Log4j扩展使用--自定义输出

• 写在前面的话

log4j支持自定义的输出。所有的输出都实现了自Appender接口。一般来说，自定义输出值需要继承AppenderSkeleton类，并实现几个方法就可以了。

写这篇博客，我主要也是想说，框架之所有被成为是一个框架，是在帮我们完成大部分的通用代码，这就有一个前提就是说它必须要有具有良好的扩张性。方便每一个使用者来扩展，当然我们也可以根据自己的喜好去改人家框架的源码，但是最实在的也是最有效的去扩展人家开源框架，在扩展的时候我们也可以参照人家原来的默认实现，这样子对于我们的学习也是一大进步。

• 一个自定义输出的例子

OK，废话不说了，现在我们开始吧。先来看一个自定义输出的例子，CountingConsoleAppender跟控制台输出类似，不同的是会统计日志输出的次数。当输出次数超出预定的值时，会做相应的业务处理，这里简单的为打印出一行提示信息，并停止输出。代码如下：

package org.linkinpark.commons.logtest;import java.util.Objects;import org.apache.log4j.AppenderSkeleton;import org.apache.log4j.spi.ErrorCode;import org.apache.log4j.spi.LoggingEvent;public class CountingConsoleAppender extends AppenderSkeleton{protected int count = 0;protected int limit = 10;/** * 关闭资源 */@Overridepublic void close(){if (this.closed){return;}this.closed = true;}/** * 这里需要使用格式化器 */@Overridepublic boolean requiresLayout(){return true;}@Overrideprotected void append(LoggingEvent event){// 1，验证，如果没有格式化器，报错，如果次数超过限制，报错if (this.layout == null){errorHandler.error("没有设置[" + name + "]日志格式化器。", null, ErrorCode.MISSING_LAYOUT);return;}if (count >= limit){errorHandler.error("输出次数[" + limit + "]达到了[" + getName() + "]的上限。", null, ErrorCode.WRITE_FAILURE);return;}// 控制台打印日志System.out.println(this.layout.format(event));// 如果配置的格式化器没有处理异常，这里打印异常栈信息if (layout.ignoresThrowable()){String[] throwableStrRep = event.getThrowableStrRep();if (Objects.nonNull(throwableStrRep)){for (String throwStr : throwableStrRep){System.out.println(throwStr);}}}// 打印日志结束，修改打印次数count++;}public int getCount(){return count;}public CountingConsoleAppender setCount(int count){this.count = count;return this;}public int getLimit(){return limit;}public void setLimit(int limit){this.limit = limit;}}

配置文件如下：

#定义输出等级和输出appenderlog4j.rootLogger=DEBUG,countingconsolelog4j.appender.countingconsole=org.linkinpark.commons.logtest.CountingConsoleAppender  #设置输出样式log4j.appender.countingconsole.layout=org.apache.log4j.PatternLayout#日志输出信息格式为log4j.appender.countingconsole.layout.ConversionPattern=[%-d{yyyy-MM-dd HH:mm:ss}]-[%t-%5p]-[%C-%M(%L)]： %m%n #控制最大输出次数log4j.appender.countingconsole.limit=3#打开4j本身的日志输出log4j.debug=true

OK，现在我们来运行下测试看下控制台输出情况，测试代码如下：

package org.linkinpark.commons.logtest;import org.apache.log4j.Logger;import org.junit.Test;/** * @创建作者： LinkinPark * @创建时间： 2016年2月23日 * @功能描述： 测试自己扩展的CountConsoleAppender */public class Log4jTest{public static Logger log = Logger.getLogger(Log4jTest.class);@Testpublic void logTest(){log.debug("debug级别的日志输出");log.debug("debug级别的日志输出1");log.debug("debug级别的日志输出2");log.debug("debug级别的日志输出3");}}

