logback自定义MessageConverter

import ch.qos.logback.classic.pattern.MessageConverter;
import ch.qos.logback.classic.spi.ILoggingEvent;
import com.alibaba.fastjson.JSON;
import com.didi.utils.log.LogUtils;
import org.apache.commons.lang3.StringUtils;
import org.slf4j.helpers.MessageFormatter;

import java.util.stream.Stream;

public class ArgumentJsonFormatLayout extends MessageConverter {
    

    @Override
    public String convert(ILoggingEvent event) {
    
        try {
    
            String cs = event.getMDCPropertyMap().get(LogUtils.CSPANID);
            if (StringUtils.isBlank(cs)) {
    
                return "";
            } else {
    
                return "cspanid=" + cs;
            }

        } catch (Exception e) {
    
            return event.getMessage();
        }
    }
}

注意conversionRule内的conversionWord，这是我们可以提取Pattern内咱们自己设定的转义符号，converterClass则填自定义的全限定类名。

<?xml version="1.0" encoding="UTF-8"?>
<configuration debug="true">
    　
    <conversionRule conversionWord="cspanid" converterClass="com.xxx.log.ArgumentJsonFormatLayout"/>

    <property name="logPattern"
              value="%cspanid[%level][%d{YYYY-MM-dd'T'HH:mm:ss.SSS+0800}][%file:%line]%X{DL_TAG:-_com_request_in}||traceid=%X{TRACE_ID:-init}||spanid=%X{SPAN_ID:-init}||uri=%X{CUR_URL:-init}||cur_service=%X{CUR_SERVICE:-init}||msg=%m%n"/>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">　　　　
        <encoder>　　　　　　
            <Pattern>${logPattern}</Pattern>
            <charset>UTF-8</charset>
        </encoder>
    </appender>
    　　
    <root level="info">　　　　
        <appender-ref ref="STDOUT"/>
    </root>
</configuration>

其实现原理在于：

从图中可以看出，整个过程中起关键作用的几个类为：AppenderAttatchableImpl，OutputStreamAppender、Encoder、Layout、OutputStream。这几类和配置文件中相应元素一一对应。logback通过类的继承和组合层层封装方法，最后通过OutputStream写入到控制台(ConsoleAppender)或者是文件(FileAppender)中。

Encode接口

  public void doEncode(E event) throws IOException {
    
    String txt = layout.doLayout(event);
    outputStream.write(convertToBytes(txt));
    outputStream.flush();
  }

public String doLayout(ILoggingEvent event) {
    
    if (!isStarted()) {
    
      return CoreConstants.EMPTY_STRING;
    }
    return writeLoopOnConverters(event);
  }

protected String writeLoopOnConverters(E event) {
    
    StringBuilder buf = new StringBuilder(128);
    Converter<E> c = head;
    while (c != null) {
    
      c.write(buf, event);
      c = c.getNext();
    }
    return buf.toString();
  }

public void start() {
    
    if(pattern == null || pattern.length() == 0) {
    
      addError("Empty or null pattern.");
      return;
    }
    try {
     
      Parser<E> p = new Parser<E>(pattern);
      if (getContext() != null) {
    
        p.setContext(getContext());
      }
      Node t = p.parse();
      this.head = p.compile(t, getEffectiveConverterMap());
      if (postCompileProcessor != null) {
    
        postCompileProcessor.process(head);
      }
      setContextForConverters(head);
      ConverterUtil.startConverters(this.head);
      super.start();
    } catch (ScanException sce) {
    
      StatusManager sm = getContext().getStatusManager();
      sm.add(new ErrorStatus("Failed to parse pattern \"" + getPattern()
          + "\".", this, sce));
    }
  }

  static {
    

    defaultConverterMap.put("d", DateConverter.class.getName());
    defaultConverterMap.put("date", DateConverter.class.getName());

    defaultConverterMap.put("r", RelativeTimeConverter.class.getName());
    defaultConverterMap.put("relative", RelativeTimeConverter.class.getName());

    defaultConverterMap.put("level", LevelConverter.class.getName());
    defaultConverterMap.put("le", LevelConverter.class.getName());
    defaultConverterMap.put("p", LevelConverter.class.getName());

    defaultConverterMap.put("t", ThreadConverter.class.getName());
    defaultConverterMap.put("thread", ThreadConverter.class.getName());

    defaultConverterMap.put("lo", LoggerConverter.class.getName());
    defaultConverterMap.put("logger", LoggerConverter.class.getName());
    defaultConverterMap.put("c", LoggerConverter.class.getName());

    defaultConverterMap.put("m", MessageConverter.class.getName());
    defaultConverterMap.put("msg", MessageConverter.class.getName());
    defaultConverterMap.put("message", MessageConverter.class.getName());

    defaultConverterMap.put("C", ClassOfCallerConverter.class.getName());
    defaultConverterMap.put("class", ClassOfCallerConverter.class.getName());

