json格式化错误

    前一片我简单说了json格式数据与对象直接的转换。
    但是有时候你会发现，出问题了，数据格式化不了，还报了错误，怎么回事？
    以下是我出现的错误：
   

Exception in thread "main" net.sf.json.JSONException: There is a cycle in the hierarchy!
	at net.sf.json.util.CycleDetectionStrategy$StrictCycleDetectionStrategy.handleRepeatedReferenceAsObject(CycleDetectionStrategy.java:97)
	at net.sf.json.JSONObject._fromBean(JSONObject.java:833)
	at net.sf.json.JSONObject.fromObject(JSONObject.java:168)
	at net.sf.json.AbstractJSON._processValue(AbstractJSON.java:265)
	at net.sf.json.JSONObject._processValue(JSONObject.java:2808)
	at net.sf.json.JSONObject.processValue(JSONObject.java:2874)
	at net.sf.json.JSONObject.setInternal(JSONObject.java:2889)
	at net.sf.json.JSONObject.setValue(JSONObject.java:1577)
	at net.sf.json.JSONObject._fromBean(JSONObject.java:934)
	at net.sf.json.JSONObject.fromObject(JSONObject.java:168)
	at net.sf.json.AbstractJSON._processValue(AbstractJSON.java:265)
	at net.sf.json.JSONArray._processValue(JSONArray.java:2514)
	at net.sf.json.JSONArray.processValue(JSONArray.java:2539)
	at net.sf.json.JSONArray.addValue(JSONArray.java:2526)
	at net.sf.json.JSONArray._fromCollection(JSONArray.java:1057)
	at net.sf.json.JSONArray.fromObject(JSONArray.java:123)
	at net.sf.json.AbstractJSON._processValue(AbstractJSON.java:237)
	at net.sf.json.JSONObject._processValue(JSONObject.java:2808)
	at net.sf.json.JSONObject.processValue(JSONObject.java:2874)
	at net.sf.json.JSONObject.setInternal(JSONObject.java:2889)
	at net.sf.json.JSONObject.setValue(JSONObject.java:1577)
	at net.sf.json.JSONObject._fromBean(JSONObject.java:934)
	at net.sf.json.JSONObject.fromObject(JSONObject.java:168)
	at net.sf.json.JSONObject.fromObject(JSONObject.java:130)
	at test.Test2.main(Test2.java:28)

    一开始的时候我也疑惑是怎么回事，之前的都可以正常格式化，为什么这样不行，后来在别人的指导下检查了代码，发现因为我使用了自身关联（我这样叫他）。
    代码如下：
   

package test;

import java.util.List;

public class ClassS {
	//id
	private int id ;
	//班级名称
	private String name;
	//拥有的学生
	private List<Student> students;
	
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public List<Student> getStudents() {
		return students;
	}
	public void setStudents(List<Student> students) {
		this.students = students;
	}
	public ClassS(int id, String name, List<Student> students) {
		super();
		this.id = id;
		this.name = name;
		this.students = students;
	} 
	
	
}

package test;

public class Student {
	//id
	private int id ;
	//名称
	private String name;
	//年龄
	private int age;
	//性别
	private String gender;
	//住址
	private String address;
	//身份证id
	private String cdCard;
	//所属班级
	private ClassS classs;
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	public String getGender() {
		return gender;
	}
	public void setGender(String gender) {
		this.gender = gender;
	}
	public String getAddress() {
		return address;
	}
	public void setAddress(String address) {
		this.address = address;
	}
	public String getCdCard() {
		return cdCard;
	}
	public void setCdCard(String cdCard) {
		this.cdCard = cdCard;
	}
	public ClassS getClasss() {
		return classs;
	}
	public void setClasss(ClassS classs) {
		this.classs = classs;
	}
	public Student(int id, String name, int age, String gender, String address,
			String cdCard) {
		super();
		this.id = id;
		this.name = name;
		this.age = age;
		this.gender = gender;
		this.address = address;
		this.cdCard = cdCard;
	}
}

 public static void main( String[] args ){
    
    	Student stu1 = new Student(1, "goodboy",20,"boy","我的老家在东北，家住松花江上啊", "123456789123456789");
    	Student stu2 = new Student(2, "goodboy",20,"boy","我的老家在东北，家住松花江上啊", "123456789123456789");
    	List<Student> stus = new ArrayList<Student>();
    	
    	stus.add(stu1);
    	stus.add(stu2);
    	ClassS classs = new ClassS(1,"class1",stus);
    	stu1.setClasss(classs);
    	stu2.setClasss(classs);
    	System.out.println(JSONObject.fromObject(classs));
    }

    从上面看出代码都是正确的，但是你会发现在ClassS与Student是一对多的关系，及一个班级对应了多个学生，而在Student中又对应了所属的班级：
  

//所属班级
	private ClassS classs;

   结果json在格式化ClassS 这个对象的时候在进行格式化其内部的对象时候，由于json是深度递归进行格式化的又回到了其自身，结果就成了一个死循环，问题就出现了。既然有问题肯定要解决，怎么解决呢？我的方法是过滤掉其自身的关联，json的JSONObject和JSONArray都提供了这个过滤的机制：
 

JSONObject.fromObject(object, jsonConfig)
JSONArray.fromObject(object, jsonConfig)

所以我自己写了一个过滤的类：

package test;


import net.sf.json.JsonConfig;
import net.sf.json.util.PropertyFilter;
/**
 *
 * 功能描述：过滤掉格式化数据为json格式的时候的自身关联
 * 
 */
public class FiltrateUtil {

	public static JsonConfig filtrate(final String[] str) {
		JsonConfig config = new JsonConfig();
		config.setJsonPropertyFilter(new PropertyFilter() {
			public boolean apply(Object source, String name, Object value) {
				if(str!=null&&str.length>0){
					boolean flag = false;
					for(int i = 0;i<str.length;i++){
						if (name.equals(str)) {
							flag = true;
							break;
						}
					}
					return flag;
				}else{
					return false;
				}
			}
		});
		return config;
	}
}

    然后在把测试的代码修改下，如下：
   

System.out.println(JSONObject.fromObject(classs,FiltrateUtil.filtrate(new String[]{"classs"})));

   我想要的json字符就正常的被打印出来了。

  

{"id":1,"name":"class1","students":[{"address":"我的老家在东北，家住松花江上啊","age":20,"cdCard":"123456789123456789","gender":"boy","id":1,"name":"goodboy"},{"address":"我的老家在东北，家住松花江上啊","age":20,"cdCard":"123456789123456789","gender":"boy","id":2,"name":"goodboy"}]}