Logback的配置文件查找顺序

问题
使用slf4j和logback记录日志，有这样一个需求，如果指定了一个特定logback.xml，就用指定的配置，如果没有，则系统提供一个默认的配置。

解决
classpath下放一个配置文件logback.xml，使用环境变量logback.configurationFile指定另一个配置。logback会首先使用环境变量指定的文件，如果没有指定或文件不存在，logback会使用classpath下的文件。

原理 - logback配置文件查找过程
1. 使用如下代码获取日志器

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
Logger logger = LoggerFactory.getLogger(LogTest.class);

2. slf4j首先会判断有没有初始化过

org.slf4j.LoggerFactory

public static Logger getLogger(Class<?> clazz) {
Logger logger = getLogger(clazz.getName());
...
return logger;
}

public static Logger getLogger(String name) {
ILoggerFactory iLoggerFactory = getILoggerFactory();
return iLoggerFactory.getLogger(name);
}

public static ILoggerFactory getILoggerFactory() {
if (INITIALIZATION_STATE == UNINITIALIZED) {
synchronized (LoggerFactory.class) {
if (INITIALIZATION_STATE == UNINITIALIZED) {
INITIALIZATION_STATE = ONGOING_INITIALIZATION;
performInitialization();
}
}
}
...
}

3. 如果没有初始化过，则执行初始化，初始化的过程是加载org.slf4j.impl.StaticLoggerBinder类

private final static void performInitialization() {
bind();
...
}

private final static void bind() {
...
staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
...
}

private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";

static Set<URL> findPossibleStaticLoggerBinderPathSet() {
// use Set instead of list in order to deal with bug #138
// LinkedHashSet appropriate here because it preserves insertion order during iteration
Set<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>();
try {
ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();
Enumeration<URL> paths;
if (loggerFactoryClassLoader == null) {
paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
} else {
paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);
}
while (paths.hasMoreElements()) {
URL path = paths.nextElement();
staticLoggerBinderPathSet.add(path);
}
} catch (IOException ioe) {
Util.report("Error getting resources from path", ioe);
}
return staticLoggerBinderPathSet;
}

4. 加载StaticLoggerBinder类的初始化过程

org.slf4j.impl.StaticLoggerBinder

static {
SINGLETON.init();
}

void init() {
new ContextInitializer(defaultLoggerContext).autoConfig();
}

ch.qos.logback.classic.util.ContextInitializer

final public static String GROOVY_AUTOCONFIG_FILE = "logback.groovy";
final public static String AUTOCONFIG_FILE = "logback.xml";
final public static String TEST_AUTOCONFIG_FILE = "logback-test.xml";
final public static String CONFIG_FILE_PROPERTY = "logback.configurationFile";

public void autoConfig() throws JoranException {
URL url = findURLOfDefaultConfigurationFile(true);
if (url != null) {
configureByResource(url);
} else {
Configurator c = EnvUtil.loadFromServiceLoader(Configurator.class);
if (c != null) {
try {
c.setContext(loggerContext);
c.configure(loggerContext);
} catch (Exception e) {
throw new LogbackException(String.format("Failed to initialize Configurator: %s using ServiceLoader", c != null ? c.getClass()
.getCanonicalName() : "null"), e);
}
} else {
BasicConfigurator basicConfigurator = new BasicConfigurator();
basicConfigurator.setContext(loggerContext);
basicConfigurator.configure(loggerContext);
}
}
}

public URL findURLOfDefaultConfigurationFile(boolean updateStatus) {
ClassLoader myClassLoader = Loader.getClassLoaderOfObject(this);
URL url = findConfigFileURLFromSystemProperties(myClassLoader, updateStatus);
if (url != null) {
return url;
}

url = getResource(GROOVY_AUTOCONFIG_FILE, myClassLoader, updateStatus);
if (url != null) {
return url;
}

url = getResource(TEST_AUTOCONFIG_FILE, myClassLoader, updateStatus);
if (url != null) {
return url;
}

return getResource(AUTOCONFIG_FILE, myClassLoader, updateStatus);
}

private URL findConfigFileURLFromSystemProperties(ClassLoader classLoader, boolean updateStatus) {
String logbackConfigFile = OptionHelper.getSystemProperty(CONFIG_FILE_PROPERTY);
...
}

可见，logback加载的过程是
（1）使用logback.configurationFile环境变量的设置
（2）使用classpath中的logback.groovy
（3）使用classpath中的logback-test.xml
（4）使用classpath中的logback.xml
（5）查找com.qos.logback.classic.spi.Configurator接口实现类，调用实现类的configure方法设置
（6）使用BasicConfigurator类的configure方法设置