解决spring-boot使用logback的大坑

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我们。