slf4j + log4j原理实现及源码分析
#0 系列目录#html
- 2种日志接口框架，4种日志实现框架
- jdk-logging、log4j、logback日志介绍及原理
- jcl与jul、log4j一、log4j二、logback的集成原理
- slf4j与jul、log4j一、log4j二、logback的集成原理
- slf4j、jcl、jul、log4j一、log4j二、logback大总结
- slf4j + log4j原理实现及源码分析
- Apache Log4j
- Log4j架构分析与实战
- Log4J配置文件详解
- Apache Commons Logging
- Commons Logging 架构分析
- JDK Logging
- JDK Logging 深刻分析
- Logback
- 从Log4j迁移到LogBack的理由
- Logback 深刻分析
- Apache Log4j2
- Log4j2架构分析与实战
- Log4j2配置文件详解
#1 Slf4j# ##1.1 介绍## SLF4J，即简单日志门面（Simple Logging Facade for Java）。从设计模式的角度考虑，它是用来在log和代码层之间起到门面的做用。对用户来讲只要使用slf4j提供的接口，便可隐藏日志的具体实现。这与jdbc和类似。使用jdbc也就避免了不一样的具体数据库。使用了slf4j能够对客户端应用解耦。由于当咱们在代码实现中引入log日志的时候，用的是接口，因此能够实时的更具状况来调换具体的日志实现类。这就是slf4j的做用。java
SLF4J所提供的核心API是一些接口以及一个LoggerFactory的工厂类。SLF4J提供了统一的记录日志的接口，只要按照其提供的方法记录便可，最终日志的格式、记录级别、输出方式等经过具体日志系统的配置来实现，所以能够在应用中灵活切换日志系统。数据库
配置SLF4J是很是简单的一件事，只要将与你打算使用的日志系统对应的jar包加入到项目中，SLF4J就会自动选择使用你加入的日志系统。apache
##1.2 简单使用##设计模式
/**
* Slf4j 日志门面接口 Test
* @author taomk
* @version 1.0
* @since 15-10-15 下午3:39
*/
public class Slf4jFacadeTest {
private static Logger logger = LoggerFactory.getLogger(Slf4jFacadeTest.class);
public static void main(String[] args){
if(logger.isDebugEnabled()){
logger.debug("slf4j-log4j debug message");
}
if(logger.isInfoEnabled()){
logger.debug("slf4j-log4j info message");
}
if(logger.isTraceEnabled()){
logger.debug("slf4j-log4j trace message");
}
}
}
上述Logger接口、LoggerFactory类都是slf4j本身定义的。那么，SLF4J是怎么实现日志绑定的？api
##1.3 日志绑定##性能优化
- 在应用中，经过LoggerFactory类的静态getLogger()获取logger，代码以下：
private static Logger logger = LoggerFactory.getLogger(Slf4jFacadeTest.class);
- LoggerFactory.getLogger(Slf4jFacadeTest.class)方法的源码以下：LoggerFactory.java
public static Logger getLogger(String name) {
ILoggerFactory iLoggerFactory = getILoggerFactory();
return iLoggerFactory.getLogger(name);
}
上述获取Log的过程大体分红2个阶段：服务器
- 获取ILoggerFactory的过程 (从字面上理解就是生产Logger的工厂)；
- 根据ILoggerFactory获取Logger的过程；
下面来详细说明：架构
- 1 获取ILoggerFactory的过程，
又能够分红3个过程： - 1.1 从类路径中寻找
org/slf4j/impl/StaticLoggerBinder.class类：LoggerFactory.java
private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";
private 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 = (URL) paths.nextElement();
staticLoggerBinderPathSet.add(path);
}
} catch (IOException ioe) {
Util.report("Error getting resources from path", ioe);
}
return staticLoggerBinderPathSet;
}
若是找到多个，则输出 Class path contains multiple SLF4J bindings，表示有多个日志实现与slf4j进行了绑定。下面看下当出现多个StaticLoggerBinder的时候的输出日志（简化了一些内容）：LoggerFactory.javaapp
SLF4J: Class path contains multiple SLF4J bindings. SLF4J: Found binding in [slf4j-log4j12-1.7.12.jar!/org/slf4j/impl/StaticLoggerBinder.class] SLF4J: Found binding in [logback-classic-1.1.3.jar!/org/slf4j/impl/StaticLoggerBinder.class] SLF4J: Found binding in [slf4j-jdk14-1.7.12.jar!/org/slf4j/impl/StaticLoggerBinder.class] SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation. SLF4J: Actual binding is of type [org.slf4j.impl.Log4jLoggerFactory]
- 1.2
“随机选取"一个StaticLoggerBinder.class来建立一个单例：
