使用Iterator模式将对象转成String.doc

操纵JSOM、XML、Java bean等对象时你可能最先想到访问者模式。但是使用访问者模式很难从调用代码控制回调。比如，不能有条件的从所有回调的子分支和叶子节点跳过某个分支。解决这个问题就可以使用Iterator模式遍历整个对象，生成易于开发者阅读和调试的字符串。该迭代器具备一定的通用性，我在使用XPath查找Java对象和在StackHunter中记录异常的工具中都用到了它。API本文主要介绍的两个类：StringGenerator和ObjectIterator。字符串生成器StringGenerator工具类将对象转化为字符串，使对象可读性更好。可以用它来实现类的toString方法或者把对象的字符串表达作为日志调试代码：123456789101112131415161718192021222324package com.stackhunter.util.tostring.example;import com.stackhunter.example.employee.Department;import com.stackhunter.example.employee.Employee;import com.stackhunter.example.employee.Manager;import com.stackhunter.example.people.Person;import com.stackhunter.util.tostring.StringGenerator;public class StringGeneratorExample public static void main(String args) Department department = new Department(5775, Sales).setEmployees(new Employee(111, Bill, Gates), new Employee(222, Howard, Schultz), new Manager(333, Jeff, Bezos, 75000);System.out.println(StringGenerator.generate(department);System.out.println(StringGenerator.generate(new int 111, 222, 333 );System.out.println(StringGenerator.generate(true);StringGenerator.generate()将department，数组和boolean值进行格式化输出。12345678910111213141516171819202122com.stackhunter.example.employee.Department129719f4deptId = 5775employeeList = java.util.ArrayList7037717aemployeeList0 = com.stackhunter.example.employee.Employee17a323c0firstName = Billid = 111lastName = GatesemployeeList1 = com.stackhunter.example.employee.Employee57801e5ffirstName = Howardid = 222lastName = SchultzemployeeList2 = com.stackhunter.example.employee.Manager1c4a1bdabudget = 75000.0firstName = Jeffid = 333lastName = Bezosname = SalesI39df3255object0 = 111object1 = 222object2 = 333true对象迭代器ObjectIterator使用迭代器模式遍历对象的属性，以键值对形式保存。对象中的Java bean、集合、数组及map都要进行迭代。ObjectIterator也会考虑到对象之间循环引用的处理。123456789101112131415161718192021222324package com.stackhunter.util.tostring.example;import com.stackhunter.example.employee.Department;import com.stackhunter.example.employee.Employee;import com.stackhunter.example.employee.Manager;import com.stackhunter.util.objectiterator.ObjectIterator;public class ObjectIteratorExample public static void main(String args) Department department = new Department(5775, Sales).setEmployees(new Employee(111, Bill, Gates), new Employee(222, Howard, Schultz), new Manager(333, Jeff, Bezos, 75000);ObjectIterator iterator = new ObjectIterator(some department, department);while (iterator.next() System.out.println(iterator.getName() + = + iterator.getValueAsString();通过遍历整个对象生成键值对的集合。使用getValueAsString()方法而不是toString()格式化输出。对于原始类型、包装类型、字符串、日期和枚举使用原始的toString()实现。对于其他类型输出类名和hash值。ObjectIterator.getDepth()会增加缩进，输出更易读。调用next()之前使用nextParent()缩短当前分支跳跃到下一属性。123456some department=com.stackhunter.example.employee.Department780324ffdeptId=5775employeeList=java.util.ArrayList6bd15108employeeList0=com.stackhunter.example.employee.Employee22a79c31firstName=Bill.Java对象迭代器的具体实现实现iterator模式的第一步是创建通用的迭代器接口：IObjectIterator。无论遍历的对象是Java bean、数组还是map都可以使用该接口。12345public interface IObjectIterator boolean next();String getName();Object getValue();使用该接口可以按照单一顺序依次获取当前属性的name和value。实现了IObjectIterator的类用来处理某一种类型的对象。大多数类调用getName()返回名称前缀。ArrayIterator使用了元素的索引：return name + + nextIndex + ;。属性迭代器PropertyIterator可能是最重要的迭代类。它使用Java bean introspection读取对象属性，将它们转化为键值对序列。