Mybatis的MetaObject解析

SqlSessionFactory初始化： http://donald-draper.iteye.com/2331917
Mybatis加载解析Mapper（xml）文件第一讲: http://donald-draper.iteye.com/blog/2333125
Mybatis加载解析Mapper（xml）文件第二讲： http://donald-draper.iteye.com/blog/2333191
Mybatis 解析Mapper（class）： http://donald-draper.iteye.com/blog/2333293
Mybatis的Environment解析详解： http://donald-draper.iteye.com/blog/2334133
mybatis 动态标签语句的解析（BoundSql）： http://donald-draper.iteye.com/blog/2334135

在我们前面几篇文章中，经常出现一个类为MetaObject，今天我们就来看看，MetaObject的作用在讲Environments解析的时候有这么一段代码，设置数据源的属性

public void setProperties(Properties properties)
    {
        Properties driverProperties = new Properties();
	//获取dataSource的对象描述
        MetaObject metaDataSource = SystemMetaObject.forObject(dataSource);
	//获取dataSource的所有属性，并设置
        for(Iterator i$ = properties.keySet().iterator(); i$.hasNext();)
        {
            Object key = i$.next();
            String propertyName = (String)key;
            if(propertyName.startsWith("driver."))
            {
                String value = properties.getProperty(propertyName);
                driverProperties.setProperty(propertyName.substring(DRIVER_PROPERTY_PREFIX_LENGTH), value);
            } else
            if(metaDataSource.hasSetter(propertyName))
            {
                String value = (String)properties.get(propertyName);
                Object convertedValue = convertValue(metaDataSource, propertyName, value);
                metaDataSource.setValue(propertyName, convertedValue);
            }
        }
        //设置metaDataSource的driverProperties属性
        if(driverProperties.size() > 0)
            metaDataSource.setValue("driverProperties", driverProperties);
    }

我们可以看到有这么几句代码

 MetaObject metaDataSource = SystemMetaObject.forObject(dataSource);
 metaDataSource.hasSetter(propertyName)
 metaDataSource.setValue("driverProperties", driverProperties);

来看一下SystemMetaObject

public class SystemMetaObject
{
    public static final ObjectFactory DEFAULT_OBJECT_FACTORY;
    public static final ObjectWrapperFactory DEFAULT_OBJECT_WRAPPER_FACTORY;
    public static final MetaObject NULL_META_OBJECT;
    private static class NullObject
    {
        private NullObject()
        {
        }
    }
    public static MetaObject forObject(Object object)
    {
        return MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
    }


    static 
    {
        DEFAULT_OBJECT_FACTORY = new DefaultObjectFactory();
        DEFAULT_OBJECT_WRAPPER_FACTORY = new DefaultObjectWrapperFactory();
        NULL_META_OBJECT = MetaObject.forObject(org/apache/ibatis/reflection/SystemMetaObject$NullObject, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
    }
}

从上面的SystemMetaObject，我们可以看出，SystemMetaObject返回对象的MetaObject对象，实际上通过的MetaObject的forObject方法。

//Sql包装类

public class BoundSql
{
    public BoundSql(Configuration configuration, String sql, List parameterMappings, Object parameterObject)
    {
        this.sql = sql;
        this.parameterMappings = parameterMappings;
        this.parameterObject = parameterObject;
        additionalParameters = new HashMap();
        metaParameters = configuration.newMetaObject(additionalParameters);
    }
    private String sql;
    private List parameterMappings;
    private Object parameterObject;
    private Map additionalParameters;
    private MetaObject metaParameters;
}

在Sql包装类，我们可以看到一下几句代码：

metaParameters = configuration.newMetaObject(additionalParameters);
 //Configuration新建MetaObject对象
  public MetaObject newMetaObject(Object object)
    {
        return MetaObject.forObject(object, objectFactory, objectWrapperFactory);
    }