private final static void bind() {
try {
Set<URL> staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
// 打印搜索到的全部StaticLoggerBinder日志
reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
// the next line does the binding 随机选取绑定
StaticLoggerBinder.getSingleton();
INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
// 打印最终实际绑定StaticLoggerBinder日志
reportActualBinding(staticLoggerBinderPathSet);
fixSubstitutedLoggers();
} catch (NoClassDefFoundError ncde) {
String msg = ncde.getMessage();
if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;
Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
Util.report("Defaulting to no-operation (NOP) logger implementation");
Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");
} else {
failedBinding(ncde);
throw ncde;
}
} catch (java.lang.NoSuchMethodError nsme) {
String msg = nsme.getMessage();
if (msg != null && msg.indexOf("org.slf4j.impl.StaticLoggerBinder.getSingleton()") != -1) {
INITIALIZATION_STATE = FAILED_INITIALIZATION;
Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");
Util.report("Your binding is version 1.5.5 or earlier.");
Util.report("Upgrade your binding to version 1.6.x.");
}
throw nsme;
} catch (Exception e) {
failedBinding(e);
throw new IllegalStateException("Unexpected initialization failure", e);
}
}
- 1.3 根据上述建立的StaticLoggerBinder单例，
返回一个ILoggerFactory实例：LoggerFactory.java
public static ILoggerFactory getILoggerFactory() {
if (INITIALIZATION_STATE == UNINITIALIZED) {
INITIALIZATION_STATE = ONGOING_INITIALIZATION;
performInitialization();
}
switch (INITIALIZATION_STATE) {
case SUCCESSFUL_INITIALIZATION:
// 返回绑定
return StaticLoggerBinder.getSingleton().getLoggerFactory();
case NOP_FALLBACK_INITIALIZATION:
return NOP_FALLBACK_FACTORY;
case FAILED_INITIALIZATION:
throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
case ONGOING_INITIALIZATION:
// support re-entrant behavior.
// See also http://bugzilla.slf4j.org/show_bug.cgi?id=106
return TEMP_FACTORY;
}
throw new IllegalStateException("Unreachable code");
}
因此slf4j与其余实际的日志框架的集成jar包中，都会含有这样的一个org/slf4j/impl/StaticLoggerBinder.class类文件，而且提供一个ILoggerFactory的实现。
- 2 根据ILoggerFactory获取Logger的过程：
这就要看具体的ILoggerFactory类型了，下面与Log4j集成来详细说明。
#2 Log4j介绍# Apache的一个开放源代码项目，经过使用Log4j，咱们能够控制日志信息输送的目的地是控制台、文件、GUI组件、甚至是套接口服务器、NT的事件记录器、UNIX Syslog守护进程等；用户也能够控制每一条日志的输出格式；经过定义每一条日志信息的级别，用户可以更加细致地控制日志的生成过程。这些能够经过一个配置文件来灵活地进行配置，而不须要修改程序代码。具体详细介绍，参见Log4j架构分析与实战。
#3 Slf4j与Log4j集成# ##3.1 Maven依赖##
<!-- slf4j -->
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.12</version>
</dependency>
<!-- slf4j-log4j -->
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
<version>1.7.12</version>
</dependency>
<!-- log4j -->
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.17</version>
</dependency>
##3.2 使用案例##
- 第一步：编写log4j.properties配置文件,放到类路径下
log4j.rootLogger = debug, console
log4j.appender.console = org.apache.log4j.ConsoleAppender
log4j.appender.console.layout = org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss} %m%n
