在 spring 中将 bean 注入手动实例化的 class

Question

I've got some domain classes that I'm using. There is a requirement for logging in them, but it doesn't look like spring will allow you to inject a bean into a manually instantiated object. I don't want to have to pass the logger as a parameter to constructor because this logger is a bean that I'm autowiring into most of my classes. It looks a little bit like this:

public class MainClass {
    private Logger logger;
    public MainClass(Logger logger) {
        this.logger = logger;
    }
    public void doThing() {
        //..do stuff
        DomainClass domainObject = new DomainClass(//params);
    }
}

public class DomainClass {
    //bunch of fields
    
    //field I want to autowire
    private Logger logger;
}

@Component
public class Logger {
    //stuff
}

Is there any way to achieve this without passing the logger as a parameter?

Answer 1

There is a lot of great Java logging libraries that do not require you to drag the logger around. For example, the logging in SLF4J looks like this:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class MainClass {
    private static final Logger logger = LoggerFactory.getLogger(MainClass.class);

    public MainClass() { }
    
    public void doThing(String userInput) {
        ...
        logger.info("Info message");
        logger.debug("Debug message");
        logger.error("Error message with an argument: {}", userInput);
        ...
    }
}

As you see, SLF4J does not require you to register the logger in the IoC container and instead relies on the static factory method LoggerFactory.getLogger .

I just can't emphasize how nice and configurable Java logging libraries are compared to any hand-written logger. I recommend you to use an existing and well established logging library, for example the combination SLF4J + Logback that Spring Boot uses by default.

If you can't use a third-party logging library for your use case, consider managing your MainClass and other classes like this in the IoC container together with the logger. This way you'll have the full power of Spring auto-wiring at your fingertips. Spring can't do anything about objects you create yourself. Spring operates only on the objects in its IoC container.