1.Springboot在启动的时候初始化日志监听器

org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

2.日志监听器的初始化方法

protected void initialize(ConfigurableEnvironment environment, ClassLoader classLoader) {(new LoggingSystemProperties(environment)).apply();this.logFile = LogFile.get(environment);if (this.logFile != null) {this.logFile.applyToSystemProperties();}this.loggerGroups = new LoggerGroups(DEFAULT_GROUP_LOGGERS);this.initializeEarlyLoggingLevel(environment);this.initializeSystem(environment, this.loggingSystem, this.logFile);this.initializeFinalLoggingLevels(environment, this.loggingSystem);this.registerShutdownHookIfNecessary(environment, this.loggingSystem);}

this.logFile = LogFile.get(environment)这里获取日志文件路径

3.LogFile类

public class LogFile {public static final String FILE_NAME_PROPERTY = "logging.file.name";public static final String FILE_PATH_PROPERTY = "logging.file.path";private final String file;private final String path;LogFile(String file) {this(file, (String)null);}LogFile(String file, String path) {Assert.isTrue(StringUtils.hasLength(file) || StringUtils.hasLength(path), "File or Path must not be empty");this.file = file;this.path = path;}public void applyToSystemProperties() {this.applyTo(System.getProperties());}public void applyTo(Properties properties) {this.put(properties, "LOG_PATH", this.path);this.put(properties, "LOG_FILE", this.toString());}private void put(Properties properties, String key, String value) {if (StringUtils.hasLength(value)) {properties.put(key, value);}}public String toString() {return StringUtils.hasLength(this.file) ? this.file : (new File(this.path, "spring.log")).getPath();}public static LogFile get(PropertyResolver propertyResolver) {String file = propertyResolver.getProperty("logging.file.name");String path = propertyResolver.getProperty("logging.file.path");return !StringUtils.hasLength(file) && !StringUtils.hasLength(path) ? null : new LogFile(file, path);}
}

这里会自动从application.propertie或者application.yml中获取logfile的name和path

4.LogFile方法get（静态方法）

    public static LogFile get(PropertyResolver propertyResolver) {String file = propertyResolver.getProperty("logging.file.name");String path = propertyResolver.getProperty("logging.file.path");return !StringUtils.hasLength(file) && !StringUtils.hasLength(path) ? null : new LogFile(file, path);}

1. get方法首先从springboot配置文件中获取filename和fileapath
2. 然后判断是否获取成功，如果都没有获取成功，返回null
3. 那么springboot就不会将日志写到文件
4. 如果获取一个值或全部获取就返回 一个LogFile，通过构造函数传入filename和filepath

5.LogFile核心逻辑toString()

toString是Object的方法，java中所有类都是Object的子类，LogFile重写了toString方法

    public String toString() {return StringUtils.hasLength(this.file) ? this.file : (new File(this.path, "spring.log")).getPath();}

判断filename是否有值：

1. 如果有值直接返回filename
2. 否则返回路径+spring.log

6.结论

也就是说，如果我们配置了filename和filepath，那么我们的日志文件名就是filename，路径会忽略；
如果我们只配置了路径，那么我们的日志文件名默认是spring.log，并且springboot会自动帮我们创建文件夹

所以，如果我们想指定自己日志的路径和名称，那么我们的路径里文件夹必须自己创建