    defaultConverterMap.put("M", MethodOfCallerConverter.class.getName());
    defaultConverterMap.put("method", MethodOfCallerConverter.class.getName());

    defaultConverterMap.put("L", LineOfCallerConverter.class.getName());
    defaultConverterMap.put("line", LineOfCallerConverter.class.getName());

    defaultConverterMap.put("F", FileOfCallerConverter.class.getName());
    defaultConverterMap.put("file", FileOfCallerConverter.class.getName());

    defaultConverterMap.put("X", MDCConverter.class.getName());
    defaultConverterMap.put("mdc", MDCConverter.class.getName());

    defaultConverterMap.put("ex", ThrowableProxyConverter.class.getName());
    defaultConverterMap.put("exception", ThrowableProxyConverter.class
        .getName());
    defaultConverterMap.put("throwable", ThrowableProxyConverter.class
        .getName());

    defaultConverterMap.put("xEx", ExtendedThrowableProxyConverter.class.getName());
    defaultConverterMap.put("xException", ExtendedThrowableProxyConverter.class
        .getName());
    defaultConverterMap.put("xThrowable", ExtendedThrowableProxyConverter.class
        .getName());

    defaultConverterMap.put("nopex", NopThrowableInformationConverter.class
        .getName());
    defaultConverterMap.put("nopexception",
        NopThrowableInformationConverter.class.getName());

    defaultConverterMap.put("cn", ContextNameAction.class.getName());
    defaultConverterMap.put("contextName", ContextNameConverter.class.getName());
    
    defaultConverterMap.put("caller", CallerDataConverter.class.getName());

    defaultConverterMap.put("marker", MarkerConverter.class.getName());

    defaultConverterMap.put("property", PropertyConverter.class.getName());

    
    defaultConverterMap.put("n", LineSeparatorConverter.class.getName());
  }

输出日志的最终任务会落到Enconder#doEncode身上，第一段代码位于LayoutWrappingEncoder.java中，doEncode会最终调用Layout.doLayout得到格式化的文本，然后使用outputStream输出。第二段代码位于PatternLayout.java中，doLayout会调用writeLoopOnConverters，而writeLoopOnConverters会将日志文件通过Converter链进行格式化。Converter链是怎么产生的呢？请看第三段代码，看来是通过配置的pattern得到的，大体上是通过Parser去解析我们配置的pattern("%d %-4relative [%thread] %-5level %logger{35} - %msg%n")，parser通过识别"%"来判决是关键词还是普通文本，例如[]就是普通文件，relative/msg/n等都是关键字。Node t = p.parse();将pattern转化成Node链，Node有多种例如KeywordNode。通过p.compile(t, getEffectiveConverterMap())得到Convert链，主要是将Node链转化成Convert链，PatternLayout中列出了所有的Convert与pattern中名字的对应关系(最后一段代码)，例如d代表DateConverter，level 代码LevelConverter等。

其实我们的日志信息在经过Converter链格式化之前还经过了一次格式化。我们通常会使用logger.info(“somemsg:{}”,{})，使用"{}"作为占位符，logback会将其替换成后面出现的参数，这个过程是怎么发生的呢？

  public LoggingEvent(String fqcn, Logger logger, Level level, String message,
      Throwable throwable, Object[] argArray) {
    
    this.fqnOfLoggerClass = fqcn;
    this.loggerName = logger.getName();
    this.loggerContext = logger.getLoggerContext();
    this.loggerContextVO = loggerContext.getLoggerContextRemoteView();
    this.level = level;

    this.message = message;

    FormattingTuple ft = MessageFormatter.arrayFormat(message, argArray);
    formattedMessage = ft.getMessage();

    if (throwable == null) {
    
      argumentArray = ft.getArgArray();
      throwable = ft.getThrowable();
    } else {
    
      this.argumentArray = argArray;
    }

    if (throwable != null) {
    
      this.throwableProxy = new ThrowableProxy(throwable);
      LoggerContext lc = logger.getLoggerContext();
      if (lc.isPackagingDataEnabled()) {
    
        this.throwableProxy.calculatePackagingData();
      }
    }

    timeStamp = System.currentTimeMillis();

    // ugly but under the circumstances acceptable
    LogbackMDCAdapter logbackMDCAdapter = (LogbackMDCAdapter) MDC
        .getMDCAdapter();
    mdcPropertyMap = logbackMDCAdapter.getPropertyMap();
  }

    FormattingTuple ft = MessageFormatter.arrayFormat(message, argArray);
    formattedMessage = ft.getMessage();

IloggingEvent非常重要，是log信息的携带者，其实现类为LoggingEvent，在实例化LoggingEvent时，使用MessageFormatter和FormattingTuple将{}占位符进行了替换。

原文链接：https://www.jianshu.com/p/5616844a5f68