Integrating OutputStream or Writer with Log4J Logger

Integrating OutputStream or Writer with Log4J Logger

5 April 2024 Stephan Petzl Leave a comment Tech-Help

If you’re dealing with a third-party API that requires an OutputStream or Writer and you want to integrate it with Log4J’s logging capabilities, you’re in the right place. This guide will walk you through how to redirect OutputStream or Writer to Log4J’s internal logging system.

Creating a Custom Log4J OutputStream

One of the most straightforward methods to achieve this is by creating a custom OutputStream that logs each byte written to it using a specified Log4J logger. Below is a Java class example that implements this functionality:

package jacareto.toolkit.log4j;

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


public class LogOutputStream extends OutputStream {
    private Logger logger;
    private Level level;
    private String mem;

    public LogOutputStream(Logger logger, Level level) {
        mem = "";

    public void setLogger(Logger logger) {
        this.logger = logger;

    public Logger getLogger() {
        return logger;

    public void setLevel(Level level) {
        this.level = level;

    public Level getLevel() {
        return level;

    public void write(int b) {
        byte[] bytes = new byte[1];
        bytes[0] = (byte) (b & 0xff);
        mem += new String(bytes);

        if (mem.endsWith("\n")) {
            mem = mem.substring(0, mem.length() - 1);

    public void flush() {
        logger.log(level, mem);
        mem = "";

To use this custom OutputStream, you would create an instance of LogOutputStream, passing the desired Logger and Level to its constructor. This OutputStream can now be used as a parameter for methods expecting an OutputStream, and all bytes written to it will be logged accordingly.

Using Log4j IOStreams

Alternatively, you can utilize Log4j’s IOStreams component, which provides classes that can write to a Logger while writing to another OutputStream or Writer.

OutputStream outputStream = IoBuilder

This approach is particularly useful when you need to integrate with existing systems like Appium, as demonstrated in the example below where an Appium service is started programmatically and its logs are controlled by Log4j:

final Logger logger = LogManager.getLogger(getClass());

// ... Appium configuration setup ...

// Build the Appium service
AppiumServiceBuilder builder = new AppiumServiceBuilder();
// ... builder setup ...

// Start the server with the builder
AppiumDriverLocalService service = AppiumDriverLocalService.buildService(builder);

OutputStream outputStream = IoBuilder



By creating a custom OutputStream or utilizing the IOStreams component of Log4j, you can effectively redirect output from a third-party API to the Log4J logging system. This not only allows for more centralized and consistent logging but also gives you the flexibility to handle the logging output as needed by your application.

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