测试绿条，控制台输出如下：

log4j: Parsing for [root] with value=[DEBUG,countingconsole].log4j: Level token is [DEBUG].log4j: Category root set to DEBUGlog4j: Parsing appender named "countingconsole".log4j: Parsing layout options for "countingconsole".log4j: Setting property [conversionPattern] to [[%-d{yyyy-MM-dd HH:mm:ss}]-[%t-%5p]-[%C-%M(%L)]： %m%n ].log4j: End of parsing for "countingconsole".log4j: Setting property [limit] to [3].log4j: Parsed "countingconsole" options.log4j: Finished configuring.[2016-02-25 23:42:16]-[main-DEBUG]-[org.linkinpark.commons.logtest.Log4jTest-logTest(19)]： debug级别的日志输出 [2016-02-25 23:42:16]-[main-DEBUG]-[org.linkinpark.commons.logtest.Log4jTest-logTest(20)]： debug级别的日志输出1 [2016-02-25 23:42:16]-[main-DEBUG]-[org.linkinpark.commons.logtest.Log4jTest-logTest(21)]： debug级别的日志输出2 log4j:ERROR 输出次数[3]达到了[countingconsole]的上限。

• 关于例子的解释

1，在扩展这个appender的时候，我有参照consoleAppender的实现。核心就是说实现append方法，当然我们直接继承自AppenderSkeleton类来进行的扩展，所以可以直接拿到里面的一些属性，比如layput，比如erroHandler等等

2，刚开始的写这个类的时候，我直接定义了一个limit属性，用来控制日志输出次数，直接是在代码中赋的初始值，为了方便，所以我就想写进配置文件中，但是怎么都注入不进去，控制台一直报下面这个error：

log4j:WARN Failed to set property [limit] to value "3". 

没办法，我只要打开log4j本身的日志，配置文件中设值log4j.debug=true就OK。后来终于发现我的set方法有问题，这个方法这里必须是void返回类型的，而我一般的set方法都是返回自身this，所以这里没有注入。关于log4j处理set注入我下面一节会整理到。

3，当然我们在扩展的时候直接继承ConsoleAppender自这个类也是可以的，这样子的话只需要重写append方法就够了，其他的都不需要了。我自己试了一下测试通过，代码类似，这里不做赘述了。

• 关于反射set值的另一种方式

我们经常编码，但是其实写反射的代码并不是很多，一般的在IOC框架中都是读取配置文件或者说扫描注解来获取相关key-value，返回跑下set方法的反射，就可以设值到一个对象里面去了，这样子的话就可以把一些属性的设值放入到配置文件中，实现解耦。

在以前我们是这样子编码的：

// 取出需要设置Field值的目标对象Object target = getObject(objAndProp[0]);// 该Field对应的setter方法名:set + "属性的首字母大写" + 剩下部分String mtdName = "set" + objAndProp[1].substring(0 , 1).toUpperCase() + objAndProp[1].substring(1);// 通过target的getClass()获取它实现类所对应的Class对象Class<?> targetClass = target.getClass();// 获取该属性对应的setter方法，下面这一行道出了springIOC的精髓，为什么实现XML我们每次都要提供get和set方法，除了注解的哦Method mtd = targetClass.getMethod(mtdName , String.class);// 通过Method的invoke方法执行setter方法，将config.getProperty(name)的属性值作为调用setter的方法的实参mtd.invoke(target , config.getProperty(name));

看过了log4j的源码以后，我们多了一种选择，就是使用JDK自带的PropertyDescriptor类，这个类就是按照javabean规范写的一个存储器。

该类里面有2个方法可以直接获取我们的get和set方法：setReadMethod，getWriteMethod。以后这也是一种尝试，必要的时候可以参照log4j来用这个方式跑反射。OK，我这里贴出log4j中该类的源码：

