spring--9--类型转换PropertyEditor

浅浅的花香味﹌ 2023-01-05 05:10 246阅读 0赞

# spring–类型转换`PropertyEditor` #

### 文章目录 ###

*  spring--类型转换\`PropertyEditor\`
 *  1 一个小例子
 *  2 原理
 *   *  2.1 \`TypeConverterDelegate \`的构造方法
     *  2.2 \`convertIfNecessary(@Nullable String propertyName, @Nullable Object oldValue, @Nullable Object newValue,@Nullable Class requiredType, @Nullable TypeDescriptor typeDescriptor)\`方法，类型转换
     *   *  2.2.1 根据要求类型\`requiredType\`获取默认的\`PropertyEditor\`
         *  2.2.2 进行\`String->requiredType\`的类型转换
         *  2.2.3 \`Map\`集合的类型转换
         *  2.2.4 通过构造方法进行类型转换
 *  3 \`PropertyEditorRegistrySupport\`
 *   *  3.1 \`CustomEditorHolder\`
     *  3.2 \`registerCustomEditor(Class<?> requiredType, PropertyEditor propertyEditor)\`方法，注册\`String->requiredType\`的\`PropertyEditor\`
     *  3.3\`registerCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath, PropertyEditor propertyEditor)\`方法，注册指定属性的\`PropertyEditor\`
     *  3.4 \`findCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath)\`方法， 获取指定条件的用户定义的\`PropertyEditor\`
     *   *  3.4.1 根据属性名获取\`PropertyEditor\`
         *  3.4.2 根据要求类型获取\`PropertyEditor\`
     *  3.5 \`getDefaultEditor(Class<?> requiredType)\`方法，根据要求类型获取默认的\`PropertyEditor\`
     *  3.6 \`hasCustomEditorForElement(@Nullable Class<?> elementType, @Nullable String propertyPath)\`方法，判断是否为\`propertyName\`指定了\`PropertyEditor\`（属性专用的\`PropertyEditor\`）

`PropertyEditor`位于 `JDK`的 `java.beans`包下，是 `JDK`自带的类型转换接口，其设计目的主要为 `GUI`提供支持，它可以将 `GUI`界面上输入的 `String`类型转化为我们需要的类型。

# 1 一个小例子 #

<dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>1.18.16</version>
    </dependency>
    
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-context</artifactId>
        <version>5.2.11.RELEASE</version>
    </dependency>

然后提供两个类`User`、`Role`

@Data
    @ToString
    public class User { 
    
        private String name;
        private String username;
        private String password;
        //User中有Role对象
        private Role role;
    }
    
    @Data
    @ToString
    public class Role { 
        private Integer id;
        private String rolename;
        private String description;
    }

接下来我们再编写一个属性编辑器`PropertyEditor`，重写里面的`setAsText`方法。

继承`PropertyEditorSupport`类的原因是`PropertyEditor`中有很多方法，而且那些方法都没有默认实现，如果我们实现它，那么就需要实现所有的方法。

public class StringToRolePropertyEditor extends PropertyEditorSupport { 
    
        /** * Sets the property value by parsing a given String. May raise * java.lang.IllegalArgumentException if either the String is * badly formatted or if this kind of property can't be expressed * as text. * * @param text The string to be parsed. * 我们需要将"12,lx,中国"格式字符串转化为一个Role对象 */
        @Override
        public void setAsText(String text) throws IllegalArgumentException { 
            String[] split = text.split(",");
            Role role = new Role();
            role.setId(Integer.valueOf(split[0]));
            role.setRolename(split[1]);
            role.setDescription(split[2]);
            setValue(role);
        }
    }

对应的`spring`配置文件如下

<?xml version="1.0" encoding="UTF-8"?>
    <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
    
        
        <bean id="user" class="com.lx.converter.domain.User">
            <property name="role" value="12,lx,中国"></property>
        </bean>
    
        
        <bean id="customEditorConfigurer" class="org.springframework.beans.factory.config.CustomEditorConfigurer">
            
            <property name="customEditors">
                <map>
                    <entry key="com.lx.converter.domain.Role" value="com.lx.converter.propertyEditor.StringToRolePropertyEditor"></entry>
                </map>
            </property>
        </bean>
    
    </beans>

最后编写一个启动类，测试

public class Main { 
        public static void main(String[] args) { 
    
            //声明一个可以读取xml配置文件的容器
            ClassPathXmlApplicationContext classPathXmlApplicationContext = new
                    ClassPathXmlApplicationContext();
    
            //设置配置文件
            classPathXmlApplicationContext.setConfigLocation("spring-context.xml");
    
            //刷新容器,这是spring中最重要的方法
            classPathXmlApplicationContext.refresh();
    
            User user = (User) classPathXmlApplicationContext.getBean("user");
    
            System.out.println(user);
    
        }
    }

控制台打印如下所示

User(name=null, username=null, password=null, role=Role(id=12, rolename=lx, description=中国))

自动的将`"12,lx,中国"`转化为了`Role`对象

# 2 原理 #

> 从上篇文章中我们已经知道了类型转换工作被委托给`TypeConverterDelegate`类的`convertIfNecessary`方法，下面我们就来详细的看一下该方法的源码，看看它是怎么调用`PropertyEditor`将`"12,lx,中国"`转化为了`Role`对象的

## 2.1 `TypeConverterDelegate`的构造方法 ##

> `TypeConverterDelegate`是`TypeConverterSupport`类（`BeanWrapperImpl`是它的子类）的一个字段类型，`spring`创建`BeanWrapperImpl`对象的时候会自动使用下面这个构造方法初始化`TypeConverterDelegate`

//PropertyEditor注册中心
    private final PropertyEditorRegistrySupport propertyEditorRegistry;
    
    //BeanWrapperImpl包装的对象
    @Nullable
    private final Object targetObject;
    
