探秘Spring的PropertyEditor

今天无意之中一位网友咨询一个问题让我有了深入了解一下Spring的PropertyEditor机制，虽然之前也大概知道些，但是都是知道它是什么，却不知道在Spring整个机制中它是如何执行的。今天就趁着中午闲暇时间看了一下Spring这方面的源码。所以这里主要是通过分析Spring的源码来了解PropertyEditor。

其实PropertyEditor是JDK里面的java.beans下面的类，是提供AWT进行渲染用的。Spring通过利用该接口，来实现Bean的属性转换器。我们在Spring的xml或者其他途径配置的bean属性都是字符串类型的值，但是对应到每个具体的属性是各种类型的，Spring通过各种PropertyEditor来对各个属性进行类型转换，在Spring中的PropertyEditor并不是直接实现PropertyEditor接口，是通过PropertyEditorSupport类屏蔽了一些Spring不需要的方法比如paintValue，从而对它们提供了默认的实现，所以Spring里面的PropertyEditor都是在PropertyEditorSupport的基础上实现的。那么先看看Spring到底提供了哪些PropertyEditor的实现，下面截取了通过IDE工具找到的实现类截图：

这里面基本涉及到了所有JDK提供的类型，知道spring提供哪些PropertyEditor之后，还得需要这些PropertyEditor是怎么嵌入Spring的。

PropertyEditorRegistry

这个是Spring框架内部将PropertyEditor嵌入Spring的方式，其中BeanWrapperImpl就是一个例子。先看看BeanWrapperImpl的类图：

可以看到BeanWrapperImpl其实就是PropertyEditorRegistry的子类。其实PropertyEditorRegistry是一个接口，做具体事情的是PropertyEditorRegistrySupport。在PropertyEditorRegistrySupport中存在一个方法createDefaultEditors，这个方法就是初始化Spring中默认PropertyEditor。下面看看哪些是Spring默认的PropertyEditor：

private void createDefaultEditors() {
        this.defaultEditors = new HashMap<Class<?>, PropertyEditor>(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(Pattern.class, new PatternEditor());
        this.defaultEditors.put(Properties.class, new PropertiesEditor());
        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());

        // 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);
        }
    }

执行完这个那么BeanWrapperImpl就具备上面类型的转换功能，可能上面能够转换的类型还不能满足我们的需求，那么可以通过另一种方式将PropertyEditor注入到Spring中。

PropertyEditorRegistrar

该接口只有一个方法void registerCustomEditors(PropertyEditorRegistry registry),实现该方法就可以往传入的registry添加自定义的PropertyEditor，一般情况下传入的registry是BeanWrapperImpl的实体，所以就等于将自定义的PropertyEditor注入到BeanWrapperImpl里面。现在的问题是怎么将PropertyEditorRegistrar注入到Spring中，那么就可以将自定的PropertyEditor添加到Spring中。Spring提供了一个类专门用来将第三方的PropertyEditor注入到它里面，这个类就是CustomEditorConfigurer，先看看这个类的类图：

发现它是实现了BeanFactoryPostProcessor接口，那么将会在构造完BeanDefinition之后调用方法postProcessBeanFactory,只有整个方法是嵌入Spring的唯一途经，那么看看这个方法里面做了什么事情:

<!--lang:java-->
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
    if (this.propertyEditorRegistrars != null) {
        for (PropertyEditorRegistrar propertyEditorRegistrar : this.propertyEditorRegistrars) {
            beanFactory.addPropertyEditorRegistrar(propertyEditorRegistrar);
        }
    }

    if (this.customEditors != null) {
        for (Map.Entry<String, ?> entry : this.customEditors.entrySet()) {
            String key = entry.getKey();
            Object value = entry.getValue();
            Class requiredType = null;

            try {
                requiredType = ClassUtils.forName(key, this.beanClassLoader);
                if (value instanceof PropertyEditor) {
                    if (logger.isWarnEnabled()) {
                        logger.warn("Passing PropertyEditor instances into CustomEditorConfigurer is deprecated: " +
                                "use PropertyEditorRegistrars or PropertyEditor class names instead. " +
                                "Offending key [" + key + "; offending editor instance: " + value);
                    }
                    beanFactory.addPropertyEditorRegistrar(
                            new SharedPropertyEditorRegistrar(requiredType, (PropertyEditor) value));
                }
                else if (value instanceof Class) {
                    beanFactory.registerCustomEditor(requiredType, (Class) value);
                }
                else if (value instanceof String) {
                    Class editorClass = ClassUtils.forName((String) value, this.beanClassLoader);
                    Assert.isAssignable(PropertyEditor.class, editorClass);
                    beanFactory.registerCustomEditor(requiredType, editorClass);
                }
                else {
                    throw new IllegalArgumentException("Mapped value [" + value + "] for custom editor key [" +
                            key + "] is not of required type [" + PropertyEditor.class.getName() +
                            "] or a corresponding Class or String value indicating a PropertyEditor implementation");
                }
            }
            catch (ClassNotFoundException ex) {
                if (this.ignoreUnresolvableEditors) {
                    logger.info("Skipping editor [" + value + "] for required type [" + key + "]: " +
                            (requiredType != null ? "editor" : "required type") + " class not found.");
                }
                else {
                    throw new FatalBeanException(
                            (requiredType != null ? "Editor" : "Required type") + " class not found", ex);
                }
            }
        }
    }
}

发现它都调用了beanFactory的方法将配置的PropertyEditorRegistrar和customEditors注入到Spring的bean工厂里面。我们只要往spring容器中注入一个CustomEditorConfigurerBean，然后设置propertyEditorRegistrars和customEditors属性就可以将自定义的PropertyEditor注入到Spring中了。当然你也可以不用CustomEditorConfigurer类，自定义一个类实现BeanFactoryPostProcessor接口，然后按照CustomEditorConfigurer类似的路径，也可以达到目的。下面给出一个自定义PropertyEditor大致流程：

<!--lang:java-->
public class CustomPropertyEditor extends PropertyEditorSupport {

@Override
public void setAsText(String text) throws IllegalArgumentException {
    super.setAsText(text);
}

@Override
public Object getValue() {
    return super.getValue();
}
}

上面是定义了一个自定义PropertyEditor,下面需要将这个PropertyEditor注入到Spring里面中：

<!--lang:xml-->
<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
    <property name="customEditors">
        <map>
            <entry key="com.xx.foo.FooPojo" value="com.xx.foo.CustomPropertyEditor"/>
        </map>
    </property>
</bean>

Spring都会调用setAsText方法将该属性的字面值设置进来，然后需要根据你规定的规范转换成对应的属性对象。

下面再给出一个ClassEditor的实现，大家可以仿照这个来实现：

<!--lang:java-->
public class ClassEditor extends PropertyEditorSupport {

private final ClassLoader classLoader;


/**
 * Create a default ClassEditor, using the thread context ClassLoader.
 */
public ClassEditor() {
    this(null);
}

/**
 * Create a default ClassEditor, using the given ClassLoader.
 * @param classLoader the ClassLoader to use
 * (or {@code null} for the thread context ClassLoader)
 */
public ClassEditor(ClassLoader classLoader) {
    this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());
}


@Override
public void setAsText(String text) throws IllegalArgumentException {
    if (StringUtils.hasText(text)) {
        setValue(ClassUtils.resolveClassName(text.trim(), this.classLoader));
    }
    else {
        setValue(null);
    }
}

@Override
public String getAsText() {
    Class clazz = (Class) getValue();
    if (clazz != null) {
        return ClassUtils.getQualifiedName(clazz);
    }
    else {
        return "";
    }
}

}