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  2. How do I redirect log4j output to my HttpServletResponse output stream?

How do I redirect log4j output to my HttpServletResponse output stream?

由 不打扰是莪最后的温柔 提交于 2019-12-04 06:22:39

This was an interesting problem. The key thing is to write your own appender. I looked up the in built org.apache.log4j.ConsoleAppender code for inspiration. I have tested this in my tomcat and verified that it works. I used log4j-1.2.17 (hopefully shouldn't matter)

1) First implement your own appender. This appender will write all log events to current thread's outputstream

package com.tstwbprj.log;

import org.apache.log4j.Layout;
import org.apache.log4j.WriterAppender;

import java.io.IOException;
import java.io.OutputStream;

public class HttpLogAppender extends WriterAppender {

    static ThreadLocal<OutputStream> streamPerHttpThread = new ThreadLocal<OutputStream>();

    public HttpLogAppender() {

    }

    public HttpLogAppender(Layout layout) {
        setLayout(layout);       //super-class method
        activateOptions();
    }

    public void setCurrentHttpStream(OutputStream stream) {
        streamPerHttpThread.set(stream);
    }


    public void activateOptions() {
        setWriter(createWriter(new CurrentHttpThreadOutStream()));
    }


    /**
     * An implementation of OutputStream that redirects to the
     * current http threads servlet output stream
     */
    private static class CurrentHttpThreadOutStream extends OutputStream {
        public CurrentHttpThreadOutStream() {
        }

        public void close() {
        }

        public void flush() throws IOException {
            OutputStream stream = streamPerHttpThread.get();
            if (stream != null) {
                stream.flush();
            }
        }

        public void write(final byte[] b) throws IOException {
            OutputStream stream = streamPerHttpThread.get();
            if (stream != null) {
                stream.write(b);
            }
        }

        public void write(final byte[] b, final int off, final int len)
                throws IOException {
            OutputStream stream = streamPerHttpThread.get();
            if (stream != null) {
                stream.write(b, off, len);
            }
        }

        public void write(final int b) throws IOException {
            OutputStream stream = streamPerHttpThread.get();
            if (stream != null) {
                stream.write(b);
            }
        }
    }
}

2) Add this appender in your log4j configuration file just like the other settings

log4j.rootLogger=DEBUG, CA, FA , HA
..
log4j.appender.HA=com.tstwbprj.log.HttpLogAppender log4j.appender.HA.layout=org.apache.log4j.PatternLayout log4j.appender.HA.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

3) Add a small piece of code in your servlet so that this appender works correctly . Here's my servlet.

import org.apache.log4j.Category;
import org.apache.log4j.Logger;
import javax.servlet.ServletOutputStream;
import java.io.IOException;

public class LogServlet extends javax.servlet.http.HttpServlet {

    private static final Logger LOG = Logger.getLogger(LogServlet.class);

    protected void doPost(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, IOException {

    }

    protected void doGet(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, IOException {
        ServletOutputStream outstream = response.getOutputStream();
        configureLogForCurrentRequest(outstream);

        LOG.info("Got request");//this is now send to the servlet output stream !!
        LOG.info("Hello!!");
        LOG.info("Done!!");
    }

    private void configureLogForCurrentRequest(ServletOutputStream outstream) {

        HttpLogAppender appender = (HttpLogAppender) LOG.getAppender("HA");
        while (appender == null) {
            Category parent = LOG.getParent();
            if (parent == null) {
                break; //This ideally shouldn't happen. Navigated all the way to root logger and still did not find appender !!..something wrong with log4j configuration setup
            }
            appender = (HttpLogAppender) parent.getAppender("HA");

        }
        appender.setCurrentHttpStream(outstream);
    }
}

Caution : This is not thoroughly tested especially with multiple servlet requests etc. Also not sure why you want to do this. Its not typical to pipe log messages to browser. Proceed with caution..:)-

Try with something like this:

Logger logger = Logger.getRootLogger();
String name = "myAppender";

Appender servletAppender = logger.getAppender(appenderName);
OutputStream out = response.getOutputStream();

if (servletAppender == null) {
    servletAppender = new WriterAppender(new PatternLayout("%d{ISO8601} %p - %m%n"), out);
    servletAppender.setName(appenderName);
    appender.setThreshold(org.apache.log4j.Level.DEBUG);
    logger.addAppender(servletAppender);
}

try {
    // Your work
    worker.work();
} finally {
    logger.removeAppender(appenderName);
    out.flush();
}

I suggest to take alternative approach and fetch log file contents to separate browser tab.

This would not require main code modification and would not destroy original page's formatting.

Some web-based log file viewers links:

Peter Mularien

Not a precise answer as such, but a better way that I have seen this handled is to write your own Appender that will collect logs in a ThreadLocal. At the time your servlet request completes, you can drain the contents of the ThreadLocal and output to the response stream however you wish.

This satisfies the (unstated) requirement of thread safety, and can fairly cleanly isolate the log4j (or other logging framework) implementation code (which should be small, using this technique) from the manipulation of the ThreadLocal, which could in theory be reused in other areas of your code.

This type of technique is used by many server-side scripting languages such as ColdFusion and others.

I won't go into the potential bugs you could cause with inappropriate use of ThreadLocal in an app server, there are techniques to manage this, along with relevant answers on SO and other sites.

Hope this answer might redirect your thinking in a slightly different direction!

标签
Spring
servlets
log4j
httpresponse
outputstream
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