上篇博客介绍了ContextInitializer类如何把框架的配置工作委托给各个Action具体实现类,这篇博客就接下来介绍一下,Action组件是如何进行配置和实际工作的

 

 

 如图所示,首先Action是一个抽象类,定义了begin()、body()、end()方法,这些方法如上篇博客所说,是给Interpreter调用的,Interpreter的endElement()方法会调用private的callEndAction()方法,然后callEndAction()方法调用实际Action的end()方法

然后针对GenericConfigurator中addInstanceRules()方法定义的每种元素,比如<appender>、<appender-ref>,都有一个对应的Action,负责对这种元素进行处理

Action组件的类别是很多的,比较常见的有LoggerAction、AppenderAction、AppenderRefAction等,下面就以AppenderAction和AppenderRefAction为例子,来说明Action的工作方式

    public void begin(InterpretationContext ec, String localName, Attributes attributes) throws ActionException {
        // We are just beginning, reset variables
        appender = null;
        inError = false;

        String className = attributes.getValue(CLASS_ATTRIBUTE);
        if (OptionHelper.isEmpty(className)) {
            addError("Missing class name for appender. Near [" + localName + "] line " + getLineNumber(ec));
            inError = true;
            return;
        }

        try {
            addInfo("About to instantiate appender of type [" + className + "]");

            appender = (Appender<E>) OptionHelper.instantiateByClassName(className, ch.qos.logback.core.Appender.class, context);

            appender.setContext(context);

            String appenderName = ec.subst(attributes.getValue(NAME_ATTRIBUTE));

            if (OptionHelper.isEmpty(appenderName)) {
                addWarn("No appender name given for appender of type " + className + "].");
            } else {
                appender.setName(appenderName);
                addInfo("Naming appender as [" + appenderName + "]");
            }

            // The execution context contains a bag which contains the appenders
            // created thus far.
            HashMap<String, Appender<E>> appenderBag = (HashMap<String, Appender<E>>) ec.getObjectMap().get(ActionConst.APPENDER_BAG);

            // add the appender just created to the appender bag.
            appenderBag.put(appenderName, appender);

            ec.pushObject(appender);
        } catch (Exception oops) {
            inError = true;
            addError("Could not create an Appender of type [" + className + "].", oops);
            throw new ActionException(oops);
        }
    }

比如配置文件

<appender name="ma" class="MyAppender" />  

当解析到这行时,就会调用AppenderAction的begin()方法,把"MyAppender"这个属性给读出来,然后根据类名创建一个MyAppender的实例

    public static Object instantiateByClassName(String className, Class<?> superClass, Context context) throws IncompatibleClassException,
                    DynamicClassLoadingException {
        ClassLoader classLoader = Loader.getClassLoaderOfObject(context);
        return instantiateByClassName(className, superClass, classLoader);
    }

    public static Object instantiateByClassNameAndParameter(String className, Class<?> superClass, Context context, Class<?> type, Object param)
                    throws IncompatibleClassException, DynamicClassLoadingException {
        ClassLoader classLoader = Loader.getClassLoaderOfObject(context);
        return instantiateByClassNameAndParameter(className, superClass, classLoader, type, param);
    }

    public static Object instantiateByClassName(String className, Class<?> superClass, ClassLoader classLoader) throws IncompatibleClassException,
                    DynamicClassLoadingException {
        return instantiateByClassNameAndParameter(className, superClass, classLoader, null, null);
    }

    public static Object instantiateByClassNameAndParameter(String className, Class<?> superClass, ClassLoader classLoader, Class<?> type, Object parameter)
                    throws IncompatibleClassException, DynamicClassLoadingException {

        if (className == null) {
            throw new NullPointerException();
        }
        try {
            Class<?> classObj = null;
            classObj = classLoader.loadClass(className);
            if (!superClass.isAssignableFrom(classObj)) {
                throw new IncompatibleClassException(superClass, classObj);
            }
            if (type == null) {
                return classObj.newInstance();
            } else {
                Constructor<?> constructor = classObj.getConstructor(type);
                return constructor.newInstance(parameter);
            }
        } catch (IncompatibleClassException ice) {
            throw ice;
        } catch (Throwable t) {
            throw new DynamicClassLoadingException("Failed to instantiate type " + className, t);
        }
    }