配置文件的详细内容参见Log4J配置文件详解。
- 第二步：代码中以下使用
private static Logger logger=LoggerFactory.getLogger(Log4jSlf4JTest.class);
public static void main(String[] args){
if(logger.isDebugEnabled()){
logger.debug("slf4j-log4j debug message");
}
if(logger.isInfoEnabled()){
logger.info("slf4j-log4j info message");
}
if(logger.isTraceEnabled()){
logger.trace("slf4j-log4j trace message");
}
}
- 补充说明：
- 1 配置文件一样能够随意放置，如log4j1原生方式加载配置文件的方式log4j1原生开发
- 2 注意二者方式的不一样：
slf4j: Logger logger=LoggerFactory.getLogger(Log4jSlf4JTest.class); log4j: Logger logger=Logger.getLogger(Log4jTest.class);
slf4j的Logger是slf4j定义的接口，而log4j的Logger是类。LoggerFactory是slf4j本身的类。
##3.3 原理分析## 先来看下slf4j-log4j12包中的内容：
- 的确是有
org/slf4j/impl/StaticLoggerBinder.class类。 - 该StaticLoggerBinder返回的ILoggerFactory类型将会是
Log4jLoggerFactory。 - Log4jLoggerAdapter就是实现了slf4j定义的Logger接口。
来看下具体过程：
- 1 获取对应的ILoggerFactory：从上面的slf4j的原理中咱们知道：ILoggerFactory是由StaticLoggerBinder来建立出来的，因此能够简单分红2个过程：
- 1.1 第一个过程：slf4j寻找绑定类StaticLoggerBinder：使用ClassLoader来加载
“org/slf4j/impl/StaticLoggerBinder.class"这样的类的url，而后就找到了slf4j-log4j12包中的StaticLoggerBinder。 - 1.2 第二个过程：建立出StaticLoggerBinder实例，
并建立出ILoggerFactory，源码以下：
StaticLoggerBinder.getSingleton().getLoggerFactory();
以slf4j-log4j12中的StaticLoggerBinder为例，建立出的ILoggerFactory为Log4jLoggerFactory。
- 2 根据ILoggerFactory获取Logger的过程，来看下Log4jLoggerFactory是如何返回一个slf4j定义的Logger接口的实例的，源码以下：
org.apache.log4j.Logger log4jLogger;
if (name.equalsIgnoreCase(Logger.ROOT_LOGGER_NAME))
log4jLogger = LogManager.getRootLogger();
else
log4jLogger = LogManager.getLogger(name);
Logger newInstance = new Log4jLoggerAdapter(log4jLogger);
- 2.1 咱们能够看到是经过log4j1的原生方式，即便用log4j1的LogManager来获取，
引起log4j1的加载配置文件，而后初始化，最后返回一个org.apache.log4j.Logger log4jLogger，参见log4j1原生开发。 - 2.2 将上述的org.apache.log4j.Logger
log4jLogger封装成Log4jLoggerAdapter，而Log4jLoggerAdapter是实现了slf4j的接口，因此咱们使用的slf4j的Logger接口实例（这里即Log4jLoggerAdapter）都会委托给内部的org.apache.log4j.Logger实例。
#4 Slf4j与Log4j源码分析# ##4.1 Slf4j初始化##
- 初始化时机：下面这行代码，会在
当前类类加载时或者显示得执行调用LoggerFactory.getLogger()方法时，触发Slf4j初始化，并绑定具体日志：Slf4jFacadeTest.java
private static Logger logger = LoggerFactory.getLogger(Slf4jFacadeTest.class);
- 初始化步骤：
- 源码1：LoggerFactory.java
public static Logger getLogger(String name) {
// 1. 初始化LoggerFactory，绑定具体日志，得到具体日志的LoggerFactory。
ILoggerFactory iLoggerFactory = getILoggerFactory();
// 2. 根据具体日志的LoggerFactory，触发具体日志的初始化并得到具体日志的Logger对象；
return iLoggerFactory.getLogger(name);
}
- 源码2：LoggerFactory.java
public static ILoggerFactory getILoggerFactory() {
// 1. 是否已经初始化了，不然进行初始化
if (INITIALIZATION_STATE == UNINITIALIZED) {
INITIALIZATION_STATE = ONGOING_INITIALIZATION;
performInitialization();
}
switch (INITIALIZATION_STATE) {
case SUCCESSFUL_INITIALIZATION:
// 2. 成功初始化，则直接得到具体日志的LoggerFactory
return StaticLoggerBinder.getSingleton().getLoggerFactory();
case NOP_FALLBACK_INITIALIZATION:
return NOP_FALLBACK_FACTORY;
case FAILED_INITIALIZATION:
throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
case ONGOING_INITIALIZATION:
// support re-entrant behavior.
