documentation for details. The instance can be customized before calling * {@link #run(String...)}. * @param resourceLoader the resource loader to use * @param primarySources the primary bean sources * @see #run(Class, String[]) * @see #setSources(Set) */
@SuppressWarnings({ "unchecked", "rawtypes" })
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 = deduceWebApplicationType();
setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class));
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
this.mainApplicationClass = deduceMainApplicationClass();
}
private WebApplicationType deduceWebApplicationType() {
if (ClassUtils.isPresent(REACTIVE_WEB_ENVIRONMENT_CLASS, null)
&& !ClassUtils.isPresent(MVC_WEB_ENVIRONMENT_CLASS, null)) {
return WebApplicationType.REACTIVE;
}
for (String className : WEB_ENVIRONMENT_CLASSES) {
if (!ClassUtils.isPresent(className, null)) {
return WebApplicationType.NONE;
}
}
return WebApplicationType.SERVLET;
}通过这段代码，我们发现了原来spring boot是通过检查当前环境中是否存在
org.springframework.web.servlet.DispatcherServlet类来判断当前是否是web环境的。
接着往下看，获得了ConfigurableEnvironment环境以后，通过后面的代码对环境进行“微调”。
通过this.configureIgnoreBeanInfo(environment);如果System中的spring.beaninfo.ignore属性为空，就把当前环境中的属性覆盖上去：
private void configureIgnoreBeanInfo(ConfigurableEnvironment environment) {
if(System.getProperty("spring.beaninfo.ignore") == null) {
Boolean ignore = (Boolean)environment.getProperty("spring.beaninfo.ignore",
Boolean.class, Boolean.TRUE);
System.setProperty("spring.beaninfo.ignore", ignore.toString());
}
}通过Banner printedBanner = this.printBanner(environment);这行代码打印出spring boot的Banner。还记得spring boot启动的时候，在控制台显示的那个图片吗？这里不作深究，继续往下看：
context = this.createApplicationContext();创建了应用上下文：
public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
+ "annotation.AnnotationConfigApplicationContext";
public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework.boot."
+ "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";
public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework."
+ "boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";
protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass;
if (contextClass == null) {
try {
switch (this.webApplicationType) {
case SERVLET:
contextClass = Class.forName(DEFAULT_WEB_CONTEXT_CLASS);
break;
case REACTIVE:
contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
break;
default:
contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
}
}
catch (ClassNotFoundException ex) {
throw new IllegalStateException(
"Unable create a default ApplicationContext, "
+ "please specify an ApplicationContextClass",
ex);
}
}
return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}通过这里我们看到，spring boot是根据不同的webApplicationType的类型，来创建不同的ApplicationContext的。
总结：通过上面的各种深挖，我们知道了spring boot 2.0中的环境是如何区分普通环境和web环境的，以及如何准备运行时环境和应用上下文。时间不早了，今天就跟大家分享到这里，下一篇文章会继续跟大家分享spring boot 2.0源码的实现。
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原文出处：https://www.cnblogs.com/lizongshen/p/9136535.html
