Why use log4j instead of system out. println?
This is the back-end small class of the monastery. Each article is shared from
[background introduction] [knowledge analysis] [common problems] [solutions] [coding practice] [extended thinking] [more discussion] [References]
Eight aspects of in-depth analysis of back-end knowledge / skills. This article shares:
[why use log4j instead of system. Out. Println?]
Hello, I'm Jiang Xinyi, a student of Shanghai Branch of it Academy. I'm an honest, pure and kind java programmer
Today, I'd like to share with you the knowledge points in deep thinking about Java task 1 on the official website of the Academy - why use log4j instead of system out. println?
1. Background introduction
Adding log records to applications is generally based on three purposes: monitoring the changes of variables in the code and periodically recording them in files for statistical analysis by other applications; Track the running track of the code as the basis for future audit; Act as a debugger in the integrated development environment and print the debugging information of the code to the file or console.
The most common way is to embed many print statements in the code, which can be output to the console or file. A better way is to construct a log operation class to encapsulate such operations, rather than filling the main body of the code with a series of print statements.
Today, with the emphasis on reusable component development, in addition to developing a reusable log operation class from beginning to end, Apache provides us with a powerful log operation package - log4j
Log4j is an open source project of Apache. By using log4j, we can control that the destination of log information transmission is console, file, GUI component, even socket server, NT event recorder, UNIX syslog daemon, etc; We can also control the output format of each log; By defining the level of each log information, we can control the log generation process in more detail. The most interesting thing is that these can be flexibly configured through a configuration file without modifying the application code.
2. Knowledge analysis
About log4j, you need to know the following points
The configuration file of log4j has the following three aspects
1. Root directory (level and destination);
2. Destination (console, files, etc.);
3. Output style
Root directory (level and destination)
log4j. rootLogger = [ level ],appenderName,…
There are four levels officially recommended, from low to high: debug, info, warn and error
Appendername is the parameter name of the destination
Destination (console, files, etc.)
log4j. appender. appenderName = fully. qualified. name. of. appender. class
Log4j provides five output destinations
Configure the format of log information
log4j. appender. appenderName. layout = fully. qualified. name. of. layout. class
Log4j provides the following four formats
3. Frequently asked questions
1. How is log4j loaded?
2. Why don't you stop after the error
3. Why use log4j instead of system out. println
4. What is the difference between rootcategory and rootlogger
4. Solutions
How is log4j loaded?
When the application starts, it will look for log4j. In the SRC directory by default If the XML configuration file does not exist, it will continue to look for log4j Properties file, as long as one of them is found, the contents of the configuration file will be loaded
Of course, it can also be loaded manually, because automatic loading will only load log4j XML or log4j Properties. If there are configuration files with different names or not placed in the root directory, they need to be loaded manually
Why doesn't error stop
Because there are not only four levels in log4j, but seven
Why use log4j instead of system OUT. PRINTLN
Programmers often encounter the situation of debugging programs outside the Java ide environment. At this time, most people will choose to use system out. Println statement is used to debug the method of outputting the value of a variable. This leads to a very troublesome problem: once one day the programmer decides not to display these systems out. Println can only comment out these garbage statements line by line. If you need to debug the variable value one day, you can only remove these comments and restore the system out. Println statement, the debugging code is full of logic code, which is neither beautiful nor conducive to search
Log4j handles this problem well, and system Out can only be output to the console, while logging tools such as log4j can configure the output target and output level to reduce the coupling between the output form and the program
What is the difference between rootcategory and rootlogger
In fact, rootlogger is a subclass of rootcategory, which is an old way of writing
5. Coding practice
Why use log4j instead of sys out_ Tencent video
6. Expand thinking
Why use slf4j instead of log4j?
Slf4j: simple logging facade for Java. It is not a specific logging solution. It only serves a variety of logging systems. According to the official statement, slf4j is a simple facade for the logging system, allowing end users to use the desired logging system when deploying their applications.
7. References
Reference 3:
Reference 2:
Reference 3:
8. More discussion
1. Difference between log4j and log4j2
Log4j is through a The properties file is used as the main configuration file, but log4j 2 has abandoned this method and adopts xml，. JSON or JSN only needs to introduce a jar package to do log4j in this way, while log4j 2 requires two cores
For log4j to take effect, we need to be on the web XML
log4j2. Just put XML in the project resource directory. Remember that a fine node is log4j2 XML instead of log4j XML, XML name can't be less than 2
The most important difference is the difference between configuration files. You can configure them according to your situation when you use them
2. How to close log4j without comments
Via console options
Immediateflush = true: indicates that all messages will be output immediately. If it is set to false, it will not be output. The default value is true.
3. XML configuration file and properties configuration file
Properties is a simple key value correspondence.
If properties can't meet your requirements, you naturally need to use XML.
XML can store complex object structures with multiple attributes.
That's all for today's sharing. You are welcome to like, forward, leave messages and make bricks~
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Author: Jiang Xinyi
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Source: Jianshu
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