// See also http://bugzilla.slf4j.org/show_bug.cgi?id=106
return TEMP_FACTORY;
}
throw new IllegalStateException("Unreachable code");
}
- 源码3：LoggerFactory.java
private final static void performInitialization() {
// 1. 绑定具体的日志实现
bind();
if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {
versionSanityCheck();
}
}
- 源码4：LoggerFactory.java
private final static void bind() {
try {
// 1. 扫描查找“org/slf4j/impl/StaticLoggerBinder.class”
Set<URL> staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
// 2. 打印找到多个“org/slf4j/impl/StaticLoggerBinder.class”的日志
reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
// the next line does the binding 随机选择绑定，类加载器随机选择。
StaticLoggerBinder.getSingleton();
INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
// 3. 打印绑定具体org/slf4j/impl/StaticLoggerBinder.class的日志
reportActualBinding(staticLoggerBinderPathSet);
fixSubstitutedLoggers();
} catch (NoClassDefFoundError ncde) {
String msg = ncde.getMessage();
if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;
Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
Util.report("Defaulting to no-operation (NOP) logger implementation");
Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");
} else {
failedBinding(ncde);
throw ncde;
}
} catch (java.lang.NoSuchMethodError nsme) {
String msg = nsme.getMessage();
if (msg != null && msg.indexOf("org.slf4j.impl.StaticLoggerBinder.getSingleton()") != -1) {
INITIALIZATION_STATE = FAILED_INITIALIZATION;
Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");
Util.report("Your binding is version 1.5.5 or earlier.");
Util.report("Upgrade your binding to version 1.6.x.");
}
throw nsme;
} catch (Exception e) {
failedBinding(e);
throw new IllegalStateException("Unexpected initialization failure", e);
}
}
- 源码5：LoggerFactory.java
private 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) {
// 1. 扫包查找“org/slf4j/impl/StaticLoggerBinder.class”
paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
} else {
paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);
}
while (paths.hasMoreElements()) {
URL path = (URL) paths.nextElement();
staticLoggerBinderPathSet.add(path);
}
} catch (IOException ioe) {
Util.report("Error getting resources from path", ioe);
}
return staticLoggerBinderPathSet;
}
- 源码6：slf4j-log4j-1.6.2.jar中StaticLoggerBinder.java
private static final StaticLoggerBinder SINGLETON = new StaticLoggerBinder();
public static final StaticLoggerBinder getSingleton() {
// 1. 返回StaticLoggerBinder实例
return SINGLETON;
}
private StaticLoggerBinder() {
// 2. StaticLoggerBinder初始化
loggerFactory = new Log4jLoggerFactory();
try {
Level level = Level.TRACE;
}cache (NoSuchFieldError nsfe) {
Util.report("This version of SLF4J requires log4j version 1.2.12 or later. See also http://www.slf4j.org/codes.html#log4j_version");
}
}
以上就是Slf4j初始化过程的源代码，其初始化过程就是绑定具体日志实现；
##4.2 Log4j初始化## 这里只在源码层级作分析，不想看源码，可直接参考具体详细流程，请参见Log4j初始化分析。
-
初始化时机：前一小节Slf4j已提到，
在iLoggerFactory.getLogger(name)时触发Log4j初始化。iLoggerFactory具体类型为：Log4jLoggerFactory。 -
初始化步骤：
- 源码1：Log4jLoggerFactory.java
public Logger getLogger(String name) {
Logger slf4jLogger = null;
// protect against concurrent access of loggerMap
synchronized (this) {
slf4jLogger = (Logger) loggerMap.get(name);
if (slf4jLogger == null) {
// 1. 获取Logej具体的Logger对象。
org.apache.log4j.Logger log4jLogger;
if(name.equalsIgnoreCase(Logger.ROOT_LOGGER_NAME)) {
log4jLogger = LogManager.getRootLogger();
} else {
log4jLogger = LogManager.getLogger(name);
}
slf4jLogger = new Log4jLoggerAdapter(log4jLogger);
loggerMap.put(name, slf4jLogger);
}
}
return slf4jLogger;
}
- 源码2：LogManager.java静态代码块初始化Log4j
static {
// By default we use a DefaultRepositorySelector which always returns 'h'.
