ofbiz中的factory模式运用

在ofbiz中大量使用了工厂模式，在使用工厂模式的同时使用了缓存模式，如如下生成数据操作工厂类

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package org.ofbiz.entity.datasource;

import java.util.HashMap;
import java.util.Map;

import org.ofbiz.base.util.Debug;
import org.ofbiz.base.util.UtilValidate;
import org.ofbiz.entity.config.model.Datasource;
import org.ofbiz.entity.config.EntityConfigUtil;

/**
 * Generic Entity Helper Factory Class
 * 数据存取帮助类工厂，此处会缓存会存放三个帮助类对象(定义在framework\entity\config\entityengine.xml)
 * 1.<group-map group-name="org.ofbiz" datasource-name="localmysql"/>
 * 2.<group-map group-name="org.ofbiz.olap" datasource-name="localmysqlolap"/>
 * 3.<group-map group-name="org.ofbiz.tenant" datasource-name="localmysqltenant"/>
 */
public class GenericHelperFactory {

    public static final String module = GenericHelperFactory.class.getName();

    // protected static UtilCache helperCache = new UtilCache("entity.GenericHelpers", 0, 0);
    //静态缓存对象
    protected static Map<String, GenericHelper> helperCache = new HashMap<String, GenericHelper>();

    public static GenericHelper getHelper(GenericHelperInfo helperInfo) {
        GenericHelper helper = helperCache.get(helperInfo.getHelperFullName()); //查找帮助类是否存在于缓存中

        if (helper == null) { // don‘t want to block here 同步块，防止重复定义
            synchronized (GenericHelperFactory.class) {
                // must check if null again as one of the blocked threads can still enter
                helper = helperCache.get(helperInfo.getHelperFullName());
                if (helper == null) {
                    try {
                        Datasource datasourceInfo = EntityConfigUtil.getDatasource(helperInfo.getHelperBaseName());//获取数据源定义

                        if (datasourceInfo == null) {
                            throw new IllegalStateException("Could not find datasource definition with name " + helperInfo.getHelperBaseName());
                        }
                        String helperClassName = datasourceInfo.getHelperClass();
                        Class<?> helperClass = null;

                        if (UtilValidate.isNotEmpty(helperClassName)) {
                            try {
                                ClassLoader loader = Thread.currentThread().getContextClassLoader();
                                helperClass = loader.loadClass(helperClassName); //加载帮助类。此处是GenericHelperDAO
                            } catch (ClassNotFoundException e) {
                                Debug.logWarning(e, module);
                                throw new IllegalStateException("Error loading GenericHelper class \"" + helperClassName + "\": " + e.getMessage());
                            }
                        }

                        Class<?>[] paramTypes = new Class<?>[] {GenericHelperInfo.class};
                        Object[] params = new Object[] {helperInfo};

                        java.lang.reflect.Constructor<?> helperConstructor = null;

                        if (helperClass != null) {
                            try {
                                helperConstructor = helperClass.getConstructor(paramTypes);//构造方法
                            } catch (NoSuchMethodException e) {
                                Debug.logWarning(e, module);
                                throw new IllegalStateException("Error loading GenericHelper class \"" + helperClassName + "\": " + e.getMessage());
                            }
                        }
                        try {
                            helper = (GenericHelper) helperConstructor.newInstance(params);
                        } catch (IllegalAccessException e) {
                            Debug.logWarning(e, module);
                            throw new IllegalStateException("Error loading GenericHelper class \"" + helperClassName + "\": " + e.getMessage());
                        } catch (InstantiationException e) {
                            Debug.logWarning(e, module);
                            throw new IllegalStateException("Error loading GenericHelper class \"" + helperClassName + "\": " + e.getMessage());
                        } catch (java.lang.reflect.InvocationTargetException e) {
                            Debug.logWarning(e, module);
                            throw new IllegalStateException("Error loading GenericHelper class \"" + helperClassName + "\": " + e.getMessage());
                        }

                        if (helper != null)
                            helperCache.put(helperInfo.getHelperFullName(), helper);//放入缓存
                    } catch (SecurityException e) {
                        Debug.logError(e, module);
                        throw new IllegalStateException("Error loading GenericHelper class: " + e.toString());
                    }
                }
            }
        }
        return helper;
    }
}