slf4j日誌框架繫結機制
摘要：
一、環境搭建
我們以log4j為例，探尋slf4j與log4j的繫結過程。
1.Java類
public class Slf4jBind {
public static void main(String[] args) {
Logger LOGGER = L...
一、環境搭建
我們以log4j為例，探尋slf4j與log4j的繫結過程。
1.Java類
public class Slf4jBind {
public static void main(String[] args) {
Logger LOGGER = LoggerFactory.getLogger(Slf4jBind.class);
LOGGER.info("slf4j hello world");
}
}
2.log4j.properties檔案
來自 https://docs.oracle.com/cd/E29578_01/webhelp/cas_webcrawler/src/cwcg_config_log4j_file.html
log4j.rootLogger=ERROR,stdout
log4j.logger.com.endeca=INFO
# Logger for crawl metrics
log4j.logger.com.endeca.itl.web.metrics=INFO
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%p\t%d{ISO8601}\t%r\t%c\t[%t]\t%m%n
3.Maven的pom.xml中新增如下依賴
<dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-log4j12</artifactId> <version>1.6.4</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-api</artifactId> <version>1.6.4</version> </dependency> <dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.16</version> </dependency>
其中，slf4j-api定義介面的包。
slf4j-log4j12為slf4j與logj之間的橋接包。
log4j為log4j的具體實現包。
4.單步跟蹤
在main函式處打斷點，debug模式下單步跟蹤即可
二、slf4j繫結log4j流程
main函式中呼叫LoggerFactory.getLogger時，將依次執行下面的函式。
slf4j對LoggerFactroy的說明如下：
LoggerFactory是為各個日誌API生成Logger的幫助類。
如log4j，logback，jdk 1.4 logging，也支援NOPLogger，SimpleLogger。
LoggerFactory內部封裝了ILoggerFactory，具體的ILoggerFactory與LoggerFactory在編譯期(complile time)繫結。
public static Logger getLogger(Class clazz) {
return getLogger(clazz.getName());
}
public static Logger getLogger(String name) {
ILoggerFactory iLoggerFactory = getILoggerFactory();
return iLoggerFactory.getLogger(name);
}
/
* ILoggerFactory instance is bound with this class at compile time.
*
* @return the ILoggerFactory instance in use
*/
public static ILoggerFactory getILoggerFactory() {
if (INITIALIZATION_STATE == UNINITIALIZED) {
INITIALIZATION_STATE = ONGOING_INITILIZATION;
performInitialization();
}
switch (INITIALIZATION_STATE) {
case SUCCESSFUL_INITILIZATION:
return StaticLoggerBinder.getSingleton().getLoggerFactory();
case NOP_FALLBACK_INITILIZATION:
return NOP_FALLBACK_FACTORY;
case FAILED_INITILIZATION:
throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
case ONGOING_INITILIZATION:
// support re-entrant behavior.
// See also http://bugzilla.slf4j.org/show_bug.cgi?id=106
return TEMP_FACTORY;
}
throw new IllegalStateException("Unreachable code");
}
private final static void performInitialization() {
//對具體的日誌框架實現是否僅有一個進行check
singleImplementationSanityCheck();
//與具體的日誌框架繫結
bind();
if (INITIALIZATION_STATE == SUCCESSFUL_INITILIZATION) {
//版本依賴相容性check
versionSanityCheck();
}
}
1.對具體的日誌框架實現是否僅有一個進行check。
1).判斷classpath下是否有多個org/slf4j/impl/StaticLoggerBinder.class
2).如果有多個，將提示有多個SLF4J的繫結包。
private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";
private static void singleImplementationSanityCheck() {
try {
ClassLoader loggerFactoryClassLoader = LoggerFactory.class
.getClassLoader();
Enumeration paths;
if (loggerFactoryClassLoader == null) {
paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
} else {
paths = loggerFactoryClassLoader
.getResources(STATIC_LOGGER_BINDER_PATH);
}
// use Set instead of list in order to deal withbug #138
// LinkedHashSet appropriate here because it preserves insertion order during iteration
Set implementationSet = new LinkedHashSet();
while (paths.hasMoreElements()) {
URL path = (URL) paths.nextElement();
implementationSet.add(path);
}
if (implementationSet.size() > 1) {
Util.report("Class path contains multiple SLF4J bindings.");
Iterator iterator = implementationSet.iterator();
while(iterator.hasNext()) {
URL path = (URL) iterator.next();
Util.report("Found binding in [" + path + "]");
}
Util.report("See " + MULTIPLE_BINDINGS_URL + " for an explanation.");
}
} catch (IOException ioe) {
Util.report("Error getting resources from path", ioe);
}
}
2.繫結具體的日誌框架
根據找到的org/slf4j/impl/StaticLoggerBinder.class，執行StaticLoggerBinder.getSingleton()進行繫結。
private final static void bind() {
try {
// the next line does the binding
StaticLoggerBinder.getSingleton();//繫結
INITIALIZATION_STATE = SUCCESSFUL_INITILIZATION;
emitSubstituteLoggerWarning();
} catch (NoClassDefFoundError ncde) {
String msg = ncde.getMessage();
if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
INITIALIZATION_STATE = NOP_FALLBACK_INITILIZATION;
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_INITILIZATION;
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. or 2.0.x");
}
throw nsme;
} catch (Exception e) {
failedBinding(e);
throw new IllegalStateException("Unexpected initialization failure", e);
}
}
因為具體的日誌框架為log4j，StaticLoggerBinder返回的為Log4j的LoggerFactory。
private final ILoggerFactory loggerFactory;
private StaticLoggerBinder() {
loggerFactory = new Log4jLoggerFactory();
try {
Level level = Level.TRACE;
} catch (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");
}
}
經過以上步驟，slf4j將於具體的日誌框架log4j繫結。
如下所示，ILoggerFactory實際返回型別為Log4jLoggerFactory。
並呼叫Log4jLoggerFactory中的getLogger方法返回Log4jLogger用於log寫入。
