JSONObject转换分析

net.sf.json.JSONObject采用反射的方式，对POJO进行转换。JSONObject类实现了JSON、Map和Comparable接口，如下：

class JSONObject extends AbstractJSON implements JSON, Map, Comparable

以Person类为例分析源码：

public class Person {
    private String name;
    private int age;
    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;
    }
}

fromObject(Object object)方法：

代码实现主要在JSONObject defaultBeanProcessing(Object bean, JsonConfig jsonConfig)方法中，如下：

 private static JSONObject defaultBeanProcessing(Object bean, JsonConfig jsonConfig) {
      Class beanClass = bean.getClass(); //获取类对象 class net.sf.json.mytest.Person
      PropertyNameProcessor propertyNameProcessor = jsonConfig.findJsonPropertyNameProcessor( beanClass );      
      Collection exclusions = jsonConfig.getMergedExcludes( beanClass );//需要排除的类信息，[metaClass, declaringClass, class]
      JSONObject jsonObject = new JSONObject();
      try{
         PropertyDescriptor[] pds = PropertyUtils.getPropertyDescriptors( bean );//获取POJO类的描述信息，包括属性、属性对应的读写操作，
//[java.beans.PropertyDescriptor[name=age; propertyType=int; readMethod=public int net.sf.json.mytest.Person.getAge(); writeMethod=public void net.sf.json.mytest.Person.setAge(int)], 
//java.beans.PropertyDescriptor[name=class; propertyType=class java.lang.Class; readMethod=public final native java.lang.Class java.lang.Object.getClass()], 
//java.beans.PropertyDescriptor[name=name; propertyType=class java.lang.String; readMethod=public java.lang.String net.sf.json.mytest.Person.getName(); writeMethod=public void net.sf.json.mytest.Person.setName(java.lang.String)]]
         PropertyFilter jsonPropertyFilter = jsonConfig.getJsonPropertyFilter();
         //循环对所有属性进行处理
         for( int i = 0; i < pds.length; i++ ){
            boolean bypass = false;
            //获取属性名
            String key = pds[i].getName();
            if( exclusions.contains( key ) ){
               continue;
            }

            if( jsonConfig.isIgnoreTransientFields() && isTransientField( key, beanClass ) ){
               continue;
            }

            Class type = pds[i].getPropertyType();
            try { pds[i].getReadMethod(); }
            catch( Exception e ) {
               // bug 2565295
               String warning = "Property '" + key + "' of "+ beanClass+" has no read method. SKIPPED";
               fireWarnEvent( warning, jsonConfig );
               log.info( warning );
               continue;
            }
            if( pds[i].getReadMethod() != null ){
               Object value = PropertyUtils.getProperty( bean, key );
               if( jsonPropertyFilter != null && jsonPropertyFilter.apply( bean, key, value ) ){
                  continue;
               }
               JsonValueProcessor jsonValueProcessor = jsonConfig.findJsonValueProcessor(
                     beanClass, type, key );
               if( jsonValueProcessor != null ){
                  value = jsonValueProcessor.processObjectValue( key, value, jsonConfig );
                  bypass = true;
                  if( !JsonVerifier.isValidJsonValue( value ) ){
                     throw new JSONException( "Value is not a valid JSON value. " + value );
                  }
               }
               if( propertyNameProcessor != null ){
                  key = propertyNameProcessor.processPropertyName( beanClass, key );
               }
               //设置属性值
               setValue( jsonObject, key, value, type, jsonConfig, bypass );
            }else{
               String warning = "Property '" + key + "' of "+ beanClass+" has no read method. SKIPPED";
               fireWarnEvent( warning, jsonConfig );
               log.info( warning );
            }
         }
        .......
      }catch( JSONException jsone ){
         removeInstance( bean );
         fireErrorEvent( jsone, jsonConfig );
         throw jsone;
      }catch( Exception e ){
         removeInstance( bean );
         JSONException jsone = new JSONException( e );
         fireErrorEvent( jsone, jsonConfig );
         throw jsone;
      }
      return jsonObject;
   }

toBean(JSONObject object, Class clazz)方法

由JSONObject对象转为对应类实例，关键代码在Object toBean( JSONObject jsonObject, JsonConfig jsonConfig )中，如下：

 public static Object toBean( JSONObject jsonObject, JsonConfig jsonConfig ) {

if( jsonObject == null || jsonObject.isNullObject() ){
         return null;
      }