package org.apache.log4j.config;import org.apache.log4j.Appender;import org.apache.log4j.Level;import org.apache.log4j.Priority;import org.apache.log4j.helpers.LogLog;import org.apache.log4j.helpers.OptionConverter;import org.apache.log4j.spi.OptionHandler;import org.apache.log4j.spi.ErrorHandler;import java.beans.BeanInfo;import java.beans.IntrospectionException;import java.beans.Introspector;import java.beans.PropertyDescriptor;import java.io.InterruptedIOException;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;import java.util.Enumeration;import java.util.Properties;/** * General purpose Object property setter. Clients repeatedly invokes * {@link #setProperty setProperty(name,value)} in order to invoke setters * on the Object specified in the constructor. This class relies on the * JavaBeans {@link Introspector} to analyze the given Object Class using * reflection. *  * <p> * Usage: *  * <pre> * PropertySetter ps = new PropertySetter(anObject); * ps.set("name", "Joe"); * ps.set("age", "32"); * ps.set("isMale", "true"); * </pre> *  * will cause the invocations anObject.setName("Joe"), anObject.setAge(32), * and setMale(true) if such methods exist with those signatures. * Otherwise an {@link IntrospectionException} are thrown. *  * @author Anders Kristensen * @since 1.1 */public class PropertySetter{protected Object obj;protected PropertyDescriptor[] props;/** * Create a new PropertySetter for the specified Object. This is done * in prepartion for invoking {@link #setProperty} one or more times. *  * @param obj *            the object for which to set properties */public PropertySetter(Object obj){this.obj = obj;}/** * Uses JavaBeans {@link Introspector} to computer setters of object to be * configured. */protected void introspect(){try{BeanInfo bi = Introspector.getBeanInfo(obj.getClass());props = bi.getPropertyDescriptors();}catch (IntrospectionException ex){LogLog.error("Failed to introspect " + obj + ": " + ex.getMessage());props = new PropertyDescriptor[0];}}/** * Set the properties of an object passed as a parameter in one * go. The <code>properties</code> are parsed relative to a * <code>prefix</code>. *  * @param obj *            The object to configure. * @param properties *            A java.util.Properties containing keys and values. * @param prefix *            Only keys having the specified prefix will be set. */public static void setProperties(Object obj, Properties properties, String prefix){new PropertySetter(obj).setProperties(properties, prefix);}/** * Set the properites for the object that match the * <code>prefix</code> passed as parameter. *  *  */public void setProperties(Properties properties, String prefix){int len = prefix.length();for (Enumeration e = properties.propertyNames(); e.hasMoreElements();){String key = (String) e.nextElement();// handle only properties that start with the desired frefix.if (key.startsWith(prefix)){// ignore key if it contains dots after the prefixif (key.indexOf('.', len + 1) > 0){// System.err.println("----------Ignoring---["+key// +"], prefix=["+prefix+"].");continue;}String value = OptionConverter.findAndSubst(key, properties);key = key.substring(len);if (("layout".equals(key) || "errorhandler".equals(key)) && obj instanceof Appender){continue;}//// if the property type is an OptionHandler// (for example, triggeringPolicy of org.apache.log4j.rolling.RollingFileAppender)PropertyDescriptor prop = getPropertyDescriptor(Introspector.decapitalize(key));if (prop != null && OptionHandler.class.isAssignableFrom(prop.getPropertyType()) && prop.getWriteMethod() != null){OptionHandler opt = (OptionHandler) OptionConverter.instantiateByKey(properties, prefix + key, prop.getPropertyType(), null);PropertySetter setter = new PropertySetter(opt);setter.setProperties(properties, prefix + key + ".");try{Method writeMethod = prop.getWriteMethod();System.out.println("woqu=" + writeMethod);prop.getWriteMethod().invoke(this.obj, new Object[] { opt });}catch (IllegalAccessException ex){LogLog.warn("Failed to set property [" + key + "] to value \"" + value + "\". ", ex);}catch (InvocationTargetException ex){if (ex.getTargetException() instanceof InterruptedException || ex.getTargetException() instanceof InterruptedIOException){Thread.currentThread().interrupt();}LogLog.warn("Failed to set property [" + key + "] to value \"" + value + "\". ", ex);}catch (RuntimeException ex){LogLog.warn("Failed to set property [" + key + "] to value \"" + value + "\". ", ex);}continue;}setProperty(key, value);}}activate();}/** * Set a property on this PropertySetter's Object. If successful, this * method will invoke a setter method on the underlying Object. The * setter is the one for the specified property name and the value is * determined partly from the setter argument type and partly from the * value specified in the call to this method. *  * <p> * If the setter expects a String no conversion is necessary. * If it expects an int, then an attempt is made to convert 'value' * to an int using new Integer(value). If the setter expects a boolean, * the conversion is by new Boolean(value). *  * @param name *            name of the property * @param value *            String value of the property */public void setProperty(String name, String value){if (value == null){return;}name = Introspector.decapitalize(name);PropertyDescriptor prop = getPropertyDescriptor(name);// LogLog.debug("---------Key: "+name+", type="+prop.getPropertyType());if (prop == null){LogLog.warn("No such property [" + name + "] in " + obj.getClass().getName() + ".");}else{try{setProperty(prop, name, value);}catch (PropertySetterException ex){LogLog.warn("Failed to set property [" + name + "] to value \"" + value + "\". ", ex.rootCause);}}}/** * Set the named property given a {@link PropertyDescriptor}. *  * @param prop *            A PropertyDescriptor describing the characteristics *            of the property to set. * @param name *            The named of the property to set. * @param value *            The value of the property. */public void setProperty(PropertyDescriptor prop, String name, String value) throws PropertySetterException{Method setter = prop.getWriteMethod();if (setter == null){throw new PropertySetterException("No setter for property [" + name + "].");}Class[] paramTypes = setter.getParameterTypes();if (paramTypes.length != 1){throw new PropertySetterException("#params for setter != 1");}Object arg;try{arg = convertArg(value, paramTypes[0]);}catch (Throwable t){throw new PropertySetterException("Conversion to type [" + paramTypes[0] + "] failed. Reason: " + t);}if (arg == null){throw new PropertySetterException("Conversion to type [" + paramTypes[0] + "] failed.");}LogLog.debug("Setting property [" + name + "] to [" + arg + "].");try{setter.invoke(obj, new Object[] { arg });}catch (IllegalAccessException ex){throw new PropertySetterException(ex);}catch (InvocationTargetException ex){if (ex.getTargetException() instanceof InterruptedException || ex.getTargetException() instanceof InterruptedIOException){Thread.currentThread().interrupt();}throw new PropertySetterException(ex);}catch (RuntimeException ex){throw new PropertySetterException(ex);}}/** * Convert <code>val</code> a String parameter to an object of a * given type. */protected Object convertArg(String val, Class type){if (val == null){return null;}String v = val.trim();if (String.class.isAssignableFrom(type)){return val;}else if (Integer.TYPE.isAssignableFrom(type)){return new Integer(v);}else if (Long.TYPE.isAssignableFrom(type)){return new Long(v);}else if (Boolean.TYPE.isAssignableFrom(type)){if ("true".equalsIgnoreCase(v)){return Boolean.TRUE;}else if ("false".equalsIgnoreCase(v)){return Boolean.FALSE;}}else if (Priority.class.isAssignableFrom(type)){return OptionConverter.toLevel(v, Level.DEBUG);}else if (ErrorHandler.class.isAssignableFrom(type)){return OptionConverter.instantiateByClassName(v, ErrorHandler.class, null);}return null;}protected PropertyDescriptor getPropertyDescriptor(String name){if (props == null){introspect();}for (int i = 0; i < props.length; i++){if (name.equals(props[i].getName())){return props[i];}}return null;}public void activate(){if (obj instanceof OptionHandler){((OptionHandler) obj).activateOptions();}}}

• 总结

Log4j源码还是写的不错的，特别是一些小巧的设计，比如hashtable的性能提升，比如layout引入了解释器模式等等，都是值得我们借鉴的，在扩展性方面也写的挺好。

如果有必要的我们可以自己重写一个appender来实现我们自己的特定功能，OK，先这样子吧。