slf4j如何打印java异常堆栈信息throwable对象

If e is an Exception, and you would like to log an exception at the ERROR level, you must add an accompanying message. For example,

logger.error("some accompanying message", e);

You might legitimately argue that not all exceptions have a meaningful message to accompany them. Moreover, a good exception should already contain a self explanatory description. The accompanying message may therefore be considered
redundant.

While these are valid arguments, there are three opposing arguments also worth considering. First, on many, albeit not all occasions, the accompanying message can convey useful information nicely complementing the description contained
in the exception. Frequently, at the point where the exception is logged, the developer has access to more contextual information than at the point where the exception is thrown. Second, it is not difficult to imagine more or less generic messages, e.g. "Exception
caught", "Exception follows", that can be used as the first argument for error(String msg, Throwable t) invocations. Third, most log output formats display the message on a line, followed by the exception
on a separate line. Thus, the message line would look inconsistent without a message.

In short, if the user were allowed to log an exception without an accompanying message, it would be the job of the logging system to invent a message. This is actually what the throwing(String
sourceClass, String sourceMethod, Throwable thrown)method in java.util.logging package does. (It decides on its own that accompanying message is the string "THROW".)

It may initially appear strange to require an accompanying message to log an exception. Nevertheless, this is common practice in all log4j derived systems such as java.util.logging, logkit, etc. and of course log4j itself.
It seems that the current consensus considers requiring an accompanying message as a good a thing (TM).

String s = "Hello world";
try {
  Integer i = Integer.valueOf(s);
} catch (NumberFormatException e) {
  logger.error("Failed to format {}", s, e);
}

will print the NumberFormatException with its stack trace as expected. The java compiler will invoke the error
method taking a String and two Object arguments. SLF4J, in accordance with the programmer‘s most probable intention, will interpret NumberFormatException instance as a throwable instead of an unused Object parameter.
In SLF4J versions prior to 1.6.0, the NumberFormatException instance was simply ignored.

If the exception is not the last argument, it will be treated as a plain object and its stack trace will NOT be printed. However, such situations should not occur in practice.