上篇博客介绍了logback是怎么对接slf4j的，简言之，就是通过下面这行代码

slf4j委托具体实现框架的StaticLoggerBinder来返回一个ILoggerFactory，从而对接到具体实现框架上

这篇博客就接下来介绍一下，logback的StaticLoggerBinder类是怎么创建ILoggerFactory的

在图中可以看到，在logback里，ILoggerFactory的实现类是LoggerContext

logback的StaticLoggerBinder类实现了LoggerFactoryBinder接口，这个接口有两个方法

/**
 * An internal interface which helps the static {@link org.slf4j.LoggerFactory} 
 * class bind with the appropriate {@link ILoggerFactory} instance. 
 * 
 * @author Ceki Gülcü
 */
public interface LoggerFactoryBinder {

    /**
     * Return the instance of {@link ILoggerFactory} that 
     * {@link org.slf4j.LoggerFactory} class should bind to.
     * 
     * @return the instance of {@link ILoggerFactory} that 
     * {@link org.slf4j.LoggerFactory} class should bind to.
     */
    public ILoggerFactory getLoggerFactory();

    /**
     * The String form of the {@link ILoggerFactory} object that this 
     * <code>LoggerFactoryBinder</code> instance is <em>intended</em> to return. 
     * 
     * <p>This method allows the developer to interrogate this binder's intention
     * which may be different from the {@link ILoggerFactory} instance it is able to 
     * yield in practice. The discrepancy should only occur in case of errors.
     * 
     * @return the class name of the intended {@link ILoggerFactory} instance
     */
    public String getLoggerFactoryClassStr();
}

其中比较重要的是getLoggerFactory()方法，其实自定义的StaticLoggerBinder类不实现这个接口也是可以的，只要能保证提供getLoggerFactory()方法，并返回一个ILoggerFactory就可以了

下面就来具体地看看StaticLoggerBinder类的代码：

首先，该类必须有一个getSingleton()方法，来返回该类的单例

public class StaticLoggerBinder implements LoggerFactoryBinder {
    public static String REQUESTED_API_VERSION = "1.7.16";
    static final String NULL_CS_URL = "http://logback.qos.ch/codes.html#null_CS";
    private static StaticLoggerBinder SINGLETON = new StaticLoggerBinder();
    private static Object KEY = new Object();
    private boolean initialized = false;
    private LoggerContext defaultLoggerContext = new LoggerContext();
    private final ContextSelectorStaticBinder contextSelectorBinder = ContextSelectorStaticBinder.getSingleton();

    private StaticLoggerBinder() {
        this.defaultLoggerContext.setName("default");
    }

    public static StaticLoggerBinder getSingleton() {
        return SINGLETON;
    }

以上代码用了比较简单的单例模式，提供getSingleton()方法是对接slf4j的强制要求

然后这个类用了一个static块来保证初始化

    void init() {
        try {
            try {
                //委托ContextInitializer类对defaultLoggerContext进行初始化
                (new ContextInitializer(this.defaultLoggerContext)).autoConfig();
            } catch (JoranException var2) {
                Util.report("Failed to auto configure default logger context", var2);
            }

            if (!StatusUtil.contextHasStatusListener(this.defaultLoggerContext)) {
                StatusPrinter.printInCaseOfErrorsOrWarnings(this.defaultLoggerContext);
            }
            //对ContextSelectorStaticBinder类进行初始化
            this.contextSelectorBinder.init(this.defaultLoggerContext, KEY);
            this.initialized = true;
        } catch (Exception var3) {
            Util.report("Failed to instantiate [" + LoggerContext.class.getName() + "]", var3);
        }

    }

    static {
        SINGLETON.init();
    }

这个初始化方法init()里做了2件事:

第一件事是委托ContextInitializer类对defaultLoggerContext进行初始化。这里如果找到了任一配置文件，就会根据配置文件去初始化LoggerContext，如果没找到，会使用默认配置。关于LoggerContext是怎么根据配置文件进行配置的，在后面的博客中介绍，这里先略过

第二件事是对ContextSelectorStaticBinder类进行初始化

    public void init(LoggerContext defaultLoggerContext, Object key) throws ClassNotFoundException, NoSuchMethodException, InstantiationException, IllegalAccessException, InvocationTargetException {
        if (this.key == null) {
            this.key = key;
        } else if (this.key != key) {
            throw new IllegalAccessException("Only certain classes can access this method.");
        }

        String contextSelectorStr = OptionHelper.getSystemProperty("logback.ContextSelector");
        if (contextSelectorStr == null) {
            this.contextSelector = new DefaultContextSelector(defaultLoggerContext);
        } else if (contextSelectorStr.equals("JNDI")) {
            this.contextSelector = new ContextJNDISelector(defaultLoggerContext);
        } else {
            this.contextSelector = dynamicalContextSelector(defaultLoggerContext, contextSelectorStr);
        }

    }

如果系统参数中配置了JNDI，这里会得到一个ContextJNDISelector，实际应用中，一般会得到一个DefaultContextSelector，并且把已经初始化完成的defaultLoggerContext传给新创建的这个DefaultContextSelector

经过上面的步骤，StaticLoggerBinder的init()方法就走完了，接下来就会调用到关键的getLoggerFactory()方法

    public ILoggerFactory getLoggerFactory() {
        if (!this.initialized) {
            return this.defaultLoggerContext;
        } else if (this.contextSelectorBinder.getContextSelector() == null) {
            throw new IllegalStateException("contextSelector cannot be null. See also http://logback.qos.ch/codes.html#null_CS");
        } else {
            return this.contextSelectorBinder.getContextSelector().getLoggerContext();
        }
    }

可以看到，这里有2条分支，如果initialized是false，那么会直接返回defaultLoggerContext。否则就委托刚才提到的ContextSelectorStaticBinder返回一个ContextSelector（一般就是DefaultContextSelector），然后由ContextSelector来返回LoggerContext

public class DefaultContextSelector implements ContextSelector {
    private LoggerContext defaultLoggerContext;

    public DefaultContextSelector(LoggerContext context) {
        this.defaultLoggerContext = context;
    }

    public LoggerContext getLoggerContext() {
        return this.getDefaultLoggerContext();
    }

    public LoggerContext getDefaultLoggerContext() {
        return this.defaultLoggerContext;
    }

    public LoggerContext detachLoggerContext(String loggerContextName) {
        return this.defaultLoggerContext;
    }

    public List<String> getContextNames() {
        return Arrays.asList(this.defaultLoggerContext.getName());
    }

    public LoggerContext getLoggerContext(String name) {
        return this.defaultLoggerContext.getName().equals(name) ? this.defaultLoggerContext : null;
    }
}

总结一下这个过程：
1、StaticLoggerBinder在加载的时候，会去读取配置文件，并根据配置文件对LoggerContext进行初始化
2、然后初始化ContextSelectorStaticBinder，在这个类内部new一个DefaultContextSelector，并把第一步中配置完毕的LoggerContext传给DefaultContextSelector
3、调用getLoggerFactory()方法，直接返回第一步中配置的LoggerContext，或者委托DefaultContextSelector类返回LoggerContext

博客来源：

由于使用的版本较高，本文稍有不同，新版本中也见到很多新的编程思想

读logback源码系列文章（二）——提供ILoggerFactory