log4j打印抛出异常时堆栈内容

JAVA 中为快速定位，抛出异常的位置。往往需要在日志中描述。

log.err("异常出错点", e); 会将出错时的堆栈信息显示出来。

package com.jqyc.jase;

import static org.junit.Assert.*;

import org.apache.log4j.Logger;
import org.junit.Test;

public class TestError {

Logger log = Logger.getLogger(TestError.class);

@Test
public void test() {
String s = null;
try {
s.length();
} catch (Exception e) {
log.error( "出错日志"+s, e);
// 当然如果你懒得想提示信息的话，直接这样log.error("", e);

System.out.println(e.getMessage());
}
}

}
[com.jqyc.jase.TestError.test(TestError.java:18)] - 出错日志null
java.lang.NullPointerException
at com.jqyc.jase.TestError.test(TestError.java:16)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
printStackTrace()方法的意思是：在命令行打印异常信息在程序中出错的位置及原因。  但只会在标准输出stdout 与stderr中进行输出，无法将内容输出到日志。
需要日志中进行输出，直接打印e对象。