    
    /** * Create a new TypeConverterDelegate for the given editor registry and bean instance. * @param propertyEditorRegistry the editor registry to use * @param targetObject the target object to work on (as context that can be passed to editors) */
    public TypeConverterDelegate(PropertyEditorRegistrySupport propertyEditorRegistry, @Nullable Object targetObject) { 
        //实际上就是BeanWrapperImpl
        this.propertyEditorRegistry = propertyEditorRegistry;
        //BeanWrapperImpl包装的对象
        this.targetObject = targetObject;
    }

## 2.2 `convertIfNecessary(@Nullable String propertyName, @Nullable Object oldValue, @Nullable Object newValue,@Nullable Class<T> requiredType, @Nullable TypeDescriptor typeDescriptor)`方法，类型转换 ##

/** * Convert the value to the required type (if necessary from a String), * for the specified property. * @param propertyName name of the property * @param oldValue the previous value, if available (may be {@code null}) * @param newValue the proposed new value * @param requiredType the type we must convert to * (or {@code null} if not known, for example in case of a collection element) * @param typeDescriptor the descriptor for the target property or field * @return the new value, possibly the result of type conversion * @throws IllegalArgumentException if type conversion failed */
    @SuppressWarnings("unchecked")
    @Nullable
    public <T> T convertIfNecessary(@Nullable String propertyName, @Nullable Object oldValue, @Nullable Object newValue,
                                    @Nullable Class<T> requiredType, @Nullable TypeDescriptor typeDescriptor) throws IllegalArgumentException { 
    
        // Custom editor for this type?
        //从用户自定义的PropertyEditor中找到可以将String->requiredType的PropertyEditor,见3.4
        PropertyEditor editor = this.propertyEditorRegistry.findCustomEditor(requiredType, propertyName);
    
        ConversionFailedException conversionAttemptEx = null;
    
        /*****************************使用conversionService转换*****************************/
        // No custom editor but custom ConversionService specified?
        //获取统一转换服务
        ConversionService conversionService = this.propertyEditorRegistry.getConversionService();
        /** * 只有从用户自定义的PropertyEditor找不到合适的 * 才会尝试使用ConversionService进行类型转换 */
        if (editor == null && conversionService != null && newValue != null && typeDescriptor != null) { 
            //得到待转换值的属性描述
            TypeDescriptor sourceTypeDesc = TypeDescriptor.forObject(newValue);
            /** * 判断统一转换服务能否实现sourceTypeDesc->typeDescriptor的转换 * sourceTypeDesc：待转换值的类型描述 * typeDescriptor：要求类型的类型描述 */
            if (conversionService.canConvert(sourceTypeDesc, typeDescriptor)) { 
                try { 
                    return (T) conversionService.convert(newValue, sourceTypeDesc, typeDescriptor);
                }
                catch (ConversionFailedException ex) { 
                    // fallback to default conversion logic below
                    conversionAttemptEx = ex;
                }
            }
        }
    
        Object convertedValue = newValue;
    
        /********************************使用PropertyEditor转换*****************************/
        // Value not of required type?
        /** * 此处两种情况进入类型转换过程 * 1.用户自定义了String->requiredType转换的PropertyEditor * 2.requiredType和现有的值的类型不匹配 */
        if (editor != null || (requiredType != null && !ClassUtils.isAssignableValue(requiredType, convertedValue))) { 
            //String->Collection
            if (typeDescriptor != null && requiredType != null && Collection.class.isAssignableFrom(requiredType) &&
                convertedValue instanceof String) { 
                //获取类型的元素类型（array，collection，stream中存的元素类型）
                TypeDescriptor elementTypeDesc = typeDescriptor.getElementTypeDescriptor();
                if (elementTypeDesc != null) { 
                    Class<?> elementType = elementTypeDesc.getType();
                    //元素类型为clazz或枚举
                    if (Class.class == elementType || Enum.class.isAssignableFrom(elementType)) { 
                        //将逗号分隔的字符串转化为字符串数组
                        convertedValue = StringUtils.commaDelimitedListToStringArray((String) convertedValue);
                    }
                }
            }
            if (editor == null) { 
                /** * 从默认的PropertyEditor中找到可以将String->requiredType转换的PropertyEditor， * 见2.2.1 */
                editor = findDefaultEditor(requiredType);
            }
            //使用PropertyEditor转换String->requiredType，见2.2.2
            convertedValue = doConvertValue(oldValue, convertedValue, requiredType, editor);
        }
        /*******************************************************************************/
    
        boolean standardConversion = false;
    
