最近在写一个logback的kafka appender，无意中发现spring-boot在使用logback时的一个坑
用ConsoleAppender.java来举例，假设在logback.xml中使用了该appender，那么这个类的相关的初始化方法都会调两次，如start()方法
打断点进行debug，第一次进入start()方法如下：
可以看到所有的调用链（除了自己代码的方法）都是logback或者slf4j相关的比较正常
当跳过该断点时又会进入以此这个方法，看下调用链：
可以看到这次的初始化是由spring-boot发起的，所以这样logback初始化一次，然后spring-boot初始化一次，一共两次
我们现在可以将spring-boot的初始化去掉
debug代码可以发现LoggingApplicationListener.java这个监听器主要是用来初始化spring-boot的日志系统，现在目的将该listener在启动之前去掉
spring-boot的启动代码为：
new SpringApplicationBuilder(Launcher.class).application().run(args);
在SpringApplicationBuilder.java的构造方法打断点进行跟踪，
进入SpringAppication.java会发现该类中的代码：
private void initialize(Object[] sources) {
if (sources != null && sources.length > 0) {
this.sources.addAll(Arrays.asList(sources));
}
this.webEnvironment = deduceWebEnvironment();
setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class));
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
this.mainApplicationClass = deduceMainApplicationClass();
}
第八行应该就是注册监听器的地方了，继续往下跟踪，进入以下方法：
private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type,
Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
// Use names and ensure unique to protect against duplicates
Set<String> names = new LinkedHashSet<String>(
SpringFactoriesLoader.loadFactoryNames(type, classLoader));
List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
继续进入loadFactoryNames()方法，核心就在这里了
public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
try {
Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
List<String> result = new ArrayList<String>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
String factoryClassNames = properties.getProperty(factoryClassName);
result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
}
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
"] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}
FACTORIES_RESOURCE_LOCATION这个常量的值为META-INF/spring.factories，
打开该文件可以发现：
# 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 # 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.context.web.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.liquibase.LiquibaseServiceLocatorApplicationListener,\ org.springframework.boot.logging.ClasspathLoggingApplicationListener,\ org.springframework.boot.logging.LoggingApplicationListener # Environment Post Processors org.springframework.boot.env.EnvironmentPostProcessor=\ org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\ org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor # 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.NoUniqueBeanDefinitionFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer # FailureAnalysisReporters org.springframework.boot.diagnostics.FailureAnalysisReporter=\ org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter
ApplicationListener应该就是我们需要修改的地方了，去掉org.springframework.boot.logging.LoggingApplicationListener就可以了，我们可以在代码里面覆盖一份这块代码从而实现去掉这行，但是实际得再跑一遍，发现还是一样初始化两次
问题出在
Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
这块代码是将所有的META-INF/spring.factories都读取过来了然后进行合并，所以说哦这个META-INF/spring.factories只能增加内容，但是不能去掉某些内容，没办法了只能在代码初始化了所有的listener之后再将listener去掉，
具体代码如下（启动spring-boot的main方法中）：
SpringApplicationBuilder builder = new SpringApplicationBuilder(Launcher.class);
Set<ApplicationListener<?>> listeners = builder.application().getListeners();
for (Iterator<ApplicationListener<?>> it = listeners.iterator(); it.hasNext();) {
ApplicationListener<?> listener = it.next();
if (listener instanceof LoggingApplicationListener) {
it.remove();
}
}
builder.application().setListeners(listeners);
builder.run(args);
PS：其实log初始化两次并无伤大雅，关键是遇到了问题总是想解决下或者了解下原理
以上为个人经验，希望能给大家一个参考，也希望大家多多支持。
