本文从SpringBoot源码分析 配置文件的加载原理和配置文件的优先级跟入源码之前，先提一个问题：SpringBoot 既可以加载指定目录下的配置文件获取配置项，也可以通过启动参数（VM Options）传入配置项，为什么通过启动参数传入的配置项会“顶掉”配置文件中的配置？示例：application.yml
<strong>server.port</strong>: 8888
spring.profiles.active: dev
application-dev.yml
spring.think: hello
在IDEA中使用命令行配置项VM Options
-D<strong>server.port</strong>=5555
如下图：
启动结果：
Tomcat started on <strong>port</strong>(s): 5555 (http) with context path ''
同时在application.yml 和 启动参数（VM options）中设置server.port， 最终采用了 启动参数 中的值。
下面开始从main函数启动处，跟入SpringBoot源码，看看SpringBoot是如何处理的。系统说明JDK：1.8SpringBoot 版本： 2.0.2.RELEASEIDE： IntelliJ IDEA 2017跟入源码正文
#ApplicationConfigLoadFlow.java
public static void main(String[] args) {
SpringApplication.<strong>run</strong>(ApplicationConfigLoadFlow.class, args);
}
从SpringApplication.run函数开始，一个方法一个方法的跟入源码。需要跟入的方法给与注释或高亮。IDEA 快捷键：进入方法: Ctrl + 鼠标左键光标前进/后退： Ctrl + Shirt + 右方向键/左方向键依次跟入源码：
#SpringApplication.java
return <strong>run</strong>(new Class<?>[] { primarySource }, args)
#SpringApplication.java
return new SpringApplication(primarySources).<strong>run</strong>(args);
#SpringApplication.java
public ConfigurableApplicationContext <strong>run</strong>(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);
//跟入
ConfigurableEnvironment environment =<strong> prepareEnvironment</strong>(listeners,
applicationArguments);
configureIgnoreBeanInfo(environment);
configureIgnoreBeanInfo(environment);
进入public ConfigurableApplicationContext run(String... args) 方法后，我们重点看prepareEnvironment这个方法。这个方法之前的源码的从类名和源码注释上知道stopWatch用于计时，上下文context还未初始化，listeners监听器存储了EventPushlingRunListener。通过IDEA 一行行debug可以看到是在prepareEnvironment方法执行后，server.port 配置项才被加载入 environment 环境配置中。如下图所示。注意：配置文件中的配置还未载入，请先接着往后看。
因此，我们重新打断点跟入prepareEnvironment方法。
#SpringApplication.java
private ConfigurableEnvironment prepareEnvironment(
SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments) {
// Create and configure the environment
//跟入
ConfigurableEnvironment environment =<strong> getOrCreateEnvironment</strong>();
configureEnvironment(environment, applicationArguments.getSourceArgs());
同样的套路，通过debug发现实在getOrCreateEnvironment方法执行后得到server.port的值
#SpringApplication.java
private ConfigurableEnvironment getOrCreateEnvironment() {
if (this.environment != null) {
return this.environment;
}
if (this.webApplicationType == WebApplicationType.SERVLET) {
//跟入
return new<strong> StandardServletEnvironment</strong>();
}
虚拟机启动参数的加载 是在StandardServletEnvironment 的实例化过程中完成的。跟入StandardServletEnvironment的实例化过程之前，大家需要先了解Java模板模式。看一下StandardServletEnvironment的类继承关系图（通过IDEA 右键 类名 --> Diagrams --> Show Diagrams Popup 即可显示下图）
抽象父类AbstractEnvironment的实例化方法中，调用了可由子类继承的customizePropertySources方法。
#AbstractEnvironment.java
public AbstractEnvironment() {
//跟入
<strong>customizePropertySources</strong>(this.propertySources);
if (logger.isDebugEnabled()) {
logger.debug("Initialized " + getClass().getSimpleName() + " with PropertySources " + this.propertySources);
}
}
实体化的过程中回过头来调用了子类StandardServletEnvironment的customizePropertySources方法
#StandardServletEnvironment.java
protected void<strong> customizePropertySources</strong>(MutablePropertySources propertySources) {
propertySources.addLast(new StubPropertySource(SERVLET_CONFIG_PROPERTY_SOURCE_NAME));
propertySources.addLast(new StubPropertySource(SERVLET_CONTEXT_PROPERTY_SOURCE_NAME));
if (JndiLocatorDelegate.isDefaultJndiEnvironmentAvailable()) {
propertySources.addLast(new JndiPropertySource(JNDI_PROPERTY_SOURCE_NAME));
}
//跟入
super.<strong>customizePropertySources</strong>(propertySources);
}
又调用了父类StandardEnvironment的customizePropertySources方法
#StandardEnvironment.java
protected void<strong> customizePropertySources</strong>(MutablePropertySources propertySources) {
//跟入
propertySources.addLast(new MapPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, <strong>getSystemProperties</strong>()));