1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556public class PropertyIterator implements IObjectIterator private final Object object;private final PropertyDescriptor properties;private int nextIndex = -1;private PropertyDescriptor currentProperty;public PropertyIterator(Object object) this.object = object;try BeanInfo beanInfo = Introspector.getBeanInfo(object.getClass();properties = beanInfo.getPropertyDescriptors(); catch (RuntimeException e) throw e; catch (Exception e) throw new RuntimeException(e.getMessage(), e);Overridepublic boolean next() if (nextIndex + 1 = properties.length) return false;nextIndex+;currentProperty = propertiesnextIndex;if (currentProperty.getReadMethod() = null | class.equals(currentProperty.getName() return next();return true;Overridepublic String getName() if (currentProperty = null) return null;return currentProperty.getName();Overridepublic Object getValue() try if (currentProperty = null) return null;return currentProperty.getReadMethod().invoke(object); catch (RuntimeException e) throw e; catch (Exception e) throw new RuntimeException(e.getMessage(), e);数组迭代器ArrayIterator通过反射得到数组的长度，进而检索每个数据元素。ArrayIterator不关心从getValue()方法返回值的具体细节。它们一般情况下被传递给PropertyIterator。123456789101112131415161718192021222324252627282930313233343536public class ArrayIterator implements IObjectIterator private final String name;private final Object array;private final int length;private int nextIndex = -1;private Object currentElement;public ArrayIterator(String name, Object array) = name;this.array = array;this.length = Array.getLength(array);Overridepublic boolean next() if (nextIndex + 1 = length) return false;nextIndex+;currentElement = Array.get(array, nextIndex);return true;Overridepublic String getName() return name + + nextIndex + ;Overridepublic Object getValue() return currentElement;集合迭代器CollectionIterator与ArrayIterator非常相似。使用java.lang.Iterable调用它的Iterable.iterator()方法初始化内部迭代器。Map迭代器MapIterator遍历java.util.Map的entry。它并不深入到每个entry的键值对，这个工作由MapEntryIterator类完成。12345678910111213141516171819202122232425public class MapIterator implements IObjectIterator private final String name;private Iterator entryIterator;private Map.Entry currentEntry;private int nextIndex = -1;public MapIterator(String name, Map map) = name;this.entryIterator = map.entrySet().iterator();Overridepublic boolean next() if (entryIterator.hasNext() nextIndex+;currentEntry = (Entry) entryIterator.next();return true;return false;.Map Entry迭代器MapEntryIterator处理java.util.Map的单个entry。它只返回两个值：entry的键和值。与ArrayIterator及其他的类似，如果是复杂类型的话，它的结果可能最终传递给PropertyIterator，作为Java bean处理。根迭代器RootIterator返回单个元素初始节点。可以把它想成XML文件的根节点。目的是发起整个遍历过程。整合ObjectIterator类作为门面角色（Facade），包装了所有的遍历逻辑。它根据最后一次getValue()的返回值类型决定哪个IObjectIterator的子类需要实例化。当子迭代器在内部创建时它在栈中保存当前迭代器的状态。它也暴露了getChild()和getDepth()方法为调用者展示当前进度。12345678910111213141516171819202122232425262728293031323334private IObjectIterator iteratorFor(Object object) try if (object = null) return null;if (object.getClass().isArray() return new ArrayIterator(name, object);if (object instanceof Iterable) return new CollectionIterator(name, (Iterable) object);if (object instanceof Map) return new MapIterator(name, (Map) object);if (object instanceof Map.Entry) return new MapEntryIterator(name, (Map.Entry) object);if (isSingleValued(object) return null;return new PropertyIterator(object); catch (RuntimeException e) throw e; catch (Exception e) throw new RuntimeException(e.getMessage(), e);字符串生成器的实现已经看到如何遍历对象中的所有属性。最后的工作就是让输出