      Class beanClass = jsonConfig.getRootClass();//类对象
      Map classMap = jsonConfig.getClassMap();

      if( beanClass == null ){
         return toBean( jsonObject );
      }
      if( classMap == null ){
         classMap = Collections.EMPTY_MAP;
      }

      Object bean = null;
      try{
         if( beanClass.isInterface() ){
            if( !Map.class.isAssignableFrom( beanClass ) ){
               throw new JSONException( "beanClass is an interface. " + beanClass );
            }else{
               bean = new HashMap();
            }
         }else{
            bean = jsonConfig.getNewBeanInstanceStrategy() //实例化
                  .newInstance( beanClass, jsonObject );
         }
      }catch( JSONException jsone ){
         throw jsone;
      }catch( Exception e ){
         throw new JSONException( e );
      }

      Map props = JSONUtils.getProperties( jsonObject );//属性及属性类型  {name=class java.lang.String, age=class java.lang.Integer}
      PropertyFilter javaPropertyFilter = jsonConfig.getJavaPropertyFilter();
      for( Iterator entries = jsonObject.names( jsonConfig )
            .iterator(); entries.hasNext(); ){
         String name = (String) entries.next();//属性名
         Class type = (Class) props.get( name );//属性类型
         Object value = jsonObject.get( name );//属性值，如果属性值为null,返回的值为JSONNull对象
         if( javaPropertyFilter != null && javaPropertyFilter.apply( bean, name, value ) ){
            continue;
         }
         String key = Map.class.isAssignableFrom( beanClass )
               && jsonConfig.isSkipJavaIdentifierTransformationInMapKeys() ? name
               : JSONUtils.convertToJavaIdentifier( name, jsonConfig );
         PropertyNameProcessor propertyNameProcessor = jsonConfig.findJavaPropertyNameProcessor( beanClass );
         if( propertyNameProcessor != null ){
            key = propertyNameProcessor.processPropertyName( beanClass, key );
         }
         //设置实例对象的属性（处理各种情况）
         try{
            if( Map.class.isAssignableFrom( beanClass ) ){//Map对象单独处理
               // no type info available for conversion
               if( JSONUtils.isNull( value ) ){
                  setProperty( bean, key, value, jsonConfig );
               }else if( value instanceof JSONArray ){
                  setProperty( bean, key, convertPropertyValueToCollection( key, value, jsonConfig, name,
                        classMap, List.class ), jsonConfig );
               }else if( String.class.isAssignableFrom( type ) || JSONUtils.isBoolean( type )
                     || JSONUtils.isNumber( type ) || JSONUtils.isString( type )
                     || JSONFunction.class.isAssignableFrom( type ) ){
                  if( jsonConfig.isHandleJettisonEmptyElement() && "".equals( value ) ){
                     setProperty( bean, key, null, jsonConfig );
                  }else{
                     setProperty( bean, key, value, jsonConfig );
                  }
               }else{
                  Class targetClass = resolveClass(classMap, key, name, type);
                  JsonConfig jsc = jsonConfig.copy();
                  jsc.setRootClass( targetClass );
                  jsc.setClassMap( classMap );
                  if( targetClass != null ){
                     setProperty( bean, key, toBean( (JSONObject) value, jsc ), jsonConfig );
                  }else{
                     setProperty( bean, key, toBean( (JSONObject) value ), jsonConfig );
                  }
               }
            }else{
               PropertyDescriptor pd = PropertyUtils.getPropertyDescriptor( bean, key );//获取属性描述 
//java.beans.PropertyDescriptor[name=name; propertyType=class java.lang.String; readMethod=public java.lang.String net.sf.json.mytest.Person.getName(); writeMethod=public void net.sf.json.mytest.Person.setName(java.lang.String)]
               if( pd != null && pd.getWriteMethod() == null ){
                  log.info( "Property '" + key + "' of "+ bean.getClass()+" has no write method. SKIPPED." );
                  continue;
               }

