Mybatis3源码分析(12)-Sql解析执行-MetaObject

MetaObject

MetaObject类相当于一个工具类，Mybatis在sql参数设置和结果集映射里经常使用到这个对象。下面来详细分析一下这类。

这个类有四个属性，其中两个基本不用看。

//原始的对象
private Object originalObject;
//对原始对象的一个包装
private ObjectWrapper objectWrapper;

//这两个属性基本不用，因为在Mybatis中都找不到ObjectWrapperFactory的有效实现类
private ObjectFactory objectFactory;
private ObjectWrapperFactory objectWrapperFactory;

再看他的方法

//构造方法
private MetaObject(Object object, ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory) {
this.originalObject = object;
this.objectFactory = objectFactory;
this.objectWrapperFactory = objectWrapperFactory;

if (object instanceof ObjectWrapper) {
this.objectWrapper = (ObjectWrapper) object;
} else if (objectWrapperFactory.hasWrapperFor(object)) {
this.objectWrapper = objectWrapperFactory.getWrapperFor(this, object);
} else if (object instanceof Map) {
this.objectWrapper = new MapWrapper(this, (Map) object);
} else if (object instanceof Collection) {
this.objectWrapper = new CollectionWrapper(this, (Collection) object);
} else {
this.objectWrapper = new BeanWrapper(this, object);
}
}

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

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

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

public Class<?> getSetterType(String name) {
return objectWrapper.getSetterType(name);
}

public Class<?> getGetterType(String name) {
return objectWrapper.getGetterType(name);
}

public boolean hasSetter(String name) {
return objectWrapper.hasSetter(name);
}

public boolean hasGetter(String name) {
return objectWrapper.hasGetter(name);
}

//从originalObject获取属性值
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);
}
}

//设置originalObject属性值
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; // don't instantiate child path if value is null
} else {
metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
}
}
metaValue.setValue(prop.getChildren(), value);
} else {
objectWrapper.set(prop, value);
}
}

//应该是对collection的操作
public void add(Object element) {
objectWrapper.add(element);
}

//应该是对collection的操作
public <e> void addAll(List<e> list) {
objectWrapper.addAll(list);
}

从上面代码中可以看出MetaObject主要是封装了originalObject对象，提供了get和set的方法用于获取和设置originalObject的属性值。其中originalObject最主要的有三种类型:

Map类型
Collection类型
普通的java对象，有get和set方法的对象
getValue和setValue中的name参数支持复杂的属性访问：例如user.cust.custId,user.acts[0].acctId！

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 {
//这里相当于递归调用，直到最后一层。例如user.cust.custId
//第一次递归cust.custId
//第二次递归custId，这个就是真正访问要返回的
return metaValue.getValue(prop.getChildren());
}
} else {
return objectWrapper.get(prop);
}
}

getValue,setValue,add,addAll方法都是委托objectWrapper对象实现的。下面详细分析objectWrapper对象。

MapWrapper

public Object get(PropertyTokenizer prop) {
if (prop.getIndex() != null) {//accts[0]这种方式
Object collection = resolveCollection(prop, map);
return getCollectionValue(prop, collection);
} else {//userId这种方式
return map.get(prop.getName());
}
}

public void set(PropertyTokenizer prop, Object value) {
if (prop.getIndex() != null) {<span style="font-family: Arial, Helvetica, sans-serif; font-size: 12px;">//accts[0]这种方式</span>
Object collection = resolveCollection(prop, map);
setCollectionValue(prop, collection, value);
} else {//userId这种方式
map.put(prop.getName(), value);
}
}

集合类的不支持

public void add(Object element) {
throw new UnsupportedOperationException();
}

public <E> void addAll(List<E> element) {
throw new UnsupportedOperationException();
}

CollectionWrapper

最支持如下两个方法，其他不支持

public void add(Object element) {
object.add(element);
}

public <E> void addAll(List<E> element) {
object.addAll(element);
}

BeanWrapper

public Object get(PropertyTokenizer prop) {
if (prop.getIndex() != null) {//accts[0]方式
Object collection = resolveCollection(prop, object);
return getCollectionValue(prop, collection);
} else {//userId方式,反射
return getBeanProperty(prop, object);
}
}

public void set(PropertyTokenizer prop, Object value) {
if (prop.getIndex() != null) {//accts[0]方式
Object collection = resolveCollection(prop, object);
setCollectionValue(prop, collection, value);
} else {//userId方法，反射
setBeanProperty(prop, object, value);
}
}

集合类的不支持

public void add(Object element) {
throw new UnsupportedOperationException();
}

public <E> void addAll(List<E> list) {
throw new UnsupportedOperationException();
}

小结

MetaObject是一个工具类，提供了类似ognl和jstl这样的方式去访问map,collection及javabean。
这里举三个例子来总结MetaObject最主要的功能

对Map操作

<span style="white-space:pre">		</span>Map<String,Object> map=new HashMap<>();
MetaObject metaObject=MetaObject.forObject(map, objectFactory, objectWrapperFactory);

metaObject.setValue("UESR_ID", "123412");
//相当于执行了map.put("USER_ID","123412");

Object obj=metaObject.getValue("UESR_
4000
ID");
//相当于执行了map.get("UESR_ID");

对JavaBean操作

<span style="white-space:pre">		</span>User user=new User();
MetaObject metaObject=MetaObject.forObject(user, objectFactory, objectWrapperFactory);

metaObject.setValue("userId", "123412");
//相当于执行了user.setUserId("123412");

Object obj=metaObject.getValue("userId");
//相当于执行了user.getUserId

对Collection操作

<span style="white-space:pre">	</span>List<Object> list=new ArrayList<>();
MetaObject metaObject=MetaObject.forObject(list, objectFactory, objectWrapperFactory);

metaObject.add("ashan");
//相当于执行了list.add("ashan");

metaObject.getValue("[0]");
//相当于执行了list.get(0);