propertySources.addLast(new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));
}
通过IDEA 的变量监听功能，可以看到正是StandardEnvironment类的getSystemProperties()方法获取到了之前设置的虚拟机启动参数server.port的值。继续跟进去
#AbstractEnvironment.java
public Map<String, Object><strong> getSystemProperties</strong>() {
try {
//跟入
return (Map) System.<strong>getProperties</strong>();
#System.java
public static Properties <strong>getProperties</strong>() {
SecurityManager sm = getSecurityManager();
if (sm != null) {
sm.checkPropertiesAccess();
}
return <strong>props</strong>;
我们搜索一下有没有什么地方初始化 props
#System.java
private static Properties <strong>props</strong>;
private static native Properties <strong>initProperties</strong>(Properties props);
发现了静态方法 initProperties，从方法名上即可知道在类被加载的时候 就初始化了 props， 这是个本地方法，继续跟的话需要看对应的C++代码。回到StandardEnvironment类的customizePropertySources方法
#StandardEnvironment.java
protected void<strong> customizePropertySources</strong>(MutablePropertySources propertySources) {
//SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME: systemProperties
//跟入
propertySources.addLast(new<strong> MapPropertySource</strong>(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties()));
propertySources.addLast(new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));
}
#MutablePropertySources.java
/**
* Add the given property source object with lowest precedence.
* 添加属性源，并使其优先级最低
*/
public void <strong>addLast</strong>(PropertySource<?> propertySource) {
再看一下MutablePropertySources的类注释
* <p>Where <em>precedence</em> is mentioned in methods such as {@link #addFirst}
* and {@link #addLast}, this is with regard to the order in which property sources
* will be searched when resolving a given property with a {@link PropertyResolver}.
*
* addFist 和 add Last 会设置属性源的优先级，
* PropertyResolver解析配置时会根据优先级使用配置源
*
* @author Chris Beams
* @author Juergen Hoeller
* @since 3.1
* @see PropertySourcesPropertyResolver
*/
public class MutablePropertySources implements PropertySources {
问题2：此时我们已经看到虚拟机的启动参数先添加到系统当中，那么后面添加进来的Property Source属性源的优先级是否比 SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME(systemProperties) 属性源的优先级高呢？回到SpringApplication的prepareEnvironment方法
同样的debug套路发现listeners.environmentPrepared执行后，application.yml 和 application-dev.yml 两个配置文件的配置项都被加载完成，所以我们继续跟入environmentPrepared方法在跟入environmentPrepared方法之前，需要了解Java事件监听机制跟入environmentPrepared中的源码
#SpringApplicationRunListeners.java
public void<strong> environmentPrepared</strong>(ConfigurableEnvironment environment) {
for (SpringApplicationRunListener listener : this.listeners) {
//跟入
listener.<strong>environmentPrepared</strong>(environment);
}
}
#EventPublishingRunListener.java
public void<strong> environmentPrepared</strong>(ConfigurableEnvironment environment) {
//广播ApplicationEnvrionmentPreparedEvnet事件
//跟入
this.initialMulticaster.<strong>multicastEvent</strong>(new ApplicationEnvironmentPreparedEvent(
this.application, this.args, environment));
}
#SimpleApplicationEventMulticaster.java
public void<strong> multicastEvent</strong>(ApplicationEvent event) {
//跟入
<strong> multicastEvent</strong>(event, resolveDefaultEventType(event));
}
@Override
public void <strong>multicastEvent</strong>(final ApplicationEvent event, @Nullable ResolvableType eventType) {
ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
//注意此时 getApplicationListeners(event, type) 返回结果
//包含 监听器 *ConfigFileApplicationListener*
for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
Executor executor = getTaskExecutor();
if (executor != null) {
executor.execute(() -> invokeListener(listener, event));
}
else {
//跟入
<strong> invokeListener</strong>(listener, event);
}
}
}
#SimpleApplicationEventMulticaster.java
/**
* Invoke the given listener with the given event.