下面来看MetaObject
//MetaObject,设置Class实例属性

public class MetaObject
{
    private Object originalObject;//原始Obeject
    private ObjectWrapper objectWrapper;//包装后的Object
    private ObjectFactory objectFactory;
    private ObjectWrapperFactory objectWrapperFactory;
    private MetaObject(Object object, ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory)
    {
        originalObject = object;
        this.objectFactory = objectFactory;
        this.objectWrapperFactory = objectWrapperFactory;
        if(object instanceof ObjectWrapper)
            objectWrapper = (ObjectWrapper)object;
        else
	//如果对应objectWrapperFactory中存在，则返回object
        if(objectWrapperFactory.hasWrapperFor(object))
            objectWrapper = objectWrapperFactory.getWrapperFor(this, object);
        else
        if(object instanceof Map)
	    //Map
            objectWrapper = new MapWrapper(this, (Map)object);
        else
        if(object instanceof Collection)
	    //List
            objectWrapper = new CollectionWrapper(this, (Collection)object);
        else
	   //bean
            objectWrapper = new BeanWrapper(this, object);
    }
    //构造MetaObject
    public static MetaObject forObject(Object object, ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory)
    {
        if(object == null)
            return SystemMetaObject.NULL_META_OBJECT;
        else
            return new MetaObject(object, objectFactory, objectWrapperFactory);
    }
    public String findProperty(String propName, boolean useCamelCaseMapping)
    {
        return objectWrapper.findProperty(propName, useCamelCaseMapping);
    }
    //获取可读属性名
    public String[] getGetterNames()
    {
        return objectWrapper.getGetterNames();
    }
     //获取可写属性名
    public String[] getSetterNames()
    {
        return objectWrapper.getSetterNames();
    }
    //获取对象属性name对应的值
     public Object getValue(String name)
    {
        PropertyTokenizer prop = new PropertyTokenizer(name);
        if(prop.hasNext())
        {
            MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
            if(metaValue == SystemMetaObject.NULL_META_OBJECT)
                return null;
            else
                return metaValue.getValue(prop.getChildren());
        } else
        {
            return objectWrapper.get(prop);
        }
    }
    //给对象属性name设置value值
    public void setValue(String name, Object value)
    {
        PropertyTokenizer prop = new PropertyTokenizer(name);
        if(prop.hasNext())
        {
            MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
            if(metaValue == SystemMetaObject.NULL_META_OBJECT)
            {
                if(value == null && prop.getChildren() != null)
                    return;
                metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
            }
            metaValue.setValue(prop.getChildren(), value);
        } else
        {
            objectWrapper.set(prop, value);
        }
    }
}

下面我们先来看forObject方法

private MetaObject(Object object, ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory)
    {
        originalObject = object;
        this.objectFactory = objectFactory;
        this.objectWrapperFactory = objectWrapperFactory;
        if(object instanceof ObjectWrapper)
            objectWrapper = (ObjectWrapper)object;
        else
	//如果对应objectWrapperFactory中存在，则返回object
        if(objectWrapperFactory.hasWrapperFor(object))
            objectWrapper = objectWrapperFactory.getWrapperFor(this, object);
        else
        if(object instanceof Map)
	    //Map
            objectWrapper = new MapWrapper(this, (Map)object);
        else
        if(object instanceof Collection)
	    //List
            objectWrapper = new CollectionWrapper(this, (Collection)object);
        else
	   //bean
            objectWrapper = new BeanWrapper(this, object);
    }
    //构造MetaObject
    public static MetaObject forObject(Object object, ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory)
    {
        if(object == null)
            return SystemMetaObject.NULL_META_OBJECT;
        else
            return new MetaObject(object, objectFactory, objectWrapperFactory);
    }
//DefaultObjectWrapperFactory
 public class DefaultObjectWrapperFactory
    implements ObjectWrapperFactory
{
    public boolean hasWrapperFor(Object object)
    {
        return false;
    }
    public ObjectWrapper getWrapperFor(MetaObject metaObject, Object object)
    {
        throw new RuntimeException("The DefaultObjectWrapperFactory should never be called to provide an ObjectWrapper.");
    }
}

从MetaObject的forObject方法，我们可以看出，实际上，MetaObject的构造函数
MetaObject(Object object, ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory)是把原始Obejct包装成ObjectWrapper（ObjectWrapper，MapWrapper，CollectionWrapper，BeanWrapper）,
如果ObjectWrapperFactory已经包装了Object，则通过ObjectWrapperFactory的 getWrapperFor(MetaObject metaObject, Object object)返回相应的ObjectWrapper。

