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

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。

// 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。

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();}}
}