熟悉的一行代码

 private static final Log logger = LogFactory.getLog(Application.class);

这一行代码就是使用了LogFactory日志框架，对类Application，进行日志输出。

 private static final 这里表示虚拟机启动后就创建一个最终的日志对象Log logger

创建对象的方式

    public static Log getLog(Class clazz) throws LogConfigurationException {
        return getFactory().getInstance(clazz);
    }
 

 public static LogFactory getFactory() throws LogConfigurationException {
        // Identify the class loader we will be using
        ClassLoader contextClassLoader = getContextClassLoaderInternal();

        if (contextClassLoader == null) {
            // This is an odd enough situation to report about. This
            // output will be a nuisance on JDK1.1, as the system
            // classloader is null in that environment.
            if (isDiagnosticsEnabled()) {
                logDiagnostic("Context classloader is null.");
            }
        }

        // Return any previously registered factory for this class loader
        LogFactory factory = getCachedFactory(contextClassLoader);
        if (factory != null) {
            return factory;
        }

        if (isDiagnosticsEnabled()) {
            logDiagnostic(
                    "[LOOKUP] LogFactory implementation requested for the first time for context classloader " +
                    objectId(contextClassLoader));
            logHierarchy("[LOOKUP] ", contextClassLoader);
        }

这种创建对象的方式也可以运用到其他相似的场景

这里看上去创建的是单例对象，以减少日志调用对内存的使用，使日志调用只需生成一个对象。

Log接口和传参：

public interface Log {

    /**
     * Logs a message with debug log level.
     *
     * @param message log this message
     */
    void debug(Object message);

    /**
     * Logs an error with debug log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void debug(Object message, Throwable t);

    /**
     * Logs a message with error log level.
     *
     * @param message log this message
     */
    void error(Object message);

    /**
     * Logs an error with error log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void error(Object message, Throwable t);

    /**
     * Logs a message with fatal log level.
     *
     * @param message log this message
     */
    void fatal(Object message);

    /**
     * Logs an error with fatal log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void fatal(Object message, Throwable t);

    /**
     * Logs a message with info log level.
     *
     * @param message log this message
     */
    void info(Object message);

    /**
     * Logs an error with info log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void info(Object message, Throwable t);

    /**
     * Is debug logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than debug.
     *
     * @return true if debug is enabled in the underlying logger.
     */
    boolean isDebugEnabled();

    /**
     * Is error logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than error.
     *
     * @return true if error is enabled in the underlying logger.
     */
    boolean isErrorEnabled();

    /**
     * Is fatal logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than fatal.
     *
     * @return true if fatal is enabled in the underlying logger.
     */
    boolean isFatalEnabled();

    /**
     * Is info logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than info.
     *
     * @return true if info is enabled in the underlying logger.
     */
    boolean isInfoEnabled();

    /**
     * Is trace logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than trace.
     *
     * @return true if trace is enabled in the underlying logger.
     */
    boolean isTraceEnabled();

    /**
     * Is warn logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than warn.
     *
     * @return true if warn is enabled in the underlying logger.
     */
    boolean isWarnEnabled();

    /**
     * Logs a message with trace log level.
     *
     * @param message log this message
     */
    void trace(Object message);

    /**
     * Logs an error with trace log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void trace(Object message, Throwable t);

    /**
     * Logs a message with warn log level.
     *
     * @param message log this message
     */
    void warn(Object message);

    /**
     * Logs an error with warn log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void warn(Object message, Throwable t);
}
 

public interface Log {

    /**
     * Logs a message with debug log level.
     *
     * @param message log this message
     */
    void debug(Object message);

    /**
     * Logs an error with debug log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void debug(Object message, Throwable t);

    /**
     * Logs a message with error log level.
     *
     * @param message log this message
     */
    void error(Object message);

    /**
     * Logs an error with error log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void error(Object message, Throwable t);

    /**
     * Logs a message with fatal log level.
     *
     * @param message log this message
     */
    void fatal(Object message);

    /**
     * Logs an error with fatal log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void fatal(Object message, Throwable t);

    /**
     * Logs a message with info log level.
     *
     * @param message log this message
     */
    void info(Object message);

    /**
     * Logs an error with info log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void info(Object message, Throwable t);

    /**
     * Is debug logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than debug.
     *
     * @return true if debug is enabled in the underlying logger.
     */
    boolean isDebugEnabled();

    /**
     * Is error logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than error.
     *
     * @return true if error is enabled in the underlying logger.
     */
    boolean isErrorEnabled();

    /**
     * Is fatal logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than fatal.
     *
     * @return true if fatal is enabled in the underlying logger.
     */
    boolean isFatalEnabled();

    /**
     * Is info logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than info.
     *
     * @return true if info is enabled in the underlying logger.
     */
    boolean isInfoEnabled();

    /**
     * Is trace logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than trace.
     *
     * @return true if trace is enabled in the underlying logger.
     */
    boolean isTraceEnabled();

    /**
     * Is warn logging currently enabled?
     * <p>
     * Call this method to prevent having to perform expensive operations
     * (for example, <code>String</code> concatenation)
     * when the log level is more than warn.
     *
     * @return true if warn is enabled in the underlying logger.
     */
    boolean isWarnEnabled();

    /**
     * Logs a message with trace log level.
     *
     * @param message log this message
     */
    void trace(Object message);

    /**
     * Logs an error with trace log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void trace(Object message, Throwable t);

    /**
     * Logs a message with warn log level.
     *
     * @param message log this message
     */
    void warn(Object message);

    /**
     * Logs an error with warn log level.
     *
     * @param message log this message
     * @param t log this cause
     */
    void warn(Object message, Throwable t);
}

调用示例：

   logger.error("bean init error " + clazz == null ? "null" : clazz.getName(), ex);

输出日志的级别分别有：

warn

trace

info

fatal

error

debug

等等