* 调用对应事件的监听者
* @param listener the ApplicationListener to invoke
* @param event the current event to propagate
* @since 4.1
*/
protected void <strong>invokeListener</strong>(ApplicationListener<?> listener, ApplicationEvent event) {
ErrorHandler errorHandler = getErrorHandler();
if (errorHandler != null) {
try {
doInvokeListener(listener, event);
}
catch (Throwable err) {
errorHandler.handleError(err);
}
}
else {
//跟入
<strong> doInvokeListener</strong>(listener, event);
}
}
private void<strong> doInvokeListener</strong>(ApplicationListener listener, ApplicationEvent event) {
try {
//跟入
listener.<strong>onApplicationEvent</strong>(event);
}
#ApplicationListener.java
//实现接口的监听器当中，有并跟入ConfigFileApplicationListener的实现
void <strong>onApplicationEvent</strong>(E event);
#ConfigFileApplicationListener.java
public void<strong> onApplicationEvent</strong>(ApplicationEvent event) {
if (event instanceof ApplicationEnvironmentPreparedEvent) {
//跟入
<strong> onApplicationEnvironmentPreparedEvent</strong>(
(ApplicationEnvironmentPreparedEvent) event);
}
if (event instanceof ApplicationPreparedEvent) {
onApplicationPreparedEvent(event);
}
}
private void<strong> onApplicationEnvironmentPreparedEvent</strong>(
ApplicationEnvironmentPreparedEvent event) {
List<EnvironmentPostProcessor> postProcessors = loadPostProcessors();
postProcessors.add(this);
AnnotationAwareOrderComparator.sort(postProcessors);
for (EnvironmentPostProcessor postProcessor : postProcessors) {
//跟入：当postProcessor 为 ConfigFileApplicationListener
postProcessor.<strong>postProcessEnvironment</strong>(event.getEnvironment(),
event.getSpringApplication());
}
}
#ConfigFileApplicationListener.java
public void<strong> postProcessEnvironment</strong>(ConfigurableEnvironment environment,
SpringApplication application) {
//跟入
<strong> addPropertySources</strong>(environment, application.getResourceLoader());
}
protected void<strong> addPropertySources</strong>(ConfigurableEnvironment environment,
ResourceLoader resourceLoader) {
//environment的属性源中包含 systemProperties 属性源 即包含 server.port启动参数
RandomValuePropertySource.addToEnvironment(environment);
//跟入 load()方法
new Loader(environment, resourceLoader).<strong>load</strong>();
}
跟入load之前，需要了解java lambda表达式
#ConfigFileApplicationListener.java
public void<strong> load</strong>() {
this.profiles = new LinkedList<>();
this.processedProfiles = new LinkedList<>();
this.activatedProfiles = false;
this.loaded = new LinkedHashMap<>();
initializeProfiles();
while (!this.profiles.isEmpty()) {
Profile profile = this.profiles.poll();
load(profile, this::getPositiveProfileFilter,
addToLoaded(MutablePropertySources::addLast, false));
this.processedProfiles.add(profile);
}
//跟入
<strong>load</strong>(null, this::getNegativeProfileFilter,
addToLoaded(MutablePropertySources::addFirst, true));
addLoadedPropertySources();
}
#ConfigFileApplicationListener.java
private void<strong> load</strong>(Profile profile, DocumentFilterFactory filterFactory,
DocumentConsumer consumer) {
//getSearchLocations()默认返回：
//[./config/, file:./, classpath:/config/, classpath:/]
//即搜索这些路径下的文件
getSearchLocations().forEach((location) -> {
boolean isFolder = location.endsWith("/");
//getSearchNames()返回：application
Set<String> names = (isFolder ? getSearchNames() : NO_SEARCH_NAMES);
//跟入load(.....)