               if( pd != null ){
                  Class targetType = pd.getPropertyType();
                  if( !JSONUtils.isNull( value ) ){
                     if( value instanceof JSONArray ){  //JSONArray值
                        if( List.class.isAssignableFrom( pd.getPropertyType() ) ){
                           setProperty( bean, key, convertPropertyValueToCollection( key, value,
                                 jsonConfig, name, classMap, pd.getPropertyType() ), jsonConfig );
                        }else if( Set.class.isAssignableFrom( pd.getPropertyType() ) ){
                           setProperty( bean, key, convertPropertyValueToCollection( key, value,
                                 jsonConfig, name, classMap, pd.getPropertyType() ), jsonConfig );
                        }else{
                           setProperty( bean, key, convertPropertyValueToArray( key, value,
                                 targetType, jsonConfig, classMap ), jsonConfig );
                        }
                     }else if( String.class.isAssignableFrom( type ) || JSONUtils.isBoolean( type )  //基本类型
                           || JSONUtils.isNumber( type ) || JSONUtils.isString( type )
                           || JSONFunction.class.isAssignableFrom( type ) ){
                        if( pd != null ){
                           if( jsonConfig.isHandleJettisonEmptyElement() && "".equals( value ) ){
                              setProperty( bean, key, null, jsonConfig );
                           }else if( !targetType.isInstance( value ) ){
                              setProperty( bean, key, morphPropertyValue( key, value, type,
                                    targetType ), jsonConfig );
                           }else{
                              setProperty( bean, key, value, jsonConfig );//对String进行处理
                           }
                        }else if( beanClass == null || bean instanceof Map ){
                           setProperty( bean, key, value, jsonConfig );
                        }else{
                           log.warn( "Tried to assign property " + key + ":" + type.getName()
                                 + " to bean of class " + bean.getClass()
                                       .getName() );
                        }
                     }else{
                        if( jsonConfig.isHandleJettisonSingleElementArray() ){
                           JSONArray array = new JSONArray().element( value, jsonConfig );
                           Class newTargetClass = resolveClass(classMap, key, name, type);
                           JsonConfig jsc = jsonConfig.copy();
                           jsc.setRootClass( newTargetClass );
                           jsc.setClassMap( classMap );
                           if( targetType.isArray() ){
                              setProperty( bean, key, JSONArray.toArray( array, jsc ), jsonConfig );
                           }else if( JSONArray.class.isAssignableFrom( targetType ) ){
                              setProperty( bean, key, array, jsonConfig );
                           }else if( List.class.isAssignableFrom( targetType )
                                 || Set.class.isAssignableFrom( targetType ) ){
                              jsc.setCollectionType( targetType );
                              setProperty( bean, key, JSONArray.toCollection( array, jsc ),
                                    jsonConfig );
                           }else{
                              setProperty( bean, key, toBean( (JSONObject) value, jsc ), jsonConfig );
                           }
                        }else{
                           if( targetType == Object.class || targetType.isInterface() ) {
                              Class targetTypeCopy = targetType;
                              targetType = findTargetClass( key, classMap );
                              targetType = targetType == null ? findTargetClass( name, classMap )
                                    : targetType;
                              targetType = targetType == null && targetTypeCopy.isInterface() ? targetTypeCopy
                                    : targetType;
                           }
                           JsonConfig jsc = jsonConfig.copy();
                           jsc.setRootClass( targetType );
                           jsc.setClassMap( classMap );
                           setProperty( bean, key, toBean( (JSONObject) value, jsc ), jsonConfig );
                        }
                     }
                  }else{
                     if( type.isPrimitive() ){
                        // assume assigned default value
                        log.warn( "Tried to assign null value to " + key + ":" + type.getName() );
                        setProperty( bean, key, JSONUtils.getMorpherRegistry()
                              .morph( type, null ), jsonConfig );
                     }else{
                        setProperty( bean, key, null, jsonConfig );
                     }
                  }
               }else{
                  // pd is null
                  if( !JSONUtils.isNull( value ) ){
                     if( value instanceof JSONArray ){
                        setProperty( bean, key, convertPropertyValueToCollection( key, value,
                              jsonConfig, name, classMap, List.class ), jsonConfig );
                     }else if( String.class.isAssignableFrom( type ) || JSONUtils.isBoolean( type )
                           || JSONUtils.isNumber( type ) || JSONUtils.isString( type )
                           || JSONFunction.class.isAssignableFrom( type ) ){
                        if( beanClass == null || bean instanceof Map || jsonConfig.getPropertySetStrategy() != null || 
                            !jsonConfig.isIgnorePublicFields() ){
                           setProperty( bean, key, value, jsonConfig );
                        }else{
                           log.warn( "Tried to assign property " + key + ":" + type.getName()
                                 + " to bean of class " + bean.getClass()
                                       .getName() );
                        }
                     }else{
                        if( jsonConfig.isHandleJettisonSingleElementArray() ){
                           Class newTargetClass = resolveClass(classMap, key, name, type);
                           JsonConfig jsc = jsonConfig.copy();
                           jsc.setRootClass( newTargetClass );
                           jsc.setClassMap( classMap );
                           setProperty( bean, key, toBean( (JSONObject) value, jsc ), jsonConfig );
                        }else{
                           setProperty( bean, key, value, jsonConfig );
                        }
                     }
                  }else{
                     if( type.isPrimitive() ){
                        // assume assigned default value
                        log.warn( "Tried to assign null value to " + key + ":" + type.getName() );
                        setProperty( bean, key, JSONUtils.getMorpherRegistry()
                              .morph( type, null ), jsonConfig );
                     }else{
                        setProperty( bean, key, null, jsonConfig );
                     }
                  }
               }
            }
         }catch( JSONException jsone ){
            throw jsone;
         }catch( Exception e ){
            throw new JSONException( "Error while setting property=" + name + " type " + type, e );
         }
      }

