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当使用log4j时，如果我们调用
logger.info("123");
得到的结果只有一行，即使参数中是个Exception对象，也只是输出对象本身的信息依然只有一行。
想要像e.printStackTrace()一样输出堆栈信息，可以用logger的以下方法：
logger.error("123",e);
这种一个字符串一个Exception的方法。
参数中只有字符串参数是不行的。
参数中只有Exception参数是不行的。
示例：
public static void main(String[] args){ methodA(); } public static void
methodA(){ methodB(); } public static void methodB(){ RuntimeException e = new
RuntimeException(); logger.error("123",e); }
输出结果：
15:00:44.171 [main] ERROR Test - 123
java.lang.RuntimeException: null
at Test.methodB(Test.java:469)
at Test.methodA(Test.java:465)
at Test.main(Test.java:461)
除了上面的方法之外，还有个笨办法，就是从Exception对象中直接获得堆栈信息并输出，比如这样：
public static void main(String[] args) { methodA(); } public static void
methodA() { methodB(); } public static void methodB() { methodC(); } public
static void methodC() { Exception e = new Exception(); StackTraceElement[] arr
= e.getStackTrace(); for (int i = 0; i < arr.length; i++) {
System.out.println(" " + arr[i].toString()); } System.out.println("输出完成"); }
通过e.
getStackTrace()方法，得到的是一个StackTraceElement数组，StackTraceElement对象调用toString()方法时，得到的就是一条堆栈信息。
以上代码输出的结果是：
java.lang.Exception
Test.methodC(Test.java:16)
Test.methodB(Test.java:12)
Test.methodA(Test.java:8)
Test.main(Test.java:3)
输出完成