names.forEach(
(name) -><strong> load</strong>(location, name, profile, filterFactory, consumer));
});
}
#ConfigFileApplicationListener.java
private void<strong> load</strong>(String location, String name, Profile profile,
DocumentFilterFactory filterFactory, DocumentConsumer consumer) {
//name默认为：application，所以这个if分支略过
if (!StringUtils.hasText(name)) {
for (PropertySourceLoader loader : this.propertySourceLoaders) {
if (canLoadFileExtension(loader, location)) {
load(loader, location, profile,
filterFactory.getDocumentFilter(profile), consumer);
}
}
}
//this.propertySourceLoaders: PropertiesPropertySourceLoader,YamlPropertySourceLoader
for (PropertySourceLoader loader : this.propertySourceLoaders) {
//PropertiesPropertySourceLoader.getFileExtensions(): properties, xml
//YamlPropertySourceLoader.getFileExtensions(): yml, yaml
for (String fileExtension : loader.getFileExtensions()) {
//location: [./config/, file:./, classpath:/config/, classpath:/]
//name: application
String prefix = location + name;
fileExtension = "." + fileExtension;
//profile: null, dev
//相当于对(location, fileExtension, profile)做笛卡尔积，
//遍历每一种可能，然后加载
//加载文件的细节在loadForFileExtension中完成
<strong> loadForFileExtension</strong>(loader, prefix, fileExtension, profile,
filterFactory, consumer);
}
}
}
继续跟入 loadForFileExtension 方法，可以了解载入一个配置文件的更多细节。回到之前的load()方法
#ConfigFileApplicationListener.java
public void<strong> load</strong>() {
this.profiles = new LinkedList<>();
this.processedProfiles = new LinkedList<>();
this.activatedProfiles = false;
this.loaded = new LinkedHashMap<>();
initializeProfiles();
while (!this.profiles.isEmpty()) {
Profile profile = this.profiles.poll();
load(profile, this::getPositiveProfileFilter,
addToLoaded(MutablePropertySources::addLast, false));
this.processedProfiles.add(profile);
}
load(null, this::getNegativeProfileFilter,
addToLoaded(MutablePropertySources::addFirst, true));
//跟入
<strong>addLoadedPropertySources</strong>();
#ConfigFileApplicationListener.java
private void<strong> addLoadedPropertySources</strong>() {
//destination: 进入ConfigFileApplicationListener监听器前已有的配置
//即destination中包含 systemProperties 配置源
MutablePropertySources destination = this.environment.getPropertySources();
String lastAdded = null;
//loaded: 此次监听通过扫描文件加载进来的配置源
//loaded: application.yml, appcalition-dev.yml
List<MutablePropertySources> loaded = new ArrayList<>(this.loaded.values());
//倒序后 loaded: application-dev.yml, application.yml
Collections.reverse(loaded);
//先处理 application-dev.yml
for (MutablePropertySources sources : loaded) {
for (PropertySource<?> source : sources) {
//第一次进入: lastAdded:null
if (lastAdded == null) {
if (destination.contains(DEFAULT_PROPERTIES)) {
destination.addBefore(DEFAULT_PROPERTIES, source);
}
else {
//第一次进入: 把application-dev.yml至于最低优先级
destination.addLast(source);
}
}
else {
//第二次进入:
//让 application.yml 优先级比 application-dev.yml 低
destination.addAfter(lastAdded, source);
}
//第一次遍历结束: lastAdded: application-dev
lastAdded = source.getName();
}
}
}
执行后得到各自的优先级，如下图：
systemProperties优先级高，解析器会优先使用 systemProperties中的server.port配置项即 5555 所以最终Tomcat 启动端口是 5555从中也可以看出，如果application.yml 和 application-dev.yml中有相同的配置项，会优先采用application-dev.yml中的配置项。参考：1.SpringBoot源码分析之SpringBoot的启动过程2.SpringBoot源码分析之配置环境的构造过程3.springboot启动时是如何加载配置文件application.yml文件