这篇文章主要介绍了解决spring-boot使用logback的大坑，具有很好的参考价值，希望对大家有所帮助。如有错误或未考虑完全的地方，望不吝赐教
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 Collection extends T> getSpringFactoriesInstances(Class type, Class>[] parameterTypes, Object... args) { ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); // 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; }
继续进入loadFactoryNames()方法，核心就在这里了
public static List loadFactoryNames(Class> factoryClass, ClassLoader classLoader) { String factoryClassName = factoryClass.getName(); try { Enumeration urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) : ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION)); List result = new ArrayList(); 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 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> listeners = builder.application().getListeners(); for (Iterator
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