      return bean;
   }

 另外，Map转JSONObject问题：

会将Map结构（{key=value}）转换为json格式（“key”:"value"）。

（1）java中的null，会转换为JSONNull对象，不能直接将该对象转为String。

（2）net.sf.json.JSONObject不支持Map中key为null的情况。

举例如下：

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

import net.sf.json.JSONNull;
import net.sf.json.JSONObject;

public class Test {

    public static void main(String[] args) {
        Map<String, String> data1 = new HashMap<>();
//        data1.put(null, "empty");  //报JSON keys cannot be null.
        data1.put("ts", null);
        System.out.println(data1.toString());
        if(data1.get("ts")==null){
            System.out.println("Map中的null");
        }
        
        JSONObject jsonObject = JSONObject.fromObject(data1);
        System.out.println(jsonObject.toString());
        
        if(JSONNull.getInstance().equals(jsonObject.get("ts"))){
            System.out.println("Map中的null，转为net.sf.json.JSONObject后，变成net.sf.json.JSONNull");
        }
        Map<String, Object> map = (Map<String, Object>) JSONObject.toBean(jsonObject, HashMap.class);
        System.out.println(map.get("ts").toString());
        
//        Map<String, String> map = (Map<String, String>) JSONObject.toBean(jsonObject, HashMap.class);
//        System.out.println(map.get("ts").toString()); //java.lang.ClassCastException: net.sf.json.JSONNull cannot be cast to java.lang.String
    }

}

  结果：

{ts=null}
Map中的null
{"ts":null}
Map中的null，转为net.sf.json.JSONObject后，变成net.sf.json.JSONNull