        if (requiredType != null) { 
    
            /******************************标准类型转换**************************/
            // Try to apply some standard type conversion rules if appropriate.
            if (convertedValue != null) { 
                //属性是Object类型，则无需类型转换
                if (Object.class == requiredType) { 
                    return (T) convertedValue;
                }
                //属性是数组类型
                else if (requiredType.isArray()) { 
                    // Array required -> apply appropriate conversion of elements.
                    if (convertedValue instanceof String && Enum.class.isAssignableFrom(requiredType.getComponentType())) { 
                        convertedValue = StringUtils.commaDelimitedListToStringArray((String) convertedValue);
                    }
                    return (T) convertToTypedArray(convertedValue, propertyName, requiredType.getComponentType());
                }
                //属性是集合类型
                else if (convertedValue instanceof Collection) { 
                    // Convert elements to target type, if determined.
                    convertedValue = convertToTypedCollection(
                        (Collection<?>) convertedValue, propertyName, requiredType, typeDescriptor);
                    standardConversion = true;
                }
                /** * 属性是Map集合类型，此处只会转化Map集合key和value的类型， * 并不是将别的集合转为Map,见2.2.3 */
                else if (convertedValue instanceof Map) { 
                    // Convert keys and values to respective target type, if determined.
                    convertedValue = convertToTypedMap(
                        (Map<?, ?>) convertedValue, propertyName, requiredType, typeDescriptor);
                    standardConversion = true;
                }
                //属性为非数组类型，但填充值为数组类型，那么就取数组的第一个值进行类型转换之后填充
                if (convertedValue.getClass().isArray() && Array.getLength(convertedValue) == 1) { 
                    convertedValue = Array.get(convertedValue, 0);
                    standardConversion = true;
                }
                /** * isPrimitiveOrWrapper()判断这个类是不是一个原始类型 * （ boolean, byte, char, short, int, long, float, or double，void） * 或者包装类型 * （Boolean, Byte, Character, Short, Integer, Long, Float,Double, or Void） * 这个分支处理原始或包装类型->String类型的转换 */
                if (String.class == requiredType && ClassUtils.isPrimitiveOrWrapper(convertedValue.getClass())) { 
                    // We can stringify any primitive value...
                    return (T) convertedValue.toString();
                }
                /** * 通过构造方法进行类型转换,见2.2.4 * 直接获取转换类型类的单String参数的构造器，传入需要转换的值 * 用户在这个构造器中编写转换逻辑，spring通过这个构造器构造转换成功的对象 * 这种方式只能实现String->requiredType的转换 */
                else if (convertedValue instanceof String && !requiredType.isInstance(convertedValue)) { 
                    //requiredType不是接口，不是枚举
                    if (conversionAttemptEx == null && !requiredType.isInterface() && !requiredType.isEnum()) { 
                        try { 
                            //得到单String参数的构造器
                            Constructor<T> strCtor = requiredType.getConstructor(String.class);
                            /** * 使用这个构造器实例化一个requiredType对象 * 如此也实现了String->requiredType的转换 */
                            return BeanUtils.instantiateClass(strCtor, convertedValue);
                        }
                        catch (NoSuchMethodException ex) { 
                            // proceed with field lookup
                            if (logger.isTraceEnabled()) { 
                                logger.trace("No String constructor found on type [" + requiredType.getName() + "]", ex);
                            }
                        }
                        catch (Exception ex) { 
                            if (logger.isDebugEnabled()) { 
                                logger.debug("Construction via String failed for type [" + requiredType.getName() + "]", ex);
                            }
                        }
                    }
                    //requiredType是枚举,String->enum
                    String trimmedValue = ((String) convertedValue).trim();
                    if (requiredType.isEnum() && trimmedValue.isEmpty()) { 
                        // It's an empty enum identifier: reset the enum value to null.
                        return null;
                    }
                    convertedValue = attemptToConvertStringToEnum(requiredType, trimmedValue, convertedValue);
                    standardConversion = true;
                }
                //Number->Number
                else if (convertedValue instanceof Number && Number.class.isAssignableFrom(requiredType)) { 
                    convertedValue = NumberUtils.convertNumberToTargetClass(
                        (Number) convertedValue, (Class<Number>) requiredType);
                    standardConversion = true;
                }
            }
            //待转换的值为null,此时创建一个空容器代表已转化的值
            else { 
                // convertedValue == null
                if (requiredType == Optional.class) { 
                    convertedValue = Optional.empty();
                }
            }
    
            /***********************未转换成功才会运行下面代码**************************/
            //convertedValue是不是requiredType类型
            if (!ClassUtils.isAssignableValue(requiredType, convertedValue)) { 
                //抛出前面转换过程产生的异常
                if (conversionAttemptEx != null) { 
                    // Original exception from former ConversionService call above...
                    throw conversionAttemptEx;
                }
                //用户自定义的PropertyEditor未转换成功，此时使用conversionService进行转换
                else if (conversionService != null && typeDescriptor != null) { 
                    // ConversionService not tried before, probably custom editor found
                    // but editor couldn't produce the required type...
                    //得到待转换值的类型描述
                    TypeDescriptor sourceTypeDesc = TypeDescriptor.forObject(newValue);
                    /** * 判断统一转换服务能否实现sourceTypeDesc->typeDescriptor的转换 * sourceTypeDesc：待转换值的类型描述 * typeDescriptor：要求类型的类型描述 */
                    if (conversionService.canConvert(sourceTypeDesc, typeDescriptor)) { 
                        //统一转换服务转换类型
                        return (T) conversionService.convert(newValue, sourceTypeDesc, typeDescriptor);
                    }
                }
                //后面都是异常相关处理
                // Definitely doesn't match: throw IllegalArgumentException/IllegalStateException
                StringBuilder msg = new StringBuilder();
                msg.append("Cannot convert value of type '").append(ClassUtils.getDescriptiveType(newValue));
                msg.append("' to required type '").append(ClassUtils.getQualifiedName(requiredType)).append("'");
                if (propertyName != null) { 
                    msg.append(" for property '").append(propertyName).append("'");
                }
                if (editor != null) { 
                    msg.append(": PropertyEditor [").append(editor.getClass().getName()).append(
                        "] returned inappropriate value of type '").append(
                        ClassUtils.getDescriptiveType(convertedValue)).append("'");
                    throw new IllegalArgumentException(msg.toString());
                }
                else { 
                    msg.append(": no matching editors or conversion strategy found");
                    throw new IllegalStateException(msg.toString());
                }
            }
        }
    
        //抛出前面转换过程产生的异常
        if (conversionAttemptEx != null) { 
            if (editor == null && !standardConversion && requiredType != null && Object.class != requiredType) { 
                throw conversionAttemptEx;
            }
            logger.debug("Original ConversionService attempt failed - ignored since " +
                         "PropertyEditor based conversion eventually succeeded", conversionAttemptEx);
        }
    