在来看获取属性值方法，这里我们来看BeanWrapper

public String findProperty(String propName, boolean useCamelCaseMapping)
{
     return objectWrapper.findProperty(propName, useCamelCaseMapping);
}

//BeanWrapper

public class BeanWrapper extends BaseWrapper
{
    private Object object;
    private MetaClass metaClass;//Object对应的Class
    public BeanWrapper(MetaObject metaObject, Object object)
    {
        super(metaObject);
        this.object = object;
        metaClass = MetaClass.forClass(object.getClass());
    }
    //获取属性name对应的属性名
      public String findProperty(String name, boolean useCamelCaseMapping)
    {
        return metaClass.findProperty(name, useCamelCaseMapping);
    }
}

//MetaClass

public class MetaClass
{
    private Reflector reflector;
    public static MetaClass forClass(Class type)
    {
        return new MetaClass(type);
    }
     private MetaClass(Class type)
    {
        //获取type类型对应的Reflector
        reflector = Reflector.forClass(type);
    } 
    //获取name对应的属性名，useCamelCaseMapping是否是驼峰命名
     public String findProperty(String name, boolean useCamelCaseMapping)
    {
        if(useCamelCaseMapping)
            name = name.replace("_", "");
        return findProperty(name);
    }
    public String findProperty(String name)
    {
        StringBuilder prop = buildProperty(name, new StringBuilder());
        return prop.length() <= 0 ? null : prop.toString();
    } 
     private StringBuilder buildProperty(String name, StringBuilder builder)
    {
        PropertyTokenizer prop = new PropertyTokenizer(name);
        if(prop.hasNext())
        {
            String propertyName = reflector.findPropertyName(prop.getName());
            if(propertyName != null)
            {
                builder.append(propertyName);
                builder.append(".");
                MetaClass metaProp = metaClassForProperty(propertyName);
                metaProp.buildProperty(prop.getChildren(), builder);
            }
        } else
        {
            String propertyName = reflector.findPropertyName(name);
            if(propertyName != null)
                builder.append(propertyName);
        }
        return builder;
    }
}

//Reflector

public class Reflector
{
    private static boolean classCacheEnabled = true;
    private static final String EMPTY_STRING_ARRAY[] = new String[0];
    private static final Map REFLECTOR_MAP = new ConcurrentHashMap();
    private Class type;
    private String readablePropertyNames[];
    private String writeablePropertyNames[];
    private Map setMethods;
    private Map getMethods;
    private Map setTypes;
    private Map getTypes;
    private Constructor defaultConstructor;
    private Map caseInsensitivePropertyMap;
    //构造Class对应的Reflector
    public static Reflector forClass(Class clazz)
    {
        if(classCacheEnabled)
        {
	    //如果缓存状态开启，则从REFLECTOR_MAP获取对应的Reflector，没有则重新构建
            Reflector cached = (Reflector)REFLECTOR_MAP.get(clazz);
            if(cached == null)
            {
                cached = new Reflector(clazz);
                REFLECTOR_MAP.put(clazz, cached);
            }
            return cached;
        } else
        {
            return new Reflector(clazz);
        }
    }
    private Reflector(Class clazz)
    {
        readablePropertyNames = EMPTY_STRING_ARRAY;//可读属性名
        writeablePropertyNames = EMPTY_STRING_ARRAY;//可写属性名
        setMethods = new HashMap();//Set方法Map
        getMethods = new HashMap();//Get方法Map
        setTypes = new HashMap();
        getTypes = new HashMap();
        caseInsensitivePropertyMap = new HashMap();//大小写不敏感属性Map
        type = clazz;
        addDefaultConstructor(clazz);
        addGetMethods(clazz);
        addSetMethods(clazz);
        addFields(clazz);
        readablePropertyNames = (String[])getMethods.keySet().toArray(new String[getMethods.keySet().size()]);
        writeablePropertyNames = (String[])setMethods.keySet().toArray(new String[setMethods.keySet().size()]);
        //将可读属性添加大小写不敏感属性Map
	String arr$[] = readablePropertyNames;
        int len$ = arr$.length;
        for(int i$ = 0; i$ < len$; i$++)
        {
            String propName = arr$[i$];
            caseInsensitivePropertyMap.put(propName.toUpperCase(Locale.ENGLISH), propName);
        }
        //将可写属性添加大小写不敏感属性Map
        arr$ = writeablePropertyNames;
        len$ = arr$.length;
        for(int i$ = 0; i$ < len$; i$++)
        {
            String propName = arr$[i$];
            caseInsensitivePropertyMap.put(propName.toUpperCase(Locale.ENGLISH), propName);
        }

