Log4j – 在运行时重新加载日志记录级别

原文： https://howtodoinjava.com/log4j/how-to-reload-log4j-levels-on-runtime/

过多的日志记录是导致应用性能下降的常见原因。 它是最佳实践之一，可确保 Java EE 应用实现中的正确日志记录。 但是，请注意在生产环境中启用的日志记录级别。 过多的日志记录将触发服务器上的高 IO，并增加 CPU 使用率。 对于使用较旧硬件的较旧环境或处理大量并发卷的环境而言，这尤其可能成为问题。

A balanced approach is to implement a "reloadable logging level" facility to turn extra logging ON / OFF 
when required in your day to day production support.

让我们看看如何使用 Java 7 提供的WatchService完成此操作。

步骤 1）实现一个WatchService，它将监听给定 log4j 文件中的更改

下面的给定代码初始化了一个线程，该线程连续监视给定 log4j 文件（StandardWatchEventKinds.ENTRY_MODIFY）中的修改。 一旦观察到文件更改，就会调用configurationChanged()方法，并使用 DOMConfigurator.configure()方法再次重新加载 log4j 配置。

Log4jChangeWatcherService.java

package com.howtodoinjava.demo;
import java.io.IOException;
import java.nio.file.FileSystems;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardWatchEventKinds;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;
import org.apache.log4j.xml.DOMConfigurator;
public class Log4jChangeWatcherService implements Runnable
{
    private String configFileName = null;
    private String fullFilePath = null;
    public Log4jChangeWatcherService(final String filePath) {
        this.fullFilePath = filePath;
    }
    //This method will be called each time the log4j configuration is changed
    public void configurationChanged(final String file)
    {
        System.out.println("Log4j configuration file changed. Reloading logging levels !!");
        DOMConfigurator.configure(file);
    }
    public void run() {
        try {
            register(this.fullFilePath);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    private void register(final String file) throws IOException {
        final int lastIndex = file.lastIndexOf("/");
        String dirPath = file.substring(0, lastIndex + 1);
        String fileName = file.substring(lastIndex + 1, file.length());
        this.configFileName = fileName;
        configurationChanged(file);
        startWatcher(dirPath, fileName);
    }
    private void startWatcher(String dirPath, String file) throws IOException {
        final WatchService watchService = FileSystems.getDefault().newWatchService();
        //Define the file and type of events which the watch service should handle
        Path path = Paths.get(dirPath);
        path.register(watchService, StandardWatchEventKinds.ENTRY_MODIFY);
        Runtime.getRuntime().addShutdownHook(new Thread() {
            public void run() {
                try {
                    watchService.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        });
        WatchKey key = null;
        while (true) {
            try {
                key = watchService.take();
                for (WatchEvent< ?> event : key.pollEvents()) {
                    if (event.context().toString().equals(configFileName)) {
                        //From here the configuration change callback is triggered
                        configurationChanged(dirPath + file);
                    }
                }
                boolean reset = key.reset();
                if (!reset) {
                    System.out.println("Could not reset the watch key.");
                    break;
                }
            } catch (Exception e) {
                System.out.println("InterruptedException: " + e.getMessage());
            }
        }
    }
}

2）添加Log4jConfigurator，这是您的应用与 log4j 协作的接口

此类是一个工具类，它将 log4j 初始化代码和重载策略与应用代码分开。

Log4jConfigurator.java

package com.howtodoinjava.demo;
public class Log4jConfigurator 
{
    //This ensures singleton instance of configurator
    private final static Log4jConfigurator INSTANCE = new Log4jConfigurator();
    public static Log4jConfigurator getInstance()
    {
        return INSTANCE;
    }
    //This method will start the watcher service of log4j.xml file and also configure the loggers
    public void initilize(final String file) {
        try 
        {
            //Create the watch service thread and start it.
            //I will suggest to use some logic here which will check if this thread is still alive;
            //If thread is killed then restart the thread
            Log4jChangeWatcherService listner = new Log4jChangeWatcherService(file);
            //Start the thread
            new Thread(listner).start();
        } 
        catch (Exception e) 
        {
            e.printStackTrace();
        }
    }
}

3）测试 log4j 重载事件

为了测试代码，我记录了两个两个语句：一个调试级别和一个信息级别。 两个语句在循环 2 秒后都被记录。 我将在一些日志语句之后更改日志记录级别，并且应该重新加载 log4j。

log4j-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">
  <appender name="console" class="org.apache.log4j.ConsoleAppender"> 
    <param name="Target" value="System.out"/> 
    <layout class="org.apache.log4j.PatternLayout"> 
      <param name="ConversionPattern" value="[%t] %-5p %c %x - %m%n"/> 
    </layout> 
  </appender> 
  <root> 
    <priority value ="info" />  
    <appender-ref ref="console" /> 
  </root>
</log4j:configuration>

Log4jConfigReloadExample.java

package com.howtodoinjava.demo;
import org.apache.log4j.Logger;
public class Log4jConfigReloadExample 
{
    private static final String LOG_FILE_PATH = "C:/Lokesh/Setup/workspace/Log4jReloadExample/log4j-config.xml";
    public static void main(String[] args) throws InterruptedException 
    {
        //Configure logger service
        Log4jConfigurator.getInstance().initilize(LOG_FILE_PATH);
        //Get logger instance
        Logger LOGGER = Logger.getLogger(Log4jConfigReloadExample.class);
        //Print the log messages and wait for log4j changes
        while(true)
        {
            //Debug level log message
            LOGGER.debug("A debug message !!");
            //Info level log message
            LOGGER.info("A info message !!");
            //Wait between log messages
            Thread.sleep(2000);
        }
    }
}
Output:
[main] INFO  com.howtodoinjava.demo.Log4jConfigReloadExample  - A info message !!
[main] INFO  com.howtodoinjava.demo.Log4jConfigReloadExample  - A info message !!
[main] INFO  com.howtodoinjava.demo.Log4jConfigReloadExample  - A info message !!
[main] INFO  com.howtodoinjava.demo.Log4jConfigReloadExample  - A info message !!
[main] INFO  com.howtodoinjava.demo.Log4jConfigReloadExample  - A info message !!
Log4j configuration file changed. Reloading logging levels !!
[main] DEBUG com.howtodoinjava.demo.Log4jConfigReloadExample  - A debug message !!
[main] INFO  com.howtodoinjava.demo.Log4jConfigReloadExample  - A info message !!
[main] DEBUG com.howtodoinjava.demo.Log4jConfigReloadExample  - A debug message !!
[main] INFO  com.howtodoinjava.demo.Log4jConfigReloadExample  - A info message !!

祝您学习愉快！