        //返回转换成功的值
        return (T) convertedValue;
    }

### 2.2.1 根据要求类型`requiredType`获取默认的`PropertyEditor` ###

/** * Find a default editor for the given type. * @param requiredType the type to find an editor for * @return the corresponding editor, or {@code null} if none */
    @Nullable
    private PropertyEditor findDefaultEditor(@Nullable Class<?> requiredType) { 
        PropertyEditor editor = null;
        if (requiredType != null) { 
            // No custom editor -> check BeanWrapperImpl's default editors.
            //根据类型获取BeanWrapperImpl中默认的PropertyEditor，见3.5
            editor = this.propertyEditorRegistry.getDefaultEditor(requiredType);
            if (editor == null && String.class != requiredType) { 
                // No BeanWrapper default editor -> check standard JavaBean editor.
                /** * 获取后缀约定的PropertyEditor * 什么叫后缀约定，举个例子，现在要将String->Role,那么我们可以直接将这个PropertyEditor * 命名为RoleEditor，不需要自己注册到容器中，容器会自动根据约定获取到这个PropertyEditor * 在这个方法里面会自动的将requiredType拼接上Editor， * 然后反射实例化得到这个PropertyEditor，要求它们在同一个包下 */
                editor = BeanUtils.findEditorByConvention(requiredType);
            }
        }
        return editor;
    }

### 2.2.2 进行`String->requiredType`的类型转换 ###

/** * Convert the value to the required type (if necessary from a String), * using the given property editor. * @param oldValue the previous value, if available (may be {@code null}) * @param newValue the proposed new value * @param requiredType the type we must convert to * (or {@code null} if not known, for example in case of a collection element) * @param editor the PropertyEditor to use * @return the new value, possibly the result of type conversion * @throws IllegalArgumentException if type conversion failed */
    @Nullable
    private Object doConvertValue(@Nullable Object oldValue, @Nullable Object newValue,
                                  @Nullable Class<?> requiredType, @Nullable PropertyEditor editor) { 
    
        Object convertedValue = newValue;
    
        /** * 非String类型->requiredType * 调用PropertyEditor的setValue方法和getValue方法 * 也就是说我们可以重写这两个方法对这个非String类型干点啥 */
        if (editor != null && !(convertedValue instanceof String)) { 
            // Not a String -> use PropertyEditor's setValue.
            // With standard PropertyEditors, this will return the very same object;
            // we just want to allow special PropertyEditors to override setValue
            // for type conversion from non-String values to the required type.
            try { 
                editor.setValue(convertedValue);
                Object newConvertedValue = editor.getValue();
                if (newConvertedValue != convertedValue) { 
                    convertedValue = newConvertedValue;
                    // Reset PropertyEditor: It already did a proper conversion.
                    // Don't use it again for a setAsText call.
                    editor = null;
                }
            }
            catch (Exception ex) { 
                if (logger.isDebugEnabled()) { 
                    logger.debug("PropertyEditor [" + editor.getClass().getName() + "] does not support setValue call", ex);
                }
                // Swallow and proceed.
            }
        }
    
        Object returnValue = convertedValue;
    
        /** * String数组类型->requiredType（String类型） * 仅适用于之前未经PropertyEditor转换的数组 */
        if (requiredType != null && !requiredType.isArray() && convertedValue instanceof String[]) { 
            // Convert String array to a comma-separated String.
            // Only applies if no PropertyEditor converted the String array before.
            // The CSV String will be passed into a PropertyEditor's setAsText method, if any.
            if (logger.isTraceEnabled()) { 
                logger.trace("Converting String array to comma-delimited String [" + convertedValue + "]");
            }
            //字符串数组转化为逗号分隔的字符串
            convertedValue = StringUtils.arrayToCommaDelimitedString((String[]) convertedValue);
        }
    
        /** * String类型->requiredType * 使用PropertyEditor的setAsText方法进行类型转换 */
        if (convertedValue instanceof String) { 
            if (editor != null) { 
                // Use PropertyEditor's setAsText in case of a String value.
                if (logger.isTraceEnabled()) { 
                    logger.trace("Converting String to [" + requiredType + "] using property editor [" + editor + "]");
                }
                String newTextValue = (String) convertedValue;
                //调用setAsText方法进行类型转换
                return doConvertTextValue(oldValue, newTextValue, editor);
            }
            //要求类型就是String，无需转换，直接使用
            else if (String.class == requiredType) { 
                returnValue = convertedValue;
            }
        }
    
        return returnValue;
    }
    
    
    /** * Convert the given text value using the given property editor. * @param oldValue the previous value, if available (may be {@code null}) * @param newTextValue the proposed text value * @param editor the PropertyEditor to use * @return the converted value */
    private Object doConvertTextValue(@Nullable Object oldValue, String newTextValue, PropertyEditor editor) { 
        try { 
            /** * 先保存旧值，旧值是用属性的getter方法获取的 * 相当于属性的默认值 * newTextValue则是用户在xml中定义的经过BeanDefinitionValueResolver初步处理的值 */
            editor.setValue(oldValue);
        }
        catch (Exception ex) { 
            if (logger.isDebugEnabled()) { 
                logger.debug("PropertyEditor [" + editor.getClass().getName() + "] does not support setValue call", ex);
            }
            // Swallow and proceed.
        }
        //最终在此处调用了setAsText()方法进行类型转换
        editor.setAsText(newTextValue);
        //调用getValue()方法将转换成功的值取出来
        return editor.getValue();
    }

> `spring`底层使用`PropertyEditor`的`setAsText()`方法进行类型转换，然后再调用`getValue()`方法将转换成功的值取出。

### 2.2.3 `Map`集合的类型转换 ###

> 此种情况针对用户在`xml`中配置的`<map>`标签处理

private Map<?, ?> convertToTypedMap(Map<?, ?> original, @Nullable String propertyName,
                                        Class<?> requiredType, @Nullable TypeDescriptor typeDescriptor) { 
    
        //要转换的类型非Map集合，跳过转换，因为此处只能进行key和value的转换
        if (!Map.class.isAssignableFrom(requiredType)) { 
            return original;
        }
    