    }
     public String findPropertyName(String name)
    {
        return (String)caseInsensitivePropertyMap.get(name.toUpperCase(Locale.ENGLISH));
    }
}

回到MetaObject，再看获取属性get与setName的属性

 public String[] getGetterNames()
    {
        return objectWrapper.getGetterNames();
    }
    public String[] getSetterNames()
    {
        return objectWrapper.getSetterNames();
    }

再看BeanWrapper的相应方法，并委托给MetaClass对应方法
//BeanWrapper

public String[] getGetterNames()
    {
        return metaClass.getGetterNames();
    }

    public String[] getSetterNames()
    {
        return metaClass.getSetterNames();
    }

// MetaClass对应方法委托给Reflector
  

 public String[] getGetterNames()
    {
        return reflector.getGetablePropertyNames();
    }

    public String[] getSetterNames()
    {
        return reflector.getSetablePropertyNames();
    }

再看Reflector

public String[] getGetablePropertyNames()
    {
        return readablePropertyNames;
    }

    public String[] getSetablePropertyNames()
    {
        return writeablePropertyNames;
    }

实际上返回的是可读与可写属性名字符串组；
回到MetaObject，再看获取对象属性name对应的值
   

 public Object getValue(String name)
    {
        PropertyTokenizer prop = new PropertyTokenizer(name);
        if(prop.hasNext())
        {
            MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
            if(metaValue == SystemMetaObject.NULL_META_OBJECT)
                return null;
            else
                return metaValue.getValue(prop.getChildren());
        } else
        {
            return objectWrapper.get(prop);
        }
    }

//BeanWrapper

public Object get(PropertyTokenizer prop)
    {
        if(prop.getIndex() != null)
        {
            Object collection = resolveCollection(prop, object);
            return getCollectionValue(prop, collection);
        } else
        {
	    //获取bean对应的属性
            return getBeanProperty(prop, object);
        }
    }

通过MetaClass获取属性对应的get方法，调用相应的方法

private Object getBeanProperty(PropertyTokenizer prop, Object object)
    {
        Invoker method = metaClass.getGetInvoker(prop.getName());
        return method.invoke(object, NO_ARGUMENTS);
        throw ExceptionUtil.unwrapThrowable(t);
        throw new ReflectionException((new StringBuilder()).append("Could not get property '").append(prop.getName()).append("' from ").append(object.getClass()).append(".  Cause: ").append(t.toString()).toString(), t);
    }

//BaseWrapper

protected Object getCollectionValue(PropertyTokenizer prop, Object collection)
    {
        if(collection instanceof Map)
            return ((Map)collection).get(prop.getIndex());
        int i = Integer.parseInt(prop.getIndex());
        if(collection instanceof List)
            return ((List)collection).get(i);
        if(collection instanceof Object[])
            return ((Object[])(Object[])collection)[i];
        if(collection instanceof char[])
            return Character.valueOf(((char[])(char[])collection)[i]);
        if(collection instanceof boolean[])
            return Boolean.valueOf(((boolean[])(boolean[])collection)[i]);
        if(collection instanceof byte[])
            return Byte.valueOf(((byte[])(byte[])collection)[i]);
        if(collection instanceof double[])
            return Double.valueOf(((double[])(double[])collection)[i]);
        if(collection instanceof float[])
            return Float.valueOf(((float[])(float[])collection)[i]);
        if(collection instanceof int[])
            return Integer.valueOf(((int[])(int[])collection)[i]);
        if(collection instanceof long[])
            return Long.valueOf(((long[])(long[])collection)[i]);
        if(collection instanceof short[])
            return Short.valueOf(((short[])(short[])collection)[i]);
        else
            throw new ReflectionException((new StringBuilder()).append("The '").append(prop.getName()).append("' property of ").append(collection).append(" is not a List or Array.").toString());
    }

