在本章中，我们将讨论在使用SLF4J时获得的各种错误消息或警告以及这些消息的原因/含义。
无法加载类“org.slf4j.impl.StaticLoggerBinder”。
这是在类路径中没有提供SLF4J绑定时引起的警告。
以下是完整的警告 -
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further
details.
要解决此问题，需要添加任一日志框架绑定。本教程上一小节对此进行了解释说明。
注 - 这种情况发生在SLF4J的版本中，介于1.6.0和1.8.0-beta2之间。
No SLF4J providers were found
在slf4j-1.8.0-beta2中，上述警告更清楚地说“未找到SLF4J提供商”。
以下是完整的警告 -
SLF4J: No SLF4J providers were found.
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#noProviders for further details.
Classpath包含针对1.8之前的slf4j-api版本的SLF4J绑定
如果使用的是SLF4J 1.8版本，并且在类路径中具有以前版本的绑定但没有1.8的绑定，则会看到如下所示的警告。
SLF4J: No SLF4J providers were found.
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#noProviders for further details.
SLF4J: Class path contains SLF4J bindings targeting slf4j-api versions prior to
1.8.
SLF4J: Ignoring binding found at
[jar:file:/C:/Users/maxsu/Desktop/Latest%20Yiibai/SLF4J%20Tutorial/
slf4j-1.7.25/slf4j-jdk14-1.7.25.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#ignoredBindings for an explanation.
NoClassDefFoundError: org/apache/commons/logging/LogFactory
如果正在使用slf4j-jcl，并且类路径中只有slf4j-jcl.jar，将得到一个例外(异常)，例如下面给出的例外(异常)。
Exception in thread "main" java.lang.NoClassDefFoundError:
org/apache/commons/logging/LogFactory
at org.slf4j.impl.JCLLoggerFactory.getLogger(JCLLoggerFactory.java:77)
at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:358)
at SLF4JExample.main(SLF4JExample.java:8)
Caused by: java.lang.ClassNotFoundException:
org.apache.commons.logging.LogFactory
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 3 more
要解决此问题，需要将commons-logging.jar添加到类路径中。
Detected both jcl-over-slf4j.jar AND bound slf4j-jcl.jar on the classpath..
绑定slf4j-jcl.jar将slf4j logger的调用重定向到JCL，jcl-over-slf4j.jar将JCL logger的调用重定向到slf4j。因此，不能在项目的类路径中同时拥有这两者。如果这样做，会得到一个例外(异常)，例如下面给出的(异常)。
SLF4J: Detected both jcl-over-slf4j.jar AND bound slf4j-jcl.jar on the class
path, preempting StackOverflowError.
SLF4J: See also http://www.slf4j.org/codes.html#jclDelegationLoop for more
details.
Exception in thread "main" java.lang.ExceptionInInitializerError
at org.slf4j.impl.StaticLoggerBinder.(StaticLoggerBinder.java:71)
at org.slf4j.impl.StaticLoggerBinder.(StaticLoggerBinder.java:42)
at org.slf4j.LoggerFactory.bind(LoggerFactory.java:150)
at org.slf4j.LoggerFactory.performInitialization(LoggerFactory.java:124)
at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:412)
at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:357)
at SLF4JExample.main(SLF4JExample.java:8)
Caused by: java.lang.IllegalStateException: Detected both jcl-over-slf4j.jar
AND bound slf4j-jcl.jar on the class path, preempting StackOverflowError. See
also http://www.slf4j.org/codes.html#jclDelegationLoop for more details.
at org.slf4j.impl.JCLLoggerFactory.(JCLLoggerFactory.java:54)
... 7 more
要解决此问题，请删除其中一个jar文件。
Detected logger name mismatch
可以通过以下方式创建Logger对象：
将要创建的记录器的名称作为参数传递给getLogger()方法。
将类作为参数传递给此方法。
如果通过将类作为参数传递来创建记录器工厂对象，并且已将系统属性slf4j.detectLoggerNameMismatch设置为true，那么作为参数传递给getLogger()方法的类的名称和使用类应该相同，否则将收到以下警告 -
“Detected logger name mismatch”
请考虑以下示例。
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class SLF4JExample {
public static void main(String[] args) {
System.setProperty("slf4j.detectLoggerNameMismatch", "true");
//Creating the Logger object
Logger logger = LoggerFactory.getLogger(Sample.class);
//Logging the information
logger.info("Hi Welcome to Yiibai.com");
}
}
在这里，将slf4j.detectLoggerNameMismatch属性设置为true。使用的类的名称是SLF4JExample，传递给getLogger()方法的类名是Sample，因为它们都不相等，将会收到以下警告。
SLF4J: Detected logger name mismatch. Given name: "Sample"; computed name:
"SLF4JExample".
SLF4J: See http://www.slf4j.org/codes.html#loggerNameMismatch for an
explanation
Dec 10, 2019 10:23:00 PM SLF4JExample main
INFO: Hi Welcome to Yiibai.com
注 - 它在slf4j 1.7.9之后发生
Classpath contains multiple SLF4J bindings.
应该在类路径中只有一个绑定。如果有多个绑定，将收到一个警告，列出绑定及其位置。
假设，如果在类路径中有绑定slf4j-jdk14.jar和slf4j-nop.jar，将收到以下警告。
SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in
[jar:file:/C:/Users/Yiibai/Desktop/Latest%20Yiibai/SLF4J%20Tutorial/
slf4j-1.7.25/slf4j-nop-1.7.25.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in
[jar:file:/C:/Users/Yiibai/Desktop/Latest%20Yiibai/SLF4J%20Tutorial/
slf4j-1.7.25/slf4j-jdk14-1.7.25.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.helpers.NOPLoggerFactory]
Detected both log4j-over-slf4j.jar AND bound slf4j-log4j12.jar on the class path
要将log4j logger调用重定向到slf4j，需要使用log4j-over-slf4j.jar绑定，如果要将slf4j调用重定向到log4j，则需要使用slf4j-log4j12.jar绑定。
因此，不能在类路径中同时拥有这两者。如果这样做，将收到以下异常。
SLF4J: Detected both log4j-over-slf4j.jar AND bound slf4j-log4j12.jar on the
class path, preempting StackOverflowError.
SLF4J: See also http://www.slf4j.org/codes.html#log4jDelegationLoop for more
details.
Exception in thread "main" java.lang.ExceptionInInitializerError
at org.slf4j.impl.StaticLoggerBinder.(StaticLoggerBinder.java:72)
at org.slf4j.impl.StaticLoggerBinder.(StaticLoggerBinder.java:45)
at org.slf4j.LoggerFactory.bind(LoggerFactory.java:150)
at org.slf4j.LoggerFactory.performInitialization(LoggerFactory.java:124)
at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:412)
at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:357)
at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:383)
at SLF4JExample.main(SLF4JExample.java:8)
Caused by: java.lang.IllegalStateException: Detected both log4j-over-slf4j.jar
AND bound slf4j-log4j12.jar on the class path, preempting StackOverflowError.
See also http://www.slf4j.org/codes.html#log4jDelegationLoop for more details.
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