        /** * 判断类型requiredType的类型是不是下面7种之一 * Map、SortedMap、NavigableMap、HashMap、LinkedHashMap、TreeMap、EnumMap */
        boolean approximable = CollectionFactory.isApproximableMapType(requiredType);
        if (!approximable && !canCreateCopy(requiredType)) { 
            if (logger.isDebugEnabled()) { 
                logger.debug("Custom Map type [" + original.getClass().getName() +
                             "] does not allow for creating a copy - injecting original Map as-is");
            }
            return original;
        }
    
        //判断经过BeanDefinitionValueResolver初步解析之后的值是不是requiredType的实例
        boolean originalAllowed = requiredType.isInstance(original);
        //获取Map集合key的TypeDescriptor，未指定就返回null
        TypeDescriptor keyType = (typeDescriptor != null ? typeDescriptor.getMapKeyTypeDescriptor() : null);
        //获取Map集合value的TypeDescriptor，未指定就返回null
        TypeDescriptor valueType = (typeDescriptor != null ? typeDescriptor.getMapValueTypeDescriptor() : null);
        if (keyType == null && valueType == null && originalAllowed &&
            /** * 判断是否为propertyName指定了PropertyEditor，此时只根据属性名匹配， * 因为该方法第一个参数为null，见3.6 */
            !this.propertyEditorRegistry.hasCustomEditorForElement(null, propertyName)) { 
            return original;
        }
    
        Iterator<?> it;
        try { 
            it = original.entrySet().iterator();
        }
        catch (Throwable ex) { 
            if (logger.isDebugEnabled()) { 
                logger.debug("Cannot access Map of type [" + original.getClass().getName() +
                             "] - injecting original Map as-is: " + ex);
            }
            return original;
        }
    
        //用来存放经过类型转换的键值对的集合
        Map<Object, Object> convertedCopy;
        try { 
            if (approximable) { 
                /** * 如果original的类型为EnumMap，那么就创建一个空的和original大小相同的EnumMap * 如果original的类型为SortedMap，那么就创建一个空的和original大小相同的SortedMap * 如果original是其他类型的Map，那么就创建一个空的和original大小相同的LinkedHashMap */
                convertedCopy = CollectionFactory.createApproximateMap(original, original.size());
            }
            else { 
                //反射创建一个requiredType类型的map集合
                convertedCopy = (Map<Object, Object>)
                    ReflectionUtils.accessibleConstructor(requiredType).newInstance();
            }
        }
        catch (Throwable ex) { 
            if (logger.isDebugEnabled()) { 
                logger.debug("Cannot create copy of Map type [" + original.getClass().getName() +
                             "] - injecting original Map as-is: " + ex);
            }
            return original;
        }
    
        /**********************迭代器遍历map，转换每一个key、value*************************/
        while (it.hasNext()) { 
            Map.Entry<?, ?> entry = (Map.Entry<?, ?>) it.next();
            Object key = entry.getKey();
            Object value = entry.getValue();
            //生成键属性名
            String keyedPropertyName = buildKeyedPropertyName(propertyName, key);
            
            //递归调用convertIfNecessary()方法转换key和value
            Object convertedKey = convertIfNecessary(keyedPropertyName, null, key,
                                                     (keyType != null ? keyType.getType() : null), keyType);
            Object convertedValue = convertIfNecessary(keyedPropertyName, null, value,
                                                       (valueType!= null ? valueType.getType() : null), valueType);
            
            try { 
                //转换成功的值放入convertedCopy集合中
                convertedCopy.put(convertedKey, convertedValue);
            }
            catch (Throwable ex) { 
                if (logger.isDebugEnabled()) { 
                    logger.debug("Map type [" + original.getClass().getName() +
                                 "] seems to be read-only - injecting original Map as-is: " + ex);
                }
                return original;
            }
            //转换前和转换后的值相同为true，表示original可以直接使用，否则使用转换的值
            originalAllowed = originalAllowed && (key == convertedKey) && (value == convertedValue);
        }
        return (originalAllowed ? original : convertedCopy);
    }

生成键属性名(类似`customEditors[com.lx.converter.domain.Role]`)

private String buildKeyedPropertyName(@Nullable String propertyName, Object key) { 
        //PROPERTY_KEY_PREFIX="["
        //PROPERTY_KEY_SUFFIX="]"
        return (propertyName != null ?
                propertyName + PropertyAccessor.PROPERTY_KEY_PREFIX + key + PropertyAccessor.PROPERTY_KEY_SUFFIX :
                null);
    }

### 2.2.4 通过构造方法进行类型转换 ###

//获取String参数的构造器
    Constructor<T> strCtor = requiredType.getConstructor(String.class);
    //使用这个构造器实例化对象
    return BeanUtils.instantiateClass(strCtor, convertedValue);

如何使用呢？下面是一个小例子

@Data
    @ToString
    @NoArgsConstructor
    public class Role { 
        private Integer id;
        private String rolename;
        private String description;
    
        /** * 定义了这个构造方法，就能通过requiredType.getConstructor(String.class)获取到这个构造器了 * BeanUtils.instantiateClass(strCtor, convertedValue);实例化这个对象的时候， * 将convertedValue作为构造器参数实例化对象 */
        public Role(String text) { 
            //转换逻辑在这里写了
            String[] split = text.split(",");
            this.id = Integer.valueOf(split[0]);
            this.rolename = split[1];
            this.description = split[2];
        }
    }

注释掉配置文件中下面`bean`定义，我们自定义的`PropertyEditor`就不会生效了

<bean id="customEditorConfigurer" class="org.springframework.beans.factory.config.CustomEditorConfigurer">
        
        <property name="customEditors">
    
            <map>
                <entry key="com.lx.converter.domain.Role" value="com.lx.converter.propertyEditor.StringToRolePropertyEditor"></entry>
            </map>
        </property>
    </bean>

测试

User(name=null, username=null, password=null, role=Role(id=12, rolename=lx, description=中国))

仍能正常的进行类型转换。

# 3 `PropertyEditorRegistrySupport` #

该类非常重要，某种程度上说它是`PropertyEditorRegistry`接口的唯一实现，提供`PropertyEditor`注册和查找的功能。并且该类还是`BeanWrapperImpl`的父类

