Logging and Debugging

There are a few logging frameworks available for use in Java. Logging is preferable to printing to stdout or stderr with System.out.println() for a number of reasons:

  • Logged messages are labeled with a source name, making it easier to figure out where the logged messages are coming from.

  • Logged messages have a severity level which allows for simple filtering (e.g. disable all non-critical notices).

  • The available logger frameworks allow you to enable or disable messages from certain sources.

Sponge uses org.slf4j.Logger, not java.util.logging.Logger.

Getting a Logger

The Guice module used during the initialization of plugins has a plugin-scoped logger. This allows you to annotate a field, method, or constructor with @Inject to get the logger for your plugin, which is pre-configured with the correct plugin ID.

Note

See Main Plugin Class for information on configuring your plugin ID.

Example - Field

import com.google.inject.Inject;
import org.slf4j.Logger;

@Inject
private Logger logger;

Example - Method

private Logger logger;

@Inject
private void setLogger(Logger logger) {
    this.logger = logger;
}

Example - Constructor

// For the purpose of this example, "Banana" is the class name

private Logger logger;

@Inject
public Banana(Logger logger) {
    this.logger = logger;
}

It is recommended to set your logger in your main plugin class, as it is instantiated with the Guice injector when the plugin is loaded.

Creating a getter method for your logger in the same class in which it was set is also ideal, although optional. An example getter method is illustrated below.

public Logger getLogger() {
    return logger;
}

Emitting Messages

Emitting a message with your logger is very simple.

Note

The following example assumes that the getter method for your logger is named getLogger(), as shown in the previous section. This may differ for you depending on what you named your getter method.

getLogger().info("String");
getLogger().debug("String");
getLogger().warn("String");
getLogger().error("String");

The String is the message you wish to emit. For example:

getLogger().warn("This is a warning!");

Manipulating the Logging

Note

These techniques should only be used in very rare cases such as badly chosen logging defaults in libraries. Add a config option to disable those if you use them.

Some libraries use bad logging practices such as logging on too high a level. In these cases, you have three choices:

  1. Ask the author of that library to adjust their logging standards, as this fixes the problem at its source.

  2. Recommend your users to configure the logging using a log4j2.xml config file. Provide users with the recommended configuration additions.

  3. Configure the logging in your plugin yourself:

    ((org.apache.logging.log4j.core.Logger) LogManager.getLogger("FtpLoggingFilter")).setLevel(Level.WARN);

    This configures the log level of the FtpLoggingFilter logger to WARN. This will hide all messages that use a lower log level such as INFO and DEBUG.

    Warning

    This solution assumes that log4j2 is used as logging framework by the server, however that might not be the case for all/future implementations of the SpongeAPI.

If you have any questions regarding logging you can always ask us on IRC, Discord or the Forums.