Log4J - 运行时变量替换 [英] Log4J – Runtime variable substitution

问题描述

Log4J 似乎有一个恼人的限制 - 在运行时，变量替换似乎不起作用。

Log4J appears to have an annoying restriction – at runtime, variable substitution does not appear to work.

在此示例中

文件：Log4j.properties

File: Log4j.properties

file_pattern =％d {ISO8601}％-5p％m％n

file_pattern=%d{ISO8601} %-5p %m%n

log4j.rootLogger = DEBUG ，FileAppender

log4j.rootLogger=DEBUG, FileAppender

log4j.appender.FileAppender = org.apache.log4j.FileAppender
log4j.appender.FileAppender.layout = org.apache.log4j.PatternLayout
log4j.appender.FileAppender.layout.ConversionPattern = $ {file_pattern}
log4j.appender.FileAppender.File = log4jtest1.log

log4j.appender.FileAppender=org.apache.log4j.FileAppender log4j.appender.FileAppender.layout=org.apache.log4j.PatternLayout log4j.appender.FileAppender.layout.ConversionPattern=${file_pattern} log4j.appender.FileAppender.File=log4jtest1.log

log4j.appender .FileAppender.Threshold = ERROR

log4j.appender.FileAppender.Threshold=ERROR

log4j.properties文件中配置的FileAppender会产生正确的输出：

The FileAppender configured in the log4j.properties file produces the correct output:

文件：log4jtest1.log

File: log4jtest1.log

错误示例错误消息
FATAL Sam致命消息

ERROR Sample error message FATAL Sample fatal message

如果我尝试在运行时创建FileAppender

If I attempt to create a FileAppender at runtime

import org.apache.log4j.FileAppender;
import org.apache.log4j.Level;
import org.apache.log4j.Logger;
import org.apache.log4j.PatternLayout;

public class Main {
    static final Logger logger = Logger.getLogger(Main.class);

    public static void main(String[] args) throws Exception {

        FileAppender appender = new FileAppender();
        appender.setFile("log4test2.log");

        PatternLayout pl = new PatternLayout("${file_pattern}");

        appender.setLayout(pl);
        appender.setName("log4jtest2");
        appender.setThreshold(Level.ERROR);
        appender.activateOptions(); 
        logger.addAppender(appender);

        logger.trace("Sample trace message");
        logger.debug("Sample debug message");
        logger.info("Sample info message");
        logger.warn("Sample warn message");
        logger.error("Sample error message");
        logger.fatal("Sample fatal message");
    }
}

Te输出

文件：log4jtest2.log

File: log4jtest2.log

$ {file_pattern} $ {file_pattern}

${file_pattern}${file_pattern}

任何人都可以解释问题是什么以及如何解决？

Can anyone explain what is the problem and how can it be fixed?

相关问题：申请是否可以访问ResourceBundle以读取要替换的变量？

Related question: Can an application access the ResourceBundle in order to read variables intended to be substituted?

推荐答案

变量替换是 PropertyConfigurator 不是PatternLayout。如果查看代码，则永远不要定义file_pattern应该是什么。但为什么你需要代码中的变量替换？只需这样做：

Variable substitution is a feature of PropertyConfigurator not of PatternLayout. If you look at your code, you never define what file_pattern should be. But why would you need variable substitution in code? Just do this:

 PatternLayout pl = new PatternLayout("%d{ISO8601} %-5p %m%n");

如果你想在其他地方引用该字符串，只需将其设为常量。

If you want to reference that string somewhere else, just make it a constant.

编辑：您必须阅读属性对象，但PropertyConfigurator可以使用属性对象而不是文件，因此您可以加载它，执行您需要执行的操作并将其传递给PropertiesConfigurator，因此您只有一个配置路径。

You will have to read the properties object, but PropertyConfigurator can take a properties object instead a file, so you could load that, do what you need to do and pass it on to the PropertiesConfigurator, so you only have one configuration path.

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