以上代码其实就是根据"MyAppender"这个类名来创建了一个类实例并返回,这种写法使得创建类实例的动作延迟到框架运行期间,实现动态创建实例,很值得学习

之后就是把"ma"属性调用setAppenderName()方法,赋给刚创建的这个MyAppender实例

最后关键性的一步,就是把这个初始化完毕的MyAppender放到InterpretationContext的AppenderBag里,至于有什么用,我们接下来看AppenderRefAction就会明白

    public void begin(InterpretationContext ec, String tagName, Attributes attributes) {
        // Let us forget about previous errors (in this object)
        inError = false;

        // logger.debug("begin called");

        Object o = ec.peekObject();

        if (!(o instanceof AppenderAttachable)) {
            String errMsg = "Could not find an AppenderAttachable at the top of execution stack. Near [" + tagName + "] line " + getLineNumber(ec);
            inError = true;
            addError(errMsg);
            return;
        }

        AppenderAttachable<E> appenderAttachable = (AppenderAttachable<E>) o;

        String appenderName = ec.subst(attributes.getValue(ActionConst.REF_ATTRIBUTE));

        if (OptionHelper.isEmpty(appenderName)) {
            // print a meaningful error message and return
            String errMsg = "Missing appender ref attribute in <appender-ref> tag.";
            inError = true;
            addError(errMsg);

            return;
        }

        HashMap<String, Appender<E>> appenderBag = (HashMap<String, Appender<E>>) ec.getObjectMap().get(ActionConst.APPENDER_BAG);
        Appender<E> appender = (Appender<E>) appenderBag.get(appenderName);

        if (appender == null) {
            String msg = "Could not find an appender named [" + appenderName + "]. Did you define it below instead of above in the configuration file?";
            inError = true;
            addError(msg);
            addError("See " + CoreConstants.CODES_URL + "#appender_order for more details.");
            return;
        }

        addInfo("Attaching appender named [" + appenderName + "] to " + appenderAttachable);
        appenderAttachable.addAppender(appender);
    }

代码的含义一目了然,由于<appender-ref>元素肯定是嵌套在<logger>里,所以ec.peekObject()方法取出的就是刚刚创建的Logger实例,接下来就从InterpretationContext的AppenderBag中取出来,然后调用setAppender()方法,把Appender赋给Logger。如果设置了多个<appender-ref>,那么这些Appender都会被赋给Logger

其他如LoggerAction、RootLoggerAction的代码也是类似的,而且方法体都不大,就不重复叙述了,大家可以自己去看

阅读了这部分源代码,我觉得颇有体会,主要学习到以下2个设计的思路:
1、通过在配置文件中指定类名,然后调用ClassLoader的方法,可以在程序运行期间动态地创建类的实例
2、利用一个Context,来保存配置期间的类实例和变量,把不同的元素给串联起来

其实看完ContextInitializer类,再来深入地看Action类的实现,是很简单的。但是读这部分代码,却让我觉得心情很愉悦,因为代码中包含的设计思想,不仅仅是用在logback框架中,对我们自己程序的配置怎么写也很有指导意义。掌握了这种设计思路,有助于写出更具灵活性和可扩展性的程序

本系列博客到本篇为止,已经对logback的整体框架有了一个high level的了解。去配置使用logback框架是毫无困难的了,而且如果日志模块出现了问题,要定位也是非常简单的了。不过接下来我们还要继续探究一下更深层次的代码,下一篇博客介绍一下Appender类中的Encoder,是怎么将ILoggingEvent实际记录成日志的

 

备注:

  源博客中有第八章,查看了内容发现与现有的程序差别过大, 这里不再转载

  现版本写入日志是由 OutputStreamAppender 类中的方法通过 writeBytes 方法通过 OutputStream 直接添加的

 

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(七)——配置的实际工作类Action
posted on 2021-06-22 11:06  幂次方  阅读(544)  评论(0编辑  收藏  举报