先来看`PropertyEditorRegistry`接口，该接口提供了三个方法

public interface PropertyEditorRegistry { 
    
        //注册String->requiredType转换的PropertyEditor
        void registerCustomEditor(Class<?> requiredType, PropertyEditor propertyEditor);
    
        //注册指定属性的PropertyEditor
        void registerCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath, PropertyEditor propertyEditor);
    
        //获取指定条件的PropertyEditor
        @Nullable
        PropertyEditor findCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath);
    
    }

接下来看一下`PropertyEditorRegistrySupport`的属性

public class PropertyEditorRegistrySupport implements PropertyEditorRegistry { 
    
        //统一转换服务，spring自建的一套类型转换
        @Nullable
        private ConversionService conversionService;
    
        //是否允许使用默认的PropertyEditor
        private boolean defaultEditorsActive = false;
    
        private boolean configValueEditorsActive = false;
        
        //默认的PropertyEditor
        @Nullable
        private Map<Class<?>, PropertyEditor> defaultEditors;
    
        //覆盖的PropertyEditor，优先级比defaultEditors的高
        @Nullable
        private Map<Class<?>, PropertyEditor> overriddenDefaultEditors;
    
        //用户注册的String->requiredType的PropertyEditor
        @Nullable
        private Map<Class<?>, PropertyEditor> customEditors;
    
        /** * 映射关系为propertyName->PropertyEditor * 保存用户单独为某个属性的类型转换注册的PropertyEditor */
        @Nullable
        private Map<String, CustomEditorHolder> customEditorsForPath;
    
        /** * 缓存 * 当完成String->xxxType的类型转换后，就会将xxxType和其使用的PropertyEditor缓存到这里面 */
        @Nullable
        private Map<Class<?>, PropertyEditor> customEditorCache;
    }

## 3.1 `CustomEditorHolder` ##

`CustomEditorHolder`是`PropertyEditorRegistrySupport`的嵌套类

/** * Holder for a registered custom editor with property name. * Keeps the PropertyEditor itself plus the type it was registered for. */
    private static final class CustomEditorHolder { 
    
        private final PropertyEditor propertyEditor;
    
        //注册类型
        @Nullable
        private final Class<?> registeredType;
    
        private CustomEditorHolder(PropertyEditor propertyEditor, @Nullable Class<?> registeredType) { 
            this.propertyEditor = propertyEditor;
            this.registeredType = registeredType;
        }
    
        private PropertyEditor getPropertyEditor() { 
            return this.propertyEditor;
        }
    
        @Nullable
        private Class<?> getRegisteredType() { 
            return this.registeredType;
        }
    
        /** * 该方法会获取保存的propertyEditor对象,只不过要遵循如下条件 * 1.如果用户未指定注册类型，就直接返回保存的propertyEditor对象 * 2.如果用户指定了注册类型，也指定了要求类型，那么只有当这两个类型之间有继承关系时才会返回保存的propertyEditor对象 * 3.如果用户指定了注册类型，但是未指定要求类型，那么只有当注册类型不是Collection和数组时才会返回保存的propertyEditor对象 * 4.其他情况均返回null */
        @Nullable
        private PropertyEditor getPropertyEditor(@Nullable Class<?> requiredType) { 
            // Special case: If no required type specified, which usually only happens for
            // Collection elements, or required type is not assignable to registered type,
            // which usually only happens for generic properties of type Object -
            // then return PropertyEditor if not registered for Collection or array type.
            // (If not registered for Collection or array, it is assumed to be intended
            // for elements.)
            if (this.registeredType == null ||
                (requiredType != null &&
                 (ClassUtils.isAssignable(this.registeredType, requiredType) ||
                  ClassUtils.isAssignable(requiredType, this.registeredType))) ||
                (requiredType == null &&
                 (!Collection.class.isAssignableFrom(this.registeredType) && !this.registeredType.isArray()))) { 
                return this.propertyEditor;
            }
            else { 
                return null;
            }
        }
    }

## 3.2 `registerCustomEditor(Class<?> requiredType, PropertyEditor propertyEditor)`方法，注册`String->requiredType`的`PropertyEditor` ##

@Override
    public void registerCustomEditor(Class<?> requiredType, PropertyEditor propertyEditor) { 
       registerCustomEditor(requiredType, null, propertyEditor);
    }

> 该方法中调用`3.3`中的方法来完成注册功能

## 3.3`registerCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath, PropertyEditor propertyEditor)`方法，注册指定属性的`PropertyEditor` ##

@Override
    public void registerCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath, PropertyEditor propertyEditor) { 
        //两个条件不能都为null
        if (requiredType == null && propertyPath == null) { 
            throw new IllegalArgumentException("Either requiredType or propertyPath is required");
        }
        //指定了属性名
        if (propertyPath != null) { 
            if (this.customEditorsForPath == null) { 
                this.customEditorsForPath = new LinkedHashMap<>(16);
            }
            //注册，两种条件的注册位置是不一样的
            this.customEditorsForPath.put(propertyPath, new CustomEditorHolder(propertyEditor, requiredType));
        }
        //未指定属性名
        else { 
            if (this.customEditors == null) { 
                this.customEditors = new LinkedHashMap<>(16);
            }
            //注册
            this.customEditors.put(requiredType, propertyEditor);
            //清空缓存
            this.customEditorCache = null;
        }
    }

>  *  如果指定了属性名，直接注册到`customEditorsForPath`集合中
>  *  如果未指定属性名，直接注册到`customEditors`集合中

## 3.4 `findCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath)`方法， 获取指定条件的用户定义的`PropertyEditor` ##

@Override
    @Nullable
    public PropertyEditor findCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath) { 
        Class<?> requiredTypeToUse = requiredType;
    
