如何使用Java反射將數組寫入XML文件？

Question

我正在嘗試編寫一個小型庫，它將Java對象通過反射轉換為XML。 我已經完成了大部分工作，但在嘗試迭代數組時遇到了錯誤。

以下是我用於測試的域對象：

在Company.java中：

import java.util.List;

import com.google.common.collect.Lists;

public class Company
{
    public Employee employeeArray[];

    public Employee[] getEmployeeArray()
    {
        return employeeArray;
    }

    public void setEmployeeArray(Employee[] employeeArray)
    {
        this.employeeArray = employeeArray;
    }
}

在Employee.java中：

public class Employee
{
    public String firstName;
    public String lastName;

    public Employee() {}

    public Employee(String firstName, String lastName)
    {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName()
    {
        return firstName;
    }

    public void setFirstName(String firstName)
    {
        this.firstName = firstName;
    }

    public String getLastName()
    {
        return lastName;
    }

    public void setLastName(String lastName)
    {
        this.lastName = lastName;
    }
}

庫的核心（ObjectXMLWriter.java）：

import java.io.StringWriter;
import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.util.List;

import javax.xml.transform.OutputKeys;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;

import org.apache.log4j.Logger;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;

import com.falcondev.web.DOMFactory;
import com.google.common.collect.Lists;

public class ObjectXMLWriter
{
    private static final Logger logger = Logger.getLogger(ObjectXMLWriter.class);

    private String fileLocation;
    private Object object;
    private boolean shouldOverride;

    public ObjectXMLWriter(String fileLocation, Object object) {
        this.fileLocation = fileLocation;
        this.object = object;
        this.shouldOverride = false;
    }

    public ObjectXMLWriter(String fileLocation, Object object, boolean shouldOverride) {
        this(fileLocation, object);
        this.shouldOverride = shouldOverride;
    }

    public String getFileLocation()
    {
        return fileLocation;
    }

    public void setFileLocation(String fileLocation)
    {
        this.fileLocation = fileLocation;
    }

    public Object getObject()
    {
        return object;
    }

    public void setObject(Object object)
    {
        this.object = object;
    }

    public boolean isShouldOverride()
    {
        return shouldOverride;
    }

    public void setShouldOverride(boolean shouldOverride)
    {
        this.shouldOverride = shouldOverride;
    }

    public boolean saveObject() throws Exception {

        boolean saveSuccessful = false;
        Document document = createDocument();

        TransformerFactory transformerFactory = TransformerFactory.newInstance();
        Transformer transformer = transformerFactory.newTransformer();
        transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");

        StringWriter stringWriter = new StringWriter();
        StreamResult result = new StreamResult(stringWriter);
        DOMSource source = new DOMSource(document);
        transformer.transform(source, result);
        String xmlString = stringWriter.toString();

        //print XML
        System.out.println("Here's the xml:\n\n" + xmlString);

        //TODO save XML file
        return saveSuccessful;
    }

    private Node createNode(Document document, Object object) throws Exception {
        Node node = document.createElement(getObjectClassName(object));
        logger.trace("NODE: " + node);

        if(node != null) {
            //create children nodes from object fields
            List<Field> fields = Lists.newArrayList(object.getClass().getFields());
            for(Field field: fields) {
                Object obj = field.get(object);
                logger.trace("OBJECT: " + obj);

                if(obj == null || !checkInstantiability(obj) || field.getType().isAssignableFrom(List.class) || field.getType().isArray()) {
                    logger.debug("ATTEMPTING TO CREATE NODE FOR FIELD: " + field.getName());
                    logger.debug("FIELD TYPE: " + field.getType());

                    //TODO add types as needed
                    String fieldValue = "";
                    if(List.class.isAssignableFrom(field.getType())) { //TODO check if object is iterable instead
                      //TODO figure out how to iterate through iterable's
                    }
                    else if(field.getType().isArray()) {
                        Object array = field.get(obj); //Fails here
                        int length = Array.getLength(array);
                        for (int i = 0; i < length; i++) {
                          System.out.println(Array.get(array, i));
                          node.appendChild(createNode(document, Array.get(array, i)));
                        }
                    }
                    else if(field.getType() == Class.class) {
                        fieldValue = obj.toString().replaceFirst("class ", "");
                    }
                    else {
                        fieldValue = obj.toString();
                    }
                    logger.debug("FIELD OBJECT VALUE: '" + fieldValue + "'");

