使用Logback在每个日志文件的开头添加一个表达式

Question

I have a project which has multiple modules. and every module has its own logback.xml. Their appender is rolling files. How can I add a specific expression at the start of every output log file?

Answer 1

You can add a header to every file created by a (Rolling)FileAppender .

The following configuration will achieve this:

<appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>target/test.log</file>
    <encoder class="ch.qos.logback.core.encoder.LayoutWrappingEncoder">
        <layout class="ch.qos.logback.classic.PatternLayout">
            <fileHeader>Your File Header ...</fileHeader>
            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
        </layout>
    </encoder>
</appender>

With the above configuration the following text will appear before the first line written to the given file by any instance of the Logback subsystem:

Your File Header ...

Note: if your application is restarted (or Logback is re initialised within your application) then the PatternLayout will write that header again so you could end up with a log file having the following content:

Your File Header ...
11:11:59.589 [main] INFO  o.g.sandbox.logback.LogbackTest - hello!
Your File Header ...
11:12:12.352 [main] INFO  o.g.sandbox.logback.LogbackTest - hello!

To avoid this you could write your own Layout or Appender. Here's a simple example extending FileAppender :

public class FileAppenderWithHeader extends FileAppender {

    private String header;

    public String getHeader() {
        return header;
    }

    public void setHeader(String header) {
        this.header = header;
    }

    @Override
    public void openFile(String fileName) throws IOException {
        super.openFile(fileName);
        File activeFile = new File(getFile());
        if (activeFile.exists() && activeFile.isFile() && activeFile.length() == 0) {
            write((header + "\n").getBytes());
        }
    }

    private void write(byte[] byteArray) throws IOException {
        if (byteArray == null || byteArray.length == 0)
            return;

        lock.lock();
        try {
            super.getOutputStream().write(byteArray);
            if (super.isImmediateFlush()) {
                super.getOutputStream().flush();
            }
        } finally {
            lock.unlock();
        }
    }
}

You would configure this like so:

<appender name="FILE" class="org.glytching.sandbox.logging.FileAppenderWithHeader">
    <file>target/test.log</file>
    <header>Your File Header ...</header>
    <encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
        <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
        </pattern>
    </encoder>
</appender>

And every file created by this appender would start with this line (regardless of how many times the appender is started/stopped):

Your File Header ...