        /***************************属性名匹配PropertyEditor********************************/
        //根据属性名查找PropertyEditor
        if (propertyPath != null) {  
            if (this.customEditorsForPath != null) { 
                // Check property-specific editor first.
                //第一次获取
                PropertyEditor editor = getCustomEditor(propertyPath, requiredType);
                if (editor == null) { 
                    List<String> strippedPaths = new ArrayList<>();
                    //剥离属性名中[]再次获取
                    addStrippedPropertyPaths(strippedPaths, "", propertyPath);
                    for (Iterator<String> it = strippedPaths.iterator(); it.hasNext() && editor == null;) { 
                        String strippedPath = it.next();
                        editor = getCustomEditor(strippedPath, requiredType);
                    }
                }
                if (editor != null) { 
                    return editor;
                }
            }
            if (requiredType == null) { 
                /** * 这个方法会被子类重写 * BeanWrapperImpl可以通过内省找到属性名对应的类型 */
                requiredTypeToUse = getPropertyType(propertyPath);
            }
        }
    
        /************************类型匹配PropertyEditor*******************************/
        // No property-specific editor -> check type-specific editor.
        //找不到就根据类型匹配PropertyEditor
        return getCustomEditor(requiredTypeToUse);
    }

> 属性名匹配是优先的

### 3.4.1 根据属性名获取`PropertyEditor` ###

/** * Get custom editor that has been registered for the given property. * @param propertyName the property path to look for * @param requiredType the type to look for * @return the custom editor, or {@code null} if none specific for this property */
    @Nullable
    private PropertyEditor getCustomEditor(String propertyName, @Nullable Class<?> requiredType) { 
        //先根据名字获取
        CustomEditorHolder holder =
            (this.customEditorsForPath != null ? this.customEditorsForPath.get(propertyName) : null);
        //然后根据要求类型获取
        return (holder != null ? holder.getPropertyEditor(requiredType) : null);
    }

> 通过该方法获取`PropertyEditor`会经过两次筛选

### 3.4.2 根据要求类型获取`PropertyEditor` ###

/** * Get custom editor for the given type. If no direct match found, * try custom editor for superclass (which will in any case be able * to render a value as String via {@code getAsText}). * @param requiredType the type to look for * @return the custom editor, or {@code null} if none found for this type * @see java.beans.PropertyEditor#getAsText() */
    @Nullable
    private PropertyEditor getCustomEditor(@Nullable Class<?> requiredType) { 
        //用户自定义的PropertyEditor会被放到customEditors属性中
        if (requiredType == null || this.customEditors == null) { 
            return null;
        }
        // Check directly registered editor for type.
        PropertyEditor editor = this.customEditors.get(requiredType);
        if (editor == null) { 
            //从缓存中获取
            // Check cached editor for type, registered for superclass or interface.
            if (this.customEditorCache != null) { 
                editor = this.customEditorCache.get(requiredType);
            }
            //缓存中获取不到
            if (editor == null) { 
                // Find editor for superclass or interface.
                //遍历用户自定义的PropertyEditor
                for (Iterator<Class<?>> it = this.customEditors.keySet().iterator(); it.hasNext() && editor == null;) { 
                    Class<?> key = it.next();
                    //能够转换的类型是要求类型父类或父接口
                    if (key.isAssignableFrom(requiredType)) { 
                        //得到这个PropertyEditor
                        editor = this.customEditors.get(key);
                        // Cache editor for search type, to avoid the overhead
                        // of repeated assignable-from checks.
                        if (this.customEditorCache == null) { 
                            this.customEditorCache = new HashMap<>();
                        }
                        //缓存
                        this.customEditorCache.put(requiredType, editor);
                    }
                }
            }
        }
        return editor;
    }

> 方法流程：
> 
>  *  首先尝试直接根据要求类型获取`PropertyEditor`
>  *  如果获取不到，再尝试从缓存中获取`PropertyEditor`
>  *  如果缓存中也获取不到，就用户自定义的`PropertyEditor`，只要这个`PropertyEditor`能够转换的类型是要求类型父类或父接口，就使用当前`PropertyEditor`，并缓存，方便下次快速得到该要求类型的`PropertyEditor`

## 3.5 `getDefaultEditor(Class<?> requiredType)`方法，根据要求类型获取默认的`PropertyEditor` ##

/** * Retrieve the default editor for the given property type, if any. * <p>Lazily registers the default editors, if they are active. * @param requiredType type of the property * @return the default editor, or {@code null} if none found * @see #registerDefaultEditors */
    @Nullable
    public PropertyEditor getDefaultEditor(Class<?> requiredType) { 
        //如果这个字段值为false，表明不使用默认的PropertyEditor
        if (!this.defaultEditorsActive) { 
            return null;
        }
        /** * 重写的默认的PropertyEditor，优先级大于默认PropertyEditor * BeanWrapper初始化的时候，会调用ResourceEditorRegistrar的registerCustomEditors方法, * 将spring定义的默认的PropertyEditor注册到BeanWrapper的overriddenDefaultEditors中 * ResourceEditorRegistrar是spring默认属性编辑器注册商，实现了PropertyEditorRegistrar接口 */
        if (this.overriddenDefaultEditors != null) { 
            PropertyEditor editor = this.overriddenDefaultEditors.get(requiredType);
            //找到了符合条件的PropertyEditor，直接返回
            if (editor != null) { 
                return editor;
            }
        }
        //未找到就创建默认的PropertyEditor
        if (this.defaultEditors == null) { 
            createDefaultEditors();
        }
        return this.defaultEditors.get(requiredType);
    }
    
    
    /** * Actually register the default editors for this registry instance. * 这些是常见的基本的PropertyEditor，非常多，里面包括了基本数据类型的转换 */
    private void createDefaultEditors() { 
        this.defaultEditors = new HashMap<>(64);
    