// 1. 初始化Logger仓库，并添加一个RootLogger实例，默认日志级别为DEBUG。
Hierarchy h = new Hierarchy(new RootLogger((Level) Level.DEBUG));
repositorySelector = new DefaultRepositorySelector(h);
/** Search for the properties file log4j.properties in the CLASSPATH. */
String override =OptionConverter.getSystemProperty(DEFAULT_INIT_OVERRIDE_KEY, null);
// 2. 检查系统属性log4j.defaultInitOverride，若是该属性被设置为false，则执行初始化；不然（只要不是false，不管是什么值，甚至没有值，都是不然），跳过初始化。
// if there is no default init override, then get the resource
// specified by the user or the default config file.
if(override == null || "false".equalsIgnoreCase(override)) {
String configurationOptionStr = OptionConverter.getSystemProperty(
DEFAULT_CONFIGURATION_KEY,
null);
String configuratorClassName = OptionConverter.getSystemProperty(
CONFIGURATOR_CLASS_KEY,
null);
URL url = null;
// 3. 把系统属性log4j.configuration的值赋给变量resource。若是该系统变量没有被定义，则把resource赋值为"log4j.properties"。注意：在apache的log4j文档中建议使用定义log4j.configuration系统属性的方法来设置默认的初始化文件是一个好方法。
// if the user has not specified the log4j.configuration
// property, we search first for the file "log4j.xml" and then
// "log4j.properties"
if(configurationOptionStr == null) {
url = Loader.getResource(DEFAULT_XML_CONFIGURATION_FILE);
if(url == null) {
url = Loader.getResource(DEFAULT_CONFIGURATION_FILE);
}
} else {
try {
// 4. 试图把resource变量转化成为一个URL对象url。若是通常的转化方法行不通，就调用org.apache.log4j.helpers.Loader.getResource(resource, Logger.class)方法来完成转化。
url = new URL(configurationOptionStr);
} catch (MalformedURLException ex) {
// so, resource is not a URL:
// attempt to get the resource from the class path
url = Loader.getResource(configurationOptionStr);
}
}
// If we have a non-null url, then delegate the rest of the
// configuration to the OptionConverter.selectAndConfigure
// method.
if(url != null) {
LogLog.debug("Using URL ["+url+"] for automatic log4j configuration.");
try {
// 5. 若是url以".xml"结尾，则调用方法DOMConfigurator.configure(url)来完成初始化；不然，则调用方法PropertyConfigurator.configure(url)来完成初始化。若是url指定的资源不能被得到，则跳出初始化过程。
OptionConverter.selectAndConfigure(url, configuratorClassName,
LogManager.getLoggerRepository());
} catch (NoClassDefFoundError e) {
LogLog.warn("Error during default initialization", e);
}
} else {
LogLog.debug("Could not find resource: ["+configurationOptionStr+"].");
}
} else {
LogLog.debug("Default initialization of overridden by " +
DEFAULT_INIT_OVERRIDE_KEY + "property.");
}
}
- 源码3：OptionConverter.java
public static void selectAndConfigure(URL url, String clazz, LoggerRepository hierarchy) {
Configurator configurator = null;
String filename = url.getFile();
// 1. 若是url以".xml"结尾，则调用方法DOMConfigurator.configure(url)来完成初始化；不然，则调用方法PropertyConfigurator.configure(url)来完成初始化。若是url指定的资源不能被得到，则跳出初始化过程。
if(clazz == null && filename != null && filename.endsWith(".xml")) {
clazz = "org.apache.log4j.xml.DOMConfigurator";
}
if(clazz != null) {
LogLog.debug("Preferred configurator class: " + clazz);
configurator = (Configurator) instantiateByClassName(clazz,