                    //TODO check for annotation to choose whether to create element or attribute
                    Element element = document.createElement(field.getName());
                    element.setTextContent(fieldValue);
                    node.appendChild(element);
                }
                else {
                    logger.debug("ATTEMPTING TO CREATE OBJECT NODE FOR FIELD: " + field.getName());
                    node.appendChild(createNode(document, obj));
                }
            }
        }

        return node;
    }

    private Document createDocument() throws Exception {
        Document document = DOMFactory.create();
        if(checkInstantiability(object)) {
            Node rootNode = createNode(document, object);
            document.appendChild(rootNode);
        }
        else {
            logger.error("CANNOT SAVE UNINSTANTIABLE OBJECT. DOCUMENT MUST HAVE A ROOT NODE");
        }

        return document;
    }

    private String getObjectClassName(Object object) {
        return object.getClass().getSimpleName().toLowerCase();
    }

    private boolean checkInstantiability(Object object) {
        try
        {
            object.getClass().newInstance();
        }
        catch (InstantiationException exception)
        {
            return false;
        }
        catch (IllegalAccessException exception)
        {
            return false;
        }

        return true;
    }
}

和測試驅動程序（Test.java）：

public class Test
{
    public static void main(String[] args) throws Exception
    {
        Employee[] employees = new Employee[3];
        employees[0] = new Employee("Tom", "C");
        employees[1] = new Employee("Paul", "E");
        employees[2] = new Employee("George", "A");
        Company company = new Company();
        company.setEmployeeArray(employees);

        new ObjectXMLWriter("resources/test2.xml", company).saveObject();
    }
}

運行此代碼時出現的錯誤如下：

Exception in thread "main" java.lang.IllegalArgumentException: Can not set [Lcom.falcondev.orm.test.Employee; field com.falcondev.orm.test.Company.employeeArray to [Lcom.falcondev.orm.test.Employee;
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:146)
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:150)
    at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:37)
    at sun.reflect.UnsafeObjectFieldAccessorImpl.get(UnsafeObjectFieldAccessorImpl.java:18)
    at java.lang.reflect.Field.get(Field.java:358)
    at com.falcondev.orm.ObjectXMLWriter.createNode(ObjectXMLWriter.java:116)
    at com.falcondev.orm.ObjectXMLWriter.createDocument(ObjectXMLWriter.java:149)
    at com.falcondev.orm.ObjectXMLWriter.saveObject(ObjectXMLWriter.java:74)
    at com.falcondev.orm.test.Test.main(Test.java:37)

我意識到這是相當多的代碼發布。 我嘗試了以下示例，如本文所示（http://stackoverflow.com/questions/2200399/iterating-over-arrays-by-reflection/2200493#2200493），它工作正常，所以必須有一些微妙的區別從簡單的例子和​​我在上面的代碼中做的事情。

簡單測試有效：

public class Test
{

    public static void main(String[] args) throws Exception
    {
        Employee[] employees = new Employee[3];
        employees[0] = new Employee("Tom", "C");
        employees[1] = new Employee("Paul", "E");
        employees[2] = new Employee("George", "A");
        Company company = new Company();
        company.setEmployeeArray(employees);

        Field field = company.getClass().getField("employeeArray");
        if (field.getType().isArray()) {
          Object array = field.get(company);
          int length = Array.getLength(array);
          for (int i = 0; i < length; i++) {
            System.out.println(Array.get(array, i));
          }
        }
    }
}

至於技術信息，我使用的是這個例子：windows 7，eclipse 3.7，jdk 1.6.0_26，log4j 1.2.16，apache commons-lang3-3.0.1和google guava 10.0

過去幾周我一直試圖讓這個工作，所以任何幫助將不勝感激。

編輯：

為了將來參考和其他用途，解決問題的相關代碼如下：

boolean shouldSaveFieldValue = true;

if(... || field.getType() == String.class) {
  ..    
  else if(field.getType().isArray()) {
    Object array = field.get(object);
    ...
    shouldSaveFieldValue = false;
  }

  if(shouldSaveFieldValue) {
    ...
    Element element = document.createElement(field.getName());
    element.setTextContent(fieldValue);
    node.appendChild(element);
  }
}

Answer 1

首先，我建議使用庫而不是自己編輯庫。 我對JAXB非常幸運，這不需要第三方jar。 對於你的問題，我認為你只是在變量名稱“object”和“obj”之間感到困惑：

Company object = a Company
Field field = [ Company.Employee[] employeeArray ]
Employee[] obj = object.employeeArray

無法計算field.get（obj），因為該參數不是Company類型。 這是相關的代碼：

private Node createNode(Document document, Object object) {
    ..
    List<Field> fields = Lists.newArrayList(object.getClass().getFields());
        for(Field field: fields) {
        ...
        Object obj = field.get(object);
        ...
        else if(field.getType().isArray()) {
            Object array = field.get(obj); //Fails here

Answer 2

更改：

public Employee employeeArray[];

至：

public transient Employee employeeArray[];

據我所知，數組不是可序列化的，我認為這就是為什么你有一個員工列表和一個數組。 因此，這告訴序列化程序在序列化對象時忽略employee數組。