        // Simple editors, without parameterization capabilities.
        // The JDK does not contain a default editor for any of these target types.
        this.defaultEditors.put(Charset.class, new CharsetEditor());
        this.defaultEditors.put(Class.class, new ClassEditor());
        this.defaultEditors.put(Class[].class, new ClassArrayEditor());
        this.defaultEditors.put(Currency.class, new CurrencyEditor());
        this.defaultEditors.put(File.class, new FileEditor());
        this.defaultEditors.put(InputStream.class, new InputStreamEditor());
        this.defaultEditors.put(InputSource.class, new InputSourceEditor());
        this.defaultEditors.put(Locale.class, new LocaleEditor());
        this.defaultEditors.put(Path.class, new PathEditor());
        this.defaultEditors.put(Pattern.class, new PatternEditor());
        this.defaultEditors.put(Properties.class, new PropertiesEditor());
        this.defaultEditors.put(Reader.class, new ReaderEditor());
        this.defaultEditors.put(Resource[].class, new ResourceArrayPropertyEditor());
        this.defaultEditors.put(TimeZone.class, new TimeZoneEditor());
        this.defaultEditors.put(URI.class, new URIEditor());
        this.defaultEditors.put(URL.class, new URLEditor());
        this.defaultEditors.put(UUID.class, new UUIDEditor());
        this.defaultEditors.put(ZoneId.class, new ZoneIdEditor());
    
        // Default instances of collection editors.
        // Can be overridden by registering custom instances of those as custom editors.
        this.defaultEditors.put(Collection.class, new CustomCollectionEditor(Collection.class));
        this.defaultEditors.put(Set.class, new CustomCollectionEditor(Set.class));
        this.defaultEditors.put(SortedSet.class, new CustomCollectionEditor(SortedSet.class));
        this.defaultEditors.put(List.class, new CustomCollectionEditor(List.class));
        this.defaultEditors.put(SortedMap.class, new CustomMapEditor(SortedMap.class));
    
        // Default editors for primitive arrays.
        this.defaultEditors.put(byte[].class, new ByteArrayPropertyEditor());
        this.defaultEditors.put(char[].class, new CharArrayPropertyEditor());
    
        // The JDK does not contain a default editor for char!
        this.defaultEditors.put(char.class, new CharacterEditor(false));
        this.defaultEditors.put(Character.class, new CharacterEditor(true));
    
        // Spring's CustomBooleanEditor accepts more flag values than the JDK's default editor.
        this.defaultEditors.put(boolean.class, new CustomBooleanEditor(false));
        this.defaultEditors.put(Boolean.class, new CustomBooleanEditor(true));
    
        // The JDK does not contain default editors for number wrapper types!
        // Override JDK primitive number editors with our own CustomNumberEditor.
        this.defaultEditors.put(byte.class, new CustomNumberEditor(Byte.class, false));
        this.defaultEditors.put(Byte.class, new CustomNumberEditor(Byte.class, true));
        this.defaultEditors.put(short.class, new CustomNumberEditor(Short.class, false));
        this.defaultEditors.put(Short.class, new CustomNumberEditor(Short.class, true));
        this.defaultEditors.put(int.class, new CustomNumberEditor(Integer.class, false));
        this.defaultEditors.put(Integer.class, new CustomNumberEditor(Integer.class, true));
        this.defaultEditors.put(long.class, new CustomNumberEditor(Long.class, false));
        this.defaultEditors.put(Long.class, new CustomNumberEditor(Long.class, true));
        this.defaultEditors.put(float.class, new CustomNumberEditor(Float.class, false));
        this.defaultEditors.put(Float.class, new CustomNumberEditor(Float.class, true));
        this.defaultEditors.put(double.class, new CustomNumberEditor(Double.class, false));
        this.defaultEditors.put(Double.class, new CustomNumberEditor(Double.class, true));
        this.defaultEditors.put(BigDecimal.class, new CustomNumberEditor(BigDecimal.class, true));
        this.defaultEditors.put(BigInteger.class, new CustomNumberEditor(BigInteger.class, true));
    
        // Only register config value editors if explicitly requested.
        if (this.configValueEditorsActive) { 
            StringArrayPropertyEditor sae = new StringArrayPropertyEditor();
            this.defaultEditors.put(String[].class, sae);
            this.defaultEditors.put(short[].class, sae);
            this.defaultEditors.put(int[].class, sae);
            this.defaultEditors.put(long[].class, sae);
        }
    }

## 3.6 `hasCustomEditorForElement(@Nullable Class<?> elementType, @Nullable String propertyPath)`方法，判断是否为`propertyName`指定了`PropertyEditor`（属性专用的`PropertyEditor`） ##

/** * Determine whether this registry contains a custom editor * for the specified array/collection element. * @param elementType the target type of the element * (can be {@code null} if not known) * @param propertyPath the property path (typically of the array/collection; * can be {@code null} if not known) * @return whether a matching custom editor has been found */
    public boolean hasCustomEditorForElement(@Nullable Class<?> elementType, @Nullable String propertyPath) { 
        //this.customEditorsForPath里面保存了用户指定的属性专用的PropertyEditor
        if (propertyPath != null && this.customEditorsForPath != null) { 
            //遍历属性专用的PropertyEditor
            for (Map.Entry<String, CustomEditorHolder> entry : this.customEditorsForPath.entrySet()) { 
                /** * 先根据属性名匹配 * 然后调用getPropertyEditor(elementType)方法根据类型匹配,见3.1 */
                if (PropertyAccessorUtils.matchesProperty(entry.getKey(), propertyPath) &&
                    entry.getValue().getPropertyEditor(elementType) != null) { 
                    return true;
                }
            }
        }
        // No property-specific editor -> check type-specific editor.
        //否则就从类型专用的PropertyEditor查询
        return (elementType != null && this.customEditors != null && this.customEditors.containsKey(elementType));
    }

> 该方法先根据属性匹配`PropertyEditor`，如果能找到，直接返回`true`。如果找不到在根据类型匹配