// MetaClass的获取属性的Get方法，委托给Reflector
 

public Invoker getGetInvoker(String name)
    {
        return reflector.getGetInvoker(name);
    }

再看Reflector
从get方法Map中，获取对应的Invoker

public Invoker getGetInvoker(String propertyName)
    {
        Invoker method = (Invoker)getMethods.get(propertyName);
        if(method == null)
            throw new ReflectionException((new StringBuilder()).append("There is no getter for property named '").append(propertyName).append("' in '").append(type).append("'").toString());
        else
            return method;
    }

回到MetaObject，再看给对象属性name设置value值
   

public void setValue(String name, Object value)
    {
        PropertyTokenizer prop = new PropertyTokenizer(name);
        if(prop.hasNext())
        {
            MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
            if(metaValue == SystemMetaObject.NULL_META_OBJECT)
            {
                if(value == null && prop.getChildren() != null)
                    return;
                metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
            }
            metaValue.setValue(prop.getChildren(), value);
        } else
        {
            objectWrapper.set(prop, value);
        }
    }

//BeanWrapper
   

public void set(PropertyTokenizer prop, Object value)
    {
        if(prop.getIndex() != null)
        {
            Object collection = resolveCollection(prop, object);
            setCollectionValue(prop, collection, value);
        } else
        {
	    //设置bean属性
            setBeanProperty(prop, object, value);
        }
    }

    //获取bean属性对应的set方法，并设置
      

private void setBeanProperty(PropertyTokenizer prop, Object object, Object value)
    {
        try
        {
            Invoker method = metaClass.getSetInvoker(prop.getName());
            Object params[] = {
                value
            };
            try
            {
                method.invoke(object, params);
            }
           
        }
    }

// MetaClass，获取set方法，委托给Reflector
  

 public Invoker getSetInvoker(String name)
    {
        return reflector.getSetInvoker(name);
    }
再看Reflector，从setMethods方法Map中，获取Invoker
public Invoker getSetInvoker(String propertyName)
    {
        Invoker method = (Invoker)setMethods.get(propertyName);
        if(method == null)
            throw new ReflectionException((new StringBuilder()).append("There is no setter for property named '").append(propertyName).append("' in '").append(type).append("'").toString());
        else
            return method;
    }

总结：
从上面可以看出，MetaObject是用于包装Object的，将Object包装成ObjectWrapper，MapWrapper，CollectionWrapper，BeanWrapper等，ObjectWrapper是对应属性及方法的一个包装类，MetaObject获取obejct的属性值，以及给属性设置，是委托给
ObjectWrapper，ObjectWrapper在委托给MetaClass，MetaClass最后交Reflector；Reflector包含Obejct的属性名，set方法，以及get方法的描述，MetaObject获取属性值和设置属性值方法，本质上是，从Reflector获取属性对应方法的Invoker，然后invoke。下面一节，我们讲Reflector。


//PropertyTokenizer

public class PropertyTokenizer
    implements Iterable, Iterator
{
   
    private String name;
    private String indexedName;
    private String index;
    private String children;
    public PropertyTokenizer(String fullname)
    {
        int delim = fullname.indexOf('.');
        if(delim > -1)
        {
            name = fullname.substring(0, delim);
            children = fullname.substring(delim + 1);
        } else
        {
            name = fullname;
            children = null;
        }
        indexedName = name;
        delim = name.indexOf('[');
        if(delim > -1)
        {
            index = name.substring(delim + 1, name.length() - 1);
            name = name.substring(0, delim);
        }
    }
    public String getName()
    {
        return name;
    }
    public String getIndex()
    {
        return index;
    }
    public String getIndexedName()
    {
        return indexedName;
    }
    public String getChildren()
    {
        return children;
    }
    public boolean hasNext()
    {
        return children != null;
    }
    public PropertyTokenizer next()
    {
        return new PropertyTokenizer(children);
    }
    public void remove()
    {
        throw new UnsupportedOperationException("Remove is not supported, as it has no meaning in the context of properties.");
    }

    public Iterator iterator()
    {
        return this;
    }
    public volatile Object next()
    {
        return next();
    }

}