log4j及slf4j使用

log4j是apache基金会开源的一个Java项目输出日志的解决方案，slf4j仅仅是一个为Java程序提供日志输出的统一接口，并不是一个具体的日志实现方案，就比如JDBC一样，只是一种规则而已，所以单独的slf4j是不能工作的，必须搭配其他具体的日志实现方案例如log4j或log4j2。

使用log4j步骤如下：

1. pom文件引入slf4j.

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-log4j12</artifactId>
    <version>1.7.5</version>
</dependency>

   2. 在src/main/resources目录下添加log4j.properties文件。

### 设置###
log4j.rootLogger = error,stdout,D,E

### 输出信息到控制抬 ###
log4j.appender.stdout = org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target = System.out
log4j.appender.stdout.layout = org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern = [%-5p] %d{yyyy-MM-dd HH:mm:ss,SSS} method:%l%n%m%n

### 输出DEBUG 级别以上的日志到=E://logs/error.log ###
log4j.appender.D = org.apache.log4j.DailyRollingFileAppender
log4j.appender.D.File = D://logs/log.log
log4j.appender.D.Append = true
log4j.appender.D.Threshold = DEBUG 
log4j.appender.D.layout = org.apache.log4j.PatternLayout
log4j.appender.D.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss}  [ %t:%r ] - [ %p ]  %m%n

### 输出ERROR 级别以上的日志到=E://logs/error.log ###
log4j.appender.E = org.apache.log4j.DailyRollingFileAppender
log4j.appender.E.File =D://logs/error.log 
log4j.appender.E.Append = true
log4j.appender.E.Threshold = ERROR 
log4j.appender.E.layout = org.apache.log4j.PatternLayout
log4j.appender.E.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss}  [ %t:%r ] - [ %p ]  %m%n

  3. 添加TestLogger class

package com.jzd1997;

import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class TestLogger {
    Logger logger = LoggerFactory.getLogger(TestLogger.class);
    @Test
    public void testLogger(){
        try{
            // 记录debug级别的信息
            logger.debug("This is debug message.");
            // 记录info级别的信息
            logger.info("This is info message.");
            // 记录error级别的信息
            logger.error("This is error message.");
            int divider = 10;
            int divided = 0;
            divider = divider/divided;
        }catch(Exception e){
            logger.error(e.getMessage(),e);
        }
    }
}

  4. 以前在系统中捕获到Exception后，我出于简便，直接e.printStackTrace()，但是这种情况就会出现

错误堆栈只出现在控制台，不会出现在错误log中，不方便我们控制日志级别。查询资料后发现可以通过

下面的代码来把错误输出到log文件中。

        try{
        }catch(Exception e){
            logger.error(e.getMessage(),e);
        }

5. 运行测试后会找到错误日志文件。

2019-07-03 17:08:08  [ main:0 ] - [ ERROR ]  This is error message.
2019-07-03 17:08:08  [ main:4 ] - [ ERROR ]  / by zero
java.lang.ArithmeticException: / by zero
	at com.jzd1997.TestLogger.testLogger(TestLogger.java:21)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)