Configurator.class,
null);
if(configurator == null) {
LogLog.error("Could not instantiate configurator ["+clazz+"].");
return;
}
} else {
configurator = new PropertyConfigurator();
}
configurator.doConfigure(url, hierarchy);
}
##4.3 Log4j输出日志## 这里只在源码层级作分析，不想看源码，可直接参考具体详细流程，请参见Log4j输出日志。
- 预处理： 当调用Log4j的方法（如：debug(String, Throwable)、info(String, Throwable)）输出日志时，首先对日志信息进行预处理。
- 源码1：Category.java（
Logger继承自Category.java）
public void info(Object message) {
// 1. 根据全局日志等级threshold进行判断，若是日志等级低于threshold，不输出日志。
if(repository.isDisabled(Level.INFO_INT))
return;
// 2. 根据当前logger配置的日志等级level进行判断，若是日志等级低于当前logger配置的日志等级，不输出日志。
if(Level.INFO.isGreaterOrEqual(this.getEffectiveLevel()))
// 3. 将日志信息封装成LoggingEvent对象。
forcedLog(FQCN, Level.INFO, message, null);
}
- 源码2：Category.java
protected void forcedLog(String fqcn, Priority level, Object message, Throwable t) {
// 1. 将LoggingEvent对象分发给全部的Appender。
callAppenders(new LoggingEvent(fqcn, this, level, message, t));
}
public void callAppenders(LoggingEvent event) {
int writes = 0;
for(Category c = this; c != null; c=c.parent) {
// Protected against simultaneous call to addAppender, removeAppender,...
synchronized(c) {
if(c.aai != null) {
// 2. 将LoggingEvent对象分发给全部的Appender。
writes += c.aai.appendLoopOnAppenders(event);
}
if(!c.additive) {
break;
}
}
}
if(writes == 0) {
repository.emitNoAppenderWarning(this);
}
}
public int appendLoopOnAppenders(LoggingEvent event) {
int size = 0;
Appender appender;
if(appenderList != null) {
size = appenderList.size();
for(int i = 0; i < size; i++) {
appender = (Appender) appenderList.elementAt(i);
appender.doAppend(event);
}
}
return size;
}
- 输出日志前还有两道工序须要处理：
Filter处理和日志信息格式化。
- 源码1：
public synchronized void doAppend(LoggingEvent event) {
if (closed) {
LogLog.error("Attempted to append to closed appender named ["
+ name + "].");
return;
}
if (!isAsSevereAsThreshold(event.getLevel())) {
return;
}
// Filter处理
Filter f = this.headFilter;
FILTER_LOOP: while (f != null) {
// 1. 有三种返回值 DENY、ACCEPT、NEUTRAL，DENY表示丢弃当前日志信息，ACCEPT表示输出当前日志信息，NEUTRAL表示继续下一个Filter。Filter只能在XML配置文件中使用，Properties文件中不支持。
switch (f.decide(event)) {
case Filter.DENY:
return;
case Filter.ACCEPT:
break FILTER_LOOP;
case Filter.NEUTRAL:
f = f.getNext();
}
}
this.append(event);
}
public void append(LoggingEvent event) {
// Reminder: the nesting of calls is:
//
// doAppend()
// - check threshold
// - filter
// - append();
// - checkEntryConditions();
// - subAppend();
if(!checkEntryConditions()) {
return;
}
subAppend(event);
}
protected void subAppend(LoggingEvent event) {
// 2. 日志信息格式化：对日志进行格式化处理。
this.qw.write(this.layout.format(event));
if (layout.ignoresThrowable()) {
String[] s = event.getThrowableStrRep();
if (s != null) {
int len = s.length;
for (int i = 0; i < len; i++) {
this.qw.write(s[i]);
this.qw.write(Layout.LINE_SEP);
}
}
}
if (shouldFlush(event)) {
// 3. 将日志信息输出至目的地（文件、数据库或网格）
this.qw.flush();
}
}
- 若想了解Log4j的日志异步输出实